View Full Version : Timing/Dieseling problems on my sbc

08-26-2009, 02:46 PM
To start i have a small block chevy 355 with vortec heads-around 440HP and torque. The motor was dynoed and given a set of recommendations. I'm having tons of problems as far as throttle response, idling, and just the feeling of not having the power.
Everything is brand new- Holley 770 Street avenger with stock jets, HEI dizzy with the 65000 volt coil ontop. Brand new spark plugs(champion RS12YC), new mallory 8mm plugs.

The motor calls for 32 degrees of total timing. The idle is set at an average of 12-14 degrees. The timing is completely set, but when i turn off the ignition the motor will diesel and keep turning over after i turn the key off. Which in turn is a sign of in proper timing. The idle mixture screws were adjusted with vacuum gauge. There are lighter springs in the dizzy to make the advance kick in quicker.

So my question is, why is it still dieseling and lacking all this power. I should be able to roast my tires and feel like i have 440hp. I have no low end torque and when i snap the throttle while driving there will be a slight delay. I run 93 octane gas and it passes through 2 fuel filters. Are my jets too large and need to be smaller? And this motor has less than a 1000 miles on it. Its brand new
Here are pictures of my #1 spark plug that i just replaced 2 days ago.

Here is the number 2 plug that has been in the motor for about a month

08-26-2009, 02:57 PM
"The timing is completely set, but when i turn off the ignition the motor will diesel and keep turning over after i turn the key off. Which in turn is a sign of in proper timing."

Don't know where you got that idea. Dieseling is normally a symptom of too much initial advance. Also, by looking at the plugs you're running rich as hell.

08-26-2009, 03:03 PM
i read that dieseling is the result of the timing being off. But i have it set dead on 32 degrees. So i need to back the timing off?

08-26-2009, 03:08 PM
You mentioned the motor was dynoed...did it run okay on the dyno? If it did, we can narrow your problem down to what changed between now and then. As for the plugs, the #1 porcelain looks white and the strap isn't discolored. The mixture is probably close in that cylinder. The soot on the #2 threads could just be from the plugs not being tight enough or it could be rich. Its hard to tell from the photos. The better indicator of lean/rich is the part of the porcelain buried down deep in the plug. It is hard to see without a good light...or cutting the threaded portion of the plug away. You want a tan-colored ring down near the base of the porcelain.

08-26-2009, 03:11 PM
yeah it looks like your running of the main metering circuit at an idle=your throttle blades are open too far and your bypassing the idle circuit.Now this does have a direct relation to timing and your current dizzy setup,I would put one medium spring and one light to get your total advance to come in at 3400-3500,go back and set you initial at 18*,now here is how they are all tied in the more you advance you initial timing the higher the idle speed therefore letting you turn back your idle speed screw(not the mixture screws)and closing throttle blades enough to start using the idle circuit.try this and tell us what happens

08-26-2009, 03:12 PM
I would start with retarding the timing.

But also making sure that you are not off a tooth, I'm going to assume your aware of the old school bring to TDC on #1 and making sure the rotor and #1 plug wire on cap are aligned procedure.

The hesitation could aslo be a carb issue, mainly the accelerator pump being adjusted wrong.

What size is your cam? Compression ratio? Do you have any clue as to the size of your jets you have now?

I'm running a 67(I believe) jet in my 625 Demon, on my 383, with aluminum heads and a .500 lift cam...

Those plugs do say your running pretty rich, that 770 may be too much.

08-26-2009, 03:13 PM
Its a crate motor built by yearone, they send a spec sheet and dyno results. So im hoping it ran good on the dyno lol. The #1 plug was changed 2 days ago and #2 has been in there for a month. At certain times when idling it smells like fuel pretty bad. So i might need to de jet the carb., buy all new plugs and go from there. But still want to see if i can solve the problem before going to that next step. Thanks for all the help guys-If need be, i will try and get a better picture of that #1 plug

08-26-2009, 03:14 PM
Did a quick search. Here was another guy having your same problem with an Avenger carb...

08-26-2009, 03:17 PM
I would start with retarding the timing.

But also making sure that you are not off a tooth, I'm going to assume your aware of the old school bring to TDC on #1 and making sure the rotor and #1 plug wire on cap are aligned procedure.

The hesitation could aslo be a carb issue, mainly the accelerator pump being adjusted wrong.

What size is your cam? Compression ratio? Do you have any clue as to the size of your jets you have now?

I'm running a 67(I believe) jet in my 625 Demon, on my 383, with aluminum heads and a .500 lift cam...

Those plugs do say your running pretty rich, that 770 may be too much.

Yeah i did it the old fashioned way. Set to TDC and drop dizzy in untill its aligned with #1.
The cam is a 520 lift, 218/228 duration
9:5:1 compression
Jet size i have no idea yet. I will need to pull the bowls and look

08-26-2009, 03:23 PM
Did a quick search. Here was another guy having your same problem with an Avenger carb...

My RPMS do drop quite a bit from park to drive, but i like the idle at around 800when in drive. So it is back to the timing issue thats causing it. But i have many other problems aswell

08-26-2009, 03:49 PM
It seems that too much vacuum advance (springs too light) and the throttle blade issue were the keys (primaries open too far to maintain a decent idle) for that guy.

You could always contact year one for some help...

08-26-2009, 05:14 PM
If you had a REAL cam, that carb would work better, you have too much carb for one, meaning as much as you need to lean it out might not completely solve your problem, you need about a 650. That cam is tiny, not much bigger than some stockers.

See what jets you have and if you have a jet kit, go down 2 jettings at a time and see if it helps. From there you should also start out by screwing the idle mixture screws in all the way and then pull out by 1 and a half to start. As mentioned, you need to make sure timing is in effect with the throttle blades being as close to shut as possible, that tame cam should be able to idle about 5-600 RPM no problem. You might need to lower the squirter size as well.

Like i said, it is allot of carb for a 350 with such a mild cam, you need to really detune that thing to get it right.

08-26-2009, 10:12 PM
One day ill beef this motor up :)

So i need to possibly make sure the timing is correct. Turn idle screws 1 1/2 turn, de jet the carb and throw some heavier springs on the initial side? What do you guys mean the throttle blades as close as they can be to being shut? So your saying look down the carb while its at full advance and see how far open the butterflies are?

08-26-2009, 10:17 PM
okay, my jet sizes are primary-72 and secondary-75

08-27-2009, 01:44 AM
ok here ya go the throttle blades at an idle are what we are talking about so here are some pics to help you out
pic#1 throttle blades completely shut idle speed screw backed all the way out the car will not run like this.
pic#2 blades open part throttle notice the long skinny slot on the venturi wall? that is you idle transfer slot the next pics will deal with this
pic#3 are the throttle blades at the preferred setting at an idle notice the the slot is showing about the same length as it is in width(it looks square)now do you have to pull the carb off the set this? no this is just showing you where the carb should have been baselined before it was installed and adjusted from there.Now you can have too much of this slot showing and it will bypass the idle circuit on the carb and run off the primary metering circuit this is not a good thing it will run way rich and have slugish response from the carb even stumbling and falling on it's face when you wack the throttle under load. there is a direct relationship between initial timing and how your idle circuit works on your the one thing you haven't told us is do you have a vacuum advance on your dizzy if so explain how you go about setting your initial timing.edit i just realized you carb choice and as stated above it is way too big a 650 would of been a much better choice,so year one sold this crate to you carb to pan or was it just a long block?

08-27-2009, 02:39 AM
nice pics, that explains a lot. I can easily take my carb off if i need too.

Long block. The only thing i needed to add was carb, fuel pump, front accessories.

HEI dizzy was vacuum adv., i have the old fashioned timing light, so this is being done with a timing tape.

08-27-2009, 02:41 AM
But they recommend a holley 750 double pumper? I have the same motor with a basic holley 750, not a street avenger, and it runs like a damn beast!! Different carb which means different settings, but i still would know when this motor is running good because it moves a 4000 pound camaro verrrrry easily with 18inch wide wheels.

08-27-2009, 02:49 AM
so your just running the mechanical advance or both?if your running the vac. you are unplugging the vac advance to set your initial?if so what does your timing jump to when you plug it back in?

08-27-2009, 02:57 AM
nono, when setting the initial its with it hooked up, then when setting total its plugged.
But i havent set the initial yet completely because to be honest i didn't know where to start and set it at. So the total timing is set

08-27-2009, 03:19 AM
just making sure I know alot of guys that set it backwards.ah so there is part of you issue set your initial first then the advance as far as sbc's go i like 18* at an idle and nothing over 36 total,now is your vac advance adjustable I.E small set screw in the module itself to control the amount it advances?

08-27-2009, 07:06 AM
If you turn the key off, there is no power going to the distributor. How can timing make the motor diesel if there is no spark going to the plugs? Usually carbon build up is the culprit. To much compression. That's how diesel's work, by compression.

08-27-2009, 04:06 PM
incorrect timing is what causes dieseling in chevys perspective.

08-27-2009, 04:07 PM
just making sure I know alot of guys that set it backwards.ah so there is part of you issue set your initial first then the advance as far as sbc's go i like 18* at an idle and nothing over 36 total,now is your vac advance adjustable I.E small set screw in the module itself to control the amount it advances?

That i am unsure of. I will check when i get home from school. It should be like an allen right?

08-27-2009, 04:14 PM
yup most of them you unplug the vac. line and stick the allen in the port to adjust

08-27-2009, 06:45 PM
i have 2 66 jets from another holley carb. Is that too small?

08-27-2009, 07:40 PM
And just to clear things up, initial timing is the vac adv. hooked up and is checked at idle in park. Ive been reading that it should be around 15-18 on a mild SBC. Now if i check total timing and set it at 32 degrees and the initial is too low then i need a heavier spring on that side?
When im all said and done im going to buy all new plugs because i might have fouled these out too much to get any readings

08-27-2009, 08:27 PM
springs will not change the total advance only adjusting to vac advance will,the springs set when the mechanical kicks in,you should have full advance at 3400-3600rpm,and yes you will go though alot of plugs jetting the crab.Right now the thing to do is set the initial at 16-18 then run the rpms up to see how much total advance your dizzy is giving you.

08-27-2009, 10:03 PM
ill throw a timing light on it tomorrow. thanks for your help

08-28-2009, 01:06 AM
the cause of your dieseling is most likely carbon buildup in the combustion chamber as somewhat indicated by the spark plugs, along with too rich an idle mixture setting. That combined with the extra timing advance at idle is causing extra heat in the combustion chamber, and if you have a fast idle like > 900 when you shut down will further increase the likelihood of dieseling. If you're going to run more timing than the standard 8-10 degrees at idle via vacuum advance, which is okay, you have to lean out the idle mixture.

regarding no low end performance,
first check the fuel level in the bowls and make sure it's at the bottom of the sight plug when car is on level ground. Stick with the stock jets if you are at sea level, if you're at 1000' or greater then consider going lower on the main jets. Once you get the dieseling problem corrected and the idle mixture correct, then there are systematic steps to take to tune the carb properly, regarding the accelerator pump cam, vacuum secondary springs, and power valve sizes.

And to set the record straight on setting timing, if you didn't read the above links,
you disconnect the vacuum advance and have the motor less than 800 rpm for idle. This assures the rpm is low enough so there is no mechanical advance happening, and there is no vacuum advance happening because it's disconnected.
So now you rotate the distributor and get 8 to 10 degrees BTDC with your timing light. In your case set it to 8.
Now what will happen is the distributor's mechanical advance will give you 24 degrees more advance, this happens when the rpms are high enough when the weights in the distributor overcome the springs holding them. The weights fly outward under centrifugal force and allow the distributor plate inside to move 24 degrees in the direction opposite the rotor spins, giving advance. base timing of 8 deg. plus 24 deg mechanical advance = traditional 32 deg total timing. If you set base timing at 10, then that gives you 34 degrees total timing at rpm. Typically you play with the base timing between 8 - 10, and typically never more than 6 - 12, to adjust what the total advance would be, depending on what cylinder heads and cam your running, some like more advance, some don't. This is all with no vacuum advance on the distributor. Once timing is set, then you hook up the vacuum line from the distributor to the carb. You can't set timing with the vacuum advance hooked up because any fluctuation in vacuum at idle, and there is, will adjust the timing as you are trying to adjust it. Then as you adjust timing, that will alter the vacuum signal being provided to the distributor. And vacuum advance works on the principle of vacuum, at low rpm or high rpm/light load there is high vacuum which gives you more timing advance which increases fuel economy. At full throttle or any rpm high load there is no or less vacuum, therefore no or much less vacuum advance on the distributor. For this reason, the vacuum advance should never give you more timing than the base timing setting plus the amount of mechanical advance the distributor gives.

08-28-2009, 02:41 AM
wow thank you, i read your post and ill look over those sites thoroughly. When in park the idle is at 1100-1200 and then in drive it jumps down to 750-800. Which should not happen.
I'm going to set the timing tomorrow and let you know how fast total kicks in etc..

I have the vacuum gauge that hooks up to the carb. Do i set the idle mixture screws with it in park idling or while in drive idling?

08-28-2009, 01:11 PM
Having the idle at 1100 or better was probably a big factor causing the dieseling when you shut down.
first set the idle with car in park, out of gear. Adjust the throttle stop screw counterclockwise, lowering the idle down to 700-800. Disconnect the throttle cable if you have to, sometimes the throttle cable brackets aren't set right and will prevent the carb throttle from closing all the way- you want the throttle cable at idle to be pushing against the carb throttle closing it, ensuring idle.
Then check and make sure the fuel level is at the bottom of the sight glass on both bowls on the carb. Everything on the carb works off fuel level in the bowl, so you need to have that correct before you try to tune anything else otherwise you'll be wasting your time. Now adjust the idle mixture screws, do this by richening it by turning CCW until the idle drops and/or motor stumbles. Remember where this position is. Then turn mixture screw CW leaning it out, idle will raise up and sound good, but keep going CW leaning it till rpms drop and motor starts to stumble. remember this position. Now you know the point where max lean is, and max rich is, on the idle mixture screw. Turn it back out CCW to the middle position, halfway between those two points. Whether you start by going CCW (rich) first then CW (lean), or lean first then rich, does not matter.
Do this for both idle mix screws, one at a time. Both should start out at approximately 1-1/2 turns from seated, this is the factory setting and will allow any engine to start and run at fast idle.
if it is connected, then what will happen is what i described above, your mixture adjusting will affect the timing, which will then affect how the engine idles through adjusting the timing at idle, and it'll seem like nothing is happening when you're turning the idle mixture screws, that's because the distributor is compensating via timing for the change in mixture to keep the engine running... you'll find yourself having to turn the mixture screws a lot to get anything to happen. With vacuum advance disconnected, you should only have to turn the mixture screws no more than 1 turn in either direction to notice a change in idle.

Once you get the idle mix positions known at idle out of gear, then since you have an auto trans you can drop the car in gear with parking brake on, with vacuum advance disconnected, and readjust the idle mix screws with having a little bit of load on the engine. But you should find nearly the same setting, if anything you'll want it slightly richer like 1/8 to no more than 1/4 of a turn. Then reconnect vacuum advance, run the car on the street and can see where to tune from there.

I mention disconnecting the vacuum advance because that ensures you have no vacuum advance happening. If the vac. line from distributor is connected to the carb, then at low idle the distributor actually does not see vacuum because of the way the vacuum connection is on the carb, it is blocked by the throttle blades. But you mentioned the 1100 rpm idle along with 14 dBTDC timing so without seeing it first hand i'm not sure what's happening. So you want to be sure.
Only when you crack the throttle open does the ported vacuum connection on the carb see vacuum, providing vacuum advance at throttle positions above idle technically. Some people hook the vacuum advance up to full time manifold vacuum, and that's a whole other debate, and once you get your idlle set and dieseling cured, may prove useful in tuning low end performance. But for now, do one step at a time, tuning one variable at a time. You should know what you are tuning and why, and know what the expected outcome should be. You should NOT be tuning the carb under the premise of "let me turn this and see what happens", that is the wrong way to tune and unless you're really lucky will take you down the path of problems and poor performance.

And, the distributor vacuum advance is a bonus, not a necessity. What advance the distributor provides mechanically, based soley on rpm, is sufficient. I recommend disconnecting the vacuum advance and plugging off the ported vacuum nipple on the carb with a buna-n cap for the time being. That will remove one tuning variable. Get the car running good and strong without the vacuum advance, then connect it to gain fuel economy and adjust the vacuum advance canister as necessary.

couple more:

08-28-2009, 01:15 PM
Ok i was just outside trying to time it.
Vacuum plugged and idling at 800ish i got it to 9-10 degrees. I couldn't get a good look at the RPM gauge but full timing probably came in around 2000-2100 RPMS. It was reading 31-32 which is too high with initial. But if i wasn't trying to get the initial and mechanical together then the total timing would be set.

I was having a ton of problems with the idle. When i adjust the idle screw and snap the throttle the RPMS increase greatly unless i pull back on the throttle bracket. So the bracket is getting stuck.
So ill try and put it in different words. When i set the idle to 800 and snap the throttle, it doesn't return back to 800. It will jump to 1100-1000 RPMS unless i pull back on the bracket then it returns to 800. I squirted some wd40 solvent on the springs but its still doing it.

I also reset the idle mixture screws to 1 1/2 turn out.

The full timing is coming in too quick possibly and i need to put back the heavier springs. When i turned off the truck it didn't diesel or back fire so that's a good thing.

08-28-2009, 01:18 PM
I also forgot to mention. Now that i set the idle to around 800, when in drive its idling around 400 which is pretty low

08-28-2009, 02:09 PM
ok, good, sounds like progress.

the throttle backet setups always seem to be a problem, so don't sweat it for now. Just disconnect the throttle cable from the carb and work the carb by hand manually. The throttle on the carb should snap closed to idle on it's own, if it doesn't then you have a throttle spring or throttle shaft problem on the carb.

like is said, set the idle to 800 rpm out of gear, in park. Get timing at 8 degrees BTDC, with vacuum advance disconnected. Now, once timing is set, at 800 rpm, adjust the idle mix screws like i described above.
Then drop it in gear with brake on, idle should only drop 100-200 rpm. if it is still dropping a lot, first adjust both idle mix screws richer by 1/8 of a turn CCW at a time, to to more than 1/2 turn CCW. If that doesn't help raise the idle rpm in gear back to a normal 600-650 rpm, then try going a little bit leaner on the idle mix screws just to see if it helps. If adjusting the idle mixture doesn't help, then set the idle mix screws back to the initial 1-1/2 turns out, go back to park and adjust timing 1-2 degrees advance so now your at 10 dBTDC base timing, and repeat the in-gear check and mixture adjustment, no luck then advance again to 12 dBTDC and see.
Now once you get to 14-16 dBTDC base timing and still have no luck, go to the other side of the initial 8 dBTC, try 6 dBTDC and see. If all that doesn't work, only then would i consider the 770cfm avenger carb to big for your engine. The engine isn't producing a strong enough vacuum signal at that low of an rpm because of having too small a cubic inch engine compared to the high cfm carb- this is where/why you would want a 650cfm carb. Otherwise the tradeoff is, you can use the carb and reap the hp at high rpms but sacrifice idle quality, or idle altogether and have an idle > 1000 rpm.
Fortuntely, you have a smaller intake duration cam. i think you said 210/218 was the cam? this typically provides good vacuum because it closes the intake valve earlier on the upstroke and if the lobe separation angle (LSA) on it isn't low then you should be able to tune idle.

08-28-2009, 02:18 PM
Alright ill go give that a shot. I went to grab lunch and when i got home, threw it into park and it idled at 900-950 and it dieseled when i turned the ignition off...

08-30-2009, 12:42 AM
i bought 69 size jets today. Im going to give them a shot and i need to buy another timing tape because they all keep falling off......

09-17-2009, 04:02 PM
Well i know its been a while since ive updated this. My truck got into a minor accident and ive been working on that for the past month. The jets got installed and it seems to be running a tad bit better. I don't smell as much gas fumes coming out of the exhaust. I check the total timing today and its almost dead on at 32-33 degrees. The initial im unsure of because my 6th timing tape came off the other day. I am now at 1 1/2 turns out on the idle mixtures. When the motor is idling less than 900 in park and i turn it off it never diesels. Now because of my shitty stall in my tranny, i have to crank the idle up because in drive it idles around 400 and that isn't good. I took another picture of the plug that i originally showed you guys before. I will be buying a new set of plugs this week.

82 camaro
09-20-2009, 10:05 AM
Most the time the motor will diesel due to the throttle blades being opened to far to make the car idle. I would set initial timing with the vac adv unplugged from the dist. at around 15 deg then total timing with vacuum line unplugged also at 30-32 deg Now hook up the vacuum adv line and set the carb mixture screw with the car in gear use your vacuum gauge. By setting your initial timing with the vacuum line hooked to the dist. when you put the car in gear you will loose your vacuum and the car does not want to run in gear also when you step on the gas you have no vacuum adv and your timing is I am guessing around 4-5 deg adv this pulls a lot of power from the motor.

09-20-2009, 07:55 PM
alright ill try that out tomorrow. thanks

Lonnies Performance
09-27-2009, 11:52 AM
The throttle plates too far open primarily causes this.

As mentioned above, spark timing does not cause dieseling, because when the key is off, there is no spark (you cannnot create a spark without power to the coil) & therefore no ignition timing. That is why it is caulled dieseling, because diesels do not need spark plugs to run......

Actually at times more initial advance will increase the idle speed & allow you to close the throttle plates to get the idle back down. This will choke off the engine somewhat & can stop dieseling.

09-28-2009, 12:21 AM
So more initial can be achieved by lightning up on the springs or making them tighter on what side of the distributor? Because right now the total timing is where its suppose to be

Lonnies Performance
09-28-2009, 02:20 PM
Springs are the centrifugal advance.

Initial is done by rotating the distributor.

After the best initial is determined, you need to limit the total advance by the advance weight shape or with the limiting slot/bushing in the distriutor.

09-28-2009, 04:54 PM
Rotate the distributor to where, how much initial is best initial? Turn it until the motor starts chocking then let off and move back? That would be the best initial, then look at total timing and see where its at. Adjust by putting different bushings? untill total timing is achieved. Then adjust the idle back down because the initial is so high, then adjust the idle mixture screws?

09-29-2009, 09:40 AM
First, you need a timing light with a dial (unless you have a damper with all of the degrees marked). Put the car in gear (with someone on the brakes!) and set your idle to 700-800RPM. With that mild cam, it can probably go even lower. Unhook your vacuum advance line. Set your timing light dial to 12 degrees. Check it at the damper. If the zero line on the damper and the zero line on the pointer don't line up, you need to loosen your hold-down for the distributer and turn it until the lines do line up. After you adjust, you may need to reset your idle (if you had to advance the timing at all, the idle RPM probably went up). Now you are at 12 degrees initial advance!

Now, put it in park and see how much the idle changes (it shouldn't be much...maybe 200 RPM). Check the timing. It should stll be at 12 degrees BTDC. Bring the RPMs up to 1500 and check the timing and then again at 2000 RPM. This will tell you if your mechhanical advance is coming on too quickly (vacuum line still disconnected!). With that mild cam, you don't want your full mechanical to come on untill 3000-3500 RPM. If it is all in (about 32 degrees) by 2000RPM, change your weights and springs in the distributer to slow down the mechanical advance.

Vacuum advance is another beast. Get this stuff sorted out first and then we can discuss vacuum advance.

09-29-2009, 11:01 AM
I havent read through all the pages but from the 1st page I know that your running rich. Back both sides of the carb down 2 jet sizes to start out with.

Also, you are running 93 octane, correct? You better be.


And set the idle to around 700-750. 800 is too high.

You want the easy way to make it stop dieseling when you shut it off? Cut it off while its still in gear.

09-29-2009, 04:18 PM
I'm going to go purchase one of those dial lights. Im fucking tired of this timing tape bs. Yes i do run 93 octane and i went down to 69 size jets already. So i went down maybe 4 sizes. I put a fresh set of plugs in yesterday and i will see if its still running rich with the new plugs. When i re time the motor when i get my new light, im going to throw another set of plugs in again.

I really do appreciate all the help you guys are giving

82 camaro
10-18-2009, 09:55 AM
Did you fix it?

10-18-2009, 10:55 AM
no not yet, my motor broke a pressed in rocker arm stud, so the whole top end is off the motor. I bought a nice dial back timing light too
should all be fixed by next week

11-03-2009, 11:18 PM
I see lots of conflicting answers in this thread, but some of it is good/correct info. Now lets try to sort out some of the good and bad here.

First, what's the max rpm you're spinning this motor too? I'm guessing you could get by with a smaller carb (650-670) with that 355.

Second, on the timing issue, all the guys saying to disconnect your vac advance prior to setting initial timing are 100% correct. You absolutely must do this. If initial timing is set with vac advance connected to FULL manifold vacuum, you will lose most of that timing as soon as you jump on the throttle hard from a dead stop (causing low/no vacuum condition), causing your dead spot at initial acceleration due to super low timing.

Many vac advance canisters will not deliver full advance without at least 15" of vacuum, some even 18". You can buy other canisters that will allow full advance at lower vacuum readings to suit your cam (I'd guess your idle vacuum is around ~14"?), but until you have the right canister, I would NOT recommend using a full manifold vacuum source for your distrubitor. Now, before some of you guys get all excited and yell at me (yes, I know this can work well in some setups), the last thing you want for idle quality is constant timing fluctuation at idle.... and that is exactly what you'll get unless your advance canister gives full advance by whatever your normal vac reading is at idle (both in and out of gear). This is likely the cause for his wild drop in idle speed from park to drive, because in park/neutral there is more vacuum, allowing more of the advance to kick in and provide for a higher idle speed. Then, when shifting into gear, vacuum drops, advance drops, and idle speed drops. This is because the advance canister is not providing full and/or consistant advance at ALL vacuum readings seen at idle (both in and out of gear). All of these issues are due to not having proper initial advance set withOUT any input from vac advance!

So for the time being, use a ported (above the throttle plates) vacuum source for your advance (this will keep the vac advance shut off at idle, but active at high vacuum, low rpm cruise).....but not until AFTER setting your initial advance. Keep the vac advance totally disconnected while setting your initial timing. Somewhere between 10-14 degrees initial (with NO vac advance input) will likey be perfect for your setup.

Your motor should idle fine at 700-800rpm with that cam and 10-14 degrees of initial (no vac input) advance.

Once all that is sorted out, we can look at your total timing advance for proper rpm and number. And once that is correct, if you are still unhappy with idle quality or engine temps, you can look at changing your advance canister to one that will allow full and consistant advance at your idle vacuum reading.

Next up will be tuning the carb for your setup. I'd stick with a vacuum secondary.

Hopefully, by getting your timing issues sorted out, you've been able to restore the carb to more reasonable settings. Idle mixture screws will likely need to be right around the factory settings of 1-1/2 turns out from fully seated (this is where I'd start, then adjust in 1/8 or 1/4 increments, CW will lean, CCW will richen). You're tuning for best vacuum reading here, and it's best done in gear. If you're still fouling plugs and not able to notice any changes in idle speed or vacuum by adjusting these screws, then your throttle blades may still be open too far, thus exposing too much of the idle transfer slot (more than .040" is going to cause a problem). You want the blades to be as close to seated as possibile. You have a couple of options for drawing more air at idle without opening the primary blades too far. First, you can try opening the rear blades a bit (you'll probably have to remove the carb to get at the adjustment screw), and close the primaries to compensate. If you've got both the front and rear blades open as far as possibile without exposing either of the transfer slots and the idle speed is still too low, then you can drill small holes in the throttle blades. This will allow air to flow even when the blades are shut. Start with very small holes, like 3/32" or 1/8", and drill them in the primary throttle plates in the middle of the blade section between the idle transfer slot side of the throttle bore and the throttle shaft. You should now be able to close the blades more, correcting the relationship between the blades and the transfer slot.

Once your idle circuit is set, if you still have problems with hard acceleration from a dead stop you can look at the accelerator pump circuit. First thing to check is that there is zero lash between the pump arm and the adjustment screw. You want them to be in constant contact, with no noticeable up-and-down play at all, but not so tight that you can't move the arm (on the pump side) side-to-side. This is an important adjustment, because you don't want it too loose (poor performance), or too tight (damage may occur to the pump diaphram, throttle could get jammed open, or pump may be too far into it's travel to deliver a full shot leading to poor performance).

Once you're sure the adjustment is correct, you need to determine if whatever hard acceleration issues you're having are due to a lean or a rich condition. If lean, you'll likely get a backfire, or just a studder/stumble right at the exact moment you hit the gas from an idle or very light throttle cruise. If rich, it'll probably just feel lazy, or maybe bog a moment after hitting the gas. Either way, you'll have to look at changing shot nozzle size (bigger number if you're too lean, smaller number if you're too rich) and pump cam selection. There are dozens of combinations you can use here, with Holley shot nozzles ranging in size from something like #19 or #21 all the way to #52, then you have (I beleive) 9 different pump cams for the standard 30cc pump, and 2 more for the 50cc pump. If you find that the biggest nozzles and biggest cams still leave you with a lean stumble, you may find the 50cc pump kit conversion helpful. You'll likely need a 1/2" spacer plate to get the larger pump to clear your intake though.

Once all of that is sorted out, you can start looking at things like power valve selection and main jetting. These two go hand-in-hand, IMO, and you really need a vacuum gage mounted in the vehicle to properly tune these circuits for best vacuum and best MPG/driveability/power.

Hope that helps.

11-04-2009, 11:51 AM
Just reading thru all the posts, and something got me wondering something.

What are you setting the gaps on your plugs??