Timing/Dieseling problems on my sbc
08-27-2009, 06:06 AM
#21
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, 03:07 PM
#23
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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, 06:40 PM
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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
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, 07:27 PM
#27
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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.
08-28-2009, 12:06 AM
#29
http://en.wikipedia.org/wiki/Dieseling
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.
http://www.corvette-restoration.com/.../Timing101.pdf
http://www.bronco.com/cms/node/31
http://www.corvette-restoration.com/...c_Adv_Spec.pdf
http://www.holley.com/data/Products/...99R10219-3.pdf
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.
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.
http://www.corvette-restoration.com/.../Timing101.pdf
http://www.bronco.com/cms/node/31
http://www.corvette-restoration.com/...c_Adv_Spec.pdf
http://www.holley.com/data/Products/...99R10219-3.pdf
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, 01:41 AM
#30
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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?
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, 12:11 PM
#31
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.
AND DO THIS WITH THE VACCUM ADVANCE DISCONNECTED!
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:
http://www.lbfun.com/warehouse/tech_..._explained.pdf
http://65corvette.nonethewiser.net/t...um_advance.pdf
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.
AND DO THIS WITH THE VACCUM ADVANCE DISCONNECTED!
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:
http://www.lbfun.com/warehouse/tech_..._explained.pdf
http://65corvette.nonethewiser.net/t...um_advance.pdf
Last edited by 1 FMF; 08-28-2009 at 12:50 PM.
08-28-2009, 12:15 PM
#32
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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.
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, 01:09 PM
#34
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.
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.
Last edited by 1 FMF; 08-28-2009 at 01:18 PM.
08-28-2009, 01:18 PM
#35
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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...
09-17-2009, 03:02 PM
#37
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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.
09-20-2009, 09:05 AM
#38
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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.
Last edited by 82 camaro; 09-20-2009 at 09:12 AM.
09-27-2009, 10:52 AM
#40
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.
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.
Last edited by Lonnies Performance; 09-27-2009 at 11:03 AM.