Timing/Dieseling problems on my sbc
09-27-2009, 11:21 PM
#41
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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
09-28-2009, 03:54 PM
#43
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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, 08:40 AM
#44
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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.
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, 10:01 AM
#45
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.
EDIT:
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.
Also, you are running 93 octane, correct? You better be.
EDIT:
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, 03:18 PM
#46
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I'm going to go purchase one of those dial lights. Im ******* 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
I really do appreciate all the help you guys are giving
10-18-2009, 09:55 AM
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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
should all be fixed by next week
11-03-2009, 10:18 PM
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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.
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.
Last edited by RPM WS6; 11-03-2009 at 10:29 PM.
