I have a sbc 400. some times when I shut it off it kicks backwards and spits up. Is it too far advanced or too retarded? I am working on taking this car off the strip back to the streets. I feel the timing is going to need a good going over to get things right!! ain:
slow down the idle speed as much as possible and readjust the idle mixture.. you will have to go back and forth a few times between idle speed and idle mixture to get it perfect.
you may also want.. to get the engine up to operating temps.. and ONLY if you have done it before and know how to do without destroying your motor... at 2500 to 3500 RPMs.. trickle some water thru the carb.. or find a small vacuum hose that has manifold vacuum and dip the end into a 8 or 10 ounce container of water.. so it sucks it in slowly.. while you keep the Revs up.. if you do it at too low a speed you will DESTROY YOUR MOTOR.. with a hydraulic lock where water fills a combustion chamber and the piston comes up and cannot compress the full chamber..
i have a friend whos car kicks back as he turns the key off sometimes.
his will only turn the crank about 1/2 a turn.. i blame the ignition module.. its keeping the coil primary grounded when the engine is not running. and as the key turns off. the primary winding collapses causing a spark.. .i have a feeling its could also be the ignition switch. but he has not let me do some diagnostic work ..
so it depends .. is your engine going several turns backwards or only a partial turn in either direction.
We found cars built during the early emissions era had a big problem with carbon build up related to the EGR system. This with a combination of a fast/rich idle may be the problem. We had a product at the GM dealership called top end cleaner that was used quite often. If after checking carb and ignition you still have a problem, I would find a GM dealer for this stuff. I think other companies make it now too. If you try the stuff follow the instructions, have a helper and do it outside. The exhaust will stink and smoke like crazy.
It's an MSD ignition with a three-stage retarder in the center stage there's a 4% retard module in it and I don't know how that system works ! I've been told that if you don't have a zero in those spots that that box can cause a lot of trouble, I have no idea what's going on with it but it runs hot. the motor does after probably about 4 or 5 miles. I've done just about everything to it, I got dual fans on it, A aluminum radiator, but I think it's all to do with the timing! I got to figure it out, if I could I'd take that retarded box out of line with the MSD ignition ! thanks for any help you can give me.
I did not get a chance to work on it. I need to clean off the fly wheel so the MSD timing tape will stick good. Then I'll check the timing on it aster finding TDC. I will keep you up dated when I can.:thumbup:
To do it properly, you need to limit your curve to about 16° of mechanical advance, set your initial at about 20°, and put the vacuum advance on a manifold vac port. This will give you a nice clean idle and you can close your throttle blades enough to stop the run-on. 99% of the time, run-on is caused by the throttle blades being open too far. By using a short advance, lots of initial and manifold vacuum your idle will be clean and throttle response will be crisp. I've run all my SBCs this way for over 40 years and, regardless of the cam or carb, they've all run extremely well.
Lets start with cam timing or make and part number, compression ratio, what heads, and what pistons, and squish/quench clearance which is the piston crown to deck distance and gasket thickness?????
If this was run on the strip I'd expect a large cam and a lot of advance to support that. Some of that might need to change for the street.
We have had this issue on some of the more wild builds where timing idle idle has to be pushed to achieve idle quality with a large cam. One solution is an idle kicker solenoid. Holley sells one for their carbs. They're sold for use with AC, but they also work well for applications where raised idle is needed for a large cam, but hot engine run on becomes an issue. The solenoid can be used to set running idle, then when power is removed(key turned off) the throttle closes, helping kill the motor quickly. I've had success with this.
Connecting manifold vacuum to the vac advance also helps in this regard by retarding timing when man vacuum goes away. I have had to add a delay valve in some cases though to prevent hesitation on throttle tip in when using man vacuum for vac advance.
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