drdave69

The ZZ502 in my 1966 has been running bad (doesn't want to start or idle below 900 - 1000) for a while now and I have a couple of questions. I replaced the plugs, plug wires, HEI distributor cap/rotor cap and coil. It runs better but still doesn't want to idle below 900 - 1000 rpm. All of this started happening after I changed the exhaust from 3" to 2-1/2" (side pipes). I didn't notice an immediate adverse affect after the exhaust change but a couple of weeks later I filled the fuel tank. It started this condition after that. I checked the timing and it is OK. I pulled out one of the new plugs and I suspect it is running lean since it has also started running a little hotter at idle. Would bad fuel also cause this condition? If not I plan on having the carb rebuilt.

mjdart

Maybe the fuel fill up broke lose some old sludge from the fuel tank or lines.

claysmoker

You checked the timing and it's OK. What is the timing?

midyear

vacuum leak?? that would account for a high idle, and the lean mixture would make it run hotter. check the PCV hose, and or PB hose if you have power brakes. could be one of them has come loose or gone bad.

ILBMF

Vacuum leak or carb issue or possibly fuel.

Kerrmudgeon

Choking down a 502 a half inch of exhaust will have a little effect on performane, but it should run about the same under normal driving. I'd suspect the fuel, or fuel filter after you filled it up. You don't say if the plugs were a different colour when you checked?? What colour were they, and.....were they all the same?

drdave69

The engine has a fixed single timing pointer and from what I read is 8 degrees. At approximately 800 - 900 rpms the timing marks were aligned.

drdave69

It does have a cobbled together PCV hose (two hoses with one slipped inside the other). Will spraying carb cleaner into the join while running tell me anything?

drdave69

I only checked one plug (#1) and it was a little white. I was planning on running all of the fuel out of the tank and replacing the sending unit (fuel gauge doesn't work) and I will replace the fuel filter at that time.

Pop Chevy

I changed the exhaust from 3" to 2-1/2"
you put a cork up it's butt, time to retune !

63 340HP

Yes.

​​​​​​Chase and replace every vacuum hose and potential air leak into the intake tract.

The leak can be a vacuum hose opened when the exhaust was changed, or one left to burn on the exhaust and open over subsequent operation.



Good luck.

tbarb

If there is no vacuum leak and it's lean with the installation of the smaller exhaust it's entirely possible that the OXYGEN left after the burn combined with the camshaft overlap reversion is getting sucked backed in the cylinder causing a high idle even with the throttle blades closed.

drdave69

The only vacuum hoses are the line to the choke and PCV from the rocker cover to the air cleaner base. The vacuum advance is plugged.

DansYellow66

Plugging in the vacuum advance may help it's idle, although after plugging it in you will have to back the idle speed screw off as the idle will increase. Depending on what you have for a vacuum advance can you may end up with some other issues.

IMO, 8 degrees timing with no vacuum advance is completely inadequate for an engine like that. I would want to run at least 12 to 14 degrees initial timing w/o vacuum advance. With vacuum advance (limiting vacuum advance to 8 to 10 degrees crank timing) you could run 10 degrees initial timing OK, maybe even 8.

drdave69

I just looked up the install specs for a ZZ502 (although mine is 10 years old) and it states that the timing is 10 degrees and it specifically notes to NOT use the vacuum advance on these engines.


Would reinstalling the 3 inch side pipes be an easier way to determine if the exhaust is causing these issues?

drdave69

What is the remedy to this? It will not idle below 800 - 900. And, if I run the idle to 1000 - 1100 where it idles smoother it has run-on after turning it off (it seems to actually kick back before it shuts off).

cbernhardt

So did you clean the fuel system and try new gasoline as suggested earlier? My '68 L89 would display the exact symptoms if run on poor fuel.

Charles

tbarb

[QUOTE=drdave69;1593161883]What is the remedy to this? It will not idle below 800 - 900. And, if I run the idle to 1000 - 1100 where it idles smoother it has run-on after turning it off (it seems to actually kick back before it shuts off).[/QUOTE

I would think a good carburetor tune is in order, if you are having a hard time with it find someone that can help in your area.

First you need to eliminate what the other posters suggested, look for a air leak. I don't understand why you can't use vacuum advance but for now don't worry about it because that's not your problem.

I am not surprised it runs on with a idle that high, the lack of vacuum advance and lean mixtures make a hot combustion chamber and that acts like a spark plug when the key is turned off. Emission era cars with retarded timing had a idle solenoid that retracted when the key is off and the throttle blades were allowed to close further so it would not run on.

TCracingCA

I agree with TBarb, I would first just fatten up the idle mixture screws to see if it has a positive response! I bet the initial throttle pump shot helps it!

The change of exhaust size would have less of an effect at idle speeds! You aren't putting the air thru the engine at idle speeds! Smaller exhaust would help the lower end rpms!

It would be really hard to do, but You did put the right exhaust gasket in? You aren't blocking your exhaust flow? Yes stupid questions, but you never know!

if it runs at more RPM, you don't have a fuel feed problem, but an idle support circuit in the carb could have gotten crudded up! I would pop off the bowls and blow out the metering blocks!

DansYellow66

As noted - at idle the exhaust size difference of 2-1/2 inch versus 3 inch should not have any effect on tuning the idle.