66 327/350 idle when hot

[Rich G.]

I have a 600CFM holley on the stock manifold. The carb is only 2 years old. It has an electric choke.

Starts cold great. When it warms to operating temp, it idles and runs great. But, after driving 20-25 miles on a hot day (80F) the idle drops to 400, barely stays alive and it stumbles when coming in with the throttle.

Coolant temps stay in the 190 range. I am running a mix of pump 93 and 100LL avgas. Right now I probably have 60% avgas in it.

Ideas where to look first?

Rich

[Bill I.]

Re: 66 327/350 idle when hot

Rich, after warming the car, without taking a ride, try with the air cleaner off to pull the throttle linkage toward the front. If rpms drop, tells me something is not closing as it should until really hot. Bill.

[Joe E.]

Re: 66 327/350 idle when hot

Just went thru this with a 62 340hp and it had a vaccum leak at the intake manifold. The car ran good until it got hot and the center bolts were loose and as it expanded it caused the leak, tightened the bolts and it does fine when hot. The bolts need to be tightened in sequence starting from the center working out to the ends. This Corvette experienced the same symptons as yours ran good until run for 15 miles or more then idle would fall off and want to stall.

[Duke W.]

Re: 66 327/350 idle when hot

So I guess it's not the OE carb. How about the camshaft? Vacuum advance? Numbers? Connected to a full time vacuum source?

Why are you running avgas?

Duke

[Rich G.]

Re: 66 327/350 idle when hot

Thanks for the replies. I hope Joe is right...

Duke, This just started and all that other stuff is the same. I don't know what cam is in it. It is a full time vacuum source, but disconnecting the line and plugging it makes little difference. This motor was rebuilt about 8 years ago by the previous owner and I have had it since then. To be honest when I check vacuum it doesn't seem right for a 350HP, but having said all that, this problem just showed up. It's a driver and it used to run great all the time.

I usually don't run avgas, but I was at the airport and needed gas. Figured it was like chicken soup...it couldn't hurt.

I'll check the easy stuff first and post what I find.

Rich

[Duke W.]

Re: 66 327/350 idle when hot

I don't know what cam is in it. It is a full time vacuum source, but disconnecting the line and plugging it makes little difference. To be honest when I check vacuum it doesn't seem right for a 350HP,
Rich

That's why I asked the queston. Unknown cam! Maybe the VAC is not suitable for the idle vacuum characteristics. So what's stamped on the VAC mounting bracket?

You measured idle vacuum?

vacuum @ idle speed, please.

If you have a timing light, measure the initial timing (VAC disconnected) and total idle timing (VAC connected at normal idle speed)

The above is what is needed to zero in on the problem. Anything else is just guessing.

Duke

[Jim T.]

Re: 66 327/350 idle when hot

I have a 600CFM holley on the stock manifold. The carb is only 2 years old. It has an electric choke.

Starts cold great. When it warms to operating temp, it idles and runs great. But, after driving 20-25 miles on a hot day (80F) the idle drops to 400, barely stays alive and it stumbles when coming in with the throttle.

Coolant temps stay in the 190 range. I am running a mix of pump 93 and 100LL avgas. Right now I probably have 60% avgas in it.

Ideas where to look first?

Rich

May not be a problem, easy to check and repair. Remove the distribtor cap and rotor and check the operation of the centrifical advance. If the pivot points are dirty they could prevent the centrifical advance from retarding after driving and distributor components are hot. Happened to me on my 64 when it was only 4 years old. Cleaned up the centrifical advance and ran fine.

[Timothy B.]

Re: 66 327/350 idle when hot

The tip in stumble could be vacuum leak, what vacuum does the engine make at hot idle?

The problem could also be in the holley when it gets hot vacuum pulls gas from somewhere it's not supposed to like the base plate to main body interface. Be careful how tight the four attaching manifold screws are.

Problem could also be gasoline perculation, does the engine seem to be flooding. Take air cleaner off when hot to see if gas is dripping.

FWIW, the proper adjustment procedure for the A/F emulsion screws are done with a hot engine at the desired idle speed after idle timing is correct. Always make sure distributor and vacuum advance is set before going to carburetor. L-79 350hp should make approx 14" vacuum at 750-800 idle RPM.

[Rich G.]

Re: 66 327/350 idle when hot

Just an update.

Idle vacuum is 16 inches at 750 RPM, however since my timing light broke, I can't be sure of anything else.

I will continue to follow Duke's suggestions when I get a working timing light.

The carb does not appear to be leaking internally and all bolts are tight on the manifold.

I am starting to wonder if this is related to the avgas I put in percolating earlier than the pump gas. The last time this happened I think was the only other time I used avgas. May or may not be, as I don't usually drive 20 miles at a time in hot weather, either. The last time it did this I thought it might have been dirt in the carb somewhere because it got better as quickly as it got bad, but there was a fill of pump 93 in between.

Anyway, I will post an update when I get more data.

Rich

[Clem Z.]

Re: 66 327/350 idle when hot

I have a 600CFM holley on the stock manifold. The carb is only 2 years old. It has an electric choke.

Starts cold great. When it warms to operating temp, it idles and runs great. But, after driving 20-25 miles on a hot day (80F) the idle drops to 400, barely stays alive and it stumbles when coming in with the throttle.

Coolant temps stay in the 190 range. I am running a mix of pump 93 and 100LL avgas. Right now I probably have 60% avgas in it.

Ideas where to look first?

Rich

the aluminum intake is transferring exhaust crossover heat to the carb. block the heat riser crossover in the intake and since you have a electric choke you will have no problem when the engine is cold

[Rich G.]

Re: 66 327/350 idle when hot

Clem

Thanks. I thought about that but don't know what's involved. My heat riser works fine now. Is it a matter of holding it closed, or do I have install a block plate in the manifold?

Sorry if that's a dumb question......but consider the source

Rich

[Clem Z.]

Re: 66 327/350 idle when hot

Clem

Thanks. I thought about that but don't know what's involved. My heat riser works fine now. Is it a matter of holding it closed, or do I have install a block plate in the manifold?

Sorry if that's a dumb question......but consider the source

Rich

you should wire the heat riser valve open and to block the manifold crossover i posted how to do that so do a search under my name. if you can't find it call me 724-468-5692 as i talk better than i can type.

[Rich G.]

Re: 66 327/350 idle when hot

Clem

Thanks. I found the post you were talking about and I understand.

BTW, I see by your area code that you are near Pittsburgh. Our son moved to Pittsburgh in February and we are driving from LI to there next week for his birthday. He works for KDKA-FM, the new sports station in town.

Rich

[Joe C.]

Re: 66 327/350 idle when hot

Just an update.

Idle vacuum is 16 inches at 750 RPM, however since my timing light broke, I can't be sure of anything else.

I will continue to follow Duke's suggestions when I get a working timing light.

The carb does not appear to be leaking internally and all bolts are tight on the manifold.

I am starting to wonder if this is related to the avgas I put in percolating earlier than the pump gas. The last time this happened I think was the only other time I used avgas. May or may not be, as I don't usually drive 20 miles at a time in hot weather, either. The last time it did this I thought it might have been dirt in the carb somewhere because it got better as quickly as it got bad, but there was a fill of pump 93 in between.

Anyway, I will post an update when I get more data.

Rich

Duh..............noooooooooo.

Could you imagine how many aviation disasters there would be if the partial vapor pressure of any of the 100LL components was higher than that of motor fuel!

[Rich G.]

Re: 66 327/350 idle when hot

Yeah, Joe after flying piston powered airplanes for 42 years and owning 7 airplanes, 2 of them with the Lycoming/Bendix fuel injection and having over 4000 hours as a flight instructor, I can imagine that. I agree, it doesn't make any sense. Just an obsevation and probably misguided.

Even 100LL will boil in the injector lines of the Bendix system after shut down and sometimes at idle on hot days. Which is why auto fuel (without ethanol) was approved in some airplanes with carburators, but not for fuel injected systems.

Rich