Rough idle on 1951 8BA

5

51shoeboxer

Senior Member
When I had a freshly rebuilt Holley 94 on my 1951 Ford, the idle was quite smooth. I made one 80 mile trip and the idle was very rough. I've tried adjusting the idle needles and the curb idle, but it still runs rough. I used the old trick of clamping my hand over the carburetor throat to suck out anything that might be in the idle passages; no better. I removed the idle needles and sprayed the idle ports with carburetor cleaner; no better. Off idle, the engine runs fine, but it still won't idle smoothly like it did before my trip.

I replaced the spark plugs, rotor and cap (I have Pertronix ignition, so no points); and still no difference. I did a compression test and all cylinders are 130 plus or minus 10 PSI. Anyone have ideas why my 8BA won't idle smoothly? Thanks
 
Leaking power valve in the carb base or poor mating surface in the power valve chamber where the carb sits on the base causing a vaccum leak. Some of the new valves don't seat properly without a slight modification.
 
I'm with the vacuum leak folks. I have had more than a few 'rough idle, but good performing' engine issues over the years and that most often involves a vacuum leak. Sometimes in unusual places. Use an aerosol spray or unlit propane torch and see if it smooths out when you hit a certain spot.
 
I'm in the vacuum leak camp as well.

Not sure why there is so much hate for the Petronix unit. I'd be willing to bet that those folks who have issues with them, didn't set them up correctly, used the wrong coil or other issues that are not a fault of the Petronix unit.

When the directions are followed as written, they work great.
 
Most of these replies say vacuum leak, but I strongly agree with Steve51 in saying that it sounds like the classic ruptured power valve.

Sal
 
Could be your idle mixture screws.Take both of them out and blow compressed air in screw holes and replace them.Check how many turn before you take them out.Maybe about 1 1/2 to 2 turns out?I have had this problem of rough idle and this solved my problem.
 
Steve51 said:
Leaking power valve in the carb base or poor mating surface in the power valve chamber where the carb sits on the base causing a vaccum leak. Some of the new valves don't seat properly without a slight modification.
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I think you are right. I saw a video that demonstrates how the gasket under the power valve might get hooked on a ledge on the power valve and cause a vacuum leak. I couldn't check for that, but with the engine running, I shot some carburetor cleaner on the gasket for the power valve chamber and on the vacuum connectios on the carburetor and the vacuum chamber; and saw no difference in the idle quality. I guess I'll have to pull the carburetor apart.
 
51shoeboxer said:
I think you are right. I saw a video that demonstrates how the gasket under the power valve might get hooked on a ledge on the power valve and cause a vacuum leak. I couldn't check for that, but with the engine running, I shot some carburetor cleaner on the gasket for the power valve chamber and on the vacuum connectios on the carburetor and the vacuum chamber; and saw no difference in the idle quality. I guess I'll have to pull the carburetor apart.
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When you seperate the base from the bowl, if it's wet with fuel on the vacuum side of the power valve, that petty much says a ruptured PV diaphragm.

Sal
 
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As to the ruptured PV diaphragm, if it ever backfired through the carb, it may have "blown" the PV diaphragm.
 
51shoeboxer said:
I think you are right. I saw a video that demonstrates how the gasket under the power valve might get hooked on a ledge on the power valve and cause a vacuum leak. I couldn't check for that, but with the engine running, I shot some carburetor cleaner on the gasket for the power valve chamber and on the vacuum connectios on the carburetor and the vacuum chamber; and saw no difference in the idle quality. I guess I'll have to pull the carburetor apart.
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Did you try what I suggested before you pull the carb apart?I realize you sprayed carb cleaner in ports but compressed air will thoroughly clean them.
 
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mfirth said:
As to the ruptured PV diaphragm, if it ever backfired through the carb, it may have "blown" the PV diaphragm.
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I’m no expert but having rebuilt my share of Holly 4-bbls I do know the older generations of 4150s/4160s a single backfire can knock out the power valve.
Later generations of Hollys and Demons had a mini-check valve in that vacuum circuit. I’m surprised some entrepreneur hasn’t made a retrofit for the older carbs. I believe a lot of guys running the old carbs are fighting mismatched power valve seats/gaskets and ESPECIALLY backfires.
 
There are a few sellers who have the modified power valves. I rebuilt my carb after having this same issue but my kit had the poor fitting power valve. I used a small sharp edge file to square off that rounded area on the valve that causes it not to seat properly. I also just disconnected my non functioning wipers and hooked up a vaccum gauge under the hood. Its temporary till I get around to dealing with the wipers but its a great diagnostic tool.
 
51shoeboxer said:
I think you are right. I saw a video that demonstrates how the gasket under the power valve might get hooked on a ledge on the power valve and cause a vacuum leak. I couldn't check for that, but with the engine running, I shot some carburetor cleaner on the gasket for the power valve chamber and on the vacuum connectios on the carburetor and the vacuum chamber; and saw no difference in the idle quality. I guess I'll have to pull the carburetor apart.
Click to expand...

This may be unlikely as you say it ran fine for 80 miles. I also found my carb was missing the check weight that goes in the hole between the towers where spray nozzles sits. The check ball in the accel pump was also stuck. Along with the leaky power valve it ran very rough. Since it sounds like you are going into the carb just make sure all the parts and little gaskets are there in place. I also used 220 and 400 grit sandpaper on a flat surface to get a better fit on the the carb and throttle plate mating surface. Before I started I could see slivers of light between them held metal to metal.
 
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