1978 351m question
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1978 351m question
I have a 78 Bronco that I bought with a ton of running problems. I've got most of them ironed out except one. It has a real rough idle and had pinging under load. I set back the timing, found and fixed a few vacuum leaks and replaced a leaking trans vac modulator. The pinging is gone but the idle is still rough. I'm assuming it has a worked motor since it has an Edelbrock 400 intake and a Holley 600. I have no idea what the cam specs are. I swapped the Holley with an Edelbrock Performer 600. It made some difference but not much.I also did cap,rotor, wires, fuel and air filters. When I go to accelerate it feels like a constant stumble until I get on the gas hard. It also has plenty of power. My next step is a compression test to see whats up. I'm thinking it either has a high lift/duration cam or a burnt valve/misadjusted valve. Any ideas or suggestions? Thanks for any help.
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1978 351m question
Scup, you've probably already done this, but check to see that you are getting a good squirt from accelerator pump in carb. A lean accel. pump will cause a stumble. Also adjust idle air mixture for best idle. Check dist. advance weights for sticking, and make sure vacuum advance is working, and that it is on a ported vacuum source (no vacuum until butterfly opens). Like I said, you've probably done all this, but just some things I thought of, maybe one will help. Good luck. Don
"Nothing lasts forever except natural stone and old Ford trucks." (Willie Nelson)
"Nothing lasts forever except natural stone and old Ford trucks." (Willie Nelson)
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From: PacNW
1978 351m question
It souds like you still have a vacuum leak. A hot cam will make for a lopey idle but it shouldn't continue to stumble when you gas it. If the intake has been replaced there may be a vacuum leak around one of the ports. Hook a vacuum gauge up to a port and check to see if the needle "wanders" at idle or use the old "spray the carb cleaner around 'til the idles smooths out" trick.
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From: Virginia
1978 351m question
I told a guy awhile back on this board about how I fixed my
rough idle and he found it fixed his problem too. The intake
manifold gasket that was used by whoever put the manifold on,
blocked the heat passage that runs under the carb. The only
way I found out was to take the manifold off, but another guy
had the suggestion of putting your finger on the manifold between
the two small bolts on each side of the carb. It should be very
hot compared to the rest of the manifold, because exhaust flows
through there and helps atomize the fuel in the manifold. It
makes a big difference.
rough idle and he found it fixed his problem too. The intake
manifold gasket that was used by whoever put the manifold on,
blocked the heat passage that runs under the carb. The only
way I found out was to take the manifold off, but another guy
had the suggestion of putting your finger on the manifold between
the two small bolts on each side of the carb. It should be very
hot compared to the rest of the manifold, because exhaust flows
through there and helps atomize the fuel in the manifold. It
makes a big difference.
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