351M Stalling out when braking
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351M Stalling out when braking
I have a question for those with the knowledge. I have a '78 F150 Supercab with a 351M. The motor has been bored .30 over, ported and polished, has a performance intake manifold, and cam. The motor is running great, I do need to turn the idle down a little though. My issue that I am having is when I am driving and come to a stop or even slow down below 10-15MPH the motor stalls out. When I put the transmission in neutral and coast to a stop the engine wants to stall then too. I just drove from Saginaw, MI to Jacksonville, NC and it was a bit of a pain in the a$$ when I was pulling off for gas or having to stop for traffic. I am not sure where to start with trouble shooting this issue. I was thinking it could be too low of a stall on the stock converter but I don't think that would cause the motor to stall when I was rolling to a stop in neutral. I thank you in advance for a steer in the right direction.
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Agree the converter is a potential cause. Some other areas to look are:
- Timing. The bigger cam will want more initial vacuum advance. Stock was typically 8-12 degrees. A bigger cam may work better with 15-20 degrees initial (vacuum advance disconnected and plugged. With this setting, you may have to modify the distributor so full mechanical advance does not exceed 34-26 degrees at higher RPM.
- Carb. A partially gunked up idle circuit or incorrect adjustment can cause this problem. Sometimes opening the butterflys to get a higher or even adequate idle speed with a low vacuum cam uncovers "running" ports and the idle suffers.
- Brake Booster. If it has an internal vacuum leak it could be letting excess air in when you press the brakes and leaning the mixture.
I hope the above helps.
Also, can you post cam specs?
- Timing. The bigger cam will want more initial vacuum advance. Stock was typically 8-12 degrees. A bigger cam may work better with 15-20 degrees initial (vacuum advance disconnected and plugged. With this setting, you may have to modify the distributor so full mechanical advance does not exceed 34-26 degrees at higher RPM.
- Carb. A partially gunked up idle circuit or incorrect adjustment can cause this problem. Sometimes opening the butterflys to get a higher or even adequate idle speed with a low vacuum cam uncovers "running" ports and the idle suffers.
- Brake Booster. If it has an internal vacuum leak it could be letting excess air in when you press the brakes and leaning the mixture.
I hope the above helps.
Also, can you post cam specs?
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