Hi everyone,
I have a recently purchased '78 F250 Frankenstein with a 390. It was an engine with a carb on top, not even a functioning pcv. I was having trouble with it back-firing to the point of popping the muffler like a sardine can. Because it had a very old carburetor on it, I purchased a reman 2150 and the problem went away. In the mean time, I have replaced plugs, wires, coil, cap 'n rotor, fuel pump, master cylinder and booster, pcv and breather cap.


Now my problem. It started stalling coming to stop signs and lights. At first, I was able to brake slowly and it was fine, but if I had to brake hard, it would stall, but restart. Then got to the point it wouldn't restart. I figured out that the plugs were fouled out and replaced them again.
I have since learned to tune the engine (thanks to this site [you guys are awesome]) with a vacuum gauge which leaned it out much more, but it is still stalling when I brake. Just in the driveway, I put in drive, run it forward 10 feet, hit the brakes and it stalls. It's almost like just hitting a kill switch. In gear with my foot on the brake, its fine.


This is my 4th '78-9 but this really has me stumped. Any help with something new to look at would be greatly appreciated.

 

Just a thought, but did you check the float level with the new carb?

 

Did you change the little one-way valve in the vacuum line going to the brake booster? The plastic piece that plugs into the booster that the vacuum line attaches to?

Not that that should have much to do with it, actually, just asking. Does the brake pedal get hard right after you shut it down?

Again, not that it would cause this, but the one thing you didn't change..

Does it die only on HARD stops, or ANY stops? Float level would affect only HARD stops, I'd think.

Motor mounts good?

 

Thank you Decaruby and Krewat for your replies.


I popped the air horn and the float was a bit high so I bent it down to be level with carb. Should I go a little farther?


I checked the booster check valve and it seemed o.k. Not satisfied, I disconnected it and capped the port, so the brake is always hard but no change. It does seem to me like a vacuum leak but darned if I can find one.


It seems to stall when it is coming to the stop. What would make you think of motor mounts? I know they are very jerry-rigged from p.o. putting in the 390. There is 1" plate welded to the frame under the mounts but never gave it a second thought. I had a long day at work today but will look at them tomorrow.


Thanks again guys for the thoughts. I know the wife is getting done with taking my *** to work at 5:30 a.m. and need to get this figured out.

 

auto or stick?

 

It is an automatic.

 

Not sure on your year and carb but most carbs used with auto trans have an anti-stall dashpot, do you know if you have one and it's working and adjusted properly?

 

Excellent point

 

Thanks Hiball3985


It's a '78 F-250 with the 2150. It does not have the anti-stall dashpot. All of that sort was removed at some point. It ran fine after installing the new carb, then developed this problem. Do you think that would help?


I just dug out the old carb and was going to put it back on to see what happened. Thanks to all of you, I am better at tuning the carb and may just leave old one on if this problem goes away.


What exactly does the anti-stall do, how does it work?

 

It won't hurt but it may not help, depending on what the problem is. The anti-stall dash pot is a small spring loaded diaphragm that works against the throttle linkage, it lets the throttle close slowly when you let off the gas rather then closing abruptly. They were quite common on cars/trucks with auto trans..