vacuum leak
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vacuum leak
have a 76 f250 highboy with a 390m. totally redone, new everything.....except the actual block which was bored .060 over. it has a bigger cam, nothing wild, but definately has a lope to her now. when it has a load on it while idling, the vacuum drops from about 17-20psi to 7, and the engine bogs down and/or stalls immediately. i can catch it by feather the throttle, but until it really warms up it does this. i know the motors solid, just turned 500 miles since the rebuild. my carb is a holley 650 double pumper, and the power valves are 6.5s, so i could go to a 4 on that to compensate, but its driving me crazy as to why the vacuum is so low. could the power booster leak internally and steal it? is the mech fuel pump not supplying enough? anyone ever have this problem? any suggestions? thanks
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From: Indy
you say that it does this when cold and stops when warm? it could be a choke issue.
if you are pulling the vacume from the manifold the gauge will drop when you start to accerate.it will start pulling from the carb port.
check all your lines for cracks .
if you have ports that are plugged with the little rubber nipples, ****can them. they always crack and leak.
i use a short piece of vac line and put a stainless allen head in it to seal the line.
put one in that is big enough that you have to thread it in and put a little rtv on the threads.
if you want it to look good take the theaded part and chuck it up in a drill.
take a coarce file and get the machine grooves off, then a fine file then 600 grit and polish it and they look great.
if you are pulling the vacume from the manifold the gauge will drop when you start to accerate.it will start pulling from the carb port.
check all your lines for cracks .
if you have ports that are plugged with the little rubber nipples, ****can them. they always crack and leak.
i use a short piece of vac line and put a stainless allen head in it to seal the line.
put one in that is big enough that you have to thread it in and put a little rtv on the threads.
if you want it to look good take the theaded part and chuck it up in a drill.
take a coarce file and get the machine grooves off, then a fine file then 600 grit and polish it and they look great.
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what i meant was that it does it worse when the motor is cold....like it's immediate as opposed to 3-4 seconds when its warm. and when the motor is really up to operating temp, it wont stall but when you hit the gas it acts like its gonna then jumps up real quick. like its got a lag in it. when its warm the chokes full open mechanically. i'll give redoing the carb valves with the lines capped off instead of the rubber nipple things. but i just cant see how it would leak that much from there. thanks fox
MSEE
Joined: Apr 2004
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From: Austin, TX
The power valve doesn't affect anything at idle. Don't use RTV around a vacuum source as was mentioned in the previous post. Your problem sounds like the idle fuel mixture and curb idle speed need to be adjusted.
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sorry, im not great at explaining.....it idles just fine. its when you drop into gear that the vacuum drops. when i meant it has a load at idle, i mean when its in gear, not hitting the gas. so the majority of the time im driving with both feet (and its an auto tranny).
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sorry, im not great at explaining.....it idles just fine. its when you drop into gear that the vacuum drops. when i meant it has a load at idle, i mean when its in gear, not hitting the gas. so the majority of the time im driving with both feet (and its an auto tranny).
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