What might be the problem when my car runs really rough at idle in gear with the brakes on? V8 automatic. Car runs well once I take my foot off the brake or put it in neutral with the brake on. Could my idle be too high? I have tried to back off the idle screw as low as it will go but it still seems like it's idling too fast. I can lower it with my mixture screw but then it doesn't start well when the engine is cold. I haven't set or checked the timing since I salvaged the car.
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Plug your power booster vacuum hose and tell me how it runs.
Thomas
Long time hot rodder
Packrat junk collector
'63 Avanti R2 4 speed -
My 64 Daytona was doing almost the same thing, but it was stalling when I stopped at lights. I would have to hold the gas with one foot and the brake with the other to keep it from stalling, but ran great otherwise. I adjusted the idle mixture screws and the choke and the problem was solved.
-Chris
1964 DaytonaComment
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Good call by Thomas. I don't think that people consider that the power booster diaphragm could be leaking. I've found two leaking, one on a Chevy S10 and one on an MGB.
Best regards,
PaulBest regards,
PaulComment
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Thomas and Chris, thank you for your suggestions. It would appear that I have a diaphragm leak. Once the vacum hose is plugged from the intake manifold, brakes applied in gear shows no change in the idling. Of course, I don't have power brakes any more. By the way, that vacum hose goes to the power assist unit to a tee where the vacum hose goes to a plug. Where was it supposed to go? Before I take a look at the diaphragm, I'll check that the plugged line has no
leaks. Thanks again to all.
DennisComment
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Actually your booster may be fine. My next question requires you to place your foot on the brake pedal BEFORE you start the engine - then when you start the engine please note whether the brake pedal sinks a bit or not. If the brake pedal sinks noticeably then your power booster is working fine and the problem is likely your check valve or the grommet for the check valve. Now what I want you to do is inspect the condition of the check valve and the grommet into the power booster. If your check valve doesn't fit super snug - then it can have a problem when a greater vacuum load is placed against it (as in when you press your brake pedal). The grommet itself can get hard or cracked and not seal up tight. If it fits snug then it could be the check valve. Inspect the check valve for cracks or fractures. If your brake pedal did NOT sink noticeably upon starting the engine then your power booster is bad.
Thomas
Long time hot rodder
Packrat junk collector
'63 Avanti R2 4 speedComment
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Thomas, thanks for the diagnostic tips. I'll try the test tomorrow. Today I fiddled with choke and mixture screw, and removed the vacum line to nowhere replacing it with another plug. On the test drive the car showed no roughness at idle in gear at stop. I still have a couple of problems with holding idle when it's not warmed up. I have to give it a minute and nurse it with the throttle. Do you know where that other line from the tee on the power assist unit might have gone to that was plugged? This has always been a California car as far as I know.Comment
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what size was it? I'm trying to think of what else could have been put there but am coming up dry. On an Avanti powered car that would have been for the throttle kicker and vacuum gauge (Avanti only.) Is it possible that someone swapped on a 4bbl manifold from an R-series car and forgot to plug the unused line?
nate
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55 Commander Starlight
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55 Commander Starlight
http://members.cox.net/njnagelComment
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I didn't measure it but it's probably 5/16 or 3/8". The extra 3' of line has always been plugged since I unearthed the car. Two barrel manifold.Comment
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