Stopped at light- foot on brake - Rough Idle?

[kosmo101]

Have a 50 mercury with a 350 small block... manual steering/brakes. Seems when I come to a light press the pedal somewhat firm...rpms drop to idle..car starts rough idling. Pop to neutral...rpms go up a bit and all is fine. pop in drive and with foot off brake all is ok. Only seems when I am close to or holding my foot on the brake at a light in drive.

Is this a vacuum issue since I am using the pedal to stop the car? Any help is appreciated.

thanks kevin

[chevy 37]

What are your specs on the engine? cam,heads, carb, etc.

[kosmo101]

Actually just got the car a few days ago and trying to take it all in. All manual (brakes & steering) as I am getting a workout stopping and turning the car

I dont know the specifics off the top of my head. I do know it has mild cam in it... I just want to have educated guess or verbage before I bring it down to my mechanic.

thanks

[NTFDAY]

Are you running a PCV and if so where is hooked up?

[76GMC1500]

The vacuum advance may be on the wrong port. It should be on a port that sees little or no manifold vacuum at idle and full manifold vacuum at part throttle.

Otherwise, what are you idling at in gear at a light? There should be about a 200 rpm difference in idle speeds between in gear and neutral. Your idle speed may be low enough in neutral that when in gear, the motor struggles.

[kosmo101]

In drive idle is around 650 rpm... neutral kicks up to around 750-800 off the top of my head...

not sure about pcv. working alot of hours so will try to get to it tomorrow.

thanks kevin

[kosmo101]

problem is that when at the light...foot on brake... feels like it will stall. harder i push on pedal...rougher it gets. like is the pedal effort taking all the vacuum ?

[kosmo101]

dumb question but how to tell power brakes or not?

[tyler]

this happened to me with my galaxie, i adjusted the idle air mixture and it helped, my motor wasnt getting enough fuel at idle sounds weird but it worked now the thing idles at 700 in park and 500 in gear

[kosmo101]

there is no brake booster... and tyler.. what kind of carb did you have.. mine has an old quadrajet.. dont know much about them.. could this be the source of my issue or may it be a vacuum issue?

thanks

[Nova Dose]

Hmmm, since it could be a vacuum issue, your timing may be off a few degrees also.

[1JohnnyO]

I was gonna mention that also....make sure that your 'tuneup' items are all correct before you go changing anything. Good plugs, wires, dist. and internals, ignition timing (important!), etc. When this is all set, and you still have a problem, then you can start to isolate it.

[R Pope]

I'd bet that car has a booster somewhere, maybe a remote one under the floor? The symptoms you describe can only be a leaking vacuum booster.