I've noticed that after sitting at a stoplight with my foot on the break, usually for a time the idle is smooth but then it starts getting rough or misfiring. It does not do this in park, there's no hesitation or bog on acceleration. Is this because my idle mixture screws are set too lean?

 

It could be a number of things, I would perform a no parts tune up and go over all the settings if the parts are not that old. If it was not tune related, check for vacuum leaks, especially the power booster.

 

Earlier this year after I noticed it happening when I got it on the road and I did pressure test the booster, it does hold vacuum. It was rebuilt by Karp's, I did so pre-emptively. I didn't pursue it further because other issues popped up that were more urgent, which did include a few vacuum leaks but the vacuum gauge as of last night shows a steady 19-20 inHG. Also, if the booster wasn't holding vacuum wouldn't it always be misfiring straight away? If it's been some time since braking and the booster has had some time to build up vacuum, the engine will run perfectly fine for about 30 seconds to a minute or so after I put my foot on the pedal. What else in the braking system can be checked?

I honestly wouldn't rule out a tuning issue because I've been needing to retune it to work better on the gas that I use, because this is a driving car I needed it to run on the most commonly available gas which is E10, not what it was originally tuned for. But I'm also a total amateur. In park when warmed up it idles really good right now.

 

If Karp's rebuilt your booster, I seriously doubt it is the problem. They are the best.

 

If the engine develops a rough idle but ONLY w/ your foot on the brake, and only after a select period of time (~20" - 30") there is likely a disruption in the A/F mixture ratio. Since this ONLY occurs while holding the brake pedal while the car is idling while stopped (generally after ~20" to 30") it would appear the issue is related to a brake system vacuum leak. BTW, it should idle good in park even when cold. Once the engine starts, it should idle cold &/or warm.

You haven't stated what car this is experiencing this issue. Best if you state the year & model of the car - it's terrible practice to ASSUME you are talking about 1978 Oldsmobile 98 even though that is what is written in your member name signature. Grab some additional vacuum hose, hook it to your vacuum gauge and lay the vacuum gauge on the windshield to so you can view the vacuum gauge. See if you can replicate this issue holding the brake pedal while stopped - what happens to your vacuum reading when it begins to idle rough?

 

I'm not familiar w/ a 1978 98 (if that's what car you're referring to); however, I've noted a couple times in your comments you have referred to vacuum - asking firstly in your opening statement if this is related to your A/F idle mixture screws, then again in another later post; and, I'll note you spoke of once the engine warms up it idles good. Again, it should idle good cold &/or warm. I'm thinking you're not convinced you don't have a vacuum issue - it may not be a leak, but perhaps you're not holding/maintaining an adequate vacuum and A/F idle mixture screws are not set correctly. When I set A/F mixture (w/o using a wide-band O2 sensor), I set each A/F mixture idle mixture screw "individually" (one at a time) to achieve the highest vacuum possible w/o stumbling. Do this for each idle (A/F) mixture screw - one at a time.

 

There should not be any real changes for E10. Look down the carb throat and see if you see any fuel dripping when someone is holding the brake at idle and no brake at idle. I would also check the EGR valve.

 

Didnt have time to do any real work on it today but I was able to do some testing on the road while running errands. It doesn't seem to have anything to do with the brakes, rather it doesn't like idling in drive. When I was at a stoplight it occurred to me to put it in neutral while it was misfiring, and suddenly it went away, only to come back when I put it back in drive. In the driveway when I got home I put the parking brake on with it in drive and released the brake pedal, it made no difference in engine RPMs. After pulling into the driveway and idling it didn't misfire but ran a little rough, pressing or releasing the brake made no difference but putting in park did.
Yesterday when it was cold it stalled when i put it in reverse, something it hadn't done before this latest round of adjustments. I made a lot of changes I shouldn't have because I trusted what turned out to be a broken vacuum gauge that was lying to me. The EGR valve is brand new, but the vacuum switch was bad so I capped it off at the carburetor and haven't gotten around to getting it working properly again. It seems I either need to readjust timing and/or the idle mixture when it's in drive to see if it will play nice.
Earlier this year when I was having other issues that turned out to be the EGR switch I checked extensively for vacuum leaks and found nothing.

 

Finally was able to get a vacuum gauge back on it. After getting it warmed up i threw it in drive, using the gauge I determined it was set too lean. After getting max vacuum pressure the engine smoothed out a lot with no more misfired.

 

Glad it worked out illumined.

I meant to post days ago. My ride had the same issue. I would jump on the highway 15 mile trek, car feels great. Get off wait at a light, poof, she would pop stall off.

It was air fuel mixture.