RPM Change/Idle Mixture?

[BumbleBus]

Hi. Posted this over on the Samba, but getting no love. I can't seem to find an answer with my limited searching abilities. Hoping someone here can shed a light. Thx:

When I pump my brakes my idle goes up about 100rpm (too rich right?). When I pulled a couple plugs they were pretty black and sooty. I've read that black plugs can mean a pig rich idle? My n00b question is: if I lean up my idle mixture will that solve this or am I looking into jetting as well? Doesn't a motor lean out under load/at higher RPM? I'm currently about 2 1/2 turns out on my Solex 34 TCIC progressive on the mixture screw, but if I close it any more than this I get some fairly serious stumbling off the line (009? Prog flat spot?). Thanks for any tips.

[whc03grady]

Do you mean when you hit the brakes, it brings up your idle? I think this means the booster and/or the hose between the booster and the carb has a leak.

Also, I always assumed the flat spot off the line was attributable to the 009. When my '74 bus had a 009 and a Weber progressive, it stumbled off the line. When my '67 Squareback had a 009 and the stock carbs, it stumbled off the line. What's the common denominator? Maybe it is and maybe it isn't the carb, the 009, or both but in any case for various reasons that carb/dizzy set up is generally considered the worst possible on a Type IV bus.

[BumbleBus]

Thanks for the reply Mitch. I remember reading here or on the Samba that if you pump the brakes and the RPMs go up or down it's a quick cheat to tell if you're running rich or lean?! If I just used the brakes in normal operation I notice no discernible change in RPM. Only when pumping them repeatedly. I think the black plugs are the obvious tell though. I'm just wondering if I lean up my idle it will affect the plugs etc. or does that kind of sootiness happy only at speed/load?

Someday I hope to lose the 009 & Solex prog, but it's not in the financial cards at the moment so I'm trying to make lemonade. I suppose the obvious answer is to just lean up the idle, deal with the stumble and see if the plugs are happier.

[Oregon72]

Bumble, it is my understanding that the best way to get a accurate visual test on the plugs is to go for a nice long drive at highway speed with as little idling as possible. This will get the engine good and hot and burn off whatever might be on the plugs. Pull into your driveway and shut it down immediately - absolutely no idling allowed at all. Go back and pull a couple plugs for analysis. Lick your fingers first

When you do this, you will be getting a better read on your plugs with regard to what your mixture is in the combustion chamber when you are haulin' ass down the freeway which is the most critical time. Always a good idea to make sure all tune up stuff is spot on ahead of time with regard to valve adjustment, dwell, and timing. Hope this helps.

Post up a couple pics of the plugs when you're done.

[BumbleBus]

Good stuff thanks. Will post a photo. Supposed to be nice and hot this weekend (for Montana anyway ) so that's a good time for a nice, hot test run. She's all tuned up otherwise to the best of my ability.

[LittleTrainJames]

Nice work Bumble. I want to go on that test drive

[Amskeptic]

Good stuff thanks. Will post a photo. Supposed to be nice and hot this weekend (for Montana anyway ) so that's a good time for a nice, hot test run. She's all tuned up otherwise to the best of my ability.

You are correct that *pumping the brakes lightly and quickly* will help you discern the fuel mixture. An increase in idle speed suggests, yes, that it is too rich. A small uptick in idle is OK.
Colin