The Stovebolt Site Forums The Shop Area Engine Shop black exhaust

Hi, my '50 1/2 ton with a 235 in it runs well - does not emit huge amounts of exhuast smoke just driving down the road. But, when I start it up, it leaves a nasty black carbon spot on the driveway. Ick!! What's wrong?


Thanks,
BigHank

Black smoke is usually a sign of flooding. Do you have to choke it to start it? If so how long do you keep choke on?

Yes, I use the choke to start - if it's been sitting for a long period - Actually, I don't think I get the black smoke if the truck is warmed up - I don't need the choke then. Maybe that's all it is.. Thanks!

is your idle adjustment set properly? mine did that until i got it set by adjusting it to the maxium vacuum i could get. i then re adjusting the idle speed until i had the idle speed i wanted and max vacuum. no more spots on concrete floor and clean plugs.

It idles at pretty low RPM. How do you adjust the vacuum? I assume I'll be playing around with the distributor?

if it used to run rich it could be carbon (built up in tailpipe) getting blown out the end. How old is the exhaust pipe.

you dont adjust vacuum you adjust the idle mixture screw to obtain the highest vacuum reading you can get. what your trying to acomplish is to get the proper air fuel ratio at idle.

you need to hook a vacuum gage to your wiper connection on the intake manifold. as you increase the vacuum it will change the rpm.

[if no vacuum gage is available turn the screw in or out to obtain the highest idling speed. if engine runs too fast after adjustment re adjust idle stop screw. changing idle speed may require slight reasjustment of idling mixture.
prior to making any adjustments tune engine per specs. run engine to normal operating temp and let stabilize for at least ten minutes]
this is from my gmc maintenance manual. 47-54 100-400 models.

once you have the idle speed where you want it with the highest vacuum obtained you have reached your end point.

Is exhaust smoke black or just normal condensation? That nasty black carbon spot on the driveway is probably just from it spitting water out while it is cold.