I recently started to notice a puff of smoke coming out of the exhaust when I shift gears in my 70, usually at higher RPM with heavy throttle. The motor is a GMPP 454HO with around 500-600 miles on it and has a combo of street and highway cruising miles...no track duty yet. I've heard these motors are built loose which I thought might account for the smoke. Could that be it? Could the carb be jetted to rich maybe? Is there a more serious problem? I've heard of rings sometimes taking awhile to seal, but I figured after 600 miles they would be sealed. It runs like a champ otherwise.

prob valve seals

After 500-600 miles on a brand new motor?

How old is the motor? Did ya just buy it or has it been sitting for a few years? Some of the older 454's (couple years ago) had some problems with the oil rings. Was supposed to have been fixed.

graemlins/waving.gif
Rick

what color smoke?
Black/gray = unburnt fuel OR lean condition
Blue = Oil
White = water vapor

black/gray Smoke on shifts is pretty common actually, when you let off the throttle to shift it temporary loads the engine up then goes lean for a split second when you let off the throttle suddenly. It even happens on Auto cars because of the rapid change in engine RPM. It even happens on my 96' EFI thinderbird with a shift kit in the tranny, and nice quick little puff of black/gray on the shifts under WOT.

Is the motor using oil? Do you have to add oil between oil changes?

been there, had that

My probelm was valve seals.
Teflon racing low profile clamp on types.
The locking ring slipped off the valve boss, and was riding the top of the valve stem. Basically no seal at all. 1/2 of them were like this.
Mostly on the intake valves, right where you don't want them.
Pull a couple valve springs and check the seals.

Dave

Had the same symptom when I ran no valve seals on the exhaust side.

Originally posted by flywheel:
How old is the motor? Just over a year old

Originally posted by wayner:
Is the motor using oil? Do you have to add oil between oil changes? Doesn’t really use much oil…not noticeably anyways.

Originally posted by rancherlee:


black/gray Smoke on shifts is pretty common actually The smoke is a grayish color. I’m thinking that it could be an carb tuning issue. Good to hear that its somewhat a common issue. I’ll have to do some more tuning on it when I take it to the dyno this summer.