Why does vacuum leak causes backfire through carb

That implies he has a leaky intake valve. The low vacuum can leave gas/air mix in the intake and the heat from a slight leak can fire it off.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail >

A lean mixture can burn so slowly that the charge can still be burning at overlap, when both the intake valve is partially open and the exhaust valve hasn't fully closed, before the piston reaches top dead center.

That is exactly right and was explained to me many years ago by an engine instructor in A&P school. You could cause an aircraft radial to blowback through the carburetor any time you ran the mixture back too far to the lean side. You'd see an increase in cylinder head temp. also.

Vacuum leak causes an excessively lean mixture. Lean mixture causes backfire.

That's all there is to it. If you'd grown up with carburetors, you'd know that backfiring virtually always indicates either a lean condition, a timing problem (or cross-firing in the distributor cap), or a stuck intake valve.

It practically takes a damn miracle to keep a radial FROM backfiring (and when they backfire its more like a small bomb going off because there's so much air/fuel mixture in all that plumbing), but that's another newsgroup :-)

Yeah, but that's on a cold start. When the fuel mixture is not even due to cold intake runners. Rich and lean in spots. I'm talking about leaning it out on something like a 2800 P&W with an engine at cruise power. Not something you want to do for any length of time, but I seem to remember that it was part of a runup check to verify the mixture controls after engine maintenance. This was about 35 yrs. ago, though, and the memories fade a little. Long live Round Engines.

Thanks for all of the info. He actually found a vacuum line loose and reconnected. It fixed it for a couple of days but now it is back. May be one of the other issues.

Out of curiosity, the hose was loose on the black canister near the front on the driver's side. What is this canister on a 1988 Chevy pickup?

Likely the charcoal canister or gas tank vent. When these fail, they can act like a vacuum leak also. I don't know the plumbing on the 88's, but usually the canister has a purge valve on it with a vacuum signal line and the vacuum line to the intake. That vacuum line to the intake shouldn't be sucking at idle if the canister is good. You can pinch the line closed to see if the idle changes. If it does and the canister has the round purge valve on the top, then the canister is likely worn out.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Jan/06 (More Off Road album links at bottom of the view page)