Why does vacuum leak causes backfire through carb

Just curious but buddy had his 1988 Chevy pickup with throttle body injected
350 had it start backfiring through carb. Found vac hose off. Put it back
and seems to be OK now. Why would this cause 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 in '00
88 Cherokee 235 BFG AT's
Canadian Off Road Trips Photos: Non members can still view!
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bob wrote:
>
> Just curious but buddy had his 1988 Chevy pickup with throttle body injected
> 350 had it start backfiring through carb. Found vac hose off. Put it back
> and seems to be OK now. Why would this cause backfire through carb?

"bob" > wrote in message
...
> Just curious but buddy had his 1988 Chevy pickup with throttle body
injected
> 350 had it start backfiring through carb. Found vac hose off. Put it
back
> and seems to be OK now. Why would this cause backfire through carb?
>
>

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.

"." > wrote in message news:wiRNf.344$ia3.147@fed1read08...
> "bob" > wrote in message
> ...
>> Just curious but buddy had his 1988 Chevy pickup with throttle body
> injected
>> 350 had it start backfiring through carb. Found vac hose off. Put it
> back
>> and seems to be OK now. Why would this cause backfire through carb?
>>
>>
>
> 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.

bob wrote:

> Just curious but buddy had his 1988 Chevy pickup with throttle body injected
> 350 had it start backfiring through carb. Found vac hose off. Put it back
> and seems to be OK now. Why would this cause backfire through carb?
>
>

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.

gfulton wrote:

> "." > wrote in message news:wiRNf.344$ia3.147@fed1read08...
>
>>"bob" > wrote in message
...
>>
>>>Just curious but buddy had his 1988 Chevy pickup with throttle body
>>
>>injected
>>
>>>350 had it start backfiring through carb. Found vac hose off. Put it
>>
>>back
>>
>>>and seems to be OK now. Why would this cause backfire through carb?
>>>
>>>
>>
>>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.
>
>

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 :-)

"Steve" > wrote in message
...
> gfulton wrote:
>
>> "." > wrote in message news:wiRNf.344$ia3.147@fed1read08...
>>
>>>"bob" > wrote in message
...
>>>
>>>>Just curious but buddy had his 1988 Chevy pickup with throttle body
>>>
>>>injected
>>>
>>>>350 had it start backfiring through carb. Found vac hose off. Put it
>>>
>>>back
>>>
>>>>and seems to be OK now. Why would this cause backfire through carb?
>>>>
>>>>
>>>
>>>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.
>
> 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.

bob wrote:
>
> "bob" > wrote in message
> ...
> > Just curious but buddy had his 1988 Chevy pickup with throttle body
> > injected 350 had it start backfiring through carb. Found vac hose off.
> > Put it back and seems to be OK now. Why would this cause backfire through
> > carb?
> >
> 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 http://www.imagestation.com/album/pi...?id=2115147590
(More Off Road album links at bottom of the view page)