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What causes sputter/backfire thru the carb?

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J

Junkyard Dawg

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Discussion starter · #1 ·
I'm having a problem on my V8 S-10. When ever I'm cruising around not getting on it the engine runs fine.

But as soon as I stomp on it from a dead stop the engine sputters and sometimes backfires thru the carb....up until around 3000-3500 rpms and then it smoothens out.

Timing is set at 18 advanced and comes in around 3k, I did try richening up the carb a bit thinking it was too lean but that didn't seem to work. It's a Holley 600 carb 4160 series.

Any pointers? Thanks.
 
#2 ·
sounds like you need to work on the accell pump tuning. Try diff. cam or larger discharge nozzle
the backfire thing sounds like its leaning out.
 
Discussion starter · #5 ·
Thanx for that link. Something I have checked into....if I look down inside the carb and depress the throttle I can see fuel shoot from the nozzles. Also I noticed if the engine is cold it seems to not do it, but once it warms up then it does. Does this sound like it could still be the accelerator pump?
 
#6 ·
Popping/backfire under Hard acceleration is deffinitely a lean condition.
Take a vacuum reading, and replace the power valve with the approiate value.
If you have over 12" or so of vacuum at idle, the stock 6.5 should be fine. If it's under 12", try a power valve about 1/2 the total vacuum.
Power valves are cheap, and simple to change.
(A cat food can works great to catch te fuel from the bowel when you remove one of the bottom screws)
If the power valve doesn't do the trick, then look into the acc. pump, pump cams, and shooter sizes.
Check out the holley site for instructions how to adjust the acc. pump.
 
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#8 ·
I had the same problem. I played with power valves, accelerator pump cams and squirters. Finally had to jump up 3 sizes in main jets to get it to go away.
Steve
 
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Discussion starter · #9 ·
What's wierd is it's never done this before until recently.

I'm kinda thinking it has something to do with underhood temps as it gets pretty hot under the hood....hot enuff to bake a chicken in. :eek: I was thinking perhaps the fuel is boiling away or something. :confused:
 
#10 ·
best thing to do is keep a notebook and start trying small changes one at a time until you get it dialed in. I find it helpfull to write everything down that I change or else I forget half the stuff I did. Could be some vaporlock, do you have mechanical pump? if so you can try some 100LL av gas in it because that stuff will not vapor lock on you, just to see if it makes a diff. An electric pump pushing from the back will also eliminate vapor lock
 
#11 ·
Hi I just recently had the same problem on a new engine rebuild. I chased around with fuel issues and different carbs thinking I had a lean problem and finally found out it is a distributor that is acting up. The engine seemed to run fairly smooth at idle but you could notice the difference once I changed the distributor. To many possibilities.
 
Discussion starter · #12 ·
You know I was wondering.....is it possible that the rear secondaries are not working? I mean it only happens when you floor the pedal with the engine under load...which is the only time a vacuum secondary carb opens up...that and when you push on the throttle you can see fuel squirt from the nozzles located inside the primaries?
 
#13 ·
You might want to also check for a vacuum leak everywhere there could be one. I've had that cause a backfire. If your carb doesn't have the "blow proof" power valve or the kit added to have the same protecting effect, one good backfire can blow the power valve. Or a vacuum leak is something that could develop that wasn't there before. Something else to look at is the choke (if you have one hooked up). If you do, the fuel mixture while cold & running good could be covering up one of these other problems that you see when warmed up. Good Luck..... Odie.
 
Discussion starter · #14 ·
Well yesterday I took both fuel bowls off. cleaned the insides of them, replaced the old 6.5 powervalve with an 8.5 one and reinstalled using a new carb to manifold gasket.

Took it for a spin, at first it operated fine, but as soon as I got a bit further down the road it did it again...same symptoms. It's not until like 2500-3000 rpms when it smoothens out.

100 LL av gas? Where do I get that?

My fuel pump is a mechanical one but my fuel tank builds up enuff pressure to help push the fuel out. If I don't open my fuel cap before disconnecting the fuel line it will actually start pushing fuel out! :eek:
 
#15 ·
My car did the same thing and it drove me crazy. I looked for vaccum leaks and changed carbs, played with timing, replaced the distributor, etc. Turned out to be something simple-spark plugs. Hope yours is this simple.
 
Discussion starter · #16 ·
Yeah...new update....thought it might be the carb so I threw on my new 750 cfm carb....same thing. (seemed like the engine had a bit more uumph though) So the only other thing I could think was maybe one or several intake bolts were loose, so I torque every one of them....we'll see if this does the trick or not.....
 
#17 ·
Did you do as Mark suggested & throw a set of plugs in it?
I don't care what the ones in there look like, if you haven't tried it you might do so just for giggles ;)
Also, measure the wire resistance to be sure you don't have one going bad.

Simple stuff first :D
 
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Wolfplace
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