Nealio240z Posted July 10, 2006 Share Posted July 10, 2006 When i hit hte 4-barrel ( Q-jet ) it pops and backfires... It idels and runs fine in 2-barrel... Whats the problem? Is the carb too big? Im running the 2.4L electric fuel pump, is it not enough fuel pressure? Is my timing off? We tried 3 different carbs, all 750cfm... Is the carb too big (sb 350) I also have my down pipes glowing red hot!!! They are made of flexable stainless steel... is this because its running to rich or too lean? Can I wrap them up in header wrap... is the steel to thin? how does everyone else limit their header heat and under hood temp... Please help Im lost... Quote Link to comment Share on other sites More sharing options...
rytherwr Posted July 10, 2006 Share Posted July 10, 2006 Sounds like you may be running pretty lean.....the give-away is the backfiring when you give it too much air (opening the the secondaries) and the headers glowing overly much (running lean will do that easily). If it is a used carb, check the metering jets, make sure you are getting fuel through the entire carb, definitely check fuel pressure after the pump and before the carb....sounds like too much air and not enough fuel to me....a guess only without knowing the rest. Quote Link to comment Share on other sites More sharing options...
Nealio240z Posted July 10, 2006 Author Share Posted July 10, 2006 Im lost... If its running lean Why does it backfire (flames) through the carb? Sometimes it blows flames out hte exhaust... Is it too big a carb? Quote Link to comment Share on other sites More sharing options...
Cruez Posted July 10, 2006 Share Posted July 10, 2006 Check your ignition timing...or the advance.. might not be working properly. Quote Link to comment Share on other sites More sharing options...
Pyro Posted July 10, 2006 Share Posted July 10, 2006 No, a Q-jet is not too big for a 350. GM put Q-jets on engines from 283 to 454. A stock 2.4L fuel pump is not good enough. A Q-jet needs 5 to 7 psi. A stock 240 runs 3 to 4 psi. Lean mixture pops out carb Retarded timing pops out carb Retarded timing makes the headers glow red Rich mixture makes the headers glow red Rich mixture pops out exhaust Re-check the firing order very closely, sounds like a 5/7 switch. What are your engine specs? Quote Link to comment Share on other sites More sharing options...
LeeMS Posted July 10, 2006 Share Posted July 10, 2006 A good working Qjet will work well on a your eng. What dist. are you using, points, HEI or and aftermarket system? Can you increase rpm without backfires or do you have to ram the throttle? basic timing in the 8-10 deg and some good advance a must[the timing mark should move up the harmonic balancer when reving to 2500 rpm] if not check for mechanical and vacuum advance functions. The secondary air gates at the top rear of the carb are held close by spring tension without the eng running and with a rod to the choke pull-off with the eng running. This too can cause a lean pop back through the carb. Check all your basics,firing order, timing, fuel delivery and pressure. Sounds like your close , good luck... Quote Link to comment Share on other sites More sharing options...
Mike C Posted July 11, 2006 Share Posted July 11, 2006 Good advice. To clarify, a lean condition will cause the backfire back up the carb (and so will several other things, but this is the most likely scenario based on your descriptions) while overly rich backfires go out the exhaust. Generally.... R&R the carb, but buy a book. A Q-jet is a fine piece of equipment, but is way more complicated that a few posts on a message board can explain. $50 should get you a book and rebuild kit. Quote Link to comment Share on other sites More sharing options...
Nealio240z Posted July 11, 2006 Author Share Posted July 11, 2006 I feel like a fool. The guy who previously had the engine had to remove the distributor to get the engine out. I trusted that everything was fine... He put the dustributor 180 degrees off !!!!! I didnt think anything of it until my firing order was mentioned... I put the engine at TDC and the dist. is firing on #6... Not #1... It runs like a scalded cat... I still havent driven it yet, had to go to work (bummer). Ill have to shake it down and make sure it is street worthy... Thanks for the help... You guys rock... Quote Link to comment Share on other sites More sharing options...
Nealio240z Posted July 11, 2006 Author Share Posted July 11, 2006 Im supprised I got to 3000 RPM... he he he Quote Link to comment Share on other sites More sharing options...
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