quadrajet exaust backfire problem.

ok so I got the willys running today. It runs great at idle but as soon as you rev it, it backfires through the exhaust. we checked the timing and its all good. I know nothing about these carburetors. the secondaries do not seem to open so I'm wondering if its acting lean. I rebuilt the carb thinking it could be the accelerator pump, but it still backfires. It idles much better now though after the rebuild. any ideas on what causes a backfire with quadrajets. the carb probably came off of a 350 engine.

 

Re: quadrajet exaust bikefire problem.

does it backfire as RPMs are going up or as soon as you let off? A lean misfire would probably be coming back through the carb instead of out the exhaust. Too rich or retarded timing can cause that.

You say you checked your timing, but how much timing did you have it at at what RPM and was the vacuum advance disconnected when you checked it? How is the idle mixture set?

 

Re: quadrajet exaust bikefire problem.

well, I checked the timing at idle with the vacuum advanced disconnected and plugged. it backfires through the exaust when you increase the Rpms quickly . if you go about it really slowly the engine will rev up without backfiring.its not backfiring through the carb though. it did that when I had the timing 180* off lol. the timing is at about 7*. my idle mixture is 3 turns out, I played with it and it did not change the backfire issue. I need a vacuum gauge to properly set it I will probably borrow one this weekend.

 

dwell, aka point gap will cause your symptoms, also low voltage will cause it as well

 

ok well that makes alot of sense. maybe it is the timing. on an hei oddfire distributor what pole is the number 1 position. it has to matter, maybe i am off by 1 . I made the number 1 pole the 1st one counterclockwise from the ignition wire...if that makes any sense. i could see if it was off since it fires at odd intervals it could cause a condition like that.

 

Ya I did the search a couple of times but i didnt find that article. it appears that my timing is correct. it looks just like warlocks.

 

Re: quadrajet exaust bikefire problem.

 

Youre right. Shouldn't be if they're adjusted properly. Three turns out sounds like a bit more than I remember.

Are these fuel screws or air screws? I don't remember on a Q-jet

 

Well i probably need to check the float level, but the carb wasnt flooding out crazy so i figured it was fine...if the level is off could it run rich but not flood. as for the mixture screws i turned them in all the way in and all the way out as well it made the exaust smell come and go but didnt fix the backfire. its wierd because u can slowly increase the throttle and the thing sounds great. but if you move the throttle like you normally would to test these things it backfires.

 

ok think im on the right track, disconnected the accellerator pump and the thing ran great. so that should mean im just running super rich, guess i will just have to play with the rods and jets and all of that stuff...yay

 

I love a Q-Jet but even after buying several books on them I still had trouble. If I can make a suggestion go to some of the after market marine inboard web sites and buy a Q-jet for the type of engine you have. I had a twin engine gas-burner boat with twin 350's that developed 360 H.P. I bought a reman. Q-jet for the specific engine that came with the correct metering rods, etc. I put it on and the only adjustment I did was set the idle. Just a thought because alot of the big offshore boats run Q-Jets and they are still rebuilding them. A Q-Jet is a great carb but it has alot of adjustments.

 

Ok I know this is an old thread, but I eventually did find the problem. I thought posting what I found could help somebody. Of course everyone was correct in suggesting a timing issue. It was the timing. The actual timing was correct, however the springs in the mechanical advance were pretty much gone. It would advance way to fast and cause the backfire problem. It runs like a top now.

 

are you disconecting the vac hose to the advance can when you check timing? another check is to pull vac on the can while it is running & watching with your timing light to make sure it is advancing properly. also pull the cap & try to rotate the mechanical advance mechanism. you should be able to rotate the rotor & watch the weights move & separate from each other. this will stick up by running in an ionized atmosphere & will cause all manner of grief. weak or broken valve spring on an exhaust valve will do the exact same thing as you describe. hope this helps a little.
andy