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Author Topic:   carb back-fire/bogging
doubletrouble
Dirt Forum Racer

Total posts: 82
posted March 17, 2003 11:07 PM  
Last few weeks we have been taking the car out to the track for a little pre-season testing.The set up seems to be getting real close,playing around with tire pressure to fine tune.However we are scratching our heads over a slight bog or back-fire as we come out of the turn and nail the gas.First thought was q-jet carb,so we had it re-kitted.We ran about 10 hot laps with no problems and then it started up again...and a few laps lated over-heated.My next thought was timming,so I borrowed a timming light from a neighboor and the timming was off the scale,advance about 20 degrees.Not sure the year of the motor we looked in books and compramised to about 14 degrees.Do you guys run advanced timming? what is a good place to start.We have a old motor We yanked out of wheel squeeling cutlas Oldsmobile 350 rocket,not sure of the year I would guess mid 70's.Do you think its a timming problem?Carb fuel mixture?I have seen cams flatten out and cause similar problems,however local car buff says highly unlikely....whats your take on this dilima

doug demoss
Dirt Roller

Total posts: 23
posted March 18, 2003 08:10 AM  
I think you have two problems,First make sure you have no vacum leaks. Second put a vacum guage on the engine and make sure the reading is steady.With a dial advance light check your total timing at 4500 rpms. It should be around 38 total.Next get a set of jets and metering rods from a early 70's q-jet carb . You are to lean ,Fatten the mixture up. 515-232-0126-Quick performance

rico 08
Dirt Forum Champ
Total posts: 1139
posted March 18, 2003 12:52 PM  
also don't go off the initial timing as there are too many things that can go wrong go with timing that way,use total timing with a timing light that has a degree knob,and use 38 only for the max.My suspicion is that you're mechanical advance is froze and not advancing.

racinrich1
Dirt Full Roller

Total posts: 46
posted March 18, 2003 09:41 PM  
Quadrajet carbs have "auxillary feed tubes"
Which feed a shot of fuel up to the secondary air flap as the secondarys kick in..It is very common to see these tubes detach and cause a momentary lean condition during transition....Too loose or too tight of a airflap adjustment can cause this as well as floatlevel adjustment...HPbooks has a good book on Rochester carbs...easy to read and understand....
Rich

david197377
Dirt Roller

Total posts: 11
posted March 19, 2003 03:58 PM  


I also run olds and i had the same prob. too. The Q-get has seconday flapper adg on it .turn the carb over and on the top side there is a set ***** that controles the spring for it. Turn it till you fell the secondary is stiffer. dont go too stiff or you will have late opening and more of a flat spot. Second of all did you change the timing gears and chain? If not do so and dont use a stock set.Use a good one like a comp came's double roller set. The stock set can'nt stand the pressure or the RPM of a raceing engine.


Gordon3E
Dirt Full Roller

Total posts: 69
posted March 31, 2003 05:07 AM  
You may want to check the fuel filters also.All of them.

doubletrouble
Dirt Forum Racer

Total posts: 82
posted March 31, 2003 10:40 PM  
Hey guys,thanks for all the pointers!We were having a hard time keeping the timming on the march suggested,so we went ahead and replaced the timming chain and gear with the double gear as suggested.The old chain was real sloppy,we also went ahead and replaced the cam and lifters with a stock spec.The car is now holding a great advanced tine @ 34 degrees final timming.The carb still doesnt fit my expations,and the three others arent any better.Even though the carbs have been kitted there is nothing left on the adjustment screws.Those guys are dumping way to much fuel and flooding at high RPM's.Think I am just gonna buy a brand new one that I can fine tune.Didnt have time to test it yet at the track,but hopefully we will do that this comming week-end.Again thanks for all yalls help,still waiting on those dirt forum decals so I can "show" my gratatude!!!

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