• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Carb issues - lean backfire

Collin

Well-Known Member
Joined
Jul 17, 2009
Messages
876
Reaction score
5
Having a TON of carb issues lately. Originally had the Carter AVS and decided I wanted to get some more power out of the car. Went to a pro-systems duel feed double pumper 4150 750 CFM. Changed out the fuel pump for better fuel PSI, and tried to run this carb. The carb was nothing but problems, the car would lean backfire and any work done to the system was tough. It was a mechanical secondary carb, and we changed IAB, power valves you name it and still had lean backfire when we hit the secondaries. Talked to a technician who stated that the car may be too heavy for a mechanical secondary carb and recommended a quick fuel carb.

Cut to ordering a Quick Fuel SS 680 vacuum secondary carb. The decision was made based on talking to reps at Holley, quick fuel and a couple other guys. Running an electric choke on this carb. The car starts up and idles beautifully. The car moves forward off clutch in 1rst gear with no hesitation and is very smooth. The car runs great right up until i hit the secondaries. Then AGAIN I am having the car lean backfire back through the carb. Changed the 6.5 power valve out for an 8.5, but this carb has power valve restrictors at #49. The car definitely is backfiring through the carb and not the exhaust, car doesnt let me operate the secondaries at all in 3rd or 4th gear. Turning the car off it diesels.

Timing right now is 20 degrees initial timing and all in at 3000 RPM is 34 degrees. Running stock jets and only change was to the power valve. Ordered a kit to changed out the flow restrictors from the power valve. Trying to get into a dyno shop that tunes carbs but VERY scarce here in CT. Running manifold vacuum at 12 inches.


Carb Specs:

High Speed Air Bleed 28
Idle Air Bleed Size 70
Material Aluminum
Model SS-Series
Primary Main Jet 68
Primary Power Valve 65
Primary Pump Nozzle Size 31
Secondaries Vacuum
Secondary Main Jet 78
Throttle Bore 1-11/16 inch

Running 85 power valve with 49 restrictors
Running Manifold Vacuum at 12 inches
Timing again is 20 intial, 34 degress all in at 3000
 

69hemibeep

Sponge Bob Square Wheels
Joined
Sep 10, 2008
Messages
22,141
Reaction score
2,344
Location
AZ Desert,
Does that carb have a vacuum pod that you can put a heavier spring into for the secondaries?
 
Last edited:

Russ69Runner

Well-Known Member
Joined
Aug 4, 2016
Messages
7,696
Reaction score
3,231
Location
Lake Wales Fl.
Sound's like Your bowl float is to lean. Not enough fuel getting to the secondary jet's. Try to enrich it. Holey carbs are good. Double pumpers are tricky but it takes a little time to figure out how rich to set it. Do you have the sight glass on the bowl's or are they closed. Open the plug while it is running and make sure it is running out pretty good on while idling. Think this will fix the problem. :D
 

droptop

Well-Known Member
Joined
Sep 11, 2008
Messages
4,000
Reaction score
724
Location
Southern Indiana
I had a similar issue once and it turned out to be an intake valve not seating properly. At lower rpm it was undetectable, but as soon as the fuel mixture was richened up via the secondaries, it would start to pop thought the carb. A quick compression test will eliminate this scenario. We're you having this issue with the original AVS?
 

Collin

Well-Known Member
Joined
Jul 17, 2009
Messages
876
Reaction score
5
The original AVS would diesel and we figured a crack in the main housing, was an EBAY clean up job.

The quick fuel is a VS and does have the sight glass for the float levels.
 

Russ69Runner

Well-Known Member
Joined
Aug 4, 2016
Messages
7,696
Reaction score
3,231
Location
Lake Wales Fl.
Holley carb adjustment on U tube. Must set the float level when running. Mine was flooding out but it was old seats plus the floats where bad. Replaced all an runs fine now. I also had back firing problems. Good luck. Like I said double pumper's are tricky. :thumbsup:
 

Collin

Well-Known Member
Joined
Jul 17, 2009
Messages
876
Reaction score
5
Ordered a spring kit and the power valve restrictor kit, going to work on it this weekend
 
Back
Top