Drag strip launch problem

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couper 340

couper 340

FABO Gold Member
FABO Gold Member
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HI HAVING PROBLEM AT THE STRIP ONLY ON THE STREET ITS FINE.
My 1970 340 duster 4 speed Hughes cam and springs duration at 50 is only 230 with 545 lift
9.5 compression , Weiand act plus intake. 3.73 rear gears 2.02 casts heads .
This is the problem when i come off the line at 3500rpm the car leaps and than fall on its face were you have to back off the gas and wait 2 sec for it to come back on . i am not sure if it's flooding or not getting fuel in the carb bowel or to much heat under hood as i have no hood scoop . I tryed a new mp fuel pump new 3,8 fuel line were it is rubber also fuel regulator and took off my Thermoquad and put on my Edelbrock 750 afb almost NO change so took off my fbo ign box and coil and reinstalled my stock mp box and coil . This also made no change . That why i was thinking carb . i did set it up with air / fuel meter but at my house. Again run's great on the street were you can spin the tire off the line.
 
Can you describe your fuel line from tank (or cell) up to the carb please.
 
get a holley double pumper for the drag strip...leave the eddy afb at home..
 
DANA i have a 3/8 sender unit with 5'' peace AN4 line to stainless 3/8 line to the pump and 10'' AN4 line to the pump than 5/16 line up to the fuel regulator to 3/8 fuel filter to 3/8 AN4 to carb. this is all new . stock gas tank mopar 80 gph pump mechanical new. I DO HAVE A Migthy DEMON 650 THAT I had not installed yet just because i wanted to see what i could get out of the other carb first . also ign is all FBO BUT REINSTALL MY MP IGN BACK ON BUT KEPT DON DISTRIBUTOR
 
Well first thing I would get that filter before the regulator. And keep your line from regulator to carb as short as possible. This allows the regulator to pickup the most instant signal from the carb. Also IMO your system is on the edge of too small. Your probably spinning that thing around 6000-6500? Anything 5/16 is a red flag!! That goes for the pump fittings also. IMO the reason you don't experience this on the street is the combination of g-force and rpm. I think you momentarily starving for fuel pal.
 
THAT'S WHAT I WAS THINKING THAT gas is not getting into the bowel fast or on launch the floats are slamming closed the valve closed and using the fuel up in bowel. i turn the line pressure up 8psi I also have tune up kits with large fuel float needle and seats i can install.


ALso do you think coming off the line at 3500 is to low
 
You can adjust the regulator to 10psi and still run out of fuel! It can't regulate it, if it don't have it!
 
Youl'll have to play around and find your sweet spot for launch rpm. But the condition you described in your original post does'nt sound like it has ANYTHING to do with launch rpm.
 
Put the demon on and try it first on the street to see if it cuts out. Make sure your float levels are correct and that you are getting the proper fuel supply.
 
you might should get rid of all the 5/16" and #4an stuff out of the fuel system first. don't those carbs depend on vacuum to keep the secondaries closed. here's my theory yours may vary. maybe when the clutch is let out the the vacuum signal goes sky high and they go shut. you might try a heavier spring on them to keep them open.
 
AN4 is too small. I'm running AN8 from the tank to the regulator. Mechanical fuel pump. Then AN6 to each bowl on a double pumper race demon 750. You need to have a fuel system that will work as you upgrade your car to go faster.
 
hi, how much timing is in engine? are you running vac advance hooked up? total timing ??
 
I would also try the Demon first, your combo seems pretty stout and I wouldn't be surprised if your Edelbrock and/or Thermoquad need some mods to work. Usually fuel delivery problems show up at the big end, there should be enough reserve fuel in the bowls to handle the launch.

But yes your fuel system can only flow as much as the smallest part and it sounds like you have different sizes throughout; it should all be the same size at least 3/8" all the way.
 
I would replace the whole fuel line with at least 1/2 inch or bigger right out of the cell. but sounds like you are sloshing the fuel bowls I had this happen to me in my old duster I had to put tubes in the fuel bowls slosh tubes
 
get a fuel pressure gauge on the car if you don't have one already.
mount an electric pump in the rear and get rid of the mechanical pump.
 
will try the demon on the car and see if this fixes it . if it doesn't it could be the fuel sloshing . will also move the fuel filter . The only way for me to get rid of the foot and a half of 5/16 is to replace the pump again that will be last because i am not running out of fuel at top end . Timing at 34 deg plus vac advance i think was 12 deg . tryed unhooking vac advance and did not change the the bog. Thank you to all . will have to do more testing
 
went your foot is on the floor there is no vacuum....so you have no vacuum advance

afb secondairies throttle blades are opened mechanically ...but there is an air door that does not allow air to flow thru the secondaires until engine rpms come up...the air door is not adjustable....can not see the car being out of gas less then one second after launch....

either the fuel is sloshing and killing the engine....or you launch rpm is not high enough for the engine too move the car....hence bogging...
 
1\2" fuel line and higher rpm. I used to launch my 71 Duster at 5300-5500 rpm; it was 440, 4-speed, and 4.10 gear. Everyone thought I was crazy to launch that high of rpm, but their cars were much lighter than mine and that's the rpm she liked. Anything less and she would fall on her nose and stumble off the line. Put some throttle to her; she'll like it!
 
will try taking off at 4500 . I have a summit racing air filer on it and some one told me not to run a air filter at the track so i took it off for one pass and didn't make a change . I do have a fuel pressure gauge on the regulator.
 
-4 an is VERY bad.

1/4" fuel line has no place on a car like yours.
 
u do not need a hood scoop, #4 is way small-should be 6 or 8 or 3/8", it is not fuel sloshing - I drop the clutch at 5000 with slicks with an AVS and run 11.7 with no scoop, super stock 340 cars run 10s with AVS and thermoquads and no scoop
 
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