[406monte]

I need some help on my 750 Dp it is a 4150 and i just rebuilt the carb and installed it and it runs really good till i get at around 2500 rpm and then it starts to surge and stutter. I don't know why this is happening i have checked everything i know to check. I starts right up and idles fine. I have checked the floats and they are good and the fule preassure is set at 6 lbs. I have 30cc pumps and 6.5 power valve and 67 primary and 72 sec jets in it. I have 31 primary and 37 sec downleg squirters. I put them in there from the 31's that were in there both front and back. Also, if i am taking off fron a dead stop and hammer it down it takes off really hard then it seems to stumble and act like it is trying to float out on me and i have to peddle it to keep it from dying on me. It has done this to me once yesterday and it tried to die on me once today as well but i kept it going. Any help would be apprecitated i want to take this to the local midnight madness and see if it picked anything up from the old 750 vac sec Holley that i had on it.

Eric

 

[576 stroker]

Are the MABs clear? , if there clogged main metering will be fluffy.

Think i'd go higher on the jetting & go try it (72/78) , is the PV opening? , float heights? , maybe try a 50cc pump in the primaries?

Any fuel filters blocked?

 

[406monte]

I checked the fule filters and they are not blocked. i may try and up the jetting to help it and see if that helps. Thanks

 

[Dr. Evil]

Ditch the power valves and jet up. I would run at least 7 psi also but I doubt thats your problem.

I would bet the power valve or valves is part of the problem but some thing else could be adding to it.

I would also suggest getting jet extentions on the secondaries if you don't allready have them. Thats not your problem but its somethin you will need. Don't forget the notched float to go with them.

 

[406monte]

I went and checked the floats again and the rear one was way off some how from the initial settings. I went up to 72 prim and have 73 sec jets in it. I checked the power valve and it is good. I went and readjusted everything and took it down the street and it ran fine now. It has just a slight hardly noticeable stutter and then it is gone. I only felt it cause i was really trying to feel for it. I barely noticed it after a few minutes. I think maybe some more jet and it will be good. Well, i am off to hit the Midnight Madness tonight and see what happens i took it down the steet with my Mickey Thompson Street E.T.'s on it and it hooked so hard and reacted so different that i can't wait till tonight. Wish me luck.

I know some take out the power valve all together and jet up. How much should i go up in the jets and i should make them squared right? I think i might try that and see what happens. I am only running the power valve in the front also.

 

[576 stroker]

Take the PV out when running on the strip & square jet , i run the 605" on the street so like to use a PV up front , keep jetting up till MPH drops off then go back two#s , pull the plugs & fine tune.

 

[406monte]

Well, i it all came back to haunt me at the track. I got two decent runs out it and then the loading up came. I was on the third pass and when i came out of the water box it died. It took me forever to get it started again. I felt like an idiot sitting there holding everyone up. I finally got it started and cleaned up the carb. I made a pass and it only ran a 12.69 but i did short shift it on that pass to. The 4th run and it died again out the box and wouldn't stay running no matter what i did. I just backed it up and took it to the pits. I pulled the screws to check the fuel level and both were right at barely dripping out the screw hole. So, what gives i have no clue as to why this is doing this. Can anyone help me on this huge issue?

I did take my Demon with me as a back up and wouldn't you know it i needed it. I wanted to redeem myself and finish strong. Well, i did and ran my fastest times with it. I had some pretty good sixties as well. I did however have on time with this carb to that died coming out of the box. I got it started and cleaned it up and ran my second fastest of the night. So, all in all i did have a good time and then came home. I just want to get this other carb to run right. I guess i need to do this one the dyno and figure it out there. I did cut a perfect light on my second pass with the Demon. I felt good about that and was the only one tonight to do that.

 

[Drag Chevette]

The main jet controls most cruise & W.O.T. fuel metering.

The main jet under cruise conditions supplies the fuel and this circuit is easiest to tune.

If @ steady speeds (2200-4000 rpm / return road rpm) the engine surges it is too lean.

looking for economy?......You can usually lean the main jets down till you get this surge at 2200 rpm & then richen up the mains by 2 jets #’s to be safe.

its a 750...try 75pmj and 83smj,....with a 5.5pv

 

[Drag Chevette]

I went up to 72 prim and have 73 sec jets in it. I checked the power valve and it is good.

DUDE, when you run a primary power valve and the secondary is blocked....you need to increase the secondary jet 8-10 sizes.

the flow from your power valve channels along with the jet flow need to be close to the jetting in the rear if the rear is plugged.

right now with a 72 prim and 73 sec jets in it and a power valve your about 8 sizes lean on the secoondary........

try jetting it 75-83 and report back.

 

[brandoz28]

a car i worked on liked about a 70-78 jetting with a 6.5pv (might be a 5.5pv been a long time since i've been in that one) might pull the pv out once at the track and go up a few sizes and see how it does

 

[576 stroker]

Check float heights & needles/seats , this is why the motors loading up , would be looking @ P float height.

 

[KylePBZ]

DUDE, when you run a primary power valve and the secondary is blocked....you need to increase the secondary jet 8-10 sizes.

the flow from your power valve channels along with the jet flow need to be close to the jetting in the rear if the rear is plugged.

right now with a 72 prim and 73 sec jets in it and a power valve your about 8 sizes lean on the secoondary........

try jetting it 75-83 and report back.

Same thing I would say.

 

[406monte]

I am going to go and jet the carb 73 prim and 83 sec and see what that does for me. Then i am going to take it back down the street and see how that works and hopefully it won't die on me on a hard load.

 

[T-flow]

Well? Any luck...............

 

[406monte]

Can't it has been raining here off and on. Did the carb but must wait to try it out.

 

[Manic Machinist]

Not knowing what kinda heads and cam you have, I will share my experience with ya. When I had my first pump gas sbc, I plugged both pv's and if I remember correctly, had 82 or 84 jets squared, larger squirters than it came outa tha box with(dont remember how big), and locked out tha timing. It ran bad ass and didnt mind being driven on the street like that either. That was a 355 with Trickflows and a flat tappet solid lifter cam with .525 lift, and approx. 244 degrees @.050" duration. Footbrakin' TH350, TCI Super Streetfighter, and 4.10 gear. It normally ran 12 teens @3200ish lbs with a best of 11.96.

You may find that you need to let out a lil more on the idle air screws. Mine liked 1-1/2 to 2 turns out.(4 corner) If its stalling as you come out of the water box, it prolly is from a sudden lack of fuel when you get off the throttle.

Hope this helps.

 

[406monte]

Well, i have a similiar set up as you do MANIC. I am running a 406 with Trick Flow heads and a hyd roller .558 lift at .246/.254 @.50 on a 112* seperation. I am running 34 degrees total and all in by 2800 rpm. I am footbraking it also with a TCI Streetfighter and a 3.73 gear and i ran in the lower 12.10's to the best Friday night in the low 12.03. My best time is also a 11.96 at 115. Wow, our times and engine combo are really close. I will try and turn the screw out some more as well and see what happens. I wonder what causes the sudden lack of fuel when i get off the throttle to cause it to die.

 

[Manic Machinist]

Im thinkin' it has enough fuel to idle and also "gently" transition to using the main jets, but when you go from 4-5-6000rpm in your burnout, and then suddenly off the throttle, there is not enough fuel coming from the idle circuit to keep the engine running. I seem to remember having asimilar problem, but that was like a long time ago and my wife says never can remember shit. lol

 

[406monte]

How did you fix the issue.

 

[Looey]

Check your pump cam position, should be in the no.1 hole. Also, is your fuel pump able to keep the bowls full?