carb stumble???

[whitey70ss]

I recently changed the carb and intake on my sbc (350 .60 over .510 lift cam 292 duration,edelbrock rpm heads 10.5-1 comp. victor jr intake 750 dp) now when I feather the accelerator to take off (4 spd) it has a slight dead spot and it has a low speed miss?? But as soon as I get past that spot in the accelerator pedal it clears right up. I was planing on going to the track sat. for test and tune, and was wondering what I should be looking for to cure this problem, I already played with the accelerator pump adjustment and checked the floats, and low speed mixers. thanks in advance.

[fla1976]

Have you read your plugs. Sounds like you need to rejet the carb.

[whitey70ss]

no I have not read the plugs. I havent had it to the track yet or any place I could wind it up and shut it down . any other suggestions.

[whitey70ss]

any idea what size jets I should be running with this combination?? I havent pulled the bowls off to see yet.

[fla1976]

I'd pull the bowls and see what it has. Then, I'd jump the size a couple and retry it.

[whitey70ss]

the fronts are 66 the rear is 72H does this sound right??

[oldandtired]

the fronts are 66 the rear is 72H does this sound right??
What is the model number? That looks too lean for a 750.

4779 came with 75/76 and 8.5 PV
4779-1 came with 70/80 and 8.5 PV
4779-2 came with 70/80 and 6.5 PV
4779-3 came with 70/73 and 6.5 PV
4779-4 came with 70/80 and 6.5 PV
4779-5 came with 70/80 and 6.5 PV (same as the dash 4)
4779-6 came with 71/80 and 6.5 PV
84779 came with 70/73 and 6.5 PV

There are also some other OEM and marine part numbers.

Dave

[whitey70ss]

I drove to the (local)speed shop today and talked to a guy that teaches a hi-po class at the local college. he told me I needed a 45 power valve I had a 65 and told me to jet it 72 in the fronts and 80 in the rears. I put it together and now its raining :cry: it does seem to have better throttle response now :roll: I guess I'll see sat.

[OneBadGMC]

That's a pretty big jet spread. Rear cylinders are going to get more fuel than the front.

Last I heard (haven't run carbs in a long time), 4 jets were the biggest spread you want to do from front to rear.

Maybe someone with better carb knowledge can comment on that.

[fla1976]

I always tried to run mine square. All four the same. I guess there is no real reason except that it was race only.

[whitey70ss]

oh there is no powervalve in the back block. does that make a difference?

[fla1976]

Without a power valve, I've been told that you have to jet up a couple to compensate. There are some good carb guys on this forum, so I'm sure they can set you straight.

[whitey70ss]

It doesnt seem like there has been that many guys on here lately?? :?: I was hoping someone could direct me in the right direction.

[cepx111]

You were definetly too lean with them 66's on the primaries.

When you added that single plane manifold you killed a ton of bottom end torque but gained alot of upper end HP, jetting it up the primaries should help out the stumble.

If it were me and I wasn't runnning a P/V I'd jet it 80 sqaure as a starting point, then after making a full pass I would read the plugs and jet up or down as needed.
let us know how it pans out.
Cp

[itsabird]

i agree with cepx, and with the front pv, you are running the carb square as you gain 6 to 8 jet sizes with it.

[whitey70ss]

I drove the car this morning, carb works 100 percent better. the stumble is gone.I am going to test and tune sat. after I make a few passes should I try gradually bringing the primary jets up in size???

[itsabird]

i would baseline the car with a few run, reading the plugs, takeing note of the mph, then go up 2 jet sizes, if mph increases, then you were a little lean, if mph drop, you are a little rich, i like to keep my jets square.

[whitey70ss]

OK guys I appreciate all the help. I'll let ya know how I make out.

[whitey70ss]

well I didnt make out real well today I only got 2 passes in and they was enough to make any judgement. the first pass was 13.78 and I missed 4th gear. then I had a leaky fuel cap problem and decided to take the exhaust off and the car ran a 13.77 and I had no shifting issues. :lol: . I went for another pass and stage the car and left the clutch and my driver side axle twisted off. :shock: Has anyone ever used superior axles?? the summit rep highly recomended them to me ... I'm still a little stumped on second pass being almost the same as the first. could it be that my engine likes a little back pressure?? I will post the time slips in order below.LET ME KNOW WHAT YOU THINK . THANKS IN ADVANCE!



RT...247
60'...2.094
330..5.791
1/8...8.807
MPH..82.31
1/4...13.787
MPH..99.03




RT...120
60'...2.058
330..5.805
1/8...8.873
MPH..80.84
1/4....13.771
MPH...101.59