carb stumble???
 

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.

stumble
 

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

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.

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

jeys
 

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

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

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.

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.

Jets
 

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

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

Valve
 

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.

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

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

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.

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???

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.

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

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