650 dp

[mustang_gt88]

I just put carb on and tuned it, the issue is when i dump the gas the car stumbles then pics back up, if i slowing push the pedal its fine. Any thoughts?

[mustang_gt88]

could it be the needle valve? the reason i ask is that the back one will not adjust enough to level the float level. Do they make longer valves? i tried adjusting the acclerator cam. it got alittle better. car runs and idels great and if i slowly give it gas it opens you great.

[859tkw]

is it auto or stick?

[mustang_gt88]

Car is a 5-speed

[859tkw]

could still be a little fat on the back i have always been a fan of vac secondary cause of that reason. if u go from a dead stop wide open thru gears is it fine?

[mustang_gt88]

I have not be able to get it on the street yet. Its been rain the past to days. When you say fat ont the back do you mean running rich? I notice when i dump it the front sprays first then the back kicks in.

[859tkw]

yea thats how its supposed to be. the stumble is prob from dumping too much fuel when u hit the back barrels it can be stressful getting one jetted right sometimes

[mustang_gt88]

so i need to change the rear jets?

[859tkw]

that would be my guess if u want u can call me and maybe i can help ya out 8597579513

[mustang_gt88]

That would be great, what time is best to call?

[859tkw]

when ever ya want im off today cause of the rain

[mustang_gt88]

thanks for the help

[mustang_gt88]

I ended up getting the back bowl adjusted in, the problem was the tang on the float was bent. The car does still have a stumble but not nearly as bad as it was. I think Im headed in the right direction. Any thoughts on what jetts to use? Larger or smaller?

[2Fast4You]

Is it a new carb or used?
If it is used have you changed the accellerator pumps on it?
That will cause that to happen or maybe just adjusting some things on there. Make sure your vacuum lines to the carb are secure or plugged as needed.

[mustang_gt88]

This is a used carb and yes i did a rebiuld kit on it. I tried adjusting the spring tension on the pump arms and nothing. Also changed the position and nothing. The Vaccum line in the back is hooked to the brake bosster, the front is plugged, should this be hooked up?
Could it be i have 30 cc pumps and need to step down to 20 ccc?

[mustang_gt88]

the carb does have a pink accellerator cam which is the second heavyiest. Should I change to lighter cam?

[85_SS_302_Coupe]

I would start from the beginning on this carb. Make sure the idle is set right, and the throttle blades are set right and you don't have too much of the transfer slot uncovered. You can play with pump cams all day and it may only mask the issue. If changing pump cams makes a difference then it sounds like you're starving for fuel at the stumble, not flooding out. Also if the jets were too big in back, i don't think it would cause a stumble unless you had some ridiculous size difference between the front and back so it was really dumping a crazy amount of fuel when the secondaries open.

What size power valve are you running and have you checked that yet?

[mustang_gt88]

I have not check power valve size, What do you mean by the throttle blades being set right? If i take the car up to 2-3k rpm and dump its fine. Also if i give it gas alittle a couple of times in a row and dump its fine.

[mustang_gt88]

I also un hooked the link between the 2 and if the fronts are dumped by them self it still stumbles, but if the back is dumped the stumble i 95% gone.

[85_SS_302_Coupe]

I also un hooked the link between the 2 and if the fronts are dumped by them self it still stumbles, but if the back is dumped the stumble i 95% gone.

So if you unhook the secondaries and it still stumbles, then it has nothing to do with the secondary jets or anything related to the secondaries at all, so you've ruled that out.

If this is an off cruise stumble, then definitely check the power valve. You're having an issue when it goes from the cruise circuits to the main jets and that's where the power valve comes in to add fuel during the transition.


As for the throttle blades, the transfer slot is a little slit that's in the back bowls that the throttle blades will partially cover. You want that adjusted so that all you can see of the slot is a perfect little square. That might not make sense until you look at it and see what i'm talking about.

Is this mechanical or vacuum advance?

[mustang_gt88]

This is mechanical, Thanks for the info

[mustang_gt88]

check this link on how to tune http://www.nastyz28.com/~ericf/tech/htune.pdf

[mustang_gt88]

i checked and i changed from 7.5 to 6.5 on the power valve, seem to be alittle better. Could there be an issue with me running a victor jr style intake?

[mustang_gt88]

So I adjusted the throttle slot and no bog in the back, put in a old 6.5 and its a little better but there is a back fire now, damn carbs.

[2Fast4You]

Put a new power valve in it, the old one my have a small tare and that would cause the back fire.

[mustang_gt88]

Im going to put a new 6.5 in, also going to change the primary jetts to 70 and the shooters to 31.

[85_SS_302_Coupe]

Man i've never had to play with all that crap just to make a car run right. Accelerator cams and shoots are things you play with to make a car faster, not just simply run correctly. I'd put that thing back to as original as you can get it and start from there.

[mustang_gt88]

I changed the shooters to 28 from 25, changed pv to 4.5 and the front primarys to 70, Stumble is gone but i think im running rich, i got the front bowl adjusted but not the back. I also have a popping or back fire some times.