Engine stumbles upon acceleration

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Engine stumbles upon acceleration

Postby v8astregt » Wed Jun 28, 2006 11:38 pm

I drove my Astre up the road for the first time since the new engine. For the most part it ran great... now it idles good, takes off good, etc. BUT... when I get to roughly 10-15 mph, the engine starts to stumble, surge, hesistate, whatever you wanna call it. I'm just guessing the speed since I don't have a speedo yet, but it was before the trans could even shift to second. It's kinda of annoying, because that's all that's left before I can drive it. What's wierd is that it does it when I'm just accelerating normally, meaning when I stomp on it, it runs and runs great! At first I thought the accelerator pump, but I replaced that with a brand new one out of another carb.

Oh, and the specs... it has a 355 with 10:1 compression, Dart Pro1 aluminum heads, RPM intake, XE268H cam, TH350 with shift kit, and a Holley 750 with a Proform main body.

Any ideas? Thanks in advance.
75 Pontiac Astre GT: 406SBC/TH350, roll bar, S10 goodies, down to 11.47 so far, more to come later.
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Postby Buscop » Wed Jun 28, 2006 11:53 pm

I'd say your over carbed. A 355 is quite happy with about 670 CFM. The jets should be about 68 to 70. My 355 with 10.5:1 compression would not run right with a 770 Street Avenger then I went to a 670 and she purrs. :lol:
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Postby v8astregt » Thu Jun 29, 2006 12:13 am

One thing I can think of is that I cut the primary jets down several sizes when I had the stock 350 under the hood. I would like to say 65, but I think it may be even less?!?! I think I'll try the bigger jets tomorrow.

I have a lot of 750's sitting around, but nothing smaller. :?
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Postby gerbsinmd » Thu Jun 29, 2006 7:31 am

I'm not exactly sure what parts are on the Holley you have but sounds to me like the squirters for the primarys may be too big. I had the same kind of thing going on with my Dad's car last week and we had to put smaller ones in on both the primary and secondarys. If you look on the squirters there is a number, smaller number is a smaller hole/amount of gas. They are where the gas "squirts" out of from the accelerator pump. Since they are held in by a single screw, easy to change and try.

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Postby Buscop » Thu Jun 29, 2006 8:13 am

Try this link, this fellow is a wizard with Holley Carbs

http://www.bob2000.com/carb.htm
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Postby marco_1978_spyder » Thu Jun 29, 2006 9:22 am

I'm not saying its timing and not fuel related, but have you poured over what your timing components are doing?

make sure that the vac advance -if your running it-

is providing timing where you need it, and that the centrifical advance (or mechanical advance if thats how you know it) is operating smoothly, and not sticking or comming on to soon or late.

I'd also retourqe the carb and manifold bolts and check for any missed vac leaks.
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Postby v8astregt » Thu Jun 29, 2006 11:55 am

I'll definitely retorque all the bolts since I haven't checked them since I broke the cam in.


And on the ignition, it's an MSD Pro-billet without any vacuum advance... it's an all new system with no miles on it.


I'm gonna go outside shortly and play with the bolts, and maybe the jets.
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Postby Buscop » Thu Jun 29, 2006 1:55 pm

v8astregt wrote:I'll definitely retorque all the bolts since I haven't checked them since I broke the cam in.


And on the ignition, it's an MSD Pro-billet without any vacuum advance... it's an all new system with no miles on it.


I'm gonna go outside shortly and play with the bolts, and maybe the jets.

I just finished setting up my carb and MSD HEI Pro Billet dist. ( no vac secondaries) and she runs real sweet.
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Postby v8astregt » Thu Jun 29, 2006 2:27 pm

I GOT HER!



When I had that carb on my last bone stock 350, for some reason I really jetted it down... I pulled out two #60 jets! :shock:

Only 12 sizes too small!!!!!

New jets are in, and she runs like a beast! :twisted:
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Postby gerbsinmd » Thu Jun 29, 2006 2:48 pm

Yep, that'd do it.....

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Postby marco_1978_spyder » Thu Jun 29, 2006 8:46 pm

rock solid.


Are you sure you dont want to run vac advance on the street???
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stumbles

Postby mldeolde » Sun Jul 09, 2006 3:11 pm

play with the springs on that distriutor. from my own persoal expierience on a simular 355 sbc in my 74 nova sleeper it can make a world of difference in the performance and i agree with the carb info given by others. i only run a slightly tweeked 650 dp holley and its a stump puller and still revs to 6500 rpm with out hesitation which is about optimal for a daily driver street car :lol:
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Postby v8astregt » Sun Jul 09, 2006 4:01 pm

I got the problem all fixed. Runs great now.




And maybe a 650 is what "should" be on there, but I have 750's laying around, not 650's, so I use what I got.
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Postby craggar » Fri Jul 14, 2006 9:43 am

The 750 isn't too big at all if it's tuned right. I'll bet if you put a 650 on it the only difference you'll find is that it's a little more responsive on the bottom end.
I have a little more cam and compression then you and I run an 850 on my 355.
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