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Old 01-04-2013, 01:50 PM
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Basic carb question, does stuttering mean lean or rich?

When I say stuttering, what I mean is when you try to give it some gas and it acts like a person trying to talk when they need to clear their throat. That's what is called stuttering with carbs, right?

I see the issue in two spots. First is whenever I try to give it the lightest throttle I can. Like when trying to maintain a cruise speed, or when trying to go from a full stop very gently. If it give it more throttle its smooth as hell. The other time I see it is when trying to gun it in too low of a gear. Like cruising at 30mph in 4th and gunning it instead of down shifting to 3rd first like you normally would. I even get a carb backfire when doing that sometimes.

The only clue my newbie brain can think of is that both of those situations involve the manifold vacuum pressure dropping rather low. I only have about 9hg manifold vacuum, and I just recently switched from a 6.5pv to a 4.5pv. I'm pretty sure this issue started AFTER that swap, but then again, so did the cold weather, so I'm not sure which is to blame for the issue.

In case it matters: It's a basically stock, 1 year old sbc 355 with the smallest thumpr cam, much better than stock aluminium heads, Edlebrock rpm air gap intake, 650 holly DP, shortie headers.

Thanks for any help.

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Old 01-04-2013, 01:58 PM
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Cold weather makes engine run lean. What about the idle and mid range circuts. Can you give it a little more accelerator pump. Usally that helps with stumble. Also is the throttle linkage opening the before the acc. Pump is moving. This could casue the type of problems you talking about.

Do you have vacum and mech advance hooked up? Done any tuning with them.
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Old 01-04-2013, 02:01 PM
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could be timing(not enough) accelerator pump or pump cam.or combination of all.
start with a little more initial timing,check float leavel.advance pump cam.

you did not mention if the car surged when at a steady cruise speed,this could be lean. Ive never had any problems with a 650 DP
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Old 01-04-2013, 02:16 PM
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Quote:
Originally Posted by vinniekq2 View Post
could be timing(not enough) accelerator pump or pump cam.or combination of all.
start with a little more initial timing,check float leavel.advance pump cam.

you did not mention if the car surged when at a steady cruise speed,this could be lean. Ive never had any problems with a 650 DP
Oh it surges like a mad man when under 2k rpm, if by surging you mean the car shacking front to back rapidly. I'm getting 4.11 in the rear this month so hopefully that will help the surging some.

My initial is over 20. I have one light and one medium spring in the dizzy with the smallest mechanical advance put it atm so it hits 36 exactly when all in at 3kish.

I'm also getting a wide band o2 sensor this month, can't wait for that! heh. So, you're saying that stuttering is a lean condition?
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Old 01-04-2013, 02:32 PM
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surging is usually lean
stuttering can be lean/timing/can setting and or adjustment,

lean can be caused by a vacuum leak
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Old 01-04-2013, 03:43 PM
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Air gap intake!
Cold manifold, cold air, puddling fuel equals stutter, stutter, stutter!!
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Old 01-05-2013, 05:50 AM
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I have on O2 sensor installed on my SBC, and the stuttering you describe is usually due to the carb going too lean. When I was using an Edelbrock 1406 I could easily see the effect of changing step-up springs because it changed the point for the cruise to power transition. When the springs were wrong,(transition too late) under acceleration I would see the AFR go high (lean), feel the stutter, and then the AFR would catch up as the rods moved to the power step.

Bruce
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Old 01-05-2013, 06:16 AM
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Quote:
Originally Posted by Zerocyde View Post

In case it matters: It's a basically stock, 1 year old sbc 355 with the smallest thumpr cam, much better than stock aluminium heads, Edlebrock rpm air gap intake, 650 holly DP, shortie headers.
Thanks for any help.
Huh????? Well, anyway, jet up 3 sizes in the primaries and call it a day.
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Old 01-05-2013, 06:44 AM
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Quote:
Originally Posted by RWENUTS View Post
Air gap intake!
Cold manifold, cold air, puddling fuel equals stutter, stutter, stutter!!
Great in warm to hot weather, anything else
I traded my RPM AirGap for a normal RPM, mucho better

AirGap + Thumper =
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Old 01-05-2013, 11:08 AM
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Originally Posted by SSedan64 View Post
Great in warm to hot weather, anything else
I traded my RPM AirGap for a normal RPM, mucho better

AirGap + Thumper =
Are you saying AirGap + Thumpr is bad all the time or just in cold weather? Truth be told I'd rather lose the thumpr than the airgap, but I'm not the most knowledgeable about this stuff yet. It just feels like I could probably find a cam that gives me just as much power as the thumpr does but with less annoying tuning and driving issues.
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Old 01-05-2013, 12:33 PM
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Quote:
Originally Posted by Zerocyde View Post
Are you saying AirGap + Thumpr is bad all the time or just in cold weather? Truth be told I'd rather lose the thumpr than the airgap, but I'm not the most knowledgeable about this stuff yet. It just feels like I could probably find a cam that gives me just as much power as the thumpr does but with less annoying tuning and driving issues.
What gear ratio in the rear end and what tranny final ratio? It sounds like it's lugging from too high a gear /too tight of a converter.

I'd check the accelerator pump linkage, too. It has to deliver fuel the instant the throttle is opened- even a small opening needs to be accompanied by a shot of fuel from the accelerator pump discharge nozzles.

20 degrees initial should be enough for the smallest of the thumpr cams, but you can easily add timing just to see if it helps. Be sure the total timing doesn't go too high when you go to drive it normally. But for testing you can add timing by simply advancing the distributor- just don't go hammering on it like that until you reset/verify your total timing.

Are you using a vacuum advance? If you are, connect it to ported vacuum, and limit it to 10 degrees.
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Old 01-05-2013, 05:04 PM
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Originally Posted by cobalt327 View Post
What gear ratio in the rear end and what tranny final ratio? It sounds like it's lugging from too high a gear /too tight of a converter.
It's manual, but how do I find final ratio? 5th(overdrive) + rear?
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Old 01-06-2013, 01:08 AM
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Quote:
Originally Posted by Zerocyde View Post
Are you saying AirGap + Thumpr is bad all the time or just in cold weather? Truth be told I'd rather lose the thumpr than the airgap, but I'm not the most knowledgeable about this stuff yet. It just feels like I could probably find a cam that gives me just as much power as the thumpr does but with less annoying tuning and driving issues.
The valve overlap of the Thumper can compound the tuning problems the AirGap can cause. May be fine in Warm/Hot weather but a nightmare when it gets cold.

Final Tranny ratio = 5th/OD ratio.
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Old 01-06-2013, 03:01 AM
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Quote:
Originally Posted by Zerocyde View Post
It's manual, but how do I find final ratio? 5th(overdrive) + rear?
What is the OD ratio (or tranny type), and what is the rear gear ratio? But if it's a manual, I'd imagine you know how to drive it to not cause the engine to lug- in other words, keeping the rpm high enough (low enough gear) that the engine stays in the power band.
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Old 01-06-2013, 10:50 AM
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Originally Posted by cobalt327 View Post
What is the OD ratio (or tranny type), and what is the rear gear ratio? But if it's a manual, I'd imagine you know how to drive it to not cause the engine to lug- in other words, keeping the rpm high enough (low enough gear) that the engine stays in the power band.
Yea, I can keep it in a lower gear to keep the rpms high enough to not lug so bad. It's just annoying as hell, especially without a full muffler, lol.

As for the tranny its a world class T5 apparently, with these numbers...

(R)2.76 - (1st)2.95 - (2nd)1.94 - (3rd)1.34 - (4th)1.00 - (5th OD)0.63

And my rear is supposed to be 2.73 based off of the car, but when doing some testing with different speeds and the corresponding rpms, then plugging those numbers into different online "Engine RPM Calculators", the numbers only ever worked if I typed in that my rear was 3.73 so now I'm not sure.
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