stumble off idle 750 DP - Page 2 - Team Camaro Tech
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post #16 of 33 (permalink) Old Aug 8th, 18, 05:44 PM Thread Starter
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Re: stumble off idle 750 DP

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Originally Posted by X-77 keith View Post
Doug, I'm not a numbers guy either. Yes my 302 had a 780 vacuum secondary. I was not happy with the carb . I had a 750 double pumper built for my car by Biggs Carbs. I know everyone says it is too big for my 302, but it runs great. This is what I'm running. "See attachment" I know that there are smart people out there making charts and YouTube videos saying what works, and what doesn't work. There is always a exception to the rules I guess. I have no bog and the car loves the combo I have.
Hey Keith,

What color pump cam is in yours?

Thanks
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post #17 of 33 (permalink) Old Aug 8th, 18, 05:45 PM Thread Starter
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Re: stumble off idle 750 DP

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Lots of info out there on pump cams like found at this link:

Holley accelerator pumps/cams | Grumpys Performance Garage
Thanks for the link, alot of info there.
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post #18 of 33 (permalink) Old Aug 8th, 18, 05:47 PM Thread Starter
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Re: stumble off idle 750 DP

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is the accelerator pump adjusted properly
Hey,

I think so, they are both fairly tight on the 15 thou feeler gauge and react immediately to the throttle.
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post #19 of 33 (permalink) Old Aug 8th, 18, 05:58 PM
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Mine has a kinda dark green cam.

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post #20 of 33 (permalink) Old Aug 9th, 18, 12:36 PM Thread Starter
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Re: stumble off idle 750 DP

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Originally Posted by X-77 keith View Post
Doug, I'm not a numbers guy either. Yes my 302 had a 780 vacuum secondary. I was not happy with the carb . I had a 750 double pumper built for my car by Biggs Carbs. I know everyone says it is too big for my 302, but it runs great. This is what I'm running. "See attachment" I know that there are smart people out there making charts and YouTube videos saying what works, and what doesn't work. There is always a exception to the rules I guess. I have no bog and the car loves the combo I have.
Hey Keith,

Do you know how much vacuum you were making? Your power valve size is 4.5 but stock would have been 6.5. From what I understand the power valve should be half of the vacuum your intake is making. I checked mine and it's making 12-13 cm at idle (700 rpm).

Thanks

Last edited by Grifter; Aug 9th, 18 at 01:20 PM.
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post #21 of 33 (permalink) Old Aug 9th, 18, 01:40 PM
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If the power valve is supposed to be about half, Jessie the guy that built was spot on. I have between 8 and 9 .

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post #22 of 33 (permalink) Old Aug 9th, 18, 06:15 PM
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Re: stumble off idle 750 DP

I had the same issue with an Hp 850, I ended up with a 50 cc in the front, just like prosystems does on most of their carbs.

I have used the white and pink cams the most

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post #23 of 33 (permalink) Old Aug 9th, 18, 07:17 PM
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Re: stumble off idle 750 DP

"It has a MSD ready to run distributor with timing at 25 initial and 43 total with no vacuum advance. It has a mild cam"

If your cam is "mild" and pulls at least 13 inches of vacuum at idle, your advance would be much higher than 25 at low speeds.

Adding vacuum advance might be what you need to get rid of the stumble.

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post #24 of 33 (permalink) Old Aug 9th, 18, 08:21 PM Thread Starter
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Re: stumble off idle 750 DP

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Originally Posted by NashvilleCat View Post
"It has a MSD ready to run distributor with timing at 25 initial and 43 total with no vacuum advance. It has a mild cam"

If your cam is "mild" and pulls at least 13 inches of vacuum at idle, your advance would be much higher than 25 at low speeds.

Adding vacuum advance might be what you need to get rid of the stumble.
I didn't set up the distributor, my engine installer did, he said it's "pre curved". But when I changed the intake I read the timing it was 25 at idle and 43 at 3000 rpm, it has the smallest limiter bushing from MSD at 18 degrees.

I had a holley Street Avenger 770 on before the DP carb and it launched fine but had a hesitation when stabbed while driving. The 770 had a 31 shooter and green cam.
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post #25 of 33 (permalink) Old Aug 9th, 18, 08:52 PM
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Re: stumble off idle 750 DP

Man that's a lot of timing. I'm surprised it will start without kickback if initial is 25. I don't know your car, but I would expect 36-38 would be max to run on the street.
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post #26 of 33 (permalink) Old Aug 10th, 18, 02:33 AM
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Re: stumble off idle 750 DP

Quote:
25 initial and 43 total with no vacuum advance
Agree with above....too much timing. I would back it down to 16* , and may need to adjust carb ?

I went to an AFR gauge so I can see real time adjustments and no need to swap plugs, make a pull, check plugs, and repeat.

I think when you back your timing down, and make those carb adjustments, you'll see more vacuum, and a better running car.

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post #27 of 33 (permalink) Old Aug 10th, 18, 05:56 AM
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Re: stumble off idle 750 DP

[QUOTE=Grifter;1783080411]I didn't set up the distributor, my engine installer did, he said it's "pre curved". But when I changed the intake I read the timing it was 25 at idle and 43 at 3000 rpm, it has the smallest limiter bushing from MSD at 18 degrees.

Not sure what the installer meant by "pre-curved". MSD RTR are very adjustable and come with different advance stop bushings and advance rate springs; do you have these? Sounds like pre-curved meant he just put it in as it was and didn't want to take the time to properly recurve it for your engine if that was necessary. If you're not running any vac advance (Beth's suggestion to get one is a good idea, but with a new DP you're not going to be able to do that since you have a secondary with a separate accelerator pump). With your "pre-curved" 25 deg at idle and 18 mechanical, 43 total is a lot of advance. I'd think you would not want to exceed 36 degrees all in. As suggested below, back off the initial timing. Generally, if you stab the throttle and it bogs, you need a bigger shooter to get more initial fuel delivery.

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post #28 of 33 (permalink) Old Aug 10th, 18, 06:10 AM
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Re: stumble off idle 750 DP

In one of the earlier postings Doug asked about the primary transfer slot exposure. If you have too much (more than about 0.02" - 0.04") you will begin having problems with too rich idle and poor off idle acceleration. Transition from idle is the purpose of the T slots and if too much is exposed the transition will not be a smooth one.

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post #29 of 33 (permalink) Old Aug 10th, 18, 06:55 PM
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Re: stumble off idle 750 DP

38 total for the street and 40 at the track is the most you should run, back it down to 20 with your 18 mechanical and you are right there. The retro does come with a vacuum advance, #8360. I believe in vacuum on the street

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post #30 of 33 (permalink) Old Aug 27th, 18, 04:52 PM Thread Starter
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Re: stumble off idle 750 DP

A quick update..

I installed a #35 shooter in the primary and changed the green pump cam to the blue one.

I first tried the shooter change alone and there was a stumble, when I put in the orange pump cam it was better but stumbled once in a while. Now with the #35 primary and #31 secondary shooters and the blue pump cam it seems the stumble is gone.

I looked at changing the secondary pump cam and keeping the orange primary cam but it seems that the stock pink secondary cam is as aggressive as the secondary will take.

Thanks

Jeff
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