Engine Stumble Question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

lts70 · 05-29-2009, 02:23 PM

My 1970 stumbles a little if I punch it from a dead stop if I ride the clutch a little and get the rpms up before taking off it does fine. Other wise it runs great. It has a built 327, HEI, Edelbrock Air gap intake, Edelbrock carb 650, headman long tube headers, 9.5 to 1 compression, Crane RV cam.

I had a similar problem with my previous engine I had in the truck (307) and the only component I swapped over was the GM HEI distributor. Could it be a counter wait spring issue? I pulled the distributor out of a stock 84 Chevy truck I believe. Is there any thing I can do to check my hunch or do you guys have some other ideas that could be the culprit?

tim21391 · 05-29-2009, 07:26 PM

is the 84 chevy dist you pulled points? i had this problem on my 72.. turned out my points werent set right

berencam · 05-29-2009, 07:40 PM

Sounds like a fuel issue to me, or mabee timing not set right. Weak accelerator pump?

**El Jay** · 05-29-2009, 07:44 PM

Accelerator pump?

The accelerator pump in my '72 (402 BB) was dried up after sitting for a couple of years when I bought it.
I've driven the truck for 3 years without replacing it; I just have to "ease" into the throttle when I roll off from a stop.
I don't know how much money I've save in tickets and tires; but it's probably a lot.

beautimus · 05-29-2009, 07:49 PM

heh heh, people beat me to it !!

there is a couple of things i look for when i have a problem like you are describing.
first, the accelerator pump. i am not familiar with the carb you have but it should be a plunger arrangement that is actuated when the throttle is opened. to check it, with the engine off, open the choke by hand and look down the barrel of the carb. then push the throttle open, you should see a jet of gas spray into the throat of the carb. It could be a weak pump, that does not spray enough, or it may need adjustment, like move the linkage to the pump to another hole in the arm.

next, look for vaccuum leaks, cracked hoses or hoses not connected.

the last option that comes to mind is a worn timing chain and gears.
when you step on the gas the slack in the chain pulls tight giving you the hesitation. there is other stuff that can cause this but thats all that comes to mind. good luck!!!

jim zag · 05-30-2009, 05:44 AM

Accelerator pump may not work instantly when you work the throttle. You may need a bigger squirter.

lts70 · 06-01-2009, 01:43 PM

I will take a look at my accelerator pump tonight and see if the squirt starts as soon as I move the throttle. That is how you check it right? If that is it I will see if there is any adjustment that can be made. Thanks guys.

mjensen · 06-01-2009, 10:56 PM

After looking into the fuel issues, I would recommend reading up on tuning that HEI for your application. I found several great articles through this website that were very helpful.

Lemield · 06-02-2009, 08:32 AM

I had the same problem on my C20 with an Edelbrock 750 CFM carb. I called Edelbrock and they suggested getting a stepup spring kit and changing out my stepup springs. I cannot recall which color spring I used but it took care of the problem. Really easy to do too. If you don't have the manual that came with your carb, you can get a copy online at Edelbrocks website.

lts70 · 06-02-2009, 04:05 PM

Quote:

Originally Posted by mjensen

After looking into the fuel issues, I would recommend reading up on tuning that HEI for your application. I found several great articles through this website that were very helpful.

Do you have a link?

mjensen · 06-02-2009, 04:10 PM

Here's what was provided to me. It's very helpful for the ripping apart and adjusting these dizzys.

http://www.rustpuppy.org/ignition2/Ignition%202b.htm

05-29-2009, 02:23 PM	#1
lts70 1970 Daily Driver Join Date: Jun 2007 Location: Glenns Ferry IDAHO Posts: 1,038	Engine Stumble Question My 1970 stumbles a little if I punch it from a dead stop if I ride the clutch a little and get the rpms up before taking off it does fine. Other wise it runs great. It has a built 327, HEI, Edelbrock Air gap intake, Edelbrock carb 650, headman long tube headers, 9.5 to 1 compression, Crane RV cam. I had a similar problem with my previous engine I had in the truck (307) and the only component I swapped over was the GM HEI distributor. Could it be a counter wait spring issue? I pulled the distributor out of a stock 84 Chevy truck I believe. Is there any thing I can do to check my hunch or do you guys have some other ideas that could be the culprit? __________________ It's not truly yours if you didn't build it yourself or if you don't do your own maintenance.

05-29-2009, 07:26 PM	#2
tim21391 Registered User Join Date: Jul 2007 Location: Salina, Kansas Posts: 1,998	Re: Engine Stumble Question is the 84 chevy dist you pulled points? i had this problem on my 72.. turned out my points werent set right __________________ 1972 C-10...402/400..flowmaster 40 series 1967 Mustang 347 stroker/C-4 1966 Mustang 289/3-speed 2013 Mustang V6/6speed w/300 ponies I may be 23, but i sure do knows 67-72s are sexy! Its not MPG its smiles per gallon! build started 11/25/08 build thread: http://67-72chevytrucks.com/vboard/s....php?p=2993796

05-29-2009, 07:40 PM	#3
berencam Registered User Join Date: Aug 2007 Location: branson mo Posts: 933	Re: Engine Stumble Question Sounds like a fuel issue to me, or mabee timing not set right. Weak accelerator pump? __________________ 69 chevy c10 350 3 speed manual wood bed build thread here r.i.p. 70 c10 auto 350 build here

05-29-2009, 07:44 PM	#4
El Jay Gone to greener pastures Join Date: May 2007 Location: Gateway to the Delta Posts: 7,354	Re: Engine Stumble Question Accelerator pump? The accelerator pump in my '72 (402 BB) was dried up after sitting for a couple of years when I bought it. I've driven the truck for 3 years without replacing it; I just have to "ease" into the throttle when I roll off from a stop. I don't know how much money I've save in tickets and tires; but it's probably a lot. __________________ '69 Chevy 1/2 T LWB Stepper: Daily Beater '72 GMC 3/4 T Fleet: Another Daily Beater '72 Plymouth Gran Coupe: ? "Ah women. They make the highs higher and the lows more frequent." Friedrich Nietzsche "Never kick a fresh turd on a hot day." Harry S. Truman GUN CONTROL: Never having to say, "I missed you." Always fire two warning shots into your attacker's chest area before putting a bullet between his eyes. Paraphrased from Louis Awerbuck Last edited by El Jay; 05-29-2009 at 07:47 PM.

05-29-2009, 07:49 PM	#5
beautimus Registered User Join Date: Mar 2002 Location: woodstock, alabama, usa Posts: 938	Re: Engine Stumble Question heh heh, people beat me to it !! there is a couple of things i look for when i have a problem like you are describing. first, the accelerator pump. i am not familiar with the carb you have but it should be a plunger arrangement that is actuated when the throttle is opened. to check it, with the engine off, open the choke by hand and look down the barrel of the carb. then push the throttle open, you should see a jet of gas spray into the throat of the carb. It could be a weak pump, that does not spray enough, or it may need adjustment, like move the linkage to the pump to another hole in the arm. next, look for vaccuum leaks, cracked hoses or hoses not connected. the last option that comes to mind is a worn timing chain and gears. when you step on the gas the slack in the chain pulls tight giving you the hesitation. there is other stuff that can cause this but thats all that comes to mind. good luck!!! __________________ i survived cancer and kidney stones, but my kids are killing me. Last edited by beautimus; 05-29-2009 at 07:53 PM.

05-30-2009, 05:44 AM	#6
jim zag ENGINE BUILDER Join Date: Apr 2006 Location: Shortsville n.y. Posts: 344	Re: Engine Stumble Question Accelerator pump may not work instantly when you work the throttle. You may need a bigger squirter. __________________ JIM ZAG

06-01-2009, 01:43 PM	#7
lts70 1970 Daily Driver Join Date: Jun 2007 Location: Glenns Ferry IDAHO Posts: 1,038	Re: Engine Stumble Question I will take a look at my accelerator pump tonight and see if the squirt starts as soon as I move the throttle. That is how you check it right? If that is it I will see if there is any adjustment that can be made. Thanks guys. __________________ It's not truly yours if you didn't build it yourself or if you don't do your own maintenance.

06-01-2009, 10:56 PM	#8
mjensen Registered User Join Date: Jun 2006 Location: Utah Posts: 1,562	Re: Engine Stumble Question After looking into the fuel issues, I would recommend reading up on tuning that HEI for your application. I found several great articles through this website that were very helpful. __________________ Mike '69 GMC 3/4 ton 4x4 Build Thread

06-02-2009, 08:32 AM	#9
Lemield Registered User Join Date: Dec 2004 Location: Arvada Colorado Posts: 1,321	Re: Engine Stumble Question I had the same problem on my C20 with an Edelbrock 750 CFM carb. I called Edelbrock and they suggested getting a stepup spring kit and changing out my stepup springs. I cannot recall which color spring I used but it took care of the problem. Really easy to do too. If you don't have the manual that came with your carb, you can get a copy online at Edelbrocks website. __________________ Larry 1949 Willys CJ2A, Kubota Turbo Diesel engine, Warn Overdrive, 11 inch brakes, 12volt, Warn M8000 Winch "Little Green" 1970 Rustoleum Roller Red C20 Custom Camper 454 TH400 "Big Red" 1971 K20 6.2 Turbo Diesel 4x4 4spd Ranger Overdrive "Pumpkin Wagon" 2005 VW Beetle Diesel 44+ MPG "Silver Bullet" Last edited by Lemield; 06-02-2009 at 08:33 AM.

06-02-2009, 04:10 PM	#11
mjensen Registered User Join Date: Jun 2006 Location: Utah Posts: 1,562	Re: Engine Stumble Question Here's what was provided to me. It's very helpful for the ripping apart and adjusting these dizzys. http://www.rustpuppy.org/ignition2/Ignition%202b.htm __________________ Mike '69 GMC 3/4 ton 4x4 Build Thread



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