stumble on acceleration - The 1947 - Present Chevrolet & GMC Truck Message Board Network

crm318 · 05-10-2007, 02:01 AM

i just aquired a 78 cheyy van that i could barley get home due to a shot accelorator pump (qjet). i rebuilt the carb and there is a day and night difference in performance. the only thing is that there is a stumble on acceloration. i have rebuilt 9 qjets successfully and everything in this one was fully referbished. the motor is all factory and i set the timing to 8 degrees. if i punch it from a stop, it misfires and falls on its face. if i feather it slightly and then give it juice, it has great power. what might cause this stumble?

Slammed70 · 05-10-2007, 03:47 AM

Bakfires eh...

Only thing I can think is ignition, pull the plugs/cap, clean them up with a wire brush. If it runs any better.....then replace them.

GREASEMONKEY72 · 05-10-2007, 04:00 AM

i agree about checking the plugs and cap and dont forget points if it has them, and make sure if your timing light light as a dial on that its at 0 and recheck the timing, and make sure the carb is adjusted right

Blazer1970 · 05-10-2007, 06:34 AM

Advance the timing a little bit.

jkeady · 05-10-2007, 07:00 AM

Did you put a new accelerator pump in it? If so, did you compare the old and the new, and did you get the linkage back in place? I'm thinking maybe you have too small of an accelerator pump shot.

piecesparts · 05-10-2007, 07:35 AM

Is it a Carter or Rochester Qjet? I had one years ago do the same basic thing and found out that the Carter built Qjet is of a lower quality than the Rochester. The secondary needles fit down into the body through a couple of brass sleeves on the Rochester (good thing), the Carter carb has no brass sleeves and the body erodes away in time and the acceleration is bad, especially in a transition from cruising to power mode. This wass ppointed out to me by an old mechanic in my town, I replaced the body and it worked great.

RustyParts · 05-10-2007, 08:39 AM

Might want to check the pickup coil in the distibutor also, the two wires were prone two breaking because of the movement of the coil by the vacuum advance.

When the engine is idling the coil is stationary but as it accelerates the advance would move the coil and the wires would break contact momentarily causing the engine to stumble.

crm318 · 05-10-2007, 11:58 AM

i should had mentioned that it has brand new plugs, cap and wires. i did replace the accelorator pump and compared them. the olp one was a little crusty. ill give the ignition on more look over. thanks for the cool advice.

Hooker · 05-10-2007, 12:08 PM

Mine Was Doing The Same Thing Until This Weekend.... Check The Vacume Advance On The Dizzy...... I Changed Mine And It Does Not Stumble Anymore!!!!!!!!!!!!!!!!!!!!!!

killer71 · 05-10-2007, 12:26 PM

Can the vaccum advance be purchased at any auto store?

Hooker · 05-10-2007, 12:43 PM

yes.... make sure you know what kind of dist you got and i would take the part in... there are so many around that it might be hard to get the right one..... i looked through 15 boxes before i found the right one.... they are a simple and easy thing to replace!

jhow66 · 05-10-2007, 01:40 PM

Pump shot is your problem. Check to make sure the pump moves as soon as the throttle blades do. Is your pump rod in the right hole? Did you set the pump rod adjustment?

72BLUZ · 05-10-2007, 10:16 PM

That Sounds Like Timing To Me. Back It Up A Little And Try It

crm318 · 05-11-2007, 10:45 AM

what hole should i use on the arm? the one closest to the carb or the other? how much does a vacuum advance cost? i will recheck the timing and give everything a good look over this weekend.

mnunn454 · 05-11-2007, 07:12 PM

Sounds to me like either

(a) the accel pump isn't squirting enough fuel in when the front throttle plates open, or
(b) the needle/jets are stuck or closing too much, such that the regular a/f mixture is too lean and the pump squirt isn't enough to overcome it.

Do you have an O2 sensor and a/f ratio gauge? If not how white are the plugs?

I'd try enriching the pump squirt and seeing if that helps.

crm318 · 05-11-2007, 10:19 PM

which hole on the accelorator pump arm gives the most shot of fuel?

jhow66 · 05-12-2007, 01:20 AM

Read this :
http://corvetteactioncenter.com/kb/q....php?qstId=444

eedgell · 05-12-2007, 05:40 PM

The hole farthest out gives the most and longest squirt of juice, because it has the longest travel

cruiserman · 11-16-2007, 06:11 PM

Quote:

Originally Posted by eedgell

The hole farthest out gives the most and longest squirt of juice, because it has the longest travel

Just looking through Qjet threads, and I found this reply. The hole closer to the pivot gives more fuel.

**VetteVet** · 11-16-2007, 06:21 PM

Try this site, I got a 404 on Jhow's site,Don't know if it's the same one.

http://www.lbfun.com/Corvette/Tech/v...ng%20Paper.pdf

05-10-2007, 02:01 AM	#1
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	stumble on acceleration i just aquired a 78 cheyy van that i could barley get home due to a shot accelorator pump (qjet). i rebuilt the carb and there is a day and night difference in performance. the only thing is that there is a stumble on acceloration. i have rebuilt 9 qjets successfully and everything in this one was fully referbished. the motor is all factory and i set the timing to 8 degrees. if i punch it from a stop, it misfires and falls on its face. if i feather it slightly and then give it juice, it has great power. what might cause this stumble? __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

05-10-2007, 04:00 AM	#3
GREASEMONKEY72 Registered User Join Date: Aug 2006 Location: Bakersfield,CA Posts: 7,893	Re: stumble on acceloration i agree about checking the plugs and cap and dont forget points if it has them, and make sure if your timing light light as a dial on that its at 0 and recheck the timing, and make sure the carb is adjusted right __________________ -steven Alte Seele S.S. "Allison" '72 C/10 LWB 350SB/TH350 - DD PITBULL IS NOT A CRIME

05-10-2007, 06:34 AM	#4
Blazer1970 Registered User Join Date: Dec 1999 Location: Old Mission, MI, USA Posts: 2,507	Re: stumble on acceloration Advance the timing a little bit. __________________ Tim

05-10-2007, 07:00 AM	#5
jkeady Registered User Join Date: Apr 2004 Location: Salem, OR Posts: 127	Re: stumble on acceloration Did you put a new accelerator pump in it? If so, did you compare the old and the new, and did you get the linkage back in place? I'm thinking maybe you have too small of an accelerator pump shot. __________________ Josh '70 Custom Camper / 20

05-10-2007, 07:35 AM	#6
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: stumble on acceloration Is it a Carter or Rochester Qjet? I had one years ago do the same basic thing and found out that the Carter built Qjet is of a lower quality than the Rochester. The secondary needles fit down into the body through a couple of brass sleeves on the Rochester (good thing), the Carter carb has no brass sleeves and the body erodes away in time and the acceleration is bad, especially in a transition from cruising to power mode. This wass ppointed out to me by an old mechanic in my town, I replaced the body and it worked great.

05-10-2007, 08:39 AM	#7
RustyParts Registered User Join Date: May 2006 Location: Wyman, Kentucky Posts: 403	Re: stumble on acceloration Might want to check the pickup coil in the distibutor also, the two wires were prone two breaking because of the movement of the coil by the vacuum advance. When the engine is idling the coil is stationary but as it accelerates the advance would move the coil and the wires would break contact momentarily causing the engine to stumble. __________________ 1972 GMC Jimmy, 4WD, 454 CI, 350 Turbo, 4" lift, 35" tires The math teacher said my son "Bubba" could be a mathmatical genius. cause he's got 13 fingers!

05-10-2007, 11:58 AM	#8
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: stumble on acceleration i should had mentioned that it has brand new plugs, cap and wires. i did replace the accelorator pump and compared them. the olp one was a little crusty. ill give the ignition on more look over. thanks for the cool advice. __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

05-10-2007, 12:08 PM	#9
Hooker aka TEX-TOY Join Date: Jan 2007 Location: Kentucky Posts: 127	Re: stumble on acceleration Mine Was Doing The Same Thing Until This Weekend.... Check The Vacume Advance On The Dizzy...... I Changed Mine And It Does Not Stumble Anymore!!!!!!!!!!!!!!!!!!!!!! __________________ 1969 Chevy C10 SWB w/ disk brakes and soon to be lowered with spindles and springs!!!! Getting a paint job like a Army Chinook Helicopter!!!! HOOKERS RULE!! BRAND NEW WIFE BOUGHT IT FOR ME AS A WEDDING GIFT!!! SHE IS THE BOMB!!!!!!

05-10-2007, 12:26 PM	#10
killer71 Registered User Join Date: May 2006 Location: at work Posts: 2,552	Re: stumble on acceleration Can the vaccum advance be purchased at any auto store? __________________ R.I.P. Killer 71 10/26/07

05-10-2007, 12:43 PM	#11
Hooker aka TEX-TOY Join Date: Jan 2007 Location: Kentucky Posts: 127	Re: stumble on acceleration yes.... make sure you know what kind of dist you got and i would take the part in... there are so many around that it might be hard to get the right one..... i looked through 15 boxes before i found the right one.... they are a simple and easy thing to replace! __________________ 1969 Chevy C10 SWB w/ disk brakes and soon to be lowered with spindles and springs!!!! Getting a paint job like a Army Chinook Helicopter!!!! HOOKERS RULE!! BRAND NEW WIFE BOUGHT IT FOR ME AS A WEDDING GIFT!!! SHE IS THE BOMB!!!!!!

05-10-2007, 01:40 PM	#12
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Re: stumble on acceleration Pump shot is your problem. Check to make sure the pump moves as soon as the throttle blades do. Is your pump rod in the right hole? Did you set the pump rod adjustment? __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

05-10-2007, 10:16 PM	#13
72BLUZ Registered User Join Date: Nov 2006 Location: lafayette, ga. Posts: 832	Re: stumble on acceleration That Sounds Like Timing To Me. Back It Up A Little And Try It __________________ Freedom is never more than one generation away from extinction. We didn't pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same.... Ronald Reagan

05-11-2007, 10:45 AM	#14
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: stumble on acceleration what hole should i use on the arm? the one closest to the carb or the other? how much does a vacuum advance cost? i will recheck the timing and give everything a good look over this weekend. __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies

05-11-2007, 07:12 PM	#15
mnunn454 Registered User Join Date: Dec 2006 Location: Rhode Island Posts: 602	Re: stumble on acceleration Sounds to me like either (a) the accel pump isn't squirting enough fuel in when the front throttle plates open, or (b) the needle/jets are stuck or closing too much, such that the regular a/f mixture is too lean and the pump squirt isn't enough to overcome it. Do you have an O2 sensor and a/f ratio gauge? If not how white are the plugs? I'd try enriching the pump squirt and seeing if that helps. __________________ 69 C-10 SWB Fleetside 454/TH400 (For Sale) http://67-72chevytrucks.com/vboard/c...ct=1299&cat=12 70 El Camino 454 TH700 29 Model A 4-DR Sedan Deluxe

05-11-2007, 10:19 PM	#16
crm318 Alright Alright Alright Join Date: Mar 2004 Location: Midland, Texas Posts: 1,672	Re: stumble on acceleration which hole on the accelorator pump arm gives the most shot of fuel? __________________ 1972 Cheyenne Chevrolet red and white 357ci, Vortec heads, GM Performance high rise intake, Comp 268H .454/.454 218/218 110 LSA Quadrajet carb, Flowtech headers, 2.25 40 series HEI, Curve kit, TH 350, 3.07 gears 275/60/15s on 15x8 rallies



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05-12-2007, 01:20 AM	#17
jhow66 Registered User Join Date: Jun 2002 Location: Knoxville Tenn. Posts: 3,058	Re: stumble on acceleration Read this : http://corvetteactioncenter.com/kb/q....php?qstId=444 __________________ 56 Chevy Bel-Air 2dr. HT (purchased new) 71 Chevy Cheyenne SWB PU (502HO) 65 GMC short bed step--work in progress and my gofer

05-12-2007, 05:40 PM	#18
eedgell It's the air up here Join Date: Mar 2007 Location: Western Colorado Posts: 710	Re: stumble on acceleration The hole farthest out gives the most and longest squirt of juice, because it has the longest travel __________________ 1971 cheyenne k10 4 speed np 205 454 BB factory a/c Never drink down stream of the herd! Eric

11-16-2007, 06:21 PM	#20
VetteVet Msgt USAF Ret Join Date: Jan 2005 Location: Kalamazoo, Michigan Posts: 8,719	Re: stumble on acceleration Try this site, I got a 404 on Jhow's site,Don't know if it's the same one. http://www.lbfun.com/Corvette/Tech/v...ng%20Paper.pdf __________________ VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative.

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