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#1 |
Alright Alright Alright
Join Date: Mar 2004
Location: Midland, Texas
Posts: 1,672
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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?
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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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#2 | |
Registered User
Join Date: Oct 2006
Location: Cheney, KS
Posts: 669
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Re: stumble on acceloration
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.
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#3 |
Registered User
Join Date: Aug 2006
Location: Bakersfield,CA
Posts: 7,893
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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
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-steven Alte Seele S.S. "Allison" '72 C/10 LWB 350SB/TH350 - DD PITBULL IS NOT A CRIME |
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#4 |
Registered User
Join Date: Dec 1999
Location: Old Mission, MI, USA
Posts: 2,510
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Re: stumble on acceloration
Advance the timing a little bit.
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Tim |
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#5 |
Registered User
Join Date: Apr 2004
Location: Salem, OR
Posts: 127
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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.
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Josh '70 Custom Camper / 20 |
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#6 |
Parts and more parts
Join Date: Nov 2003
Location: Lebo, Kansas (middle of nowhere
Posts: 6,821
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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.
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#7 |
Registered User
![]() Join Date: May 2006
Location: Wyman, Kentucky
Posts: 403
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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.
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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! |
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#8 |
Alright Alright Alright
Join Date: Mar 2004
Location: Midland, Texas
Posts: 1,672
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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.
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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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#9 |
aka TEX-TOY
Join Date: Jan 2007
Location: Kentucky
Posts: 127
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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!!!!!!!!!!!!!!!!!!!!!!
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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!!!!!! ![]() |
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#10 |
Registered User
Join Date: May 2006
Location: at work
Posts: 2,552
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Re: stumble on acceleration
Can the vaccum advance be purchased at any auto store?
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R.I.P. Killer 71 10/26/07 |
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#11 |
aka TEX-TOY
Join Date: Jan 2007
Location: Kentucky
Posts: 127
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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!
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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!!!!!! ![]() |
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#12 |
Registered User
Join Date: Jun 2002
Location: Knoxville Tenn.
Posts: 3,058
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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?
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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 |
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#13 |
Registered User
Join Date: Nov 2006
Location: lafayette, ga.
Posts: 832
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Re: stumble on acceleration
That Sounds Like Timing To Me. Back It Up A Little And Try It
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![]() It must be fought for, protected, and handed on for them to do the same.... Ronald Reagan |
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#14 |
Alright Alright Alright
Join Date: Mar 2004
Location: Midland, Texas
Posts: 1,672
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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 |
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#15 |
Registered User
Join Date: Dec 2006
Location: Rhode Island
Posts: 602
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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.
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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 |
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#16 |
Alright Alright Alright
Join Date: Mar 2004
Location: Midland, Texas
Posts: 1,672
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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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#17 |
Registered User
Join Date: Jun 2002
Location: Knoxville Tenn.
Posts: 3,058
|
Re: stumble on acceleration
__________________
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 |
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#18 |
It's the air up here
Join Date: Mar 2007
Location: Western Colorado
Posts: 710
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Re: stumble on acceleration
The hole farthest out gives the most and longest squirt of juice, because it has the longest travel
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1971 cheyenne k10 4 speed np 205 454 BB factory a/c ![]() Never drink down stream of the herd! Eric |
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#19 |
Registered User
Join Date: Dec 2006
Location: Idaho
Posts: 210
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Re: stumble on acceleration
Just looking through Qjet threads, and I found this reply. The hole closer to the pivot gives more fuel.
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72 C2500 LWB 402 Sierra Grande 05 Odyssey 08 Tundra |
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#20 |
Msgt USAF Ret
![]() ![]() Join Date: Jan 2005
Location: Kalamazoo, Michigan
Posts: 8,714
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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
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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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