Hesitation of the line and backfire. - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Jefe · 05-03-2003, 12:12 AM

Once in a while in the Nova, (see signature Holley 750 Vacume Secondarys), it will backfire when I start off too quickly...once in a great great while, and most of the time it will studder or hesitate off the line just a little. Carb is new, motor less than 2,000 miles on it. Help. Thanks guys.

Douglas.

theo70 · 05-03-2003, 12:22 AM

Ahh...I see you have the same Holley Problems I'm having. No answers for you here, but I will be watching this thread, too. By the way, how can you tell the CFM by sight on Holleys? The p/o installed it.

lngbed71 · 05-03-2003, 12:34 AM

Vacum or mechanical advance?

jef5150 · 05-03-2003, 12:34 AM

new or rebuilt carb?????the hesitation is likley a bad accelerator pump in the carb,as far as the backfire,timing too advanced,or you could have a stuck "intake" valve

bigjimzlll · 05-03-2003, 12:35 AM

check the accelerator pump discharge...with the engine off..look into the carb..and move the accellerator linkage..there should be a steady spray of fuel from the squirters..if you have a good ..squirt..then richen out the idle enrichment screws a 1/4 turn at a time..if the idle goes up..then lower the idle adjustment
screw..keep richening until the bog/lag goes away
here is a good link http://www.nastyz28.com/perftune.html#carb

dinnut · 05-03-2003, 12:39 AM

you should give it to me.

Class of 69 · 05-03-2003, 07:32 PM

These guy's are right, check the accel pump. You didn't say what the cam is. A performance cam will want a little more fuel off the line, and the backfire could be the accel pump or not enough initial timing, again a perf cam will want more timing than stock. You could have the idle set too low for a perf cam also, inducing the hesitation, and backfire. It's probably something simple, so don't start changing out whole carbs, and cams, like I see some guy's doing. Work with what you have, and ,make small changes till you get what you want.

Jefe · 05-04-2003, 01:20 AM

Thanks guys, my intuition tells me something minor also. No part replacement is needed really. Just adjustment and so forth, timing is 10 Degrees initial, mechanical advance, idle is like 900 in drive. I don't have the vacume advance hooked up on the HEI because it didn't seem to make a difference and would ping pretty bad.

* Advertised duration: 292 intake/292 exhaust
* Duration at .050 in. cam lift: 244 intake/244 exhaust
* Gross valve lift: .501 in. intake/.501 in. exhaust
* Lobe separation: 110 degrees
* RPM range: 2,500 to 6,500

* 292H Magnum Comp Cam

Douglas.

05-03-2003, 12:12 AM	#1
Jefe Registered User Join Date: Jul 2000 Location: Chandler, Arizona, USA Posts: 299	Hesitation of the line and backfire. Once in a while in the Nova, (see signature Holley 750 Vacume Secondarys), it will backfire when I start off too quickly...once in a great great while, and most of the time it will studder or hesitate off the line just a little. Carb is new, motor less than 2,000 miles on it. Help. Thanks guys. Douglas. __________________ -1968 Chevy SWB = Frame Off/383/Muncie Car 4 speed -1968 Chevy SWB = numbers matching 307/Truck 4 speed. -1966 Chevy II = 2 door No Post/130,000 Orig miles/230 6cyl/powerglide/Vintage AC -1972 Chevy Nova = 2 door/412 Small Block -1927 Ford Roadster = Homebuilt/widened 8"/350/ Custom Everything.

05-03-2003, 12:34 AM	#3
lngbed71 Nothing fits but the oil Join Date: Mar 2003 Location: Gaston, Indiana Posts: 424	Vacum or mechanical advance? __________________ Land of the free Because of the brave

05-03-2003, 12:35 AM	#5
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	check the accelerator pump discharge...with the engine off..look into the carb..and move the accellerator linkage..there should be a steady spray of fuel from the squirters..if you have a good ..squirt..then richen out the idle enrichment screws a 1/4 turn at a time..if the idle goes up..then lower the idle adjustment screw..keep richening until the bog/lag goes away here is a good link http://www.nastyz28.com/perftune.html#carb __________________ It's called "drag racing" if they called it "tic..tic..WHAM!..BANG! F*&K!!!", they'd have to keep the magazines under the counter with the other men's publications click the clicky to join the site.... http://67-72chevytrucks.com/vboard/payments.php 67 lwb..first hotrod in 25 years..540 best ET is 9.45 @ 141.44 Anderson,CA

05-03-2003, 12:39 AM	#6
dinnut Registered User Join Date: Apr 2003 Location: missouri city, tx Posts: 4,486	you should give it to me. __________________ 70 longbed 496/th400/Ford 9"

05-03-2003, 07:32 PM	#7
Class of 69 Registered User Join Date: Mar 2003 Location: Stillwater MN Posts: 1,166	These guy's are right, check the accel pump. You didn't say what the cam is. A performance cam will want a little more fuel off the line, and the backfire could be the accel pump or not enough initial timing, again a perf cam will want more timing than stock. You could have the idle set too low for a perf cam also, inducing the hesitation, and backfire. It's probably something simple, so don't start changing out whole carbs, and cams, like I see some guy's doing. Work with what you have, and ,make small changes till you get what you want. __________________ 69 GMC K10 ..some restoration required....still..

05-03-2003, 12:22 AM	#2
theo70 COMING SOON... Join Date: Feb 2003 Location: Ft Worth TX Posts: 408	Ahh...I see you have the same Holley Problems I'm having. No answers for you here, but I will be watching this thread, too. By the way, how can you tell the CFM by sight on Holleys? The p/o installed it.

05-03-2003, 12:34 AM	#4
jef5150 Inline 69 Join Date: Apr 2003 Location: valleysprings,california Posts: 1,018	new or rebuilt carb?????the hesitation is likley a bad accelerator pump in the carb,as far as the backfire,timing too advanced,or you could have a stuck "intake" valve

05-04-2003, 01:20 AM	#8
Jefe Registered User Join Date: Jul 2000 Location: Chandler, Arizona, USA Posts: 299	Thanks guys, my intuition tells me something minor also. No part replacement is needed really. Just adjustment and so forth, timing is 10 Degrees initial, mechanical advance, idle is like 900 in drive. I don't have the vacume advance hooked up on the HEI because it didn't seem to make a difference and would ping pretty bad. * Advertised duration: 292 intake/292 exhaust * Duration at .050 in. cam lift: 244 intake/244 exhaust * Gross valve lift: .501 in. intake/.501 in. exhaust * Lobe separation: 110 degrees * RPM range: 2,500 to 6,500 * 292H Magnum Comp Cam Douglas. __________________ -1968 Chevy SWB = Frame Off/383/Muncie Car 4 speed -1968 Chevy SWB = numbers matching 307/Truck 4 speed. -1966 Chevy II = 2 door No Post/130,000 Orig miles/230 6cyl/powerglide/Vintage AC -1972 Chevy Nova = 2 door/412 Small Block -1927 Ford Roadster = Homebuilt/widened 8"/350/ Custom Everything.



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