Backfire coming out of carb - The 1947 - Present Chevrolet & GMC Truck Message Board Network

jayssunm · 03-31-2003, 01:32 PM

I recently installed a Holley 600 on 350 engine in my 67 Chevy SWB. I finally did get it started thanks to several people suggestions on this board (THANKS!). But now it will not stay running. She starts then after a bout a second (For lack of a better description) seems to choke and a backfire comes out of the carb.

Any suggestions???

I have tried to advance and retard the dist ever so slightly, but seems to either not start at all or choke.

**John Fabris** · 03-31-2003, 02:18 PM

Was the engine running Ok before you added the Holley?
If I where to look at your truck, I would start with making sure that points are set correctly, then find #1 piston TopDeadCenter, set distributor into place with rotor pointing to #1, put cap on, install plug wires in correct firing order (and rotation). With all of that set, pull the Carb off, and adjust the carb so that the primary butterflys are closed such that when you look at the bottom of the carb, you will see a vertical slot, you want that slot to appear as a square when the butterflys are closed. Then adjust the two air/fuel mixure screws to be turned out 2 to 2 1/2 times. then put carb on. If you know how, fill float bowl with a little bit of gas. Then try to start engine. It should run. If not, then tell us what it is doing (do you hear engine trying to start, or just spins, starts, then soon runs rough and dies, etc...)

tom hand · 03-31-2003, 02:36 PM

If it was running ok before you put the Holley on, my first thought would be a major vacuum leak i.e. forgot to hook the power brakes back up. Many Holley's have a place for a fitting in the back of the baseplate for a fitting for power brakes that you have to put a plug in if you are not useing it, Backfires really good if you leave it out...don't ask me how I know this...lol

bigjimzlll · 03-31-2003, 04:20 PM

I just had thjis same problem...my carb was out of adjustment and was way rich...by the time I had it adjusted properly, it had fouled the plugs so bad it was sputtering and back firing. Check the plugs..see if they are black.
Jim

Southpa · 03-31-2003, 05:08 PM

The last time that happened to me the center pole in my dist. cap was busted up. Must have been a real light show inside.

jayssunm · 04-02-2003, 01:34 PM

Thanks for the input. The backfire was two different things. One the distributor was just a wee bit out of adjustments. And Two (the most important) the carb and carb gasket just barely (1/16 inch) too small for the intake.

The cure---> Purchased a metal plate that goes between the carb and the intake manifold. It is about 1/4" wider than the carb base. It was a vacuum leak that was keeping the carb from running properly! Who knew?

Just need a bolt for the air cleaner and she will be on the road again! But that is when all the work starts... new tranny, rear end gears, and fixing the rust in the cab.

dinnut · 05-01-2003, 10:52 PM

is a wire crossed?

03-31-2003, 02:18 PM	#2
John Fabris TOTY 2006 Join Date: Dec 1999 Location: Magalia CA USA Posts: 2,270	Was the engine running Ok before you added the Holley? If I where to look at your truck, I would start with making sure that points are set correctly, then find #1 piston TopDeadCenter, set distributor into place with rotor pointing to #1, put cap on, install plug wires in correct firing order (and rotation). With all of that set, pull the Carb off, and adjust the carb so that the primary butterflys are closed such that when you look at the bottom of the carb, you will see a vertical slot, you want that slot to appear as a square when the butterflys are closed. Then adjust the two air/fuel mixure screws to be turned out 2 to 2 1/2 times. then put carb on. If you know how, fill float bowl with a little bit of gas. Then try to start engine. It should run. If not, then tell us what it is doing (do you hear engine trying to start, or just spins, starts, then soon runs rough and dies, etc...) __________________ John Fabris 67 C10 SWB

03-31-2003, 02:36 PM	#3
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	If it was running ok before you put the Holley on, my first thought would be a major vacuum leak i.e. forgot to hook the power brakes back up. Many Holley's have a place for a fitting in the back of the baseplate for a fitting for power brakes that you have to put a plug in if you are not useing it, Backfires really good if you leave it out...don't ask me how I know this...lol __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

03-31-2003, 04:20 PM	#4
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	I just had thjis same problem...my carb was out of adjustment and was way rich...by the time I had it adjusted properly, it had fouled the plugs so bad it was sputtering and back firing. Check the plugs..see if they are black. Jim __________________ 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

03-31-2003, 05:08 PM	#5
Southpa Registered User Join Date: May 2000 Location: Victoria, BC, Canada Posts: 2,410	The last time that happened to me the center pole in my dist. cap was busted up. Must have been a real light show inside. __________________ 1970 GMC 1500 Custom Original 350/TH350 Victoria, BC, Canada You can wish in one hand and crap in the other. See which one gets filled first.

04-02-2003, 01:34 PM	#6
jayssunm Registered User Join Date: Jan 2003 Location: San Diego, CA Posts: 98	Thanks for the input. The backfire was two different things. One the distributor was just a wee bit out of adjustments. And Two (the most important) the carb and carb gasket just barely (1/16 inch) too small for the intake. The cure---> Purchased a metal plate that goes between the carb and the intake manifold. It is about 1/4" wider than the carb base. It was a vacuum leak that was keeping the carb from running properly! Who knew? Just need a bolt for the air cleaner and she will be on the road again! But that is when all the work starts... new tranny, rear end gears, and fixing the rust in the cab. __________________ 67 Fleetside Shortie, small window, slip & slide tranny Upgrades--small block, tilt column, disc 5 lug conversion (front and rear), Future Projects--suburban gas tank under the bed, rear roll pan, better gas mileage (currently about 6-10 MPG)

03-31-2003, 01:32 PM	#1
jayssunm Registered User Join Date: Jan 2003 Location: San Diego, CA Posts: 98	Backfire coming out of carb I recently installed a Holley 600 on 350 engine in my 67 Chevy SWB. I finally did get it started thanks to several people suggestions on this board (THANKS!). But now it will not stay running. She starts then after a bout a second (For lack of a better description) seems to choke and a backfire comes out of the carb. Any suggestions??? I have tried to advance and retard the dist ever so slightly, but seems to either not start at all or choke. __________________ 67 Fleetside Shortie, small window, slip & slide tranny Upgrades--small block, tilt column, disc 5 lug conversion (front and rear), Future Projects--suburban gas tank under the bed, rear roll pan, better gas mileage (currently about 6-10 MPG)

05-01-2003, 10:52 PM	#7
dinnut Registered User Join Date: Apr 2003 Location: missouri city, tx Posts: 4,486	is a wire crossed? __________________ 70 longbed 496/th400/Ford 9"



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