need help getting the 350 up n running - The 1947 - Present Chevrolet & GMC Truck Message Board Network

69heavychevy · 05-15-2009, 01:39 PM

my friend and i got through installing a brand new gm crate motor on his 72. but for some reason we cant get it to run right. when he puts in drive and hits the pedal it sputter and backfire through the carb. but i ease onto the pedal it will accelerate fine till 20mph or so and then start backfiring. we've checked and double checked wires, vacuum hoses, timing, and just about everything else. like i said he has a brand new gm crate motor (260hp i believe) 1-5/8 long tube headers (open, no exhaust), edelbrock performer rpm, edelbrock 1406 carb, msd street fire dizzy set to 8 degrees, th400 with posi 3.08. in park we can mash the pedal all day with no sputtering or backfire, but as soon as it goes into gear it starts acting up. any idea of what the problem could, thanks in advance for your help.

nsuengineer06 · 05-15-2009, 02:47 PM

I am no expert on the matter, but sounds to me like timing and/or carb tuning. If it's not the timing directly, you may have a crack or small hole in the vacuum advance (if so equipped). Hopefully someone with a lil more knowledge will chime in soon for ya.. Best of luck!

**americanmusc1e** · 05-15-2009, 03:30 PM

you don't have any vacuum leaks do you????
and you do have the vacuum advance hooked to ported vacuum (almost no vacuum at idle--vacuum ocurs at higher rpm)???
put a vacuum guage on it if you have one
I doubt its in the carb, its got to be ignition, did you run a full 12v to the dizzy or use the factory resistor wire on the dizzy????

cdowns · 05-15-2009, 03:48 PM

did you make sure it was at TDC on THE COMPRESSION STROKE when installing dizzy?

**americanmusc1e** · 05-15-2009, 03:52 PM

Quote:

Originally Posted by cdowns

did you make sure it was at TDC on THE COMPRESSION STROKE when installing dizzy?

I thought about that, but I don't think it'll run like that, just backfire when your trying to start it

Irishpilot · 05-15-2009, 07:13 PM

I'm also leaning towards TDC issue. Double check to make sure the RRs weren't set off of the #6 piston. If you are sure they were set off of #1 TDC then I would still reset the valve-lash.

This comes down to a process of elimination.

69heavychevy · 05-16-2009, 02:36 AM

yea we made sure dizzy was installed at tdc on #1, n the vacuum advanced is hooked up to ported vacuum nipple on the carb (the one on pass side). will a bad modulator cause any of these symptoms?? the dizzy does have adjustable vacuum advanced so we will probably start messing with that next.

05-15-2009, 02:47 PM	#2
nsuengineer06 Registered User Join Date: Feb 2009 Location: Natchitoches, Louisiana Posts: 230	Re: need help getting the 350 up n running I am no expert on the matter, but sounds to me like timing and/or carb tuning. If it's not the timing directly, you may have a crack or small hole in the vacuum advance (if so equipped). Hopefully someone with a lil more knowledge will chime in soon for ya.. Best of luck! __________________ 1967 C10 LWB 454/TH400 1971 C10 SWB 1984 C30 LWB Dually 454/4 spd 4:10 2003 GMC Sierra 2500HD Crew 4x4 6.6L Duramax/Allison

05-15-2009, 03:30 PM	#3
americanmusc1e Senior Member Join Date: Jan 2009 Location: Austin, TX Posts: 1,079	Re: need help getting the 350 up n running you don't have any vacuum leaks do you???? and you do have the vacuum advance hooked to ported vacuum (almost no vacuum at idle--vacuum ocurs at higher rpm)??? put a vacuum guage on it if you have one I doubt its in the carb, its got to be ignition, did you run a full 12v to the dizzy or use the factory resistor wire on the dizzy???? __________________ -- Josh Instagram: @americanmusc1e OLD SKOOL-- 1970 C10. 454/Th400/3.07 posi Build Thread FARM TRUCK----1949 Chevrolet 3800 Power Wagon Hauler Build Thread 1999 4wd OBS Tahoe - daily. DM me if you can't see photos i posted

05-15-2009, 03:48 PM	#4
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: need help getting the 350 up n running did you make sure it was at TDC on THE COMPRESSION STROKE when installing dizzy? __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

05-16-2009, 02:36 AM	#7
69heavychevy Registered User Join Date: Jan 2007 Location: Garland,TX Posts: 431	Re: need help getting the 350 up n running yea we made sure dizzy was installed at tdc on #1, n the vacuum advanced is hooked up to ported vacuum nipple on the carb (the one on pass side). will a bad modulator cause any of these symptoms?? the dizzy does have adjustable vacuum advanced so we will probably start messing with that next.

05-15-2009, 01:39 PM	#1
69heavychevy Registered User Join Date: Jan 2007 Location: Garland,TX Posts: 431	need help getting the 350 up n running my friend and i got through installing a brand new gm crate motor on his 72. but for some reason we cant get it to run right. when he puts in drive and hits the pedal it sputter and backfire through the carb. but i ease onto the pedal it will accelerate fine till 20mph or so and then start backfiring. we've checked and double checked wires, vacuum hoses, timing, and just about everything else. like i said he has a brand new gm crate motor (260hp i believe) 1-5/8 long tube headers (open, no exhaust), edelbrock performer rpm, edelbrock 1406 carb, msd street fire dizzy set to 8 degrees, th400 with posi 3.08. in park we can mash the pedal all day with no sputtering or backfire, but as soon as it goes into gear it starts acting up. any idea of what the problem could, thanks in advance for your help.

05-15-2009, 07:13 PM	#6
Irishpilot Registered User Join Date: Jul 2006 Location: Kunsan AB, South Korea Posts: 187	Re: need help getting the 350 up n running I'm also leaning towards TDC issue. Double check to make sure the RRs weren't set off of the #6 piston. If you are sure they were set off of #1 TDC then I would still reset the valve-lash. This comes down to a process of elimination. __________________ 1971 C-10, 406ci, restoration begun! 406ci sb; zero decked, bored .030, edelbrock aluminum e-tec heads, edelbrock performer RPM intake, comp 268 cam, comp 1.6 pro-magnum roller rockers, MSD street fire HEI, MSD 8.5mm wires, Holley Street demon 770, Hedman LT headers. 10.6:1 compression, Air Ride Tech bags on all 4-corners.



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