It's alive...almost....please help! - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Chevelle454 · 08-04-2004, 12:01 AM

Finally I have everything I need to start up my '72 C20 with the 355 I rebuilt. I know the timing is right and I have spark, the valves are set right, but it's not firing up.

It will just backfire through the carb, but not the exhaust, and it won't try to fire and turn the motor at all. I checked compression, it was at 60 PSI in cylinder #1. I don't think that's a good thing...

Any suggestions on what could be the problem with the motor? It ran before I rebuilt it, lol...

Thanks,
Scott

72 CustomCamper · 08-04-2004, 12:09 AM

My big block was backfiring through the carb with low compression numbers too. Come to find out I had a valve that wasn't seating. I reseated the valve and it brought the compression number back up to 150 psi. Try uooking a air compressor up to your compression gauge hose and pressureize the cylinder and listen for air leaks at the carb. You can do this with the valve covers off and back the rocker arms backed off adjusting them down one at a time to find out which one it is...

Chuck

**kwmech** · 08-04-2004, 12:18 AM

Valves may be too tight, and dist. may be off 180. When the timing chain dots are lined up the engine fires on #6.

Chevelle454 · 08-04-2004, 10:24 AM

The distributor is right, I adjusted the valves until there was slight drag and then an additional 3/4 turn. This is really frustrating after waiting all this time to finally start it...

dubie · 08-04-2004, 10:46 AM

If it back fires thru the carb or exhaust, your timing could be off. I had the EXACT same thing happen to me when starting my new 350. I thought i had the distributor perfectly in there but I was out just enough to make it back fire. I also loosened off the vaves by 2 turns and then re adjusted then while the truck was running. Also run thru your firing order with the plugs again. I learned the hard way to always check the simple stuff first.

Chevelle454 · 08-04-2004, 02:14 PM

I backed out all the valves so that I know for sure that they closing all the way. The #1 cylinder went from 60 psi to 90 psi, but all the rest are between 30 and 60 PSI.

I'm thinking that maybe the valves are not seating(I put in new seats for unleaded gas, and I ground the valves and the seats). Maybe I had the valves too tight before and they got bent so they don't close? How important is it to lap the valves(because I didn't do it)?

I triple checked the timing, it is right on. The only thing that I can think of for the bad compression is the heads, because the pistons, rings, and all gaskets are brand new...

I want to get new heads, but I can not spend that kind of money while in school. Does anyone have any heads that need seals and maybe valves that they want to sell for around $100?

Please help...
-Scott

Longhorn Man · 08-04-2004, 06:12 PM

I have to agree about the timing...just for fun, bring the engine to #1 TDC, and check it out.
If the engine is not cranking over very well, that could well be from a spark plug igniting when the piston is only 1/2 way up. (way out of time)...you have tried jump starting it right?
On your compression test, you removed all the plugs and blocked the throttle all the way open while you cranked it with a fully charged battery, right? Anything listed there that is not done correctly will lower the cranking compression.
If it sounds like I am treatin you like a moron, please don't take it as so...I find many times the hardest problems to solve are the ones that we don't troubleshoot starting at the basics.

glennhuff · 08-04-2004, 10:01 PM

Sounds like your cam is retarded several teeth. The intake valve is still open when the piston comes up on compression. That is why it is backfiring through the carb. That's also why cranking compression is low. unfortunately that means that you have to drop the front of the pan and pull everything off the front of the engine to check the cam timing.

Glenn

Chevelle454 · 08-04-2004, 10:59 PM

I did a leak test on it. I put a air compressor line hooked in to where the spark plug goes with a hose meant for holding up the valves. I could hear air rushing out of both the intake and the carb. I took the valve cover off to make sure that both rockers were loose.

Do you think that maybe I need to lap the valves? Or is it leaking way too much for that to do any good?

08-04-2004, 12:09 AM	#2
72 CustomCamper huh ??? Join Date: Nov 2002 Location: Fredericksburg,Va. Posts: 3,369	My big block was backfiring through the carb with low compression numbers too. Come to find out I had a valve that wasn't seating. I reseated the valve and it brought the compression number back up to 150 psi. Try uooking a air compressor up to your compression gauge hose and pressureize the cylinder and listen for air leaks at the carb. You can do this with the valve covers off and back the rocker arms backed off adjusting them down one at a time to find out which one it is... Chuck __________________ 90 Blazer Silverado 350w/ 700R4 and 32" Kelly's "I am a bomb technician. If you see me running try to keep up!"

08-04-2004, 10:24 AM	#4
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	The distributor is right, I adjusted the valves until there was slight drag and then an additional 3/4 turn. This is really frustrating after waiting all this time to finally start it... __________________ 1972 C10 1972 C20

08-04-2004, 10:46 AM	#5
dubie Registered User Join Date: May 2000 Location: Flin Flon, Manitoba, Canada Posts: 8,855	If it back fires thru the carb or exhaust, your timing could be off. I had the EXACT same thing happen to me when starting my new 350. I thought i had the distributor perfectly in there but I was out just enough to make it back fire. I also loosened off the vaves by 2 turns and then re adjusted then while the truck was running. Also run thru your firing order with the plugs again. I learned the hard way to always check the simple stuff first. __________________ My name's Tim and I'm a truckaholic My 56 Chevy shop truck build http://67-72chevytrucks.com/vboard/s...d.php?t=562795

08-04-2004, 10:59 PM	#9
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	I did a leak test on it. I put a air compressor line hooked in to where the spark plug goes with a hose meant for holding up the valves. I could hear air rushing out of both the intake and the carb. I took the valve cover off to make sure that both rockers were loose. Do you think that maybe I need to lap the valves? Or is it leaking way too much for that to do any good? __________________ 1972 C10 1972 C20

08-04-2004, 12:01 AM	#1
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	It's alive...almost....please help! Finally I have everything I need to start up my '72 C20 with the 355 I rebuilt. I know the timing is right and I have spark, the valves are set right, but it's not firing up. It will just backfire through the carb, but not the exhaust, and it won't try to fire and turn the motor at all. I checked compression, it was at 60 PSI in cylinder #1. I don't think that's a good thing... Any suggestions on what could be the problem with the motor? It ran before I rebuilt it, lol... Thanks, Scott __________________ 1972 C10 1972 C20

08-04-2004, 12:18 AM	#3
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,599	Valves may be too tight, and dist. may be off 180. When the timing chain dots are lined up the engine fires on #6.

08-04-2004, 02:14 PM	#6
Chevelle454 Registered User Join Date: Aug 2003 Location: MN Posts: 380	still not working... I backed out all the valves so that I know for sure that they closing all the way. The #1 cylinder went from 60 psi to 90 psi, but all the rest are between 30 and 60 PSI. I'm thinking that maybe the valves are not seating(I put in new seats for unleaded gas, and I ground the valves and the seats). Maybe I had the valves too tight before and they got bent so they don't close? How important is it to lap the valves(because I didn't do it)? I triple checked the timing, it is right on. The only thing that I can think of for the bad compression is the heads, because the pistons, rings, and all gaskets are brand new... I want to get new heads, but I can not spend that kind of money while in school. Does anyone have any heads that need seals and maybe valves that they want to sell for around $100? Please help... -Scott __________________ 1972 C10 1972 C20

08-04-2004, 06:12 PM	#7
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	I have to agree about the timing...just for fun, bring the engine to #1 TDC, and check it out. If the engine is not cranking over very well, that could well be from a spark plug igniting when the piston is only 1/2 way up. (way out of time)...you have tried jump starting it right? On your compression test, you removed all the plugs and blocked the throttle all the way open while you cranked it with a fully charged battery, right? Anything listed there that is not done correctly will lower the cranking compression. If it sounds like I am treatin you like a moron, please don't take it as so...I find many times the hardest problems to solve are the ones that we don't troubleshoot starting at the basics.

08-04-2004, 10:01 PM	#8
glennhuff Registered User Join Date: Oct 2003 Location: Myrtle Point Oregon Posts: 24	Sounds like your cam is retarded several teeth. The intake valve is still open when the piston comes up on compression. That is why it is backfiring through the carb. That's also why cranking compression is low. unfortunately that means that you have to drop the front of the pan and pull everything off the front of the engine to check the cam timing. Glenn



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