79 K10 Back Fire

[Kapdin]

Ok so i have not had a chanse to look into this very far but since im sitting at work and its dead i thought it be a good time to write this up. So my k10 has the 350 and as far as the motor i have changed oil and filter, air filter, spark plugs, plug wires. Thats about it, now keep in mind it is running on LP Gas not gasoline. I contribute the fact that it runs without any carb clean wats so ever after sitting for 7 years to that propaine cus no way in hell would it do that with gas in the carb. I did not have dielectric grease so that might be part but the plugs and wires snapped tight. The plug wires i took off had a diff type of rubber up on the distriputer that snapped into the holder which these new one did not but they fit very tight under the cap so i know they are all pushed down tight. And it starts and runs fine, it sure shakes but you know thats could be normal since im useto my 4cylinder yota lol. I did not notice the back fire untill a little way's down the road and then got worse from their. So either it was running on 7 and the 8 started trying to fire or it started after it got warm. Any suggestions when i get home tonight would be great.

 

[crazy4offroad]

Backfire through exhaust or intake? Intake is usually vacuum leak, or intake valve problem. Exhaust is usually ignition related, or exhaust valve problem.

 

[Kapdin]

Exhaust, im going to get a new distributer cap just for kicks and giggles.

 

[crazy4offroad]

Dont forget the rotor!!!

 

[HotRodPC]

How often and consistant is this backfire? If could just be that some fuel got loose out the exhaust, and was able to get ignited. Sometimes retarded timing causes exhaust backfire too from a late spark.

 

[Kapdin]

It's very constant and on acceleration or just constant throttle

 

[crazy4offroad]

Dont like the sound of that. Could just be a bad wire or plug, but could also be a burnt valve. Vacuum test and compression test would help a lot.

 

[Kapdin]

Well on the vacuum note the hoses are pretty old and prolli cracked but wow there is alot of them. I would love to replace them all but were do you start?

 

[HotRodPC]

Dont like the sound of that. Could just be a bad wire or plug, but could also be a burnt valve. Vacuum test and compression test would help a lot.

I'd love to be wrong, but if it was a bad plug wire or bad plug, there would be no spark to the cylinder and it would be a simple miss, but it's getting it's spark somewhere or it wouldn't backfire. It's rare, but is possible, if raw fuel is exiting one cylinder, and getting ignited in the exhaust manifold from another cylinders burn, but being in the exhaust manifold it's no longer compressed so an explosion is hard to come by. I'd have lean toward the bad vavletrain component if it's consitant. Might even been as minor as a bent pushrod, bad rocker arm, or even a bad valve adjustment. Another bad suggestion and is very very possible if it's consistant, is wiping out a cam lobe.

 

[crazy4offroad]

Since he's running propane it may be likely the unburnt fuel is being ignited in the manifold by another cylinder, propane runs hotter, and I guess it really depends which cylinder it is. 8 and 4 fire close to each other, and so does 5 and 7. By the way Lance when I spoke of a vacuum test I wasnt really meaning replacing the lines, I was talking about this...
http://www.gmsquarebody.com/forum/showthread.php?t=1570
I think a vacuum leak would cause intake backfire anyway.

 

[Kapdin]

Ok so i replaced the rotor and cap and cleaned the coil and connectors and dialectric greased all the wires. And well its starts like a fuggin drean ( like for real its sweet) the back fire is still there but here is some more info. If i just sit in park or netrual and rev it its very week back fire and not as consistan, but in rev or drive its rly bad. So just my two cents, that propain is prolli old as dirt can it get moisture in it like gas? and second after taking off the air cleaner for like the 5th time the two gaskets on the propain injection thingy-mabobber sittin on the carb have seen better days, like a chunk missing on both. Dose any of that ring a bell? I rly need to drain that gas tank and get some good fuel in but i cant seem to just shove a hose down the filler neck? is ther a screen?

 

[HotRodPC]

Since he's running propane it may be likely the unburnt fuel is being ignited in the manifold by another cylinder, propane runs hotter, and I guess it really depends which cylinder it is. 8 and 4 fire close to each other, and so does 5 and 7. By the way Lance when I spoke of a vacuum test I wasnt really meaning replacing the lines, I was talking about this...
http://www.gmsquarebody.com/forum/showthread.php?t=1570
I think a vacuum leak would cause intake backfire anyway.

Yes, do this test. This can tell if you have a burnt valve or a valve not opening etc. And yes, I do keep forgetting this is propane and might be affecting something, but Propane is for certain a gas, not LPG (Liquid Propane Gas) by the time it enters the combustion chamber, but not exactly positive how propane does things different, but I am fairly certain, propane also requires a different timing spec and for a reason. This backfire could be that reason. You could retard the timing about 2 degrees and see if it goes away.

Ok so i replaced the rotor and cap and cleaned the coil and connectors and dialectric greased all the wires. And well its starts like a fuggin drean ( like for real its sweet) the back fire is still there but here is some more info. If i just sit in park or netrual and rev it its very week back fire and not as consistan, but in rev or drive its rly bad. So just my two cents, that propain is prolli old as dirt can it get moisture in it like gas? and second after taking off the air cleaner for like the 5th time the two gaskets on the propain injection thingy-mabobber sittin on the carb have seen better days, like a chunk missing on both. Dose any of that ring a bell? I rly need to drain that gas tank and get some good fuel in but i cant seem to just shove a hose down the filler neck? is ther a screen?

I always did wonder how to spell mabobber. I'da got it wrong cuz I'd spell it mehbobber. My ex was mehbobber, but I divorced her cuz she was everyone else's bobber too.

So now that you got the little humor lift, lets take it all back down. The backfire you describe is sounding more like a valve to me. It's faint while idling, and gets worse with a strain on the motor. That is charachetistics of a valve train issue, whether it's burnt, mis adjusted or even a bad cam lobe.

 

[crazy4offroad]

An air leak around the propane injection unit may cause that, maybe, might make it run a little leaner. It sounds like this thing runs on propane and gasoline, right? You may want to put an inline fuel filter in if one isn't present, and make sure it's an AC Delco and not one of those fancy pants chrome/glass things with the nylon screen, they stop nothing. As for siphoning the gas out try a smaller OD hose like air hose or 3/8 fuel line, the flapper in there can be a bitch to get by but there shouldn't be a screen.

 

[Kapdin]

Yea so the story goes this kit is just flip a switch to change from gas to propaine so timing has ner been changed. LOLOL and hotrod as that came to my mind i turn to my girl and ask, "soooooo if you were to spell mabobber how would you do it?" lol and thats wat she gave me. And so being in park i can rev it up and its still faint not just at idle, as it idles you cant hear a thing, and would i know hear any kind of tick or any thing? Cus i sat and listened for a while and she sounds great no tick no knock.

 

[crazy4offroad]

Vacuum test, compression test, pinpoint the cylinder, then I would see if you can adjust the exhaust valve for that cylinder and eliminate it.