350 ENGINE BACKFIRES THROUGH CARB

1985 Chevrolet Silverado

Tiny

Roostermw

January, 10, 2011 AT 3:06 PM

Alright, so I got a motor problem with my boat I thought you might like to try and figure out. Nothing is easy lol. Its like a puzzle. Here we go.

The motor I am working with is a GM 350 with an Electronic Distributor. It has the standard wiring (Old School) with Resitors, etc. I only have 60 hours on them since I completely rebuilt them.

It has been 6-7 months since I was last able to start and run my boat due to work, money etc. The last time I drove my boat it ran perfectly. I went down this past weekend and here is what happened.


Port Motor: Started and at Idle would have one single backfire through carb every 1-2 seconds at Idle. Nothing had been moved or touched since our last run.
Items I checked immediately:
- Distributor Cap – Cracks etc – Checked out Perfect
- Plug Wire – Burnt or Cross Firing – Checked out Perfect

Based on these findings I decided to pull the coil wire and turn the motor over. I got the same popping noise through carb.
Items I checked Immediately:
- Pulled all Plugs and turned over motor – No noise or back firing heard
- Checked Compression on All Cylinders
Left Bank: #1 – 145lbs, #3 – 145lbs, #5 – 150lbs, #7 - 142lbs
Right Bank: #2 – 150lbs, #4 – 145lbs, #6 – 155lbs, #8 - 150lbs

Here is the tricky part. When I put my compression tester in cylinder #6 and turned over motor I could hear the back firing noise. I could not tell where it was coming from though. I decided to plug cylinder #4 with a sparkplug and when I turned the motor over you could hear and see it back firing through the carb again (the linkage on the carb was moving when you would hear the back firing noise).

So based on all the information I have it has been really tough to come to a conclusion as to what is going on with it.

You would think that if it was a blown head gasket I wouldn’t have any compression, or if anything was wrong with the valve I wouldn’t have any compression. Nothing adds up since I have good compression on all of the cylinders.

11 Answers

Tiny

Docfixit

January, 10, 2011 AT 4:16 PM

A leak down test will revil the problem. Will need leak down gague and need compressed air. When it backfires engine running is there a flame from carb?

Tiny

Roostermw

January, 10, 2011 AT 4:45 PM

No flame is present. Just the sound of compressed air. It will also not stay running at idle.

Tiny

Docfixit

January, 10, 2011 AT 5:57 PM

Pull valve cover off see if exhaust valve is opening on 4 and 6

Tiny

Roostermw

January, 10, 2011 AT 6:19 PM

Will do. What do you think the reason is that 4 and 6 seem to be tied together? Cylinder 6 causes it to make noise and cylinder 4 causes it to come out of the carb. Seperately they do nothing. This is only when the sparkplug holes are utilized. No plugs = no noise.

Tiny

Docfixit

January, 10, 2011 AT 6:27 PM

Do not know tthat's why I would do a leak down test

Tiny

Roostermw

January, 10, 2011 AT 6:31 PM

Ok. I will let you know as soon as I can get that done. Interesting huh?

Tiny

Docfixit

January, 10, 2011 AT 6:35 PM

Very would love to be there to help

Tiny

Roostermw

January, 12, 2011 AT 1:17 AM

I got a minute to look at it. Pulled of Valve Cover and noticed that Cylinder #6 - Exhaust Valve was stuck shut. I was unsucessful in freeing it. I was able to pull the push rod out without any tools. The push rod is in perfect condition. I figure that the backfiring noise was coming from #6 but when I had all the plugs pulled out it was taking the path of least resistance and shooting out of #4. When I plugged #4 it then caused it again to take the path of least resistance and go back through the carb. So I guess the next thing is that I could have a collapsed valve or a burnt cam lobe. Even with a collapsed valve I shouldnt have been able to take the push rod out by hand right? What you think?

Tiny

Docfixit

January, 12, 2011 AT 8:27 PM

Seems cam lobe is at fault seems next thing is teardown is needed

Tiny

Roostermw

January, 12, 2011 AT 8:31 PM

It seems to abrubt to be the cam though. I was hoping that the lifter collapsed. I know when we broek them in we ran them at 1500 rpm's for 15 minutes and drove them softly for the first 15 hours. Breakin should have been perfect.

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