5.7 back fires through carb but timing is good.

[rydog]

I bought a 1984 K20 auto it has never started well I would have to pour gas down the quadrajet and if would back fire out the carb for awhile but eventually start and seemed to run pretty good I drove it 15 miles to get a vin inspection for it. It seemed to run rich.

Then anytime after it sat for a bit I had to go through the whole process again of pouring gas and fire balls shooting out the carb. I verified the timing by making sure cylinder one was tdc on the compression stroke and then checked it with a timing light with the vacuum advanced unhooked 8 degrees before tdc.

I decided to throw some parts at it and put a Edelbrock 1405 on it and a eps performer intake. Again verified the timing but now it still shoots flames out the carb will start for a bit and die.

I tried loosening the distributor and twisting the base some as somebody cranked and nothing.

I bought a cheap distributor off Amazon for 50 bucks tried swapping it and got the same result.

I did a compression test all cylinders were between 138-145.

I replaced the plugs and wires to be safe even though they seemed okay.

What else should I try? Leak down test?

Is the timing chan maybe doing something weird?

 

[rydog]

Ohh and for the heck of it since it seemed like every post about back firing out the carb said timing was 180 degrees off. I flipped it 180 degrees and I would say the back firing was worse...

 

[RattleSnack]

Looks like you tried all the usual suspects...

Did you double-check your spark plug wires? Maybe you got some mixed up.

My next guess would be that maybe your timing chain is stretched or jumped a tooth?....or maybe you have a worn lobe on the camshaft?

 

[rydog]

Ohhh I did kind of eye ball the push rods and valves springs and none of them looked unusual as it cranked.

Yeah I think my next step should be tear off the front and get to the timing chain and check it out. The chain didn't seem to make any noise while it did run

 

[RattleSnack]

Before you tear into it, maybe you could try this...

Take the distributor cap off so you can see the rotor. Then, try to rock the motor back and forth by turning the crank or belt accessory by hand. That might give you an idea of how much chain slop you have.

How many miles are on the 'ol girl?

 

[rydog]

That's a good idea ill try that I had if off before when I was turning it but it's hard to turn from the top with out removing the fan I've learned to use my inspection camera to see the timing marks of whatever from underneath hahaha.

The Odometer says 98,234 I would guess 198,234. It was last on the road in 1998 until the other day when I drove it to the vin inspection.

It has trailer brake controller and she is beat up pretty bad she was in a barn but with some cattle. I'm sure she has put in some work but the price was right.

 

[legend57]

A leak down test is a good idea. That will help you know how well the rings and valves are sealing. If you have a valve that is hanging open, that would appear as a backfire. If it is high mileage, you could have quite a bit of carbon build up on the valve seats, keeping them from seating properly.
If the bottom end is sealing up OK, you may just need a valve job.

 

[rydog]

A leak down test is a good idea. That will help you know how well the rings and valves are sealing. If you have a valve that is hanging open, that would appear as a backfire. If it is high mileage, you could have quite a bit of carbon build up on the valve seats, keeping them from seating properly.
If the bottom end is sealing up OK, you may just need a valve job.

Well I did a leak down test. I only have a cheap harbor freight tester which I don't think applies as much PSI as I probably should for a test.

1 and 8 were dang near 40 percent. 3 was 35 percent. The rest were between 25 and 30. Which isn't good huh?


So seems like the valves are having trouble? It did seem like a majority of the air was going through the intake.

Timing chain has some slop but it didn't jump.

Should I try to clean it up my self with some of that grinding compound? Ive never done that before. I've pulled heads and took them to a machine shop but it's been about 20 years.

Are the part store rebuilds any good? Seems like I can get those for 5 or 600 bucks.

 

[fast 99]

Generally speaking backfire into intake is lean from whatever cause: carb, vacuum leak, ignition misfire. almost any valve a train issue, example: broken valve spring, flat cam ect.

Was the vacuum checked and if so was it steady or irregular?

 

[legopnuematic]

Vacuum gauge can tell a lot.

Has the engine ever been over revved that you know of? I’m thinking valve issue, wether it be worn/bad seats, guides (which would aide in wearing the seats), bent valves, cracked, etc. That’s just my thoughts, without seeing or hearing it that’s just speculation.

If the guides or seats are worn lapping will only really be a temporary fix.

A smoke machine stuck in a plug hole with both intake and exhaust rockers backed off would visually show if you have poor valve sealing.

 

[rydog]

Generally speaking backfire into intake is lean from whatever cause: carb, vacuum leak, ignition misfire. almost any valve a train issue, example: broken valve spring, flat cam ect.

Was the vacuum checked and if so was it steady or irregular?

Ever since swapping the Edelbrock on it I haven't had it running to put a vacuum gauge on it.

It acts just like it did with the quadrajet on it. It shot fire balls out of that carb too. I figured that was probably vacuum leaks but I guess I was wrong.

I would assume if I dumped enough gas in it and cranked on it enough like I had to with the quadrajet it would fire up and as long as I didn't let it cool down it would seem to fire right back up. Maybe because everything was warmed up and sealing better?

Ive tried timing it multiple times I know I've had it close enough to run. I verified it with a light while cranking and verified the timing mark along with the compression stroke.

 

[rydog]

Vacuum gauge can tell a lot.

Has the engine ever been over revved that you know of? I’m thinking valve issue, wether it be worn/bad seats, guides (which would aide in wearing the seats), bent valves, cracked, etc. That’s just my thoughts, without seeing or hearing it that’s just speculation.

If the guides or seats are worn lapping will only really be a temporary fix.

A smoke machine stuck in a plug hole with both intake and exhaust rockers backed off would visually show if you have poor valve sealing.

I'm not a 100 percent sure it wasn't over revved it was a company truck for an aluminum plant then it was a farm truck for tenish years and then did a lot of sitting.

 

[rydog]

At this point I've got it down to the timing chain.

I'm tempted to pull the heads and trade oriellys for a fresh set.

I guess I should measure the cam and make sure it's not the issue before.

Looking back while it was running I wish I took the vacuum gauge to it and maybe gave it a couple of seafoam treatments down the carb.

40 percent leak down is pretty unhealthy right?

 

[legopnuematic]

I take it that you have the intake off at this moment?

 

[rydog]

Ohh something else I should of mentioned is during the leak down test on two cylinders when I first tried it made a pop noise and it dropped real fast but then could reapply the test and get a reading.

Was that maybe the valve stuck and psi being applied made it unstick?