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.