I don't know what else to check. Just did a tune-up including brand new dist. with module and pick-up, and a new coil. Now I've relaced the O2 sensor (finally got that sucker out). The EGR solenoid is new, as well as the decel valve. I was checking the vacuum lines today... not too carefully, but there didn't seem to be a problem there. I'll check more carefully next weekend. I want to put a new cat on (I think the old one is blown to bits) but I don't want to blow up a third. MAF sensor and intake seem alright. I'm boggled. I did have carbon build-up on the old plugs, but the O2 sensor should solve that. I'm rambling. It's late. Any more ideas. And YES the timing is set properly.

 

First ? is it coming through the intake or exhaust. If it's through the intake, it's your timing. You may be off a tooth. Just keep rotating the distributor and drive the car. Also a worn timing chain. I'm guessing you haven't messed with the valves yet? Valves out of adjustment will cause a backfire.

 

I'm fairly sure that the actual backfire is in the cat area. Timing is advanced 2° and set at 12° BTC. Timing chain replacement is a little down the road. And what do you mean "off a tooth"? If you are talking about the distributor gear, then you'd have to be off by more than a tooth since the oil pump runs off the bottom. You'd have to be out 60°, which would be more than a tooth. And at any rate, as long as the timing's correct that shouldn't matter. I might be wrong. Should I try retarding or advancing timing and see if it goes away?

 

Have you checked the balancer to make sure the outer ring (with the timing mark) hasn't slipped? Bring #1 (front passenger side) up to TDC and make sure the balancer mark lines up with 0 degrees on the timing tab.

 

I don't think that's a problem, but I will check it. How do you know #1 is at TDC (never done this before)? Any other ideas? Throw 'em at me.

 

So what's the easiest way to get #1 to TDC?

 

remove #1 spark plug. Stick finger own to cover hole (make sure engine is cold ) Rotate by hand until you feel the air pressure pushing out your finger tip. This will be the top of the compression stroke, tdc.

If I am wrong, please correct, but this is how I was able get my distributer initial timing in.

 

Best tip to see if your lined up with number 1 is...take an old cap and cut the inside top off, leave the terminals on. Now when you put in on you will see exactly where the timing marks line up when the rotor is pointing to #1

 

Can also be done with a screwdriver- but Project85's way will give you the compression stroke. A screwdriver can tell you either exhaust or compression TDC. You can also stuff a cotton rag (not a paper towel!) into the spark plug hole; the pressure inside the cylinder will blow the rag out.

"Full" screwdriver method that I've typed up before involves putting a phillips head screwdriver into the #1 plug hole, and turning the crank clockwise using a breaker bar and socket on the big center bolt of the crank pulley. When the screwdriver stops raising, that's TDC. Might be TDC of exhaust though, so look at the balancer. If the timing mark is nowhere near the timing tab, you have TDC of exhaust. Turn the engine another complete revolution (screwdriver sinks then raises back up), that should be the compression stroke, and find TDC of that.

When the rag blows out (or you feel pressure), then put a phillips head screwdriver into the #1 plug hole. Slowly turn the crank (clockwise as looking from the front of the engine) until the screwdriver stops raising- there's TDC.

Now if I recall correctly (IIRC), you have three slots on your balancer, right? Only one of them is for TDC #1 (front passenger side). IIRC again, all three slots are the same size. (85-86 have one slot that's slightly larger then the other three to distinguish it.) So you'd have to do this anyway to make sure you timed the motor with the correct balancer mark!

 

Why would it matter if it was TDC of the compression or exhaust stroke? The timing mark should still line up with the 0°. Also I like the idea of cutting an old cap, but will that work? Maybe, I'm not sure, it's kinda hot and hard to think.

 

Cutting an old cap that's the mystery out of lining up your timing. When the rotor is directly pointing at the #1 terminal, just look at your timing mark...simple

 

Well, you don't want the ignition system firing when you're on the exhaust stroke! I'm figuring you'd be checking the balancer and the distributor alignment at the same time.

 

Tom, when the rotor is pointing to #1 the timing mark should be lined up where you think it is, say 12*, if it's way off, rotate it . Bottom line is, when the rotor is at #1 (on the right stroke) that is where your timing is at.

 

TomP: Even I know you don't want the engine firing on the exhaust stroke. I'm just saying even at TDC of the exhaust stroke, the rotor will be lined up with #1. But the distributor I'm pretty sure is lined up alright, and I don't have any current plans to play with the balancer.

mvftw: What does your name stand for? But I see what you're saying about the cap thing. That makes sense. It's a little cooler now, so my brain is functioning. And I have to still have the old cap.

Also, I was thinking. I know there's some debate about whether a faulty decel valve can cause backfire, but assuming it can, if the decel valve is working but the air is not coming into it properly, that could also cause a problem. I checked, and the decel valve performs much differently when the input is connected to the main intake, compared to when it is just open to the environment.

 

camaro_junkie, from my rebel days, my first and last name and 'f___ the world'. The cap tip is real good, I got from a forum membem (I also have a Vette), and I pass it along.