78pacer

I installed new intake and exhaust valve seals less then 3k miles ago in my '78 L-82. Everything went smooth and the engine ran fine. That was a few seasons ago. More recently, I backed from my driveway, threw her in drive, stepped on the gas and heard a loud carb backfire. The engine now hesitates on acceleration(feels like its running with the choke on, and it's not) and missfires slighty at all rpm's with an slight occasional pop sound (if you really listen closely)from the throat of the carb. I had been convinced it was the carb, (I checked the accel pump operation and it's putting out a healthy spray),but with the sudden onset of this problem with the backfire, and the ignition misfire, I'm not so certain its fuel related. Does this sound typical of a cam related or valvetrain problem? If so, how should I diagnose and proceed with tracking down this gremlin?
Any suggestions would be appreciated.
Thanks,
Mike

weimer20

A "wiped" cam lobe will manifest itself in a loud ticking valve. As the lobe gets ground down, the valve lash increases to the point that the rocker will smack the top of the valve making a bit of racket. Bent pushrod will do the same. You can check it by measuring the total valve lift at the valve with an accurate measuring device. (Dial indicator). What kind of carb are you running? Holley maybe? Reason I ask is due to the symptoms you have described. Fuel and timing issues come to mind. Not valves.

78pacer

I'm running an edlebrock q-jet #1903 hot air choke 750cfm. It is fairly new. I have a brand new MSD streetfire distributor and wires.(runs alot smoother but problem still present). I set the base timing to 12 deg btdc as a factory setup. No idea how to establish total timing.
Mike

Durango_Boy

I would be looking at other things. Check for a vacuum hose that popped off. The timing may be off, and you would be wise to check all the plugs and wires. Pull the plugs and check their color. Also pull the cap and check the rotor and contacts.

weimer20

As DB stated, I'd be looking at other areas as well. Really doesn't sound like a cam / valvetrain prob. Your carb has me stumped a bit. I rebuilt q-jets many years ago. Enough time for my recollections to fade. I'll look up that carb and see if I can identify any potential troublespots there. My thoughts are going toward ignition though. Looks like DB is thinking the same. Follow DBs advice and I will continue to pick my brain.

78pacer

Thanks Durango boy and weimer20.
The plugs are all new. The wires are MSD8mm. They are brand new, along with the distributor. Two of my old wires were cracked at the cap and arching, and the old distributor was original. Plug # 3 and 5 was wet with fuel.

weimer20

The plugs wet with fuel... Was that before the wire and dist switch or did you just find that? Ya know another thing I was thinking... Wonder if there may be a rocker that is to tight? Possibly holding a valve off the seat a bit. Just a thought. Do you have a compression tester? (this is just a thought as you had stated that the seals had been done. ) Thinking out loud here.

The Money Pit

Pull the valve covers and eyeball the rockers. Mine had a few loose, and the engine was running OK, but a bit rough.(Hard to tell when the normal idle shakes the fenders.)

When I pulled the intake I found this.

Durango_Boy

That was a 'little' rough?

78pacer

weimer20,
That's a great idea, but I did that before buying the new MSD distributor. The compression in one cylinder was 155lbs. All others were 165lbs. I squirted some oil in the cylinder and tried it again and brought it up to 165. No wet plugs after the changeover. I did put a vacuum gauge on a manifold port and am pulling between 10-12 in. Is that too low?
Mike

78pacer

Thanks moneypit.
I hope I don't have anything that resembles that. Gheesh. The car has no pollution crap and headers. It's fairly loud, but if my pushrods were like those, I think I'd be hearing a lot of rattles.

weimer20

10 - 12 is ok on the vacuum. Do you have a timing light to get a look at where you are set right now??

78pacer

I set my base timing at 12 deg BTDC as the manual suggested.

weimer20

May want to look at it again. Dist may have moved or worst case cam chain jumped. Again, thinking out loud.

Gearhead74

I bought a 64 Impala 283 when I was younger. It had a backfire through the carb at any RPM higher than 2500. Real low RPM seemed to be OK. I figured it was dwell so I bought the car.

Turned out to be a flat lobe on an exhaust valve. Apparently the spent gas couldn't get completely out of the cylinder during the exhaust cycle so it flooded into the intake manifold when the intake valve opened.

I'd suggest pulling the valve covers and looking very closely at the rocker arms while the engine is running. If it's just started to wear you won't see it, but if it's worn somewhat more it will be obvious.

78pacer

Thanks again weimer20,

I considered the fact that the timing chain may have lept a tooth or two. Is there anyway to check it without removing the front of the motor? Would\could a timing chain out of synch cause a hesitation/stumble off of idle as well?

Gearhead74,

Thanks for the reply. This backfire is barely noticable upon a steady 2-3k throttle. At idle, it isn't there, or I am too deaf to notice...*s
The engine miss is prevalant more when the engine is warm then when warming up and so is the hesitation. Weird eh?
Any thoughts?



Mike

The Money Pit

I had a Firebird Formula back in 80 that the catalytic converter plugged up and caused a similar issue. Idled OK, but as soon as you started driving would choke up the engine and backfire through the carb.

I doubt timing gear jumped teeth, That kind of stuff only happens on very high mileage, never maintained vehicles, which I hope is not the case here.

Have you checked the plugs, cap and rotor? Maybe it's something simple.

bojangies46

Make sure your timing is correct and advance is working, check intake for gasket leak, a warm intake will expand more than a cold one.

78pacer

Thanks The Money Pit,

I recently replaced the distributor with a brandy new MSD streetfire unit. I also changed plugs and new 8mm MSD wires too. I needed a distributor, so this was a great time to swap it out and perhaps solve two issues at once. Well, the engine now holds time and runs smoother, but the hesitation and missfire is still prevalant. I have stainless headers and a gutted converter. It almost seems to be running on 7.5 cylinders..*s I do appreciate the input. We'll get this somnab**ch sooner or later.

Ahhh....bojangies46,
You may be onto something. I have seen oil weeping down the pass\front side of the intake onto the front of the head. Very slight, but still there. Would a vacuum leak produce a missfire as well as hesitation? I put a vacuum gauge on a manifold port of the distributor and produced 10-12inHG and it was steady. I wouldn't mind canning that old plate-blocked off EGR manifold in lieu of a nice edelbrock performer.
Mike

Matt Gruber

pick a date on your calander:
Adjust valves if not fixed by then.