naf

iTrader: (7)

My 85 LG4 has been suffering an intermittent backfire through carb associated with an immediate stumble when tipping in the throttle from a cruise. Maybe an event every other tank or so for a few months now. Not enough throttle to bring in the secondaries. It'll act up a few times then be normal for another few 100 miles. So far I've tried the following:

Disconnected vacuum to EGR-no change
Pulled carb apart, checked float level, etc. Replaced bad TPS I discovered-would not read below 2 volts. Dwell at idle is good. Thought I had it here, but no.
New distributor last summer

WOT seems fine, at least when it's not being bad.

It's probably something basic I've missed but the infrequency has made it difficult to track down. Any ideas?

naf

iTrader: (7)

Thinking on it during my 'before leaving work run'.

Maybe canister purge? Seems to happen with change in vacuum. Stuck TVS/bad valve maybe? I'll plug the vacuum lines for a while and see if it reoccurs. Maybe check the purge valve and TVS with a vacuum guage.

Don't make me figure this out on my own. Got another problem like that already with the 87.

naf

iTrader: (7)

Ok not canister purge. Did it near the house so I was able to put the dwell meter on it and duplicate it. Dwell was hovering around 40-50 immediately before it would cough and sputter. Light throttle was fine, needed about the same amount of throttle needed to take it out of TC lockup. It has done it once before with the small amount of throttle the cruise control adds to maintain speed up a slight grade, though.

Detected a slight burble in the exhaust at idle, possibly afterfire. Idle speed was steady and dwell responded to choked airhorn. M/C solenoid clicked and verified movement with IAB screw out. Timing advance verified at idle with light.

It will probably be fine in the morning. Although the issue is becoming more frequent and longer lasting.

Help me out here. Sticking float? Passage in carb blocked? Got to be fuel delivery, right? How could I duplicate this with the ccc-qjet still in feedback mode?

Where's the guy with the razor? It's got to be something simple and I'm just too close.

scooter500

iTrader: (1)

well can't help you out w/ advice but can say my 87 also b(ar)f's a wee bit when coasting down hill at 60mph w/ cruise set on.

known problems until time later in holidays: too cool engine & single wire O2 that could be carbon coated. After raising engine temp & replacing O2 sensor then I will be trying to adjust carb using your instruction previously.

royslsbry

have you checked your coil wires and distribator cap or distrib itself. a float sticking can dump to much raw fuel in and cause it to pop through the carb. could it be bad gas.

naf

iTrader: (7)

Almost certain it's dumping too much fuel. What has me perplexed is the intermittent nature, and it seems to be getting progressively worse. Oh, got 24 mpg on the last tank and haven't seen anything below 22. I check every fill-up.

The first few symptoms were a slight hesitation/stumble with very small throttle increase, even with cruise on. This was very similar to a lean stumble and could possibly have been caused by the ecm responding, too late, to a transient rich condition-leaning the mixture too much, too late. This had me initially thinking vacuum leak, hence checking the EGR and purge canister vacuum switches, etc.

Now I'm thinking that something is allowing extra fuel to be pulled in when the vacuum in the carb increases. Runs fine under high manifold, low venturi flow-when it's being bad.

Anyway I've purchased a used q-jet this morning for $35 and will rebuild it and install. I'll take the old one apart this weekend and see if I find anything out of the ordinary.

five7kid

iTrader: (14)

Ignition. Doesn't sound like a carb problem to me.

naf

iTrader: (7)

I thought ignition early on as well but in my experience mis-fires are usually accompanied by a bouncing tach. The distributor is new since September, cap/coil/rotor new since July. Nothing in there older than a year even the wires. No evidence of mis-fire even moved some stuff around while observing engine bay in the dark, of course it won't duplicate the condition except under load. Won't hurt to check spark and ohm out the coil though.

I'll eventually need the third ccc-qjet for the '87 Z I picked up last Summer. Plan to pull the engine in it over the holidays.

camaronewbie

Check plug wires

1) Make sure they are all seated properly on the cap and on each plug - Once I had a plug wire not completely on the plug - put it back on, 2 miles down the road it came loose again - had to go inside the end and crimp it with needle nose pliers so it would be slightly tighter on the plug.

2) Check the continuity of each plug wire - Once I had a brand new set of wires where one was broken inside - drove me crazy ('cause I knew they were brand new wires) until I checked continuity on each one and found it.

My two experiences with small intermediate backfirings - good luck!

five7kid

iTrader: (14)

The tach only jumps when it's a pick-up problem. Weak coil, cap, rotor, wires wouldn't make the tach act up.

naf

iTrader: (7)

And weak spark would not fire when cylinder pressures are higher. I'll check today.

lonman89

check your timing,if its off it may cause a backfire,happened to me before

naf

iTrader: (7)

Pulled all the plugs and replaced with new. All the old ones were equally black and wet, but I did shut it down after it last acted up.

Changed cap/coil/rotor with known good parts.

Checked plug wires...#7 would not ohm. Looked like it got too hot as it was no longer as flexible. Replaced all with new.

I'll still have to put some miles on it before I know for sure, but a mis-fire on seven could cause the ecm to over-richen the mixture because it's upstream of the O2. AND seven must have been firing at idle and cruise or the dwell would never have been right and mileage would have been crappy. In fact after replacing it the dwell did not require re-adjustment.

I'm mildly optimistic and thanks will be forthcoming provided it's better.

Mike

naf

iTrader: (7)

500 + miles so far and every thing seems fine. I've got that confident feeling again, at least in the vehicle not necessarily in my diagnostic abilities.

Last Summer the #7 wire was pulled off by a piece of trash I ran over, noticed right away with the bad idle. While it was off it may have baked against the y-pipe and gained some resistance/gotten brittle. Perhaps when I pulled it loose to check it last week that was the final blow and it lost all continuity. Otherwise I had discounted the ignition components because they had all been replaced recently. But in one respect: "It's got to be something simple and I'm just too close.", I appear to have been correct.

Thanks again,

Mike

five7kid

iTrader: (14)

Glad to hear it's behaving again. So many carb problems are fixed by replacing a faulty ignition component. . .