[-SS454-]

I wasn't having any problems last year when I parked it for the winter, but right from the get go this year I am struggling with a cold start misfire. At least it was just a cold start misfire, but now it seems to be randomly backfiring at idle even with the engine warmed up. Things I have found...

- Number 2 header tube is "cold". The rest you pour water on and they hiss from the water evaporating.

- Number 8 cylinder plug wire had fallen off and was touching the header, and it burned a mark into the boot.

- Number 1 cylinder is pretty black. In fact since day 1 with this car, all the plugs always seem to be rather black.

- The backfire seems to be coming from the drivers side bank.

18/36 degrees timing.

75/80 jets. I have ran as low as 72/72 jets and never saw a change the plugs during cruising.

Vacuum advance hooked up to ported. Hooked to manifold gave me too much idle timing.

The plugs are NGK's that I put in last year.

Air temperature is around 4-5 degrees Celsius.

I do have a worry that lobe has gone down, thats my worst fears.

Thoughts or suggestions?

 

[Tom Mobley]

check the #2 plug wire for high resistance of burnt insulation

replace the #2 plug

check the dist cap for crack at the #2 contact

quickie compression check on #2. disable ignition, crack throttle open with something.

>> "75/80 jets. I have ran as low as 72/72 jets and never saw a change the plugs during cruising. " what were you expecting? your car is running on the idle transition circuit at cruise, not affected by jet changes. Not new news.

 

[-SS454-]

check the #2 plug wire for high resistance of burnt insulation

replace the #2 plug

check the dist cap for crack at the #2 contact

quickie compression check on #2. disable ignition, crack throttle open with something.

>> "75/80 jets. I have ran as low as 72/72 jets and never saw a change the plugs during cruising. " what were you expecting? your car is running on the idle transition circuit at cruise, not affected by jet changes. Not new news.

How do you adjust the AFR at idle?

 

[Tom Mobley]

with the idle mix screws, adjusts curb idle mix.

 

[Tom Mobley]

however, be aware the AFR meters don't work right at idle. They're really O2 sensors, and idle exhaust can get fresh air in it from either end of the pipe.

 

[-SS454-]

I was hoping for something else. I read to adjust the screws for best vacuum, which I did. So I think they are pretty close to correct.

 

[SWHEATON]

I wasn't having any problems last year when I parked it for the winter, but right from the get go this year I am struggling with a cold start misfire. At least it was just a cold start misfire, but now it seems to be randomly backfiring at idle even with the engine warmed up. Things I have found...

- Number 2 header tube is "cold". The rest you pour water on and they hiss from the water evaporating.

- Number 8 cylinder plug wire had fallen off and was touching the header, and it burned a mark into the boot.

- Number 1 cylinder is pretty black. In fact since day 1 with this car, all the plugs always seem to be rather black.

- The backfire seems to be coming from the drivers side bank.

18/36 degrees timing.

75/80 jets. I have ran as low as 72/72 jets and never saw a change the plugs during cruising.

Vacuum advance hooked up to ported. Hooked to manifold gave me too much idle timing.

The plugs are NGK's that I put in last year.

Air temperature is around 4-5 degrees Celsius.

I do have a worry that lobe has gone down, thats my worst fears.

Thoughts or suggestions?

=========================

1st get the plug wires corrected as suggested along with some fresh plugs to replace the blk fouled plugs & ensure theres spark going to all 8 cyls verifying dist cap etc are ok too.

Thats so you dont still have a cyl or 2 not firing or misfiring due to a bad plug wire or fouled plug causing you to chase your tail looking for other issues when its still a simple bad plug wire or fouled plug causing issues.

Then you need to clean up carb calibration because from what you describe with blk plugs its running rich.

Chk float lvls & fuel pressure to ensures its not too high.

And since you tried 72/72 jet and it seemed to run ok with that leaner jetting but didnt clean up plugs i'd drop back from current 75/80 jets to 73/78.

What are cam specs,dur @.05,gross lift,LSA?

But 1st you need to ensure your running plenty of base timing if your running an aftermarket perf cam ,approx 16-18 deg base timing works for most mild to farily hot perf cams,some real hot perf cams like 20+deg base timing.

Look for 36-38 total on avg ,you may need to shorten/reduce/recalibrate the mech adv in dist to allow for the additonal base timing the perf cam needs.

Opps,just saw you got timing right in the 1st place @ 18/36.

Also,you mentioned your worried its a cam lobe gone flat,from what you described its not that likely and is simply bad plug wire or wires problem along with carbon fouled plugs issue.

Just a heads up,since it seems your running a flat tappet cam if your not already aware you need to ensure the oil your running has a proper zddp lvl an aftermarket perf ft cam requires if your motors running a non gm aftermarket ft perf cam.

Thats to ensure the perf FT cam has a normal srv'c life,but if your running the improper oil with low zddp lvl with a perf FT cam you run the risk of premature cam/lifter failure possibly trashing the entire motor in the process.

BTW,you said you went leaner on jet and plugs didnt clean up,so question now is get a inatke vac reading at idle (in gear for auto trans) and vac reading when at steady low load part throttle cruise too along with what pv's are in prim/sec of carb too.

Then report that info back here so we see if the pv selection is in the ballpark or not and to be safe even if pv's seem in balpark i'd still replace them since its been running rich for seemingly no good reason.

Could possibly have mismatch between pv vs int vac lvl causing it to be open at slower speed cruise or could have a bad blown pv both of which could dump fuel all the time esp since you said motor has backfired at times whoich could have poped back thru carb possibly blowing pv especially if its an older design carb.

Scott

 

[-SS454-]

Thanks for the great replies...

Plan on getting some new plug wires. Was thinking about the MSD 31379. I believe the straight boots bend as I can't seem to find 135 degree boots.
http://www.summitracing.com/parts/msd-31379/media/images/make/chevrolet

Engine vacuum is around 15" at 1000 rpm and I believe the PV is a 6.5. I think this would be the correct one for a manual transmission. Carb is a 3310 with 2 metering blocks.

I am starting to believe the misfire is from fowled plugs, which could be because of bad plug wires too i think. But I really think I need to lean out the idle circuits.

I wonder, when I start the engine because the choke horn got milled off, I need to pump the throttle to keep it from stalling out upon initial start up, then hold the throttle between 1000 and 1500 rpm till the engine gets to about 120 degrees and then it'll idle by itself. Im wondering if I am causing the carb to use the squirter or primaries.

camshaft is Comp 280H, 230/230@.050, 110 LSA.

Flat tappet cam, and while I wanted some 10w-30 VR1 racing oil, its not available in Canada, so I went with Joe Gibbs racing oil.

One point I tried to lower the floats to lean it out, and the engine wouldn't run right. maybe I went too far. There are sites on the side, and I think its just above the lowest line, maybe I'll recheck.

No evidence of it backfiring through the carb. At least no sounds or smoke or smells under the hood, just a bang out the tail pipe.

 

[-SS454-]

Cleaned spark plugs, and put on new plug wires. 8 cylinders firing, no misfire/backfire at this time.

 

[-SS454-]

symptoms came back. Cyl 1 again fowls up (see picture), and cylinder 2 wasn't firing. I realized that they are right next to each other on the distributor cap so I got a new cap and rotor. Fired it up, still popping. Cleaned plugs 1, 2, 3, and 5 since I pulled those for inspect. Turned it back on, and cleared up.

I did adjust the floats down on the front bowl, seems to bring in a stumble when hard cornering though. Didn't make a diff anyways.

Makes me wonder if it is my warm up procedure.