Strange Weber IDF problem - loss of main circuit?

[Geoff2]

Oil in the pick up. In the oil was conductive in anyway & bridged the airgap, you get no spark, a misfire. Could it be that as rpms increased, crankcase blowy increased, forcing oil into the gap & stopping spark generation?

[V12MECH]

Ground is certainly important, hopefully you solved the problem.

[Firedome8]

Find a loading chassis dyno with an engine analizer with a scope, on cruse do the wire wiggle test, on full power look for ignition breakdown, inject propane in suspect cylinder to check for lean condition etc....

[Nut124]

Started missing again today while driving. Not all the time but off and on.

Took the dizzy cap off for a close inspection. There is a faint crack running from #4 electrode to the rim, bottom of the cap. Seems like there is oil in the crack.

Could this do it? Perhaps the loss of power was running on 3 of 4 cyls?

[MadBill]

Good catch! No coincidence that the 'crack' starts at the misfiring cylinder. It's 99% certain to be carbon tracking due to the secondary voltage finding a lower resistance path to ground than across the plug gap.

[chimpvalet]

Distributor components can be treacherous, as I found when broke down in a small Oregon town enroute to an event in Cali. In my instance, there was a nearly invisible burn-through trace on the rotor underside which took days to sort. I was outside town on a rural road and had to hoof it back and forth for supplies and replacements, some of which were next day arrival from Portland. I don't plan to run distributor equipped going forward but would certainly carry spares if ever I did.

Cheers
Steve

[frnkeore]

Since we are throwing ignition things out, I had a problem with my Cosworth engine that only happened under full load.

I could run 8.5K in 1st, 2nd & 3rd gear, with no perceived miss but, only 7 - 7.5K, in 4th and about 6.5K in 5th (about the same mph as 4th), the engine was missing on one cyl in those two gears but, not the others. I found that because I could see the header tubes in my mirror so, I painted spots on them. Orange VHT paint, turns dark red with heat. I replace the plug in the one cold cyl, about 3 times, with no improvement.

Since it only missed when under a full load, I couldn't figure it out so, I had to pull the engine and dyno it and in 15 minutes, including warm up. We found the answer! It was the dist. Replaced it on the dyno and it was fixed. I took the bad dist home and found that what had caused it, was a warn shaft bushing but, that surprised me, I would have thought that the miss fire, would have been random in that case but, it wasn't.

[Nut124]

frnkeore, that is very interesting. Mine has behaved very similarly. Runs perfect at low speeds but will not take throttle above 3000rpm.

So, your dist shaft had radial play due to the worn bushing?

[frnkeore]

Yes, it was the bushing. That, of course effects the point gap and dwell but, why it only effected one cyl, I can not say. BUT, mine did it all the time, under load.

[V12MECH]

Worn shaft is a problem for points distributor, with pick-up coil, worn shaft could damage the coil, have you replaced cracked distributor cap?

[allencr267]

Accelerator pump/s working well?
Next time it's running bad, shut it off, Look at the acc pump's stream, what it puts out & how many strokes until dry, just sputters a drop or3 from the jets.
Tap the pump with a hammer, side to side and along its center line through the in&out, hard enough to easily break an egg, not hard enough to break a Mason jar. It's piston & valves are in a straight line, so tapping the inlet towards the outlet & reverse is more important IMO. That helps sticky piston, doesn't help valves, valve springs or debris problems very much.

Next time it's running OK, shut it off, Look at the acc pump's stream, what it puts out & how many strokes until dry, just sputters a drop or 2 from the jets.

Significantly different?

Do it on only one carb, that's all that's necessary, also, leave it with the throttle propped open for it to dry out/evaporate after.
Good Luck.
IHTFS.
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Oh shit, I forgot, the acc pump jet isn't very visible on some Webers, I'm sorry. Never mind.

[Geoff2]

Worn shaft with p/up coil dist. Other than the wear itself, the only problem it could cause is a no spark condition if the reluctor tip makes contact with the p/up coil.

[Nut124]

Tonite I have replaced all ignition components: ICM, coil, pickup, cap, rotor, wires. Plugs previosly.

Went for a test drive. Initially, no improvement. Perfect under light load up to 3000rpm. Will not pull when given throttle.

Took the main jet assys out for a quick inspecion. Saw nothing, put them back in. Now, it would take throttle and ran fine. Thought problem solved.

Went for a longer test drive, 20+miles, multiple brief WOT pulls. Ran fine until on the way back, stepped on it and it cut out and back to no power.

Scratching my head here, just speculating:

Pump jets too big? fouling plugs when stepping on it? The problem has always started 5-15sec into a WOT pull.

Should I run a new wire from alternator battery terminal to coil to see if that makes a difference?

Can a spack plug foul such that it works OK on light throttle but not at WOT?

On the way home tried stepping on it close to home hoping to identify miss firing cyl by header temp. Sprayed some CRC on primaries. Inconclusive; seemed like #4 was colder.

Any ideas?

NOTE: 3 weeks ago this same setup ran on a road course race track, hard, for 3 x 20min sessions, 10 laps ea, w/o any troble. ??

[allencr267]

All these WAG prove 2 things, too many have been extremely nice with time & knowledge appeasing them, and a little knowledge is a dangerous thing.
IHTFS?

[MadBill]

Can a spark plug foul such that it works OK on light throttle but not at WOT?...

Definitely. Cutting out under load is the trademark of fouled plugs. They can sometimes be burned clean by opening the throttle to the point of misfire then backing off a little and holding it there. After a few seconds ease the pedal down again, hopefully to find it accepting more throttle than before. Continue the Italian tune-up until progress ceases or the engine runs cleanly at W.O.T. to redline.

This said, it's highly unlikely that plug fouling could come and go as you describe your issue.