Strange Weber IDF problem - loss of main circuit?

[chevyfreak]

Keep a 2L container with water, next time it happens cool the ign coil and test again. I had a coil do the same to me as your conditions. Runs fine and then boom. Can drive nice on low rpms. Then it cools and works fine again,

Oil filled canister and over the years it went dry, was still running for 28years, just not full power. Cant complain about the service life it gave.

Chevy freak.

[Nut124]

Now it is doing it at idle in my garage. Runs on 3 cyls. #4 not firing. Removed the plug. It looks good. Seems to spark OK off the head when given ground. Spark from coil seems very strong, jumps over 1/2". Replaced the ICM. Replaced #4 plug - no improvement. New plug wires on the way.

Tonite I'm goind to swap some plug cables, dribble gas into #4 bbl and try to figure out why it is not firing.

Any suggestions?

[Nut124]

Trickled fuel into #4 BBL while idling and rpm picked right up.

Plugged idle circuit?

If the idle/progression circuit is not functioning at all, to the point that there is no combustion in that cylinder at all at say up to 3000rpm, does that affect intake air flow, booster venturi signal such that the main circuit does not start pulling when expected?

Trying to understand why the plugged idle circuit would disable main circuit as well as it seems. ??

[Nut124]

Cleaned up and fixed the #4 idle/progresssion circuit. It now idles perfertly and all 4 idle screws respond to adjustment nicely. Runs awesome in the neighborhood.

Went for a test drive; nothing, no power above 3000rpm, like a rev limiter.

I have replaced the ICM. Left to replace are coil, the capacitor and the magnetic pickup.

What does the capacitor do in the ignition?

What do you think? Someone suggested coil. Need to get a replacement.

[V12MECH]

Aside from the float drop and #4 idle circuit problems, I would suggest you look carefully at the distributor pick-up coil, 700-800 ohms ,ck. it hot, is the insulation frayed ? The capacitor is critical with points, on your car it may only be for radio static, disconnect it and see what happens, if it starts then do the old ignition coil check by pulling the cable out a 1/4 inch or so and if it stays running with a nice fat spark jumping the distance, if you only get the cable away from the tower just a little and it cuts out and it's original it's easy swap to just try another coil. I will watch for any further posts . Remember the old line, most carb problems are ignition.

[Nut124]

V12Mech, thanks for the advice. Also, thanks to all who replied.

Tonite, I replaced the entire coil assy; coil, ICM, condenser, but not the pickup. I had previously tested the pickup for resistance, cold, it was 758ohms. Have no replacement at hand.

I expected problem solved. Not so. It ran perfect up to 300orpm, then no power, no acceleration. The AFR number did not change. Almost like a very slow, delayed combustion.

So, whats' left? Could be the pickup. Or ground on coil assy, or engine ground.

I added a new ground wire on the coil heat sink. For the 1st 1/2mile, no improvement, but then it started running normal. ?? Drove for some 10miles with plenty WOT and it ran great. Then, pulled over and disconnected the extra ground wire, expecting trouble to return. It did not. Still runs OK.

Weird electrical problem? Perhaps the pickup is flaky? I will order a new one.

[chimpvalet]

Is there any chance your cam belt has jumped a tooth? Timing off by just this amount can put a hard limit on RPM range, rather like a soft rev limiter.

[Geoff2]

What dist are you running? And does it use vac adv? With Chrys & GM HEI, the pick up moves when VA activates & de-activates. This flexes the wires to the p/up. After a lot of miles, the wires break from flexing, no spark. With every one of these dists I fit to the engine, I fit new more flexible wires, with a longer length so that the flexing is spread over a longer distance.

[Nut124]

What dist are you running? And does it use vac adv? With Chrys & GM HEI, the pick up moves when VA activates & de-activates. This flexes the wires to the p/up. After a lot of miles, the wires break from flexing, no spark. With every one of these dists I fit to the engine, I fit new more flexible wires, with a longer length so that the flexing is spread over a longer distance.

Cam tming has not changed.

The dizzy does have vac advance and it does move the pickup. I have not seen any obvious damage to the pickup but I must admit I have not taken it out yet for a close exam. Getting a new pickup today.

Could a weak engine ground do this?

[MadBill]

What dist are you running? And does it use vac adv? With Chrys & GM HEI, the pick up moves when VA activates & de-activates. This flexes the wires to the p/up. After a lot of miles, the wires break from flexing, no spark. With every one of these dists I fit to the engine, I fit new more flexible wires, with a longer length so that the flexing is spread over a longer distance.

2X. I had one like much like this long ago. It ran fine until above a certain throttle opening, then misfired and cut out. Turned out that as the points plate moved due to the dropping vacuum, a wire was shorting out.

[V12MECH]

The pickup coil wires can crack under the insulation where they enter coil body, attach the ohm meter to pickup and apply vacuum to advance unit with small hand pump tester( mighty-vac) and watch reading, no vacuum hand pump? Move wires with tip of pocket screw driver. If ohm meter suddenly shows open circuit, move wires back to get a reading , add a dab of quit- set epoxy , when cured, re-check if looks good, road test and order part. This is an old GM hei trick, temporary. Another thing to do with ohm meter deal is heat pick up with hair dryer, or heat gun just to check that off list if wires pass test. Hang in there!

[V12MECH]

Another thought, earlier in your post you stated the AFR went 15-16, and had 4 psi on gage. If the ignition turns out to be a dead end, you need to do a volume test for fuel flow, a collapsed fuel pick up filter or "sock" in the tank might be a possibility, psi could be o.k., but volume maybe off, perhaps debris in tank ?

[allencr267]

3000 cruise in 4th?
Bet it'll be higher in 3rd, higher still in 2nd, and guess what in 1st!
Pleaase, I'll beg you, cruise at 2-2500 in 4th for a while, 10-20 secs, then get on the throttle.
It'll go much higher then 3000 until the float bowls drain, then it'll run at limited rpm, lean AFR, limited by the amount of fuel received thru that POS pump.
Give it a hammer wack, it might temp fix it

[modok]

I've had a similar case where there was rubber chunk floating around in the fuel tank. Sometimes it would be blocking the feed, sometimes not. Was another great story about a piece of carbon paper in the fuel tank doing the same thing and reportedly invisible.
BUT that was EFI so the problem would come and go near instantly.
We have discussed how the fuel bowls hold 5-10 seconds worth of fuel and how that would act.
The fuel pressure gauge needs to be visible while driving to catch these problems.
if it is not visible to the driver, attach a longer hose.

[Nut124]

Engine runs awesome again since Thu nite. I just do not know why and for how long.

Last nite I received a new magnetic pickup. Took the dizzy out and removed the old pickup. Unfortunately, did not check pickup gap before removal, but recall the pickup hold down screws seemed loose upon removal.

Examined the pickup. Nothing obvious wrong. Looks great. Could not get resistance to change from 758ohms no matter how I manipulated the pickup wires. There was some engine oil in the dizzy and in the pickup.

The new, replacement pickup had problems; the cable to the ICM was too short and the flex wires to the pickup were too long to the point that I could not tuck them in such that they would avoid being hit by the rotor. Put it back in the box.

Cleaned up the old pickup, put it it, set gap at .020. Set timing to 37 max, 10 at idle, w/o the vac advance. Runs awesome again.

Right now I have no root cause identified. It started running OK 1/2 mile after I added a ground wire to the ICM heat sink on Thu nite. Been running great ever since, some 50+miles of spirited driving. The heat sink is supposed to be frame ground by the studs it mounts to. The studs do have paint on them.

Have not replaced the plug wire set.