runs on 3 cylinders under load

[trickdog60]

'64 bug, 40hp. I've cleaned the carb, checked plugs and timing and fuel pump. Compression and valves OK. It continues to seem to back down to running on 3 cylinders as soon as any load is put on the engine. Any ideas? Please ask further questions to help me in diagnosis. I don't want to just start tearing into things.

[Dougy Dee]

Bad plug giving up under load.

[Aussiebug]

Agreed, the most likely cause is a bad plug.

I've had brand new Bosch plugs do that straight out of the box. Changed to NGK B5HS - the direct equivalent to the Bosch W8AC (or B5ES for the long thread heads) and never had a problem since.
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Rob
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[trickdog60]

Plugs are cheap and easy to try, so will give that a shot. Do bad plug wires give the same symptom? If I replace the plug wires, should I replace them with copper or carbon?

[dcheek]

I had a similar problem. Would run fine when cold, then as soon as it warmed up and the choke was off- no power. You would think it was fuel related. Drove me crazy, to the point that I parked it for the winter to gather my thoughts. I replaced and checked everything twice, except one item- the spark plug wires, which were a brand new set. I figured they had to be okay right? Wrong! One of them would short out when it got hot. I tossed the entire set and replace again and bingo. Been running great ever since. Moral to the story: Just because it's new doesn't necessarily mean it works.

Your problem has to be a plug wire or a plug. An easy way to check is to start the engine and pull one wire off at a time, then drive the car. If you see no difference under load you've found the bad plug.

Dave
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1961 Bug
1965 Bug
1966 Sunroof Bug
1966 Westy
1976 914 2.0
1989 911 Carrera
1981 Diesel Rabbit Pick Up
1982 Diesel Vanagon
2014 Golf (last year German mfr for NA)
2019 Jetta Gli, 35th Anniversary Edition, 6 speed

[drscope]

Wait until dark and then open the hood and crank it up. Look for any signs of arcing through the plug wires.

It may help to do this when it's humid out.

You could also take off your shoes and then reach in and grab a hold of each plug wire individually. If you have a bad wire going to ground you'll find it!

Don't assume new parts are OK. They usually aren't.

You mentioned a lot of stuff including timing, put not points. If the points are closing up this can be the result.

Also check your distributor cap. Make sure it isn't cracked or that there isn't any carbon tracks building up in or on it.

It may sound silly, but also clean up the top of the coil. Crud can build up there and if the humidity is just right that crud can become very conductive and arc to ground on the metal side of the coil.
_________________
Mother Nature is a Mean Evil Bitch!

[trickdog60]

Great suggestions. I'll try them all....except the one where I hold the plug wires while standing in water in my bare feet!!! I may use an ohm meter instead.

[trickdog60]

Thanks to you all. It was a bad plug. I switched the Bosch for NGK, and she runs like a top. Also, cleaned the top of the coil, checked the distributor cap and rotor, and vizualed the wires at night. Bad plug failed under load.javascript:emoticon('')

[cseay1]

I'm glad I'm not the only one that has given up on Bosch plugs in favor of NGK. Seems like every month we get a customer come into the shop with a running rough situation ..

Me: "How long since you last put plugs in it?"
Customer: "I just put plugs in it over the weekend, didn't help."
Me: "What plugs did you put in it."
Customer: "The Bosch plugs with the dual electrodes."

Put a set of NGK's in it, problem solved.

What does puzzle me is why the Bosch's have issues?
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Chris

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