engine and drivetrain Still Trying To Diagnose Poor Running

[Guest Anonymous]

A couple of weeks ago I posted looking for diagnostic assistance with my rough-running engine. The story: engine ran ok with Solex, but needed to pass emissions inspection - HC and CO tests. Set timing, adjusted valves and took it to a good BMW mech to have him precheck it. Came back immediately with diagnosis Solex bad - leaks around shaft, not worth fixing. Ok, I have a Weber 32/36 DGAV waiting for rebuild and rejet. Got the jets (C.D. prescription) and rebuild kit. Rebuilt, installed, fired it up. Fired after about 2 turns, very nice. Let it warm up, checked for water leaks set idle some, and shut down for the night.

Back the next evening. Start it up, it's running on 3 cylinders. After warmup it cleared but won't settle into a steady idle. It'll run at 1000-1500 rpm for a few minutes, then drop off and run real rough - definitely misfiring. Sometimes it will clear up and run smooth for a bit again (back up to 1500) then drop off again. Initial diagnosis was likely dirt in idle circuit.

Took carb apart, blew out all passages. Checked float level, operation of needle valve (bowl inlet). All looks ok. Checked base for warpage with flat edge - nothing noticable - that a gasket can't seal. Reassambled & retested. No help. Checked for vacuum leaks by spraying carb cleaner around every gasketed joint on the intake side. Nothing. Tried ether. Nothing, no change in speed.

Maybe the carb is totally shot? I have another one that I know runs and runs smoothly on a Jeep 6 cyl. Pulled it out of the Jeep, swapped out the jets, double-checked float level. Put it in the 02. Still the same.

OK, maybe it's not the carb. Drained the gas and put in 5 gal fresh with 2 containers of gas line antifreeze (yes, methanol). No change. Changed the fuel filter. No change. Changed the fuel pump for another one (used) that I have lying around. No change. Tried no filter at all, no change. Replaced the hose between the tank and the plastic line (in the trunk) and checked all clamps for seal. Pulled the sender assembly from the tank. It's clean, filter looks pristine. Put back together. Put a little vacuum on the fuel line to see if there are air leaks causing the pump to lose prime. No leaks. Put back together and tested. No change.

I'm running out of ideas. I know the distributor itself isn't perfect, I can feel a little play but I don't think it would cause this much trouble. It's so uneven, it will run smoothly for a few minutes, then go rough and slow down, almost die. If I rev it, it will spin right up and then run smoothly for a while at 1000-1500 (15 seconds to 2 or 3 minutes). Then it will get rough again and slow down.

This car has new points, rotor, cap, condenser, wires, & plugs. I've timed plenty of cars, it's timed with a strobe light so the ball is just on the edge of the view port at the recommended engine speed (I forget what that is at the moment), with vacuum line removed from dizzy. I checked mechanical and vacuum advance, they are good.

I've heard that Webers can be finicky about fuel pressure. What would be the symptoms of fuel pressure problems? Could low fuel pressure at idle speeds (below 1000 rpm) be at least part of the problem, causing rough idle as the gas in the bowl drops a bit, then picking back up as the bowl recharges? I also changed the float level 2 to 3 mm so that it will hold more fuel in the bowl. This didn't help either.

So, I'm looking for more ideas. Next will probably be electric fuel pump. Other suggestions?

[Guest Anonymous]

When it's running rough and slow, have you tried disconnecting the spark plugs one at a time? You mention that it seems to be running on 3 cylinders; with a downdraft, this would mean ignition troubles, not carb problems. Despite all the nifty new parts, I'd keep looking at ignition problems.

Can you drive it? If so, does the problem occur when you are cruising, or only at idle?

Does the exhaust smell of unburned gas, or show black smoke, when it's running rough?

Surging (alternating between slow and fast) implies a lean condition. If the mixture is very rich, on the other hand, you could get a slow, rough idle. Changing that float level could cause overly rich idle mixture, if the bowl is over-full. Webers are known to suffer when fuel pressure is too high, but I've never heard of a pump that couldn't keep up with idle, but which would allow you to rev the motor.

Double check the needle valve in the float chamber, too. Also, what temperature is the environment when you are running the car? Is it cool and damp? Icing can be a problem.

It might be a stretch, but disconnect the brake booster vacuum line and plug the manifold port. Look for other vacuum leaks, too.

Mike

[Guest Anonymous]

Mike,

Thanks for assisting. I'm starting to transfer my attention to ignition. One last thing will be to put the solex back in for a test. That would rule out Weber-specific issues. I don't plan to leave it in.

Wires would be my first suspicion on ignition issues. Poor connections, almost broken etc. Now may be the time to twitch to Pertronix, to eliminate worn distributor issues. Of course, the old pull one plug wire at a time is on the list. I pulled the one that I suspected was a problem but clearly got a second steady miss to go with the intermittent miss or weak fire.

This continues to be an interesting problem.

[Guest Anonymous]

"90% of your carburetor problems are in the ignition. Mike, keep your cotton-pickin hands off the carb until you're sure the ignition is up to snuff."

I know you've checked points, cap, rotor etc...Have you isolated the non-firing cylinder and swapped (one at a time) the spark plug, plug wire and the spark plug terminal? This latter item has been known to go bad.

Also check dwell on dizzy...timing is fine, but M10s like to be within the proper dwell range (59-64 degrees if my memory serves me.) Finally, look for vacuum leaks. If you have a vac advance dizzy, make sure the vac advance capsule doesn't have a leak. Remove vac hose from carb and suck on it. You should (1) feel resistance and (2) you should see the point plate move. Abent these, and that's at least part of your problem. Keep us posted on what you find...

Mike

[Guest Anonymous]

"Do Not Touch."

[Guest Anonymous]

I've played the same game you are many a time. I blame the mechanic for his stotty work - (me).