Jump to content
  • When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Still Trying To Diagnose Poor Running


Guest Anonymous

Recommended Posts

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?

Link to comment
Share on other sites

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

Link to comment
Share on other sites

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.

Link to comment
Share on other sites

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

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • BMW Neue Klasse - a birth of a Sports Sedan

    BMW Neue Klasse - a birth of a Sports Sedan

    Unveiling of the Neue Klasse Unveiled in 1961, BMW 1500 sedan was a revolutionary concept at the outset of the '60s. No tail fins or chrome fountains. Instead, what you got was understated and elegant, in a modern sense, exciting to drive as nearly any sports car, and yet still comfortable for four.   The elegant little sedan was an instant sensation. In the 1500, BMW not only found the long-term solution to its dire business straits but, more importantly, created an entirely new
    History of the BMW 2002 and the 02 Series

    History of the BMW 2002 and the 02 Series

    In 1966, BMW was practically unknown in the US unless you were a touring motorcycle enthusiast or had seen an Isetta given away on a quiz show.  BMW’s sales in the US that year were just 1253 cars.  Then BMW 1600-2 came to America’s shores, tripling US sales to 4564 the following year, boosted by favorable articles in the Buff Books. Car and Driver called it “the best $2500 sedan anywhere.”  Road & Track’s road test was equally enthusiastic.  Then, BMW took a cue from American manufacturers,
    The BMW 2002 Production Run

    The BMW 2002 Production Run

    BMW 02 series are like the original Volkswagen Beetles in one way (besides both being German classic cars)—throughout their long production, they all essentially look alike—at least to the uninitiated:  small, boxy, rear-wheel drive, two-door sedan.  Aficionados know better.   Not only were there three other body styles—none, unfortunately, exported to the US—but there were some significant visual and mechanical changes over their eleven-year production run.   I’ve extracted t
  • Upcoming Events

  • Supporting Vendors

×
×
  • Create New...