BMW 2002 - Bad stumbling only when going uphill

I haven't been driving my 2002 much in the last few years - maybe a few hundred miles a summer. In spite of this, it has been remarkably reliable.

When I took it out a couple weeks ago, however, it started acting up. I quickly realized that every time I started to climb a hill the car began to stumble. The longer the hill, the worse it got. Level or downhill: no problems. I found and repaired a broken float pin pivot in the rear DCOE, figuring that the float hanging improperly had caused the rear carb to flood every time I went uphill. I replaced the fuel filter as a precaution, and inspected the in-tank screen, which was very clean. Took the car out once or twice in short hops - no problems.

This morning the problem reappeared. I immediately checked the float cover repair - it is intact and holding perfectly, so the problem lies elsewhere. I checked the fuel flow from the pump (Carter electric, mounted low on the forward frame rail) - it is flowing a lot of fuel, at least on level ground. I don't have a spec on the max flow I need to feed the carbs, but there's more than enough to handle the light to moderate throttle it is stumbling under. Voltage to the pump is fine. The problem appeared with a full tank; the tank is now at about half-full.

I'm going to jack up the front end and retest the pump flow. Beyond that, I'm pretty much out of ideas. FWIW, I've run this set up for 15 years without problems; nothing that would affect the fuel system has been touched recently except as noted above. If anyone's run into similar issues or has any thoughts, I'm all ears.

Is it possible that you might have a pinhole in one of the floats?

I had a similar problem years ago that turned out to be a weak fuel pump. It tested fine on pressure, but was no good on suction. You might want to check that out. When I replaced the pump the problem went away.

Floats checked good on both carbs when I had them apart a couple weeks ago.

I'm going to check the fuel pump with the nose elevated - I planned to do that this afternoon, but 95 degree heat and my own stupidity conspired to prevent that from happening (don't ask). I am also going to check the lines for any leaks that might be allowing air in under suction.

yea.. sounds like the pump can't push fuel uphill

How about ignition ? Plugs, wires etc. Thought I would throw that out there since everyone was talking fuel, Depends on the kind of stumble I guess.

The obvious solution is to back up hills.

Back in the olden days when people would come to the shop and tell me all about their carburetor problems, I would do an ignition tune up and fix their problems 90% of the time. Just sayin.

Aside from the coil, the ignition is in good shape: I cleaned and regapped the plugs after the first incident, the points have been replaced by a Pertronix, and the cap, rotor, and wires are fairly new. While it is certainly possible that the coil or some ignition wiring is at fault, it feels much more like a fuel problem.

Does it stumble in high load conditions on level ground? IE: WOT in high gear at low speeds. If so I would check all ignition components.

My past experience with high load condition stumbles was usually caused by bad plug wires, cracked coil, or some similar problem.

Keith wrote: The obvious solution is to back up hills.

Sometimes the answer is so obvious you can't see it.