carb problem

[roncooper]

I am helping a friend get a '76 R75/6 back on the road. He just bought it off of e-bay. Apparently it had not run for a few years.

We checked the valves. All were good except the right exhaust which was very tight.

The points look good so we just set the timing.

We cleaned the carbs and put in rebuild kits including diaphragms.

It started right up. The problem is the left carb is slow to react to the throttle. We looked in the intake and the slide is fluctuating rapidly as rpm builds. It pops through the intake sometimes. We pulled the top and checked the diaphragm. There are no holes.

Any other ideas on what might be causing this problem?

Ron
91 K75RT
Several airheads in the past

[rocketman]

The diaphragm could be ripped and you might not see it, since its the diaphragm that controls the slides position I would be suspect of it. If it is leaking then the vacuum would fluctuate and cause the slide to do what you are seeing. Try switching them and see if the problem follows. Also make sure there is a good seal all around the top of the carb where the diaphragm sits. Off hand I can't think of anything else since there is no mechanical linkage to the slide.

RM

[danielroth]

when you rebuilt the carbs, did you pull the screws out of the butterfly and shaft.
to properly replace the o-ring on the butterfly shaft, great care must be taken to reinstall the assembly.
I suspect the butterfly to be in misalignment or the screws are loose from not being pinched properly. possibly the shaft boss or bushing is worn but unlikely.

[44006]

Switch the carbs to confirm problem is with carb rather than other.
They will work just fine on the wrong sides - cables are just on the
outside rather than inside

[boxerr]

Did you syncronize the carbies once back on? I know on Oilheads, the throttle body will "chatter" if out of sync.

[sumran]

How did the throttle movement feel before you mounted the carb? It should move easily and the throttle plate should close with no light around the edges. You will probably need to back off the idle adjustment screw to align the plate. Did you replace the o-ring on the throttle plate shaft? It should be replaced in a full rebuild. The metal braket that holds the shaft in (it also holds the idle adjustment screw and the return spring) should be fully tightened before the throttle plate is adjusted. If those screws are turned after the throttle plate is aligned it can drive the plate out of alignment. I assume you know about the plate bevel. Idle and enricher passages should be on the engine side of the throttle plate if correctly installed.

A couple of slide notes: The slide may have burrs and scratches that prevent smooth operation. I clean up the slide and carb body with some very fine wet/dry sandpaper. I am not trying to completely remove the scratches; I just remove the material that rises above the surface. It is easy to misalign the diaphram during installation. You probably know this, but there are keys molded into the diaphram rubber to align the assembly. If they aren't in the intended spots, they can cause vacuum leaks. When it is right, the radius on the bottom of the slide will be aligned with the venturi and the slide will rise and fall easily with the lightest touch.

I probably brought up things that you already know, but I am sharing from mistakes I have made. Hope you solve your problem soon.

[OHScot]

Valve guide sticking?

[sumran]

I would also try swapping the plugs and wires. If you have a wire or plug that is only providing fire part-time, you could be loading fuel then burning excess fuel when it does fire. Probably not the issue, but the backfiring would make me want to elimnate it as a possibility. You may want to inspect the coils for proper connection and cracks. Coils are usually not a problem on your model, but bad connections on one that has been in storage could cause intermittent problems.