High speed hesitation/missing 1995 R100RT

Lately, I noticed that my bike misses at high speed. The scenario is typically this. Engine is in fifth gear and loaded, like it would be on a slight uphill. With the tach between 3800 and 4500 rpm, if I open the throttle (for example to pass an eighteen wheeler) the bike misses, then picks back up. Up to the point when it misses and past the hesitation, I can't notice any problem. Also, if I'm going slightly downhill or even on a level road, the bike does not miss.

History.
Carbs were comletely rebuilt and resynched about 2 years ago.
More recently (3 weeks ago), the left carb started leaking. I took it off the bike, disassembled floatbowl and float, checked needle valve (fine), blew air through the fuel passages and reassembled the whole thing. Leak stopped. That's when I first noticed the hesitation problem. I also noticed the idle was lower than it used to be. I re-synched the carbs, raised the idle, replaced the Bosch spark plugs with NGK. Definite improvement, but the problem did not go away completely.

Any suggestions?

Thanks,
Marc

So, the problem seems to be the left carb, since this started happening right after the rebuild. I wonder about the diaphragm...could be it's not seated properly or has a small tear. At that RPM, you're probably still on the tapered needle/jet and starting to get input from the main jet. So your issue seems to be one of transition or immediate demand.

How much fuel is in the float bowl after you've allowed them to fill and turned off the petcock? Could be that side is running a bit lean?

The problem started after the left carb was reassembled (at least, that's when I first noticed it).
Based on what the recently removed spark plugs looked like, it doesn't appear that either cylinder is running lean, although that could be an explanation. Could a slight difference in the float tang settings cause such a problem?
When I rebuilt the carbs two years ago, I also replaced the membrane. so they're relatively new, but that doen't preclude a small tear.
Since the new spark plugs may have helped, I'm wondering whether the problem is in the ignition. Some time ago, I heard that the monolever coils were subject to moisture infiltration...or was I confusing with some other model?

I think the fuel level in the bowl of the left carb is too low. And I don't think that a momentary lean condition during accelertion is likely to show up when looking at the spark plugs.

Look where you last worked. With bike on the centerstand allow the fuel to flow to the bowls. Shut off fuel. Remove both float bowls and carefully compare the level of fuel in the bowls.

I'll get a graduated beaker so I can measure with some precision and report back.

As Paul says, check the level in the bowl. Snowbum discusses the height of the fuel in the bowl here:

https://bmwmotorcycletech.info/bingcv-2.htm

See Section 6 under "Actual Bowl Fuel Level Check".

I did it before reading Snowbum's article and I did it by checking the volume rather than the level. I marked the volume of the right bowl on the beaker (see the faint mark in the picture), then emptied the beaker and poured in the volume of the left bowl. The left bowl volume is a smidgen below the mark, meaning a smidgen less than the right. I also noticed that the left 'choke' cable was disconnected from the carburetor and I reconnected it. Both sides had a bit of crud in the bowl, and I don't know how they got past the petcock filters AND the in-line fuel filters, but apparently they did.

I will check that neither fuel line has any obstruction. I'm wondering whether the minute difference in bowl fuel height could cause the problem I'm experiencing.

Whether is may or may not cause the problem, you have discovered a discrepancy and it should be corrected. Only then will you know the answer. If the fuel level is lower in the carb, the air venturi through the throat has to work harder to raise the fuel into the throat, thus it tends to create a leaner condition.

Are you convinced that the fuel level in the bowl is correct for the right carb?

Scarletswirl said:

View attachment 79996

I did it before reading Snowbum's article and I did it by checking the volume rather than the level. I marked the volume of the right bowl on the beaker (see the faint mark in the picture), then emptied the beaker and poured in the volume of the left bowl. The left bowl volume is a smidgen below the mark, meaning a smidgen less than the right. I also noticed that the left 'choke' cable was disconnected from the carburetor and I reconnected it. Both sides had a bit of crud in the bowl, and I don't know how they got past the petcock filters AND the in-line fuel filters, but apparently they did.

I will check that neither fuel line has any obstruction. I'm wondering whether the minute difference in bowl fuel height could cause the problem I'm experiencing.

Click to expand...

Your finding is absolutely consistent with the left carburetor being lean at least at some points in the power curve. That difference in fuel volume certainly is likely to cause a lean condition in low vacuum conditions as the throttle plate opens wide. You should very carefully set the float levels in both carburetors to specifications.

It is with some embarrassment that I must admit to have found yet another problem. When I took the left carb off the bike, 3 wks ago, I took the choke lever arm off to help unhooking the cable. When I put it back on, I inadvertently had the disc 180 degrees out, so that the dimple on the shaft pointed in the wrong direction. I have corrected that and I have bent the float tang so that is now correct.

Raining cats, dogs, cows and horses, so I'm not in a position to retest, but will keep you posted as soon as I do.

Thanks,
Marc

Make sure both chokes (right and left) are full off when the lever is in the fully off position.

I'm happy to report that the problem is fixed. Whether it was the re-orienting of the enrichment circuit disc (which I believe was the primary issue) or the readjusting of the float level (probably a contributing cause), I'll leave it up to the experts to debate. As one of my favorite sayings goes, 'an effective corrective action is worth a million possible root causes'.

Thanks again to all who contributed
Marc