High speed miss

[bunkyone]

Hey all; Just returned from the RA rally, and seem to have developed a problem. While at the rally, noticed that the engine wasn't pulling like it normally does while climbing (Believe me, we did ALOT of climbing!!) I chalked this up to the higher altitude at the time, but on the return trip, I noticed it was still not pulling at higher speeds. Up to about half throttle, and 70 MPH everything seemed fine but the moment I'd try to accelerate beyond half throttle, she'd start to "hammer" (I know that's not very technical, but it's accurate) as though she was running on one cylinder. I put new plugs in, and it made no difference. Pulled the plugs earlier today, and found the shifter side plug running very close to perfect. It shows a nice even light tan color. the brake side however if completely carbon coated. It's black, and dry, not gas fouled. Before I (1) pop for a pair of carbs rebuild kits, or (2) take it to the neighborhood BMW shop (it's 'only" eighty miles away after all) I thought I'd ask this board of veteran Airheads for advise. Thanks, in advance, Dutch

[20774]

Could you provide the bike year/model and if there's any aftermarket ignition changes you've made? That would probably help formulate some thoughts on what to do/investigate.

[bunkyone]

Hey all; Sorry, the bikes an '76 R90/6. I have made no changes to the bike, but it did come with a Dyna electronic ignition system. It has run fine for the first 20 thousand miles I have owned it. Thanks for the responce, Dutch

[DennisDarrow]

How long since you checked the diaghrams in the carbs.........sounds just like that problem.......a hard roll on or demand will be answered by no power or one cylinder cutting wayyyyyyy back.......just my thoughts........Dennis

[mneblett]

Another thought -- the right coil may be on its way out, starting to fail when hot, then appearing normal once it has cooled back down. The fact that the right plug is carbon-coated suggests to me that there's nothing wrong with the right carb -- you are clearly getting fuel in the right cylinder, but it's not being burned uniformly. The fact that the plug is not wet further suggests that the plug is firing at least some of the time, but not enough for complete combustion and/or to get the combustion chamber temps up to prevent carbon build up. Try swapping the coils and do a good long/hot run like your hill climbing to see whether the problem shifts to the left side.

[JETHRIDGE]

I had the same problem years ago with a 94 R90/6. I always ran with one petcock open. Opened both and the problem went away. Same thing happened with my 84 R100 RT pulling a heavy load. Opened the other pet cock. Solved it. Now I always run with both open. Of course if you run with both open thats not the problem.

[sumran]

I agree with Dennis. Diaphrams seem like the most likely cause for what you describe. It is also very easy to check. If you manipulate the throttle and choke levers on the carb by hand, you may be able to remove the cables without loosening the adjusters. That way, you won't lose your settings. Once the cables are loose, you can remove the dome and check the diaphram.

Cleaning the idle jet and main jet assembly while you are in there is also a good idea. I need to do that on mine but I haven't had the time.:bluduh Lately, life is getting in the way of wrenching and riding time.

[lkchris]

Bing specifies regular maintenance as regards diaphragms, i. e. replace them every two years.

This is for sure the most likely problem.

After that, any diagnosis of running differences between cylinders starts with verifying valve adjustment and then checking compression.

Normally, ignition is the least likely culprit, but with aftermarket items the liklihood indeed increases.

[bunkyone]

Hey all; Thanks for the response. I'll try to cover everything suggested here; Diaphragms have not been out since I have owned this bike. I removed the float bowls last fall to clean the float chambers and jets there in, but haven't messed with the top end of the carbs. The coils likewise haven't been looked at. This was my first thought, as the bike run o.k. unless I put a load on it. I will be looking into replacement coils in the near future, as the ones on the bike appear to be original. I always run with both petcock open. I quess it's time to; rebuild the carbs, and lookinto replacement coils. Oh the jpoy of owning an older motorcycle!! Vaya con Dios, everyone, thanks again. Dutch