Stumble at full throttle

bombayduck · Mar 23, 2011

I have a 81 R100 that I've had for a couple of years. About 70K on the bike. Last fall when I crack the throttle open at 3500 and above the bike stumbles and sputters like it's starving for gas. Just getting around now to fixing it. I've adjusted the valves, put in new diaphrams, checked the fuel bowl level, checked that the timing advances ok. Also, the plugs are a nice tan color. OK so now I'm stumped. Could it be electrical? It does feels like a fuel problem. :banghead

Any ideas?
Thanks, Brad

Fourcats · Mar 23, 2011

...Apply some choke and try it. :lurk

AnnapolisAirhead · Mar 23, 2011

Double check the needle settings (I measure mine, because getting a grip on the needle to two clicks can be somewhat of a guessing game).

+1 on the carb to head tube. Idle it and spray WD40 or Carb cleaner on them. If the idle changes, you have a leak. Just replace the tubes.

Incidentally, with the bike turned off and you open the petcocks for a minute or so, then close them and carefully pull each bowl. Is the level close to identical on the left and the right from the deepest part of the bowl? On 40mm, I was told that 18-23mm is ideal. A bit lower on the 32mm carbs.

Also, is your ignition setup all stock?

kmkahuna · Mar 24, 2011

Gas cap

The two things I had to do on my 78 R100/7 was synchonize the carbs (they were WAY off after my restoration) and drill out the gas cap. (after drifting to the side of the freeway wondering why I wasn't getting any power with a full tank of fresh gas and both petcocks on...then I remembered this articlehttp://bmwmotorcycletech.info/fuelfltrs&petcocks.htm
Opened up the gas cap and vrroom! off I went...

dennisdarrow · Mar 24, 2011

Are you sure about the diaphrams?........Surely sounds like one or both of them are not working right........+1 on checking the needles and air leaks around the intake tubes.......

The valve adjustment is ok???????

I know it sounds weird but is your ignition capacitor in good shape?????.......Actually not sure if your ignition uses points but if so this condition is an indicator of spark breaking up..............

Anyway......God bless.......Dennis

PGlaves · Mar 24, 2011

PT9766 said:
Sounds like leaking ( maybe perished) carb to cylinder head hoses.

PT9766

It sounds to me like a fuel supply issue - not an air issue. Full or brisk throttle drops the vacuum suddenly which would be the least likely time to have an effect of an air leak.

I suspect diaphrams or needle/needle jet transition to main jet.

bombayduck · Mar 24, 2011

I checked everything and went for a ride. Still stumbles when I crack the thottle. I then did this" In first gear about 3000 rpm I reached down and pulled the left throttle cable where it attaches to the throttle linkage and that side speeded up. When I did the same thing on the right side, the engine bogged down and almost quit. Funny, it doesn't do that at idle, only under load. So it's narrowed down to the right carb. I checked the float level and it's good on that side. When I replaced the diaphrams last week I noticed that the hole where the needle goes into on the right carb looks different than the left carb. The left carb looks like it has a small O ring where the needle goes in but the right carb doesn't. I suspect this may be the problem. If it is missing the little O ring would this cause the stumbles on the right side? If there was an O ring there, where did it go?

lmo1131 · Mar 24, 2011

The left carb looks like it has a small O ring where the needle goes in but the right carb doesn't. I suspect this may be the problem.

I believe that the only "o" rings on R100 series carb are on the throttle shaft, the mixture jet, and two on the choke shaft.

You mentioned diaphragms, but did you actually do a complete tear down/clean/rebuild on both carbs?

Your throttle cable pull test is suspicious. It's impossible to over-emphasize synchronizing airhead carbs.

WHICH REMINDS ME: Confirm that there is a filister-head screw (with fiber washer under the screw) that closes the "vacuum port" (used for synchronizing the carburetors)? The engine will run if it is not in place, but it won't run well. This I learned one afternoon after synching my carbs, going for a ride, and then finding the screws on the bench when I got back; instantly answered my "Why is it running like crap now?" question.

dbrick · Mar 24, 2011

Is there a fuel filter in the line to the right carb? If so, does it flow as much as the left side filter?

Fourcats · Mar 24, 2011

So the answer lies here...

bombayduck · Mar 24, 2011

CORRECTION: I just looked at the left side and the right side is the same so there is no O ring in there where the needle goes in. So, I'm about to pull the right carb and take it apart to clean out the passages and check it out.

bombayduck · Mar 24, 2011

Fuel flows good. I'm going to check the vacuum port now.

bombayduck · Mar 24, 2011

Vacuum port OK. I just pulled the right carb so will rebuild it either today or tomorrow. I have the kit from Bing. I was just hesitant to do it mainly because I wanted to solve the problem another way. So I'll just do it! Darn. :violin

lmo1131 · Mar 24, 2011

I was just hesitant to do it mainly because I wanted to solve the problem another way. So I'll just do it! Darn.

Do 'em both whilst you're at it Brad. That way you'll know whats going on, and it won't take that much longer.

lmo1131 · Mar 24, 2011

So the answer lies here...

OOoo ooo o... . I've never seen a set-up like that! (I don't get out much).

How's it work? The lines coming in from the top are from the carbs? The cross-tube balances the liquid in the bottles and allows one, or the other, bottle level to change height? Easier to see and maybe less prone to "bouncing" while synching?

Or do you just pour Vodka in with the cranberry juice and share it with a friend? :stick

bombayduck · Mar 24, 2011

Problem solved with rebuilding carbs

It only took an hour from start to finish to rebuild the carb. It was really not difficult at all. For all the time I spent fiddling around beforehand I could have done both carbs. :doh

Went for a short ride and the bike has great throttle response again. Tomorrow I'll rebuild the left carb. Thanks all for the help.

Fourcats · Mar 24, 2011

Lmo1131 said:
OOoo ooo o... . I've never seen a set-up like that! (I don't get out much).

How's it work? The lines coming in from the top are from the carbs? The cross-tube balances the liquid in the bottles and allows one, or the other, bottle level to change height? Easier to see and maybe less prone to "bouncing" while synching?

Or do you just pour Vodka in with the cranberry juice and share it with a friend?

Found them doing a search on the forum...don't know how they work yet. Got the pieces, 1 foot 3/16 & 20 feet of 1/8 tubing to the bottles. Tune each side, then "sync" them, I believe. Got the tube at ACE and the bottles at BB & Beyond. Figure to mount them on a clipboard I can hang on the wall.

Friends R welcome...

kmkahuna · Mar 31, 2011

Same Problem; Bucking bronco at 70mph

I've been following this thread because I have the same problem: Restored 78 R100/7, flat top carbs, 64/32/224, Boyer Bransden MkIII electronic ignition, had the carbs rebuilt by a shop, without the needles...the problem is very, very pronounced, like a bucking bronco once I open the throttle up wide. I can "creep" up to speed, it seems, but once the throttle goes past a certain point, it's all over...I slow down, and the bike is fine along the side of the road... and the cycle repeats. Choke was tried, Carb to cylinder is tight and brand new, 150cc's every 10 seconds for each petcock, in both reserve and normal positions, vacuum ports are closed...I've done this ride 3 times now, each with the exact same symptoms...
Now I'm wondering; the mkIII electronic ignition is failing? short? diode board? I don't know how to diagnose this without being on the road going 70 mph... What do you guys think? Crack those carbs and rebuild them and buy new needles? I would just be bummed if I rebuilt the damn things again and it's an electronic issue...

Fourcats · Mar 31, 2011

The Boyer ignition went wierd on my '78. It was a simple matter of swapping it with the points set, which had been marked & saved to ease reinstalling it {should the Boyer go bad}. Have to give Pridmore the nod for that one.

dennisdarrow · Apr 1, 2011

Kurt......

For me, both in practice and reading experiences and fixes on here, it would seem to be a diaghram issue with perhaps jetting second and ignition a very distant third........Make sure that all of your "O" rings on the jets and the diaghram itself is renewed and then see what happens............Good luck.......Dennis

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