Odd occurrence at high speed and 3/4-full throttle

[tc80211]

'83 R65
So did my first longer ride >100 miles over weekend... typically highway speeds felt amazing (60-80). However I found that if I was at 75+mph and tried to twist the throttle wide open the engine would hesitate greatly and slowly accelerate.

This did not happen if I was very gradual on throttle, I could get up to 100+mph with that hesitation.

Could this be points issue? Or maybe fuel not being there when needed due to tank venting? I have a stock gas cap without the extra hole drilled in it.

Anyone experience anything similar?

[Barry]

There is the standard full throttle roll off test which sounds similar to what you were experiencing. You sustain full throttle at speed for a few seconds then back off quickly to 7/8 throttle.  If power seems to pick up as you roll the throttle off then that indicates a weak mixture.

Rolling off the throttle creates a temporary rich mixture which will improve weak jetting, go undetected if the jet is correctly sized and a cause a rich stumble if the jetting was already too rich.    

[tc80211]

The stumble happens going up to full throttle. I'll have to do some more passes to see if power increases on roll off.

When it would happen I was in traffic so i held that throttle position instead of further increasing.

[steve hawkins]

It certainly seems to be more of a carb issue.

Is your bike otherwise standard.  if so you can check your jets are all present and correct and that the needle height is correct.

My R65 was not standard, with regard to K&N air filter and exhaust silencers.  I developed a flat spot/hesitation at 3/4 throttle.  I had erroneously increased the jetting to 'account' for my changes, but in fact what cured the hesitation was to go back to a more standard jetting and then go slightly leaner - with the needle jet height - by one groove.

If you are completely standard, perhaps it might mean that your jets are worn - whatever, it is a complicated area and we need more info.

Cheers

Rev. light

[tc80211]

Jets, needles - all stock and brand new. carbs all freshly rebuilt as when i got the bike it was not in running condition...

In trouble shooting I have since raised my needle 1 notch and that made for a better mid range actually. I was on 138 main jets as per spec. i increased to 142 but this odd hesitation remains when already at around 80mph and giving it the beans.

Maybe need to go larger?

I do have aftermarket headers and muffler, so would assume im still a bit lean at 142 mains.

Also have verified no exhaust or intake leaks.

[steve hawkins]

What colour are you plugs after a sustained high speed run?

Tan is what you are looking for - dark brown to black is too rich, so you might find you are going the wrong way.  

keep an open mind.

[tc80211]

Clutch lever broke... so gotta fix that before I can try again and check plugs... when i pull plugs after normal riding they are good.

[Luca]

BMW had changed the airbox snorkels over the years.  Eventually the airbox top came with asymmetrical snorkels.  If yours are both the same, you could try ordering a replacement snorkel, PN: 13 72 1 337 501

At about $6 the snorkel is cheaper and easier to change than messing with jets.  I did and it seemed to improve a flat spot I had at about 60mph in 5th gear.  This is all from the "butt dyno" though.

P.S. When you say you need to fix the broken clutch lever, I hope you mean replace it.  If the compromised lever were to break again during operation it could put you in a serious accident.  Don't forget to get a new nylon bushing to press into the lever pivot.

[tc80211]

Yes. Part is on order from MaxBMW... def not a "fixable" part.

I have asymmetrical snorkels as it was something the previous owner already changed.

[tc80211]

Still trouble shooting this.... Definitely still getting weird engine hesitation when accelerating from 75/80mph to anything faster.... it does gradually climb and will hold just fine...

frustration has set in

[montmil]

I'll pitch in my two-cents from a possibly similar experience with my '81 R65.

In top gear and scooting along somewhat above the posted speed limit, I would occasionally experience a fall off in power that required me to drop off 15-20 mph before I could work back up to a quicker pace. All this was in top gear and no down shifting.

The final solution was fresh Bing diaphragms in the 32mm carbs. Never did really see a pin hole nor tear but the fresh parts certainly did the trick.

[tc80211]

I'll pitch in my two-cents from a possibly similar experience with my '81 R65.

In top gear and scooting along somewhat above the posted speed limit, I would occasionally experience a fall off in power that required me to drop off 15-20 mph before I could worj back up to a quicker pace. All this was in top gear and no down shifting.

The final solution was fresh Bing diaphragms in the 32mm carbs. Never did really see a pin hole nor tear but the fresh parts certainly did the trick.

Thanks Monte. I will check out the diaphragms first thing!

[fbenach]

Yup!! had the same problem a while ago and fresh diaphragms were the way to go... in my case, I felt a power cut at 50-60mph plus that sensation when you forget to open the gas valve...

Good luck!