R65 - strangled engine when warm

[baffle]

Hi all,

I've not been on the forum for years as my R65 has been so impeccably well behaved! I bought here in 2008 on 48,000, and did a full rebuild including carbs and gearbox in 2014. We've  now done 60,000 happy miles.  I do an annual fullish service (oils always changed).
Just last week, she developed a strange fault that happened as an event - it was not something that developed.
After starting and setting off in warm ambient conditions, and perhaps covering half a mile, at 4000 rpm in 4th gear and trying to accelerate to 5th, it feels like its running out of fuel - hesitating and stuttering. There is plenty of fuel in the tank. The tank has a single tap which splits into 2 on one side of the (round) air filter housing. Drop the revs and the speed down, and the engine picks up again.
At standstill, the engine revs freely.
Both plugs looks fine - if anything a little lean. I checked my valve clearances, and they haven't changed over the year - 0.2mm ex, 0.1mm in. 
Its dual coil and I re-welded the front coil bracket when I did the restore and had the frame coated, and my bracket welds are still sound.
It's points in a can ignition, and the gap was fine, although there was a pit and valley.
There is a part I don't recognise bolted on to the relay bracket that might be part of the ignition, and I need to chase the wiring so I know what it does. Aluminium shell and made by Harman Industries.

So I think it's igntion, but it could be carb. I dont really know how to find the fault -  any good ideas out there?

Having re-read what I've just written, I think I would be telling myself to clean the tank filter, as it sounds like a fuel problem!!!

thanks

[Barry]

If or when it does it again I would try loosening the fuel cap to relieve any possible vacuum that might have occurred due the internal air admittance valve in the cap not working properly.

[baffle]

Thanks for reply Barry - I remember you helped me here before.

Yes good thinking, and I did try that yesterday on a nice piece of flat road, but unfortunately there was no difference. If it were fuel starvation then it should happen at standstill too. I guess you would have to work the throttle pretty hard with the bike stood on its stand, to simulate riding a half mile fast and I'm probably not doing that 'cos it would feel like you were caning an unloaded engine.

[Tony Smith]

Split in diaphragm?

[wilcom]

Baffle,  you have a good handle for this issue.

I agree it does sound like a fuel starvation issue. I have had one issue similar to this in a 4 cyl car never on a bike. The car would run like a champ thru 1st and 2nd gear and 1/2 way thru 3rd it would just quit running till I slowed down and it would pick up again until i started to accelerate in 3rd. back to 1st and 2nd , no problem....................It was the plugs under load that stopped firing. They looked good as new, but after the dealer(I had given up) screwed in a fresh set it ran fine.

Any you "experienced" guys remember the plug testers that every shop had? They had a load or compression adjustment and as you upped the compression you could see the plug misfire  thru a little window.... the physics behind a plug sensing a load has always baffled(pun intended) me.

I still think it has to be fuel flow.

[mrclubike]

 If the fuel supply checks out good
 I would do a tune up 
points
condenser
plugs
If that doesn't fix it replace the coils (1 green Dyna coil may be cheaper than 2 singles )
Make sure the coils are getting good voltage and the wire going to the can is not shorting out or loosing continuity 
If that does not take care of it rebuild the carbs

[mrclubike]

 

Any you "experienced" guys remember the plug testers that every shop had? They had a load or compression adjustment and as you upped the compression you could see the plug misfire  thru a little window.... the physics behind a plug sensing a load has always baffled(pun intended) me.

I still think it has to be fuel flow.

Sure do but we never used it
If we had them out they were getting changed
You had to change them every 12000 miles or less any way

[skippyc]

the physics behind a plug sensing a load has always baffled(pun intended) me.

Compressed air has higher resistance so you need more voltage to get it to spark. poor plug can have opens in the electrodes or leakage to earth which cause miss fire under this higher resistance.

[donbmw]

My 82 R65 did the same to me a few years ago. Went through the hole fuel system since it seamed to run out of gas. Turns out it was the ignition wires. No problems since until the ignition unit went a few years after this. Keep this sayin the back of your min. If you thick it is a fuel problem it’s more likely ignition and the the other way.

Don

[baffle]

Thanks for all the ideas folks. I'll have a chase and let you know how I get on.
Baffle

[R65Guy]

I agree with Tony. Sounds like a bad diaphragm.


Neal

[marcmax]

When I bought mine, I got a steal on it because the previous owner couldn't solve the same problem you are having. Got it home and opened the carbs and put a bright light behind the diaphragms. No huge holes evident but the bright light showed a bunch of tiny holes that weren't very evident on first glance. The diaphragms had been replaced sometime before I bought it. Put in two new ones and its been running smooth ever since. Just my $.02

[baffle]

Just checked the diaphragms - not them sadly. I'll do a standard service at the weekend and drain the tank and check the filter - it'll put my mind at rest if nothing else.

Then I'll check the HT delivery.

[Bob_Roller]

Do you have two  separate ignition coils ??

[baffle]

Do you have two  separate ignition coils ??

Yes "Its dual coil and I re-welded the front coil bracket when I did the restore and had the frame coated, and my bracket welds are still sound."