misfire at 6500 rpm

My mk111 seems to be running fine, will idle comfortably, pick up smoothly from 1500 rpm in 3rd and generally seems to be pulling well though its the only one I've ridden so not sure excatly what I should be expecting. I don't spend much time above 5000 rpm in my typical ride pattern but have noticed that If i do hang on there is a pronounced stuttering misfire that commences around 6500 , 500 rpm before the red line. whilst completely liveable with I wonder if it indicates a significant problem that is harder to discern at lower revs. Could it be the boyer ignition, an Ht lead problem or fuel starvation ? Anyone had a similar problem?

BTw my new clutch plates and correctly adjusted clutch that I posted on previously have transformed riding the bike -snickety snick gear changes with a big )

It's a MKIII, they do not like going above 6500. Pieces start leaving the crankcase at those rpm's. Valves float etc... The only commando engine that is somewhat tolerant to 7,000 rpm is a combat 750.

What carbs are you using ? Sounds like you might be bit lean on the main jet.

Cash

delete, inadvertently posted twice

Maximum torque and horsepower are reached well before 6500 rpm, therefore it is pointless to turn the 850 motor at that rpm, unless of course you are in 4th gear and going all out for maximum top speed, it IS a long stroke motor after all, and not designed for high rpm.

But having said that, it still should not misfire. I doubt it is the boyar as any shaking or loose boyar connecting wires should have exhibited themselves long before 6000 rpm

I would suspect fuel flow, as in the petcocks are not flowing enough gas, they can get partially blocked when the rubber seals inside swell and get old. Take the gas lines off the petcocks and open them up into a container, do they flow a good steady stream of gas? Does opening the gas cap make them flow more, plugged vent hole there?

If all that ok, then try bigger main jets, what size do you have in there now, are they twin anals or single mukuni?

I have got twin amals with a replacement high flow air filter. I see that the owners manual says peak power at 5800 and not to rev past 7000. there have been some previous problems with the fuel taps so this could well be worth investigating. thanks for all replies.

Your main jets might need to go richer. The combination of ethanol type fuel, and a less restrictive air filter, could mean the jets fitted need to go up a size or 2.

As mentioned even if off peak power after 6000 850's should not misfire until valves float and then its way more noisy rough than interment misfiring. Also if not fuel flow enough it also don't tend to misfire/cut out at hi rpm, more bog down in power with surges and a bit of a rush of power as you back off. I'd suspect vibration electrical fault, trigger wires to power wires to tail lamp ground strap. Of its not unheard of for C'do to have more than one fault at once both causing similar symptoms. If this is fairly new discovery go back in mind to area last messed around and suspect electrical fualt or maybe carb balance hose or gaskets beating about loose. Oh yeah I had to rearrange my plug leads so not to arc at hi rpm via the grooves worn in rubbing the head steady. An inspection after dark can also be revealing. I liked the Boyer for finding faults, turn key on then jiggle joslte tug poke till it sparks to score a hit.

David, fit new spark plugs, even if the others aren't very old. We hear many stories about poor quality plugs.
Do you know how to do a "plug chop" to read the colour of your plugs.
AC.