Engine Breaking Down Above 4000 RPM

After a five month restoration my Commando 750 is almost on the road. One final problem that I can't seem to solve:

It is a 1971 Norton with Boyer ignition. We corrected to New Champion plugs from incorrect NGK's, It has 130/138 compression. The problem is that at 4000 rpm plus the engine breaks down. We switched the coils as one cylinder was running cold and the other cylinder started running bad. Replaced bad coil ran better buts still breaking down at around 4000 rpm. We checked the timing and found that the rotor was out by 45 degrees. We found that the bolt holding down the magnetic rotor was bottoming out and letting the rotor move. We installed washer and lock washer rotor is OK now We reset timing at 31 degrees btdc at 5000 rpm It still breaks down both high and low rpm under acceleration. Any help or suggestion for troubleshooting route much appreciated.
The restored Bike :

Check needle position. Try it one way first and you will know immediately if wrong or right. I might start by raising the needle by lowering the clip one notch.

Do you have a new cam? If so, is it the same type?

Check charging system.

Double check the ignition rotor. It's common that you need to cut the bolt down a little to secure the rotor. I'm not sure if just adding a washer would work.

Webby

I had this same problem!

Turned out it was an intermittent boyer wiring connection issue.

The problem can stem from a couple areas, either a poor boyer ground or a more likely suspect is often one of the wires going down the frame tube and through the timing cover and into the rotor area, the wires can fracture in there
and as the rpms change they can cut out and reconnect.

I think your problem is electrical and boyer wiring related, I would check and recheck and possibly consider replacing the wiring, especially if you know it to be fairly old, also inspect the wire to boyer solder connection for fracturing.

Does not sound carburation with the cutting out, gotta be electrical, just my guess.

wireing direct to battery ( jury rig 0 would tell you if theres wires deranged . Id pick condensors if Boyers run em ?Or COILS , if 6 volt .

If the spark aint blue & white and a full half inch, its no bloody use . Coils over ten years consant use are generally shot , if Lucas . Bench test Coil .
Any spade teminal not FIRM , anywhere ( ign. Swicth ) could stuff you, too.

Unless completely shot , coils tend to malfunction when warm .( read Hot ) if the oil boils in em , the insulation ( shellac ) on the bindings inside
goes all girly and stops work . Just as peculiar at times . put fingers on coil, if stinking hot, when playing up. puttemin the bin .

Did you go through the handle bar switch cluster?
The current for the spark runs through the kill switch and corrosion may under vibration be causing intermittant breaking of circuit.
When disassembling switch cluster be careful as there are tiny spings with ball bearing that can get away.

About only down side of Boyers are the trigger wire tendency to part inside insulation so All Bets Off analysis until those tossed out and upgrade installed, even if not the current issue they Will Be.

Thanks again everyone. As usual some great suggestions from this Forums members. Heading out to try some of these suggestions today.

Umm... the rotor is keyed to the crankshaft.. how can it be loose?

Fastest easiest way to check power and triggers to Boyer is key on then bump and tug all associated wires and watch/listen for a spark wtih plug laid on head to ground. I've had tail light ground strap shorting out to drop voltage to make Boyer go bunkers misfire at rpm, so can't just check direct Boyer wires and expect to find all a Commando mystery. Its possible cam chain lost tension to make Boyer go bunkers near it full adv. rpm.

maylar said:

Umm... the rotor is keyed to the crankshaft.. how can it be loose?

Click to expand...

They are referring to the electronic ignition rotor. The taper in the end of the cams tend to vary a lot and some rotors don't fit so well. Jim

Further to the Nasty Coil tirade , NASTY COILS . Lucas Coils on automobiles. nasty 4 Cyl. ones , NINE out of TEN 12 V ones over 10 yrs old WERE USELESS .
The ' Lucas Sports Coil ' was the Answer , there . THAT could produce the requitre 1/2 in. + blue / white spark. Essential to anything over 9.5 : 1 C.R.
with a decent exhaust system & Carb . The lesser inclined components being the equivilant of chokeing , farting , and haveing a paraxorism , symultaeneously.

Best avoided .