English French German Italian Spanish

Misfiring between 2000 and 4000 rpm

Submitted by andy_bone on Fri, 23/10/2020
Forums
Commando

Hi all 750 Commando users.

So here's the problem. A few months ago I noiced an occasional hesitation from the engine, nothing too serious, which cleared itself. Then with no warning the engine would hardly run at all, so much so that I didn't dare take the old girl out.

The symptons are as follow: Starts OK, (Alton, what a bit of kit!), but almost immediately backfires, hunts, strangles itself, suddenly revs up, then dies, banging fron the exhaust with black smoke puffs. Wind open the throttle and providing one can get a response, from 4000 rpm it runs fine, up to 6000 and beyond.

What have I done so far? Dismantled the single carb, which is quite new, and found it as clean as a whistle. Throttle slide free, float OK, all orifices clear. Fitted a spare pair of new plugs. No improvement.

My thought process suspects the Boyer, which I fitted as a replacement for an older Boyer about eight years ago. I think it was a Mk 3, or series 3, or something like that. But before I start throwing pound notes at the problem I would welcome any comments.

Other info: Bike has siamese exhaust, so can't tell if one cylinder is worse than another. I don't think so. Both plugs look the same, pale grey, so as they should be. Don't think it is the coils, as both would not malfunction together.

Any suggestions appreciated.

Andy Bone

Submitted by webmaster on Fri, 23/10/2020

Permalink

Andy - check the wires at the Boyer pickup they can fracture close to the point they solder to the pickup plate. 

Regards - Paul.

Submitted by alan_osborn on Fri, 23/10/2020

Permalink

I was about to say the same but also the copper wires can break inside the wire insulation.

An easy one to find this, start the bike let it tick over then with the Pick up cover off, wiggle the PU wires, it will soon dye if any are suspect.

Submitted by dave_taylor on Fri, 23/10/2020

Permalink

It's also a good sign of ignition coil failure.

Submitted by michael_sullivan on Sat, 24/10/2020

Permalink

Alan:

After the dye job, what color will it be?

(sorry, I couldn't help it.  (heh heh))

Mike

Indeed, but believe me I fought with a misfire like this years ago. I sent the whole Boyer ignition including the three year old twin coil (non Boyer) to Boyer. The ignition came back with a note explaining the ignition checked out okay but coil was beginning to fail. And they were correct. I even tried the old coil on the bike to check.

I was told that failing coils on a Commando can fail at a specific frequency and either side of it preform normally

 

Dave

Submitted by andy_bone on Mon, 26/10/2020

Permalink

Really helpful, Dave. Thanks. I have pinged an email to Boyer for their comments. My only thought is that with two seperate coils, would they both pack up together, literally on the same day? Seems a bit unlikely, unless there is an interaction between them of which I was unaware, as I thought they were to all intents and purposes two seperate circuits.

Andy

Submitted by steve_constable on Mon, 26/10/2020

Permalink

I believe the coils are wired in series with Boyer so seems conceivable that one bad coil could affect both cylinders?

Steve

Submitted by gordon_robertson on Mon, 26/10/2020

Permalink

Could be the ignition switch contacts worn and loose, a bit of engine vibration at different rev ranges causes lose of ignition and spectacular backfiring as the ignition switches on and off repeatedly.

 

Good luck with your diagnostic search.

 

Gordon

 

 

You are right there, Gordon. A few years ago I rewired the bike with a "secret" switch straight form the battery to the coils, thereby bypassing all the crappy old wiring and the ignition switch. I am thinking coils, but will test the solder joints first.

Andy

Submitted by alan_osborn on Thu, 12/11/2020

Permalink

If you have any electronic ignition on a twin cylinder bike that uses wasted spark and you have two ignition coils then if one packs up so does the other-Union work-one out all out. Ie Boyer Bransden, Pazon, Lucas RITA (and more) Usually the coils are in Series, occasionally (Lucas RITA) you might have 2 off 12V coils in parallel but with steering diodes-again any issue with either cylinder spark could be caused to either ignition coil. Or often the steering diodes.

Finally one odd fault we find with this system is-loss of spark on one cylinder-can be caused by a fault in the continuing to work cylinder coil! So change them both!

Submitted by Xanna_jeannett… on Thu, 12/11/2020

Permalink

hello now the best  way of finding a ignition fault is to take the tank off one dark night in your shed or garage or workshop and hook up a small slave tank and start the bike and let it run then turn you shed /garage or workshop  lights out and  look for any arching anywhere  in the ignition wire or wiring harness  as electric will arch to the  nearset point  and any blue or red flashing will tell you where there is a break in the wiring or on the plug wires this is a old trick  that been past down over many years  now the other thing is cracked  porcelain down the side of the electrode  of the spark plug its self can cause these problems         now stay safe  yours  anna j

 


Norton Owners Club Website by 2Toucans