920 won't rev

Hi all - any ideas - my Mk2A (upped to 920cc) is now run-in and runs a treat. I have today noticed that it is unhappy revving to over 4200 rpm - the engine just coughs - as soon as I back-off on the throttle she's fine. Cruising at 70mph equates to 3500 rpm and pulls like a train thus it's only now that I want a bit of real 'thrutch' that the problem has manifested itself. I recently fitted a pair of brand new 32mm Mk1 Amal Concentrics jetted as standard for an 828 - could it be that the main jet is a little small - I'm not sure that in my attempts to rev I'd be 'transitioning' to the main jet phase of the carburettor 'sequence'. Ignition timing (Tri-Spark Classic twin) is cock-on (I believe the Tri-Spark advance curve 'stops' above 3000 in any case).

Cheers,

Also check float bowl fuel level. Have you checked the spark plugs for colour and general appearance? Very clean or whitish plugs would confirm a lean running condition.

Well bigger main jets don't help - I went from 260's (as specified for standard 828) to 300's - no change - closing the choke slightly didn't help either - I'll try raising the needles a notch - - 

Hello Mark,

Try 220 main jets.

Regards, Colin Cheney.

Hi Mark, you have tried larger main jets, and it did not help.  You also tried closing the choke, this didn't help either, so, you are obviously not running too weak.  220 main jets, as suggested by Colin, may help.   However, it sounds like the ignition is too far retarded.  You say " it pulls like a train" at 3,500 revs.   A good Commando should just be starting to pick up at 3,000 r.p.m., and pull up to 7,000 through the gears (but not for too long!).   I don't know about the Tri-Spark ignition set up, but, 3,000 r.p.m. seems too low for "fully advanced".     Too late now, you've done it, but, why go up to 920 cc in the first place?    A well set up 750 Commando will take on an 850 any day!  Only joking, it would be  boring world if no one tried anything "different".    John.    

Mark Woodward previously wrote on Friday 6th April 2012 at 15.16hrs:

>Snip<

.............rev it to 4000 and it starts to backfire, cough and splutter though obviously with no load the throttle is not open very far (compared to when riding with a load) - this leads me to suspect an ignition problem. Plugs are new Champion N7s, H.T. Leads are new with new caps, coils are new and all wiring looks fine. The battery is in good condition. The bike starts and runs fine up to 4000 rpm. I've checked the valve and cam timing - looks fine. I cannot see how I'm going to find this problem without the aid of analysis equipment (Crypton, Bosch tuning etc.). Any ideas and any recommendations for establishments (preferably in the UK Midlands) who might be able to help ?

Hello Mark,

Do you have access to a ‘strobe’ timing light?

The reason I ask is that 25 years ago a neighbour of mine restored his 1955 Triumph TR3. When he had it finished he took it along for a session on a rolling-road, came back chuffed to bits with: “120bhp…!”

What made me wonder about the reading he’d been given, was that he said it was miss-firing above 2500rpm.

I grabbed my old (Sun) strobe & popped round. With it connected & engine running, the lamp was flashing regularly up to 2500rpm, then only occassionally.

His original coil was packing up as the revs picked up.

Replaced the coil and he was then a happy-bunny with the motor revving cleanly.

I did not want to burst his bubble, so I didn’t query how his motor had managed to produce 20% more power than original, at only half the revs….

I was half expecting him to call the following week with a 150bhp readout, but he never did.

If you can try a strobe on the bike (doesn’t need to be a fancy one), it will show you if the HT is stalling and on which circuit.

If it is an ignition problem, it may be any part of a circuit that is faulty, even something new. It wouldn’t be the first time that’s happened.

Paul

Mark Woodward previously wrote on Friday April 6th 2012 at 21.39hrs :

Hi all - a checked the timing with a decent xenon timing light with separate battery - I must admit I didn't notice whether it flickered when it started to misfire but it certainly advanced smoothly up to circa 3800 - 4000 rpm. I feel a bit guilty revving the b*lls off it parked on drive in a residential area on Good Friday - it's a bit noisy !! - although the neighbours are very forgiving. I'll try some other plugs  - I seem to be struggling finding decent ones - I've tried NGKs and Champion N7s. I'll be annoyed if it's coils, electronic ignition etc. as I've spent a lot of money on these parts recently. I'm now fairly sure that it is an ignition problem - - 

Cheers,

Hello Mark,

Frustrating isn’t it. Fishing around for an answer.

If it does turn out to be something recently purchased, perhaps the supplier will make things right, on receipt of the faulty part along with a copy of your invoice.

Thinking about your not seeing evidence of a lack of HT when the misfiring starts, while watching the xenon light….If the breakdown is on the other cylinder, then swapping the (inductive?) clamp to the other cylinders HT lead may provide evidence of the symptom.

Seeing the consistent strobe flashing, may be because you are watching the circuit that is operating properly.

Paul

So, it has happened again.  Over the years, there have been several reports of people moving away from Champion onto another type of plug and achieving instant improvements.  Having said that, I had no problems when I ran with them.

Our colleagues over the Atlantic put me onto a plug they like a lot - a Bosch Platinum 2 - Code 4306.  I bought a supply over there a while ago in Wal-Mart so I don't know if they have been superseded or if they appear in the UK.  They seem to work well anyhow.

If anyone ever comes up with a good explanation of the Champion Effect, it would be interesting to know.

Much like tyres, Norton recommended what was available at the time. Champion N7Y was good enough then for Morris Minors and the like but things move on. (oil for instance, but let's not go there) I've used the Bosch platinum 2 for a good few years after finding them relatively cheap in Wal-mart. The occasional misfire and fluffy running at tickover is a thing of the past.   

Do be wary of the overall length of the plug as the 4306 number is not well illustrated on the packet. The 4306 is 0.050" longer than a N7Y putting the spark nicely into the combustion chamber. The 4308 is shorter and puts the electrode close to where the N7Y was. There doesn't appear to be any information available on heat ranges of these plugs and I haven't seen them in the UK  Wal-mart has then in the motoring dept in packs of two so they're probably for ride on mowers!!