920 won't rev

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The interesting thing is even with the igniton problem the bike ran about perfect to 4000 rpm, good idle, easy start. Above 4 k it was like WW3, if it would go up at all.

So I guess the weakened spark was adequate for lower rpm only.
I think it was a lucky break to notice the key switch problem.
I could just as easily have missed it and torn the bike to bits looking for the problem!

Glen
 
Mysteries that have made my Cdo stumble and cough after starting ok, bad key switch connection, kill switch resistance, broken copper inside insulation, bad coil terminal, bad fuse block terminal, loose fuse internal connections, bad plugs, hidden behind trigger plate parts too close or too far apart, loose/shorting power leads to tail light or horn, carb bolts loose on manifold or bowl or balance tube leak, water in gas, alternator connections and dead battery, cam chain and tensioner poo poo. The last one was the last one on Peel who would run great right up to 49-5000 rpm indicated then just quit making power to turn higher unless roaring down Mt steeps with gravity assist. As a child I was given a puzzle that I was told was given to China prisoners who could free themselves if-when they could solve it... then later as an adult with a Commando...
 
Mark

Did you wire up the bike to run off the separate battery? i.e. wire direct to the black box & pick ups? no switch etc as Dance with shrapnel suggested?

Allways like to try this as if all is well you know were the fault maybe. If its not all well, you know its not your wiring or battery switchgear etc.

all the best Chris
 
hi mark,from your original post i would say you,re running too rich,you increased th cc by almost 100,squeazed into the same combustion chamber = more compression,the ONLY thing that determines main jet size is compression,i suggest you take it to someone that has a rolling road dyno ,or failing that you will have to resort to plug chops on a slightly uphill stretch of your fave road,you need to start with main jet first,then needle position and so on, hope this helps,iff not thats a beer you owe me
 
NOT TRUE !!! with a large increase in displacement as in a 920 conversion, a WELL done cylinder head or a big change in exhaust system will also have an effect on main jet size. another thing I have seen is some of the oxygenated fuels the are available.

chris plant said:
the ONLY thing that determines main jet size is compression,
 
Lots of things can change the needed main jet size. Generally in order of importance.

1. Compression ratio is the biggest thing. Higher compression ratio calls for a smaller jet.
2. Next would be exhaust system
3. Then camshaft and valve sizes
4. Ports, flow velocity and cylinder swirl
5. Bore- stroke ratio and rod length
6. Piston and cylinder head thermal barrier coatings [and carbon]
4. engine displacement has a very small effect if the same compression ratio is maintained. If everything else stays the same, a larger displacement will need a slightly smaller jet.

Of course all of this depends on the fuel and air density. Jim
 
Hi all - an update. Last week I lowered the Carb needles to their lowest notches (they were in the middle notch) at which point it started peeing down with rain and thus my planned test ride was aborted and then I forgot about it. I had booked the bike in today with a local bike shop. The chap was very helpful and knowledgeable, he looked at the plugs (Champion N7YCs) which were rather sooty and checked the ignition timing which was spot on. He stated that he wasn't keen on Champions as they tended to foul easily and thus he fitted NGKBP7ES plugs. Presto !! the bike then revved (on the stand) to 5500 RPM. He then took it for a test ride and on his return he stated he'd had it up to 6000 RPM and 90MPH (I'm not sure in which gear !). He said it ran quite sweetly but could ideally do with a run on a dyno for a proper setup. The plugs are now much cleaner (left hand cock-on but the right-hand perhaps a little weak - prior to this the plugs had always been very sooty). I'll be booking in for a dyno test soon at the end of which I hope to have the definitive settings for the 920 engine (main jets etc. still to be fully tested). The bike certainly seems much better - I did have NGKBP7ES plugs in prior to the change of needle change thus the combination of changes seem to have resolved most problems.

Getting there (slowly !!)

Cheers
 
Alrighty then an electrical problem mimicking a fuel issue, imagine that. Champions are the only plugs I've had so far over a life time to be bad right out of the box, twice now in Commandos for turmoil of what I'd done wrong again. Hope every one grins big time on your dyno report.
 
I'm a born pessimist - given my luck I suspect it'll churn out about 34 BHP but what the hell !!!
 
hobot said:
Alrighty then an electrical problem mimicking a fuel issue, imagine that. Champions are the only plugs I've had so far over a life time to be bad right out of the box, twice now in Commandos for turmoil of what I'd done wrong again. Hope every one grins big time on your dyno report.

Ever hear of Motorcraft, Ford Parts....when I raced my Rickman Norton in the late '70s I was given a box of Motorcraft race plugs.....50% worked!, another box...same deal...finding out our which worked was not convenient when you had to bump start the thing.....I bought Champions!..
 
Champion plugs ain't the same breed they were when we were young and carefree. Just don't buy just two and expect them to work w/o a back up at first hint of failure to fire. Any plug brand.
 
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