920 won't rev

I think I would check the valve timing before pulling the carbs around too much. I had the same symptoms on a friend's car engine a few years ago, someone had replaced the cam belt for her and after that it wouldn't rev to more than about 3000 rpm, cam was one tooth out!

Dave.

If closing the choke did not help you need to go leaner!!! bigger engine, good head work = more air flow and a stronger signal will need leaner jetting!!!

mwoo said:

- closing the choke slightly didn't help either - I'll try raising the needles a notch and let you know - -

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bill said:

If closing the choke did not help you need to go leaner!!! bigger engine, good head work = more air flow and a stronger signal will need leaner jetting!!!

mwoo said:

- closing the choke slightly didn't help either - I'll try raising the needles a notch and let you know - -

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If that be the case, feel free to lower that needle too. If one way won't work maybe the other way will. I would want to make sure that these easy things are covers before the hard things (Cam).
But dont rule out cam timing by any means.

What's happening mwoo ? any progress ?
What do the plugs look like at 4200.
AC.

320s might be too Rich . Might not be though . Dont get all pedantic, fit the ones that work .

Weak coils impeed rpms too, and give missleading plug readings.Best to set it on 100 Octane,
no dithering then .

mwoo said:

Hi Nortiboy - you say that a 260 main jet is recommended for your 920 - looking at the Norton manual that's what is specified for a standard 828 - using very 'basic' logic I think I'll go for a pair of 280s and see what happens - I also agree that the main jets may not influence the engine at 4200 thus I'll try upping the needles - I'll let you know - - -

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Who did you buy the kit from ? 260 is the starting point. If youve got fuel it will run with these jets. Bigger motor equals more suck which could mean yr rich.

What head do you have ? RH4 RH10 ?

Sounds like starvation. At 4200 you should be getting the "norton rush" when you open up.

Even with the jetting a bit off it should still go but just maybe not full open. What do the plugs look like ?

Might be time to dial in the cams if this issue has persisted since rebuild. One tooth out will do it for sure.

Hello Mark. I'm not trying to insult your abilities building bikes, but another (outside) possibility that hasn't been suggested is are the choke slides clearing the venturi when in the off position?

I had some standard MK1 Amals on my 920 last year (mine also has a big valve head), and it ran fine. You will find that it won't rev out as well at the top end of the revs with a standard head(when you get it sorted) but it should rev up past 5,500 very well, before starting to run out of puff at 6000rpm (in my experience on an otherwise standard 920). Also IIRC, main jets have a relationship with the C:R. I'm running 260s with a C:R of 9.8:1. and I think mines on the rich side.

I always think that substitution is a good idea if possible. Do you have any friends that will take their "good running" carbs off to see if the symptoms alter? I'll post you mine if you want?

Also, is this problem rev related? Is it the same in third gear as well.

It is also true that "carburettor faults" can turn out to be "electrical faults." Could you swap the coils with a known good pair?
Finally, I did have a fault with my Trispark.........could you temporarily substitute your Tri-spark for an old Boyer for example? I also have both of the above if you want me to post them?

Hi Reggie - thanks for the loan offer etc.- It's great when people offer to be so helpful but for the time being I'll 'stick with it' - just to update - the engine has only done 2000 miles since being rebuilt as a 920 by RGM (I did the gearbox) - the C.R. is circa 9:1 - I must admit I haven't checked the Cam timing (I'd assume after a £1500 rebuild that it would be OK !!!!) - in most respects I'm more than happy with how the engine behaves and to the standard of the build. I'm fairly happy that the Tri-Spark is OK. The engine pulls a treat up to circa 4000 but then coughs and backfires at about 4200 - just easing back on the throttle cures this - with the gearing I have this equates to about 85 MPH thus it's not the end of the world but seeing as I have a big motor I'd like it to use it as such. Looking at the plugs - they're not far out but possible a little rich - -

I will check the Cam timing - reference as been made to the "peaking through the exhaust port method" - can anybody advise as to what this is ? - I have access to dial gauges etc.

Cheers.

With the degree wheel in place and registered at TDC, remove the exhaust on preferably on the left side and look into the port. You can easily see the valves and their position. You may have to rotate 180 to get the right view or remove the right side pipe.

Although this is not "Dial indicator" perfect, it is a good general position. Again, at TDC, the exhaust valve is about 1/8” open (on its way to close) and intake just slightly (just noticeable) more open. If you move your crank 5 degrees either way you will see how much difference this makes. So, if you are 5 degrees off one way or the other, it will be evident.
Here is a little info from the NOC. There are other methods for finer adjustments, but this the easiest to understand.

http://www.nortonownersclub.org/support ... /camshafts

mwoo said:

the engine has only done 2000 miles since being rebuilt as a 920 by RGM.

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I would get RGM in the loop. They did the motor and have plenty of experience with the 920.

Cheers

Have you fitted new fuel taps ?.

About 100 miles after fitting new taps I had to drill out the fuel taps to allow more flow, also little pieces of the rubber seals have fouled up the orifice, ran a treat after that.

Making sure you have adequate fuel flow from the taps should certainly be checked before you do any jet changing.

Beyond that you have a larger displacement and a higher compression ratio than a stock 828. Both of these changes will create a stronger signal at the main jet and will likely mean you will need to go to a considerably smaller mainjet and maybe even a smaller needle jet. Jim

Hi all - an update - I've been fiddling about today - I lowered the needles on both carbs (weaker) to no affect. Checked the ignition timing ( I found from Tri-Spark that full advance is at 3800 rpm) - at that speed it was slightly retarded (circa 2 degrees) - I thus corrected that - it is now 28 btdc at 4000. The one thing I did find is that I can reproduce the problem with the bike stationary and in neutral i.e. 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 ?

Cheers,

Mark,

Assuming you used a strobe on the ignition timing, was the timing mark steady or jumping around? I have had an experience where AC was leaking past the rectifier causing the ignition to go of at the wrong time once the advance kicked in.

An easy test would be to power up the ignition from an external battery source and see if the problem reproduces itself.

Another test you could run is advancing the ignition and seeing if the sputtering advances; retard the ignition and see if the sputter retards. This may help point the way.

From your descriptions it sounds like it is strictly RPM related - any chance of a broken valve spring?

New plug wires? Try some old one?

Start simple, try new plugs, I have wasted a lot of time chasing problems that another set of plugs cured instantly. Jim

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 - I doubt that valve springs are the problem - they look fine to me. As I stated on a previous thread I tested a pair of (non-suppressed) NGK plugs previously fitted to the bike with a multi-meter with the test probes attached to the plug top-cap and the centre-electrode at the plug nose - I got a high resistance. I then purchased 4 x Champion N7 plugs and performed the same test, two of the plugs gave the same result as the NGKs - the second pair gave a full 'short' (as I'd expect) - these are the plugs currently fitted. I have no idea why the other plugs gave the strange high-resistance test results - as I said they are not 'suppressed' plugs (or shouldn't be !!). I intend to take the bike to a bike dealer with diagnostic test kit to see if they can find the problem as I'm fast losing patience - -

Cheers,

If rev's to 4000 then fueling is good enough to go beyond that unloaded for sure. So leves electrical like the ole motto says. Suspect broken copper inside insulation of ignition triggers or some where else shorting at 4000ish harmonics that suck voltage down to make electro brain go nutzo. Had a brake light wire do this and got Boyer goes steady again by opening the tail lens, ugh. Try running over washborad or a pot hole and look after dark for sparks and snaps while jerking tugging flicking shoving on wiring. Some electro brain ignition will spark if power or ground connection make/break and helped me find hidden fault.

When I brought my 650 ss home it would not rev above about 3500 - 4000 at which point it would cough, sputter, backfire.

The previous owner mentioned it might need a tune up and that it had been progressively running worse each year he owned it.
He won it in a raffle put on by the Ontario Vintage club. The fellow who restored it told me it never really ran properly after the resto. Herb Becker, who donated the bike to the club (as a wreck) did some work on it and said more or less the same thing.

i struggled with the usual things, carbs, ignition, back to carbs and so on. At some point in checking things I needed the ignition on. The ignition key switch is on the right and I was on the left. I turned it on then depressed the brake pedal to look at the brake light, dont know why. The brake light did not come on. Wiggled the key and Aha! the light came on.
Quickly attached a jumper wire across the key switch terminals and took off down the road, nice pure clean revs to 7 grand if wanted!
Took the key switch apart and it was full off green corrosion. All of the current for the ignition must pass thru the ignition switch and mine was like a big old resistor!

Turned out Norvil and Walridge could not supply the club with a new key switch so they bought a used one for ten bucks somewhere. The bill was in with the paperwork that came with the bike!

Long story, but I spent about a week fiddling before I found the key switch problem, so it deserves a few words.
I was quite happy to solve the problem and the bike went from one that Herb B and others said never really ran right to a real screamer.

Glen

worntorn, now that's bragging rights to preserver to solve expert stumping mystery. Steve Maney told me he limits his 920's to 7200 rpm. So missing out on a few 1000 rpm of joy.