920 won't rev

mwoo · Mar 22, 2012

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,

1up3down · Mar 22, 2012

1) Perhaps starving for fuel and coughing as the rpm's get up there?
Remove fuel lines from petcocks and open them all the way into a cup, does fuel come out nice and full?

2) Is is possible the little vent hole in the gas cap is plugged?

3) Is this is a new problem just encountered or did it pull strongly over 4200rpm during the new motor break in?

3) Yes, it is possible your jetting is not rich enough with the increased CCs, how about raising the needles one notch, try it, and if it still coughs increase the main jets two sizes and try again?

Report back as there are a number of other fixes once these are checked?

Webby03 · Mar 22, 2012

I'd be betting on the main jets. I guess 3500 RPM is a little less than half throttle, remember the main jets don't really start to kick in till more than half throttle.
If the bike feels like it "bogs down" it's probably too lean, as 1up3down mentioned try lifting the needles a notch and see if it make any difference.
There's more carb info here http://www.jba.bc.ca/Bushmans%20Carb%20 ... ml#SYNCHRO

All the best

Webby

Hobot · Mar 22, 2012

Also might bump up the float level to see if that help power range some to verify which way to go. Also see if creeping up on throttle give some pull over 4200, which also implied not engouh gas fed in. Check plugs for reference each step see what they show. Too high ignition adv can also stifle rpm climbs. Peer into the jugs to see color on surface develop or fade before and after changes too.

pete.v · Mar 22, 2012

Start raising the needle. Find out which needle you have. There are 2, one has 4 indentations(newer) and one has 2 indentation(older). Please note that these indent are not the clip groove but identifiers. You may have better luck with the 4 indent needle.
What do the plugs look like.
Don't worry about the mains until you can get over that 4200 lump and on to 6000 or so.

swooshdave · Mar 22, 2012

Luckily it's either carb or ignition.

Or maybe valves are too tight.

Or, or maybe you're not so lucky. :mrgreen:

bill · Mar 22, 2012

the 2 different needles are dependent on which spray tube it has 4 ring with the cutaway tube and 2 ring with a flat top spray tube.

pvisseriii said:
Start raising the needle. Find out which needle you have. There are 2, one has 4 indentations(newer) and one has 2 indentation(older). Please note that these indent are not the clip groove but identifiers. You may have better luck with the 4 indent needle.
What do the plugs look like.
Don't worry about the mains until you can get over that 4200 lump and on to 6000 or so.

bill · Mar 22, 2012

and here in is why I would start working on jetting issues.

mwoo said:
I recently fitted a pair of brand new 32mm Mk1 Amal Concentrics jetted as standard for an 828

myjota · Mar 22, 2012

Best analysis for main jets too small is when accelerating and you wind throttle off does it pick up a bit before slowing? Someone on here should be able to give you a starter for carb settings and jet sizes for a 920.

pete.v · Mar 22, 2012

I here ya bill, but standard got thrown out with the bath water with the 920. He does really need advice from another 920 Amal user, otherwise we're all just peck around for the truth in the dark.

bill said:
the 2 different needles are dependent on which spray tube it has 4 ring with the cutaway tube and 2 ring with a flat top spray tube.

pvisseriii said:

Start raising the needle. Find out which needle you have. There are 2, one has 4 indentations(newer) and one has 2 indentation(older). Please note that these indent are not the clip groove but identifiers. You may have better luck with the 4 indent needle.
What do the plugs look like.
Don't worry about the mains until you can get over that 4200 lump and on to 6000 or so.

Click to expand...

ML · Mar 22, 2012

Last resort if the carbs are not the problem - 1 chain pin out on the cam timing. The symptoms described are very similar to this.
Mick

mikegray660 · Mar 22, 2012

Obviously a mikuni would fix it :mrgreen: :shock:

pete.v · Mar 22, 2012

ML said:
Last resort if the carbs are not the problem - 1 chain pin out on the cam timing. The symptoms described are very similar to this.
Mick

That very thing happened with the new stage 1 cam of mine. I was pulling my hair out, swapping carb out left and right. Ended up advancing 5 then 10 degrees total (2 teeth on the cam sprocket) to attain as close to perfection as possible. I used the "peek-a-boo through the exhaust port" method.

AussieCombat · Mar 22, 2012

Yep, all of the above.
mwoo, you'r about to embark on some 'Amal experiencing', all good fun, if it doesn't get to you.
Do your self a favour, if you haven't already, and replace the top screws and the bowl screws with 'allen head',
that way you can get the slides out and the bowl off with out removing the carbs from the bike.
You don't need to remove the bowl to change main jets, but I garantee the bowls will come off sooner or later.
Check the float levels, don't rely on them being right.
Remove the drain plugs from the bowls and check fuel flow through the carb out of the bottom of the bowl.
Do you test with only one fuel tap on or both taps on ? If only one, then try with both on.
Are they new taps ? I recently fitted one new tap and notticed that the hole through the center was nowhere near as large as the old tap.
Theoretically you could run your bike up to 5,000 ish revs without any mains at all, so yes the needle setting is probably too lean.
If you end up with the Needles right up (clips in the lowest notch) and your plugs are still white, then you may consider
the next size Needle Jets.
I am not an expert.
X.... An uknown quantity.
Spert... A drip under pressure.
AC.

Nortiboy · Mar 22, 2012

mwoo said:
Hi all - any ideas - my Mk2A (upped to 920cc) quote]

I have a vested interest in this one. I have a MKIIA 920 nearing completion (trispark and 32 amals). The manufacturer of your 920 kit will have all the jet / needle / cutaway sizes for your bike. Best place to start. Drop them a line. 260 main is recommended for my 920.

Yr 920 if standard, will run out of puff approaching 6000 rpm. Mid range will improve.

Cheers

Mark

B+Bogus · Mar 23, 2012

I'm a member of the gross fuel starvation camp - I too had similar symptoms, and I was fortunate that all I had done was fit a different tank with new taps, so an easy diagnosis.
Turned both on, and 'instant power boost'.
Sticky floats would also give similar symptoms. Are the float needles an easy fit in the holders?
I struggle to believe that minor carbuettor changes would make such a significant difference, but a cam timing anomaly certainly would.

RoadScholar · Mar 23, 2012

Go to WOT then back off quickly; if you're lean you'll get a surge.

RS

mwoo · Mar 26, 2012

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 - - -

mwoo · Apr 1, 2012

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 and let you know - -

Hobot · Apr 1, 2012

Bad mixture that allows ok operation over 4000 rpm ain't so bad to keep engine from rev'ing higher, just not on good power or w/o melting piston soon after it reaches nearly full rpm. Last time this happened to me it turned out to be a combo of loose cam chain on triggering action and a worn cam lobe adding to the timing chain jump. Just saying can't forget its a Norton so faults in one area show up as symptoms somehow else. What do ya think your prize will feel like on solving this riddle?
A Brit Iron motto is if it acts like mixtuire, think electrical. Visa versa too. Took a while on sides of roads and stuck at home to get it.

920 won't rev

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