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[Presto]

Or just try a couple of main jets - one larger, one smaller.
('Plug chops' with modern fuels can be imprecise.)

[Gwilly]

Chris it seems that you've drifted into that mind set whereby you're convinced its a certain problem and can't believe the evidence before you..

You've changed electrical components twice so STOP and try a different tack...

Listen to BW, around 5000rpm the cam is running at half speed 2500rpm which doesn't sound much until you calculate that a valve is opening and closing more than 40 times per second... :0
Any weakness/ deformity or coil binding in the spring will cause valve float resulting in just your symptoms..

I would certainly get the springs measured and (spring) compression tested..

Maybe lighter valves and uprated springs would get you closer to the magic 5000rpm..

As far as fuelling is concerned just make sure that a litre can be drawn through the fuel tap within say 100 seconds.. If not she is going to cough with starvation and weakness at high revs..

Regards gwilly..

[Count Johnny]

I've come across exactly this problem (albeit not on an Enfield, but on a Reliant engined 750 Formula racing car).

In this case, it had stood in a shed for 20 years (don't ask me why the owner decided to go racing without rebuilding the engine!)and my theory that a valve spring had gone lazy was proved to be correct.

So plus 1 to Gwilly's theory.

[ChrisD]

Thanks for all the suggestions, guys on my misfiring 1996 535cc Classic. This a more complete answer, so a bit long.
I have spent hundreds of hours trying every possible carb variation – hundreds of plug chops over the past 3-4 years!. For the mikcarb I have 2 separate needles, 3 separate needle jets (P0, P2 and P4), all available main jets from 115-135 (although the 135 and P4 needle jets both result in sooty plugs while the P2 needle with clip at top or a main of 125 give me white plugs), 2 pilot jets. I am fortunate that ~15kms away there is an empty and unpopulated road up a 400m high hill about 1km long so I can run up for a minute or so again and again and there’s no-one to complain or impede. Then I can chop both plugs (one our hosts Iridium BR8EIX and one a standard BR8ES) at the top of the low-load hill and check colour etc. Also tried it with OEM tin-caddy air filter, K&N free flow or nothing too.
I have yet to fit the 32mm Mk2 Concentric from our hosts (also an Ace Café air cleaner) because the carb lacks for the air jet - both possible sizes to be ordered soon (had to manufacture an angled manifold and raise the tank 2cms so that carb will clear the tank lug).

I have four separate silencers (OEM torpedo, hitchcocks shorty and their reverse megga, large dunstall-type – goes best with the shorty and megga but far too loud) – 2 different header pipes too.
Tried 4 different pistons and with several variations of compression plates (2mm, 2+2mm, 3mm etc) over the couple of years ranging from ~6:1 up to ~10:1 – currently I’m on the 535cc in the aluminium barrel with no head gasket and viton rings in the barrel around the pushrod tubes and crank pressure of ~8.0:1.
I have tried a series of tests with OEM cams and hitchcocks performance cams and the adjustable pinion – ringing the changes in advance or retard of one or two teeth for either cam (doesn’t like a retarded inlet cam at all) and better with 1-tooth advance exhaust cam.

I also thought the vibration might be valve bounce, so tried 2 different sets of light valve springs from our hosts, with their larger and lighter valves (couldn’t afford the titanium ones though) and Samrat and rockers too - different pushrods too. Even fitted the lighter aluminium valve caps, but all those tests didn’t confirm valve bounce. With high lift cams you also have to cut down the inlet oil seal to fit otherwise it gets crushed, so lots of fine detail dremel work there.

Even tried modifying the advance/retard slot, springs and weights to get more or less advance and at different rates – that makes little difference too. Under the strobe, the advance is not smooth – in fact it advances, then retreats a little as revs rise (odd, that) then advances again with a final wild shimmying at ~4000+revs (no tachometer yet) at a present 28-30degrees BTDC (+-5 degrees or more). I suspect it is badly affected by vibration harmonics – that is why I am now focussing on electronic.

With all of this I have NEVER managed to get the bike to rev smoothly past ~4000 or so revs, in all its ~13000kms. Thack, the thought of revving it to the point which it doesn’t like and holding it there, with whatever potential damage results seems like madness – watch this space, though.
As you can see, I have tried ALL the obvious things, and am now down to the last few, larger carb, electronic ignition, big fire. Fortunately I also have a modern BMW 650 so I do get to ride (but that’s a bit like sitting on an overgrown sewing machine).
Thanks for the input – I will test some things more and revert before any further big spend.
Cheers, ChrisD

[Thack]

Good grief, you've done a hell of a lot to that engine!



In the light of this new information I'll change my advice and agree with you - a fully electronic ignition that ditches the points must be worth trying.



Your engine doesn't "like" or "dislike" anything - it's just aluminium and steel. If it's just a misfire, you won't hurt anything by holding it in the misfire zone. Having said that, it sounds like yet another plug chop would be superfluous.



I'm sorry I don't know the answer to your original question.

[Beezabryan]

Unless I've missed something, I wonder has the engine been run with a single plug?

[Presto]

Seems to me there's one thing left to do - give up! I mean to say!...

[Presto]

But I still think its a carburation problem. Fitting a MK 2 is likely only going to add to the frustration!

[Mark M]

Y'know, this is where a modern dynomometer would come in handy, not something I would usually say about our old nails, sorry, classic motorcycles! I was watching a pal of mine, a very experienced engine builder (and proper bike man,) run up a pre-war ex-Works Aston Martin engine he had just refreshed on the dyno. (He wanted to do a full rebuild but the Owner was too tight for that!) Revs had to be limited but were quite enough for decent road performance. He and the Dyno man (another friend,) felt that there was a mis-fire at revs but the lap top computer read out showed it clear as you like. The answer they both agreed, was to ditch the points and fit electronic. (Which the Owner was too tight etc.) Interestingly this engine had been converted in the 1930s to run 4 separate Amal motorcycle sidefloat carbs as there was nothing as good at the time!
REgards, Mark

[Adrian]

On the subject of Mk2 carbs and their mountings, one of Bullet Whisperer's previous projects might interest you, I saw this on the US forum earlier, it looks like no tank raising was necessary.

http://www.enfieldmotorcycles.com/forum ... 5272;image

As an alternative to Amals and Mikarb VM28s, our Cousins have been getting very good results with the TM32 flat slide Mikuni on their tuned 535 Bullets, I don't know what the availability is like where you are. If electronic ignition DOES sort it all I guess you won't need to experiment further.

Regards,

A.