Hitchcocks Forum

Guys. I wish to (hopefully) solve the misfire (1996 535cc Classic) at revs above ~4500 by fitting a BoyerBransden. The bike is twin-plugged (with our hosts dual EMGO coil) so needs the special kit at GBP171 from our hosts. Whilst that cost is painfully high, the additional local customs (20%) and VAT (14%) make it essentially unaffordable.
There is a local BB supplier, but you have to quote the BB Kit number to get the correct one.
Has any one fitted this kit for twin-plugs and can quote the kit number?
Cheers, ChrisD.

Not wishing to seem unkind in any way, but it's a bit "dodgy" asking for this info here, as helping people shop elsewhere is undermining our host's business.



However, it seems a bit over the top to blow that much money in the hope of curing a misfire. Have you done the relevant diagnostics such that you know for sure the problem can be cured by a BB? Is the coil definitely OK? The rest of the ignition system in tip top condition?



It might be worth asking Hitchcock's if other customers with your particular engine/carb/exhaust configuration have reported misfiring. I bet there's something specific wrong - condenser, points, coil, plug cap, plug, HT lead...... something. It would seem most unusual that your particular configuration simply won't run properly without a BB.



Not trying to teach you to suck eggs! Just trying to save you some dosh - especially if the BB doesn't cure it!

Chris, I've had high revs misfires and discovered that they have mostly been caused by low or intermittent voltage problems. Unfortunately the reasons are varied: 1) faulty voltage regulator 2) faulty coil 3) alternator output not high enough at high speed or lastly a faulty battery. You could try running an extra battery in parallel with the existing one to eliminate the possibility of low voltage first. REgards, Mark

Thack and MarkM. Thanks for your comments. Yes you are right, asking advice here for another supplier is an issue, BUT I have spent thousands of pounds with our hosts (yes several thousand) on this bike. I have tried 2 different coils, several new non-resistor plug caps, two different HT cable sets, two different distributors and A/R mechanisms, 2 different points sets, two different batteries (new), several new condensors and the high rev misfire is always there. I’ve set the bike up with twin coils+twin condensors, single coil and single plug, single coil and twin plug and the current dual coil and twin plug. I’ve described my attempts over the years on this site, so far with no solution that has helped. So the last resort is electronics. Mechanically the bike goes very well, running easily up to ~4000rpm at ~70mph (GPS speed) on half throttle (19T drive) so no problems there (see all my previous posts). I must admit I haven’t tried a new regulator but since the battery is always charged OK, it’s probably not necessary…..ditto for the alternator.

The second postal strike in RSA this year is now into its 8th week and I doubt I can reliably import goods this year – unless I use a courier. The cost for that, together with the ~40% surcharge due to customs etc, coupled with the potential for non-receipt, means I am loathe to spend such massive amounts of money (potentially over GBP300) for a part that I may not need. And which may never arrive (last year a local courier handling our UK passports LOST all our original UK birth certificates, so I have little trust there).
BUT, BB have an agent here, so I can acquire a part at a local cost (~GBP150+VAT) but they don’t know the kit number I need.
If I could get reliable advice and guidance here in RSA, I wouldn’t mind spending the money!
Cheers, ChrisD (Cape Town)

It's probably the carb, after changing all those parts in the ignition I doubt a bb will cure it.

It's probably the carb, after changing all those parts in the ignition I doubt a bb will cure it.

Hi Chris, could it be possible your inlet valve springs are not strong enough? Inlet valve bounce is as efficient as any rev limiter, I have seen it several times. Regards, Paul.

Chris you include lots of information. Most of it has to do with electrics. And none of the expensive changes you’ve made have solved the problem. I think the important thing you say maybe that the bike ‘goes very well, running easily up to 4000rpm on half throttle’. Beyond that it seems as if you strike trouble. This suggests that maybe the problem is not with ignition but carburation.

It sounds as if the problem cuts in as you go from needle jet to main jet. And that’s where I’d concentrate my attention. The advantage is that fixing that is likely to be a lot easier and a lot less expensive than going on fitting electrical bits that aren’t faulty. But who knows?...

I, too, am entirely unconvinced that this is an ignition problem.



Here is what I'd suggest you do. Put a piece of cloth and a spare spark plug in my pocket. Then find a long, straight, safe road and drive it into the misfiring zone and keep it there for as long as possible - a good minute or so, if you can. Then, when safe, simultaneously pull in the clutch and hit the kill switch. Come to a safe halt.



Remove the spark plug, wrap it carefully in the cloth, fit the spare and go home. Then take the spark plug outside into good daylight and take several good quality close-up photos of the electrodes and tip, making sure you capture it from all angles. Post them here and our resident experts will offer an informed view on whether it might be running a bad mixture. Remember to give the exact and complete spark plug number.



I've had high speed misfiring caused by an oil-drenched air filter when my Electra X wet-sumped; and I've had the same from running an excessively weak mixture due to ineptly set up carburettors.

LOL!! Sorry, put the cloth and spare spark plug in YOUR pocket, not mine!