Missfire problems when hot

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Mar 11, 2006
Quick rundown on the symptoms im getting.

Ridden about 150 miles yesterday with a couple of stops and after about 20 or 25 miles my bike, an 850 mk2a interstate, started missfiring and seemed to have about half the normal power but was still firing on both cyls. Then struggled on for a few miles and then the problem clears up and all back to normal for 10 or 15 miles or so and then the missfire returns. This happened about four times. The only way to keep it running was wide on the throttle which was causing the odd spit back through the air box - which it doesnt normally do.

My first thought is coils. They are the originals. The bike has boyer ign, good carbs - about 5000 miles old. Took the plugs out when i got home and they both looked fine

Been out today for a couple of miles to see if the problem is still there but its running perfect.

Anybody agree with me or have any other suggestions. Is there anyway you can check the coils or do you just have to get new ones to see if this fixes the prob?
New coils are cheap, the old ones will be well past it and are prone to overheating.
GavinJuice said:
it's a 3§Ù coil, so just check it with a meter

If mike916sp's Mk IIA is still fitted with the original Lucas equipment then it should have two 17M6 6 Volt coils (now wired in series with the Boyer ignition).
Each coil should have a primary resistance of about 1.7-1.9 Ohms, I would only expect a 17M12 (12V) coil to have a primary resistance as high as 3 Ohms (or an aftermarket dual output type).
A primary resistance check won't always identify a faulty coil.

I would guess that the fault could be due to a bad electrical connection or a faulty switch, as that is what 99% of ignition/Boyer problems appear to be, going by previous postings on that subject.
L.A.B. said:
If mike916sp's Mk IIA is still fitted with the original Lucas equipment then it should have two 17M6 6 Volt coils (now wired in series with the Boyer ignition).

Yes its still got the original Lucas coils. Wouldnt say there cheap to buy - £48.60 a pair from Norvil - but going to bite the bullet and get 2 new ones tomorrrow and see if that sorts it.

Well of into the garage now and get the tank off and old coils out. I'll let you know when the new ones are on if its fixed it.

Edited 22:30....

....Now back ... tank off and coils off. They are stamped 17 M6's with 41 72 under and 45220A under that. The resistance on the coils reads 2.2 and 2.1 ohms

The coils cab be viewed below

http://s5.photobucket.com/albums/y162/m ... 626.05.jpg
Check that your tank air vent is free...if it is blocked, will cause a vacuum in the tank and the motor will starve and cause just such symptoms...

PS...gasoline filters clean?

Well new coils fitted, all low tension wiring checked and connectors cleaned and the problem is still there.

Then cleaned out the fuel tank and all the filters and even flipped the filler cap open when i was riding. What next :?: :?:

I think the missfire seems to be more on the left cylinder but cant understand why as the plugs are both perfect colour not wet or anything like that.
Have you tried changing/swapping over the HT leads, plug caps and plugs?
Just put some old plugs in and an old set of HT leads and caps. Missfire gone. The leads on the bike were only 12 months old and the plugs had only done about 800 miles. Very strange some things :roll: Now just got to work out if its the plugs or leads that were the prob - well its only one out of four :!:
Thats a pair of coils I didnt need to buy.....Look out on ebay soon for a pair of genuine 1972 Lucas M6's :!: :!:
Thanks all for your suggestions
Its back :evil: :evil:

Been out today managed to get about a mile down the road and guess what the missfire has returned. Took the carbs off tonight and stripped them completely no blocked air passages, needles fine etc. everything is 100% clean as are the electrics - both connectors and wiring continunity- I'm running out of options fast :!: :!:

I'm now thinking it has to be the Boyer unit - its all that's left.

Anyone agree or think of anything else :?:
Been there and done it . I had the same issue two years ago. Fortunately i had access to spares from the local gang. Replaced one piece at a time until the issue was resolved by replacing the boyer. No problems since.

Hopefully you are fortunate enough to have access to a spare boyer without having to go buy and try.

Good luck
The easiest option would be to send the ignition back to Boyers for testing.

A couple of things you could try is to connect the Boyer box directly to the battery and see if that cures it, as the ignition and kill switches have been known to cause intermittent ignition problems.
Also the pickup wires can break down inside their insulation between where they are clipped to the frame down tube and where the they enter the crankcase due to the vibration. And also carefully check the soldered joints where these wires connect at the rear of the pickup plate as that is another common vibration failure point. Replace the wires and re-solder the pickup connections if there's any doubt about their condition.
Just a long shot here before you sart spending more money, are the inlet heat insulating joints still fitted?

Now working my way through all Boyer stuff bit by bit. I've been looking at the fault finding tips on the Boyer web site and it mentions resitances on stator plates but gives no readings.

I've checked mine and they are 0.00 on back of both stator coils and 0.06 across the terminals with the plate out of the bike. Does anyone know if that is about right. Imho I would have thought the reading at the terminals would be the same as at the coils :?:

Also got some new wiring to fit from Boyer unit to stator plate - just about to do that.

The plugs are flashing fine when you short the wiring to the stator so according to Boyer it is unlikely that the box is at fault :?:
Its fixed - again :!: :!:
Well fitted the new leads from the Boyer to the Stator, cleaned all the stator circuits on the back of the plate, which were pretty gunged up.
Put everything back together and it even started first kick. Strobed the timing up and went out for about 10 miles, so the engine was nice and warm, everything as smooth as silk - well comparitively apart from the normal vibes :!: :!:
Hope it dont have to reopen this thread with the 'Its back yet again' line. I want to get some miles in while the weather holds up.
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