Misfire when hot?

[brassmonkey001]

Mine started misfiring and cutting out at the weekend.
It seems to be the R/H cylinder that's suffering.
You can be going along nicely when all of a sudden power drops (by about a half I'd say ) and if you let the revs drop it will stop completely. If you leave it a few minutes it will start again fine and you can carry on for a few miles before it happens again. I found keeping at about 50mph seemed to prolong the smooth running periods, I guess it must be the most efficient speed for cooling the engine?
Now I was unsure if it was the wet getting in somewhere possibly causing a short but one thing I was aware of was the tappets were noisy. I wondered if perhaps they were so loose that when the engine got hot, the expansion of metal in the barrell etc. in conjunction with excessive valve clearance might have been sufficient to cause the valves to not open enough to allow enough mixture into the R/H cylinder.
I have since checked and adjusted the tappets but they weren't far off at all. Nor did the end-float seem excessive (What should the actual end float measure BTW?).

I took the float bowl off but couldn't see anything untoward.
I shall try swapping the leads over next.

Does anyone have any ideas?
Abrasion suggested the bean can might be packing in.
I'm thinking the coil might be breaking down but I don't have a substitute to try.

I need to get this sorted before the Wee-kend.


p.s. 1988 Para R100 GS BTW

 

[Tarka]

Sounds very much like the ignition coil breaking down when hot.
Defo that if all is well from cold or after being stopped for a few minutes follwing the misfire/cut out.

I had that problem on my old LegoBike and both my Urals too.
I just fitted a 2nd hand Jap twin output coil to cure them.

 

[(RIP) Proff]

+1 with Tarka...
Get one like Mr Jonseybop a Dyna coil as they seem to be more reliable and more robust wiv bigger sparks to kill the gremlins...........

 

[thornley]

+3
1988 fitted with early coil type? Prone to cracking and give the symptoms you describe.

 

[BillN]

+4

It's the coil - exactly the same has happened to me
eventually it will stop and not go again

But test it first before buying another
easy to test with a cheap multimeter
The resistance figures are in Haynes - if not I have them

I would not bother trying to get a secondhand K coil - just buy a new Bosch, (they have improved it since the original), or a Dyna if you like red.

Have you sorted it yet?

Good luck

 

[Mike Wilkins]

May not even be the coil

At Easter on the M5, I had similar probs. when overtaking lorries in torrential rain!!!. It was only an HT lead - one of them had a metal ferrule where it goes into the coil rather than the usual brass which had corroded. Broke off corroded bit, emery and WD40 sorted it. Got me home from Cornwall in equally torrential rain. Have changed all electrics now though

 

[Mike Wilkins]

P S

After the above incident, someone in Cornwall suggested Jetski waterproof plug caps are a good mod on a bike. Anyone have experience of this ?
Cheers
Mike

 

[BillN]

After the above incident, someone in Cornwall suggested Jetski waterproof plug caps are a good mod on a bike. Anyone have experience of this ?
Cheers
Mike

Mike, I think that the problem is that the "regular" renewal of HT leads on "older" Bikes, (and cars) is just neglected.

Especially on an Airhead, as they can last years with no problem and usual "look" OK.

But as I said in my earlier posting, there is a simple test to check if the coil is good or not

 

[DevonianScotsman]

+1 with Tarka...
Get one like Mr Jonseybop a Dyna coil as they seem to be more reliable and more robust wiv bigger sparks to kill the gremlins...........

Definitely sounds like a Gremlin problem to me.
New coil and HT leads are very likely to sort out all your woes!

 

[abrasion]

I agree that it sounds like a coil.
I was just saying that i had the same symptoms on the ribena bike. Everyone said coil.
Swapped it with the one from Stans bike. Same problem for me, no problem for her.
Swapped the beancan, no problems been fine since.

 

[brassmonkey001]

I assume this fault would also manifest itself as spluttering at high speed?

So, anyone have any nice cheap sources for coils?

 

[(RIP) pastyman]

I do agree with all that has ben suggested. but.... just in case your coiul is ok something else to check is the vent hole in the petrol cap. A blockage here will, after a while create a vacum in the tank restricting the flow of fuel causing the slow running. a short stop will allow the float bowls to refil and away you go.. only a thought and costs nothing to check.

 

[Mr K]

I do agree with all that has ben suggested. but.... just in case your coiul is ok something else to check is the vent hole in the petrol cap. A blockage here will, after a while create a vacum in the tank restricting the flow of fuel causing the slow running. a short stop will allow the float bowls to refil and away you go.. only a thought and costs nothing to check.

It's a thought: Tank-bag sealing off tank-cap breather???

 

[brassmonkey001]

Good thinking Chris & Dave but not a problem in this instance. Besides, thay would surely affect both cylinders and not just one.

 

[(RIP) Proff]

Good thinking Chris & Dave but not a problem in this instance. Besides, thay would surely affect both cylinders and not just one.

Depends if you run with one tap or two open the one furthest if you use one tap gets fuel starvation first

 

[JPR]

Elimination,dear boy,
Swopped the HT leads over yet.????

 

[alimey4u2]

All the above & check the easy cheap bits first including the plugs... Fire it up in a nice dark garage... spot the corona...

 

[lurcher]

If you do change the coil make sure it is the correct resistance 1.5 ohm anything will do as a substitute but dyna coils give a big fat spark

 

[brassmonkey001]

Is it knackered?

I bought this of ebay as a substitute/spare

It was described as fully working but I don't like the look of where it's been damaged and then glued back together(ish).

 

[DrFarkoff]

cut a ength of insulating tape in haf lengthwise and carefully wrap 2 layers over that

and then remember to vaseline inside and out so ya can get the leads back off later