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Strange 'miss' Problem

Started by Jell , Aug 11 2020 05:16 pm

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7 replies to this topic

#1 Jell

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Posted 11 August 2020 - 05:16 pm

I've got a real head-scratcher of a problem on my Mk2 850.

There is a low rev miss on one cylinder but over 3-4000 revs it clears but when the tank is off and I have a fuel feed direct to the carbs it runs perfectly. Put it back on and the miss is there again, I've tried this a few times to make sure.

It revs through great when opening the throttle. I've changed plug, cap, coil and cleaned/checked carbs.

The only thing that is bypassed by giving a direct fuel feed to the carbs is the fuel pump but as it revs through with when you open the throttle surely that can't be it can it???Anyone here had a similar problem?


#2 drewpy

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Posted 11 August 2020 - 06:15 pm

have you rode the bike without the tank?

Could happen under load

I thought you had a young daughter issue with the title  :lol:  


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#3 dablik

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Posted 12 August 2020 - 07:56 am

Check fuel lines are not kinking when you replace the tank !


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#4 Snowbird

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Posted 12 August 2020 - 09:52 am

Is the tank vent blocking and causing a vacuum that the fuel pump overcomes at higher rpm?
Just a thought.

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#5 Studley Ramrod

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Posted 12 August 2020 - 11:00 am

Might also be worth checking that there's no fuel escaping down the vacuum pipe from the pump to the inlet on No.1 cyl.  It does happen when the fuel pump diaphragm is pin-holed or worn out. Result is it dumps a glob of neat fuel down no.1 cyl straight into the cylinder causing it to miss at low revs.  It can also run straight into the lower part of the engine, a smell of the oil will confirm if petrol has got in there.

 

Also, there's an inline fuel filter that fits in the petrol spigot on the carb, sometimes the filter gets stuck in the end of the fuel pipe and then gets squashed. Prolly worth checking the end of the fuel line that connects to the carbs to see if the filter is stuck in there.


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#6 Norwegian

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Posted 12 August 2020 - 11:10 am

Now THAT's the kind of highly specialized, well into the realm of true nerdity kind of knowledge that no repair shop can ever even aspire to aquire. My hat is off, Sir!

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#7 curlylegend

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Posted 12 August 2020 - 10:06 pm

Now THAT's the kind of highly specialized, well into the realm of true nerdity kind of knowledge that no repair shop can ever even aspire to aquire. My hat is off, Sir!

 

Indeed !   What's the chances of a dealer coming up with that ?   OK, not totally impossible, but in my experience pretty much near it.


#8 MipeTDM

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Posted 13 August 2020 - 09:32 am

Could also be moisture (water) in the fuel tank due to condensing. I've had this happen to a lawnmower in the past. It cut out randomly without an apparent reason. Read a similar story about this on the dutch TDM forum indicating that the ethanol based fuel attracts moisture even easier.


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