YZF750R mis-fire? running on less than four.

[T1z3R]

this could get long so apologies to those that read it through.

before i killed the original engine in my bike and had to swap it used to run on less than four cylinders when ridden for a long time in the wet.
this didn't happen very often and was only whilst on ride outs. during my normal commute it was fine even in heavy rain.
i put this down to water/damp in my spark plugs/HT leads because a previous owner (for whatever reason) had cut down the front mudguard to right behind the forks meaning there was no protection for the engine.

i swapped the engine out about 6 weeks or so ago now and had a few dry weeks so forgot about the problem until now. but now its running on less than four almost all the time.
no matter if its wet or dry, a long or short run. it does it from idle and all the way through the gearbox and its driving me mad.
i can hear it firing on four randomly whilst sitting at idle and you can feel it firing randomly on four whilst riding.

ive butchered a hugger from an SV650 and extended my cut down mudguard in an attempt to eliminate that possible cause and have replaced the spark plug caps and plugs. ive also cut back the HT leads so ive got a good solid connection between them and the plug caps but yet i keep getting this problem.

it feels like an electrical problem to me but i could be wrong?
the last time i took all the plugs out one at a time to test for spark they all sparked fine so basically im stuck and looking for some advice if anyone has any to offer.

not as long as i thought it would be but thanks for reading anyway
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[bish777]

have you tried swapping the coils and seeing if the problem migrates? if you swap the coils and it is the falt, then cylinders 2 or 3 will play up.

If you swap the coils and its still cylinder 4 then you need to look at the carb for cylinder 4. The float height is probably wrong or its stuck open, flooding the cylinder.
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GSXR1100J

[Paddy Blake]

I would say the HT lead is shorting to the head or frame so try put something in to keep the leads away from where they can earth.

Paddy.

[T1z3R]

i havent tried swapping the coils mate but will give it a go and see.

ill also try isolating the HT leads so they cant earth as well.

thanks for the replies
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duck my sick!

[Robert2014]

Hi there if the bike is only doing this when it is warmed up it mean's u have a Burnt Valve only way to find out is to do a Comerishon Test Hope's this Help's
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If you require a mechanic then I am your man. Bike collection and recovery service available. Don't let a monkey work on your bike, call the family run business on 07778 873 184 01505684994 https://www.rdbikes.co.uk [email protected]

[Robert2014]

Hi there if the bike is only doing this when it is warmed up it mean's u have a Burnt Valve only way to find out is to do a Comerishon Test Hope's this Help's
____________________
If you require a mechanic then I am your man. Bike collection and recovery service available. Don't let a monkey work on your bike, call the family run business on 07778 873 184 01505684994 https://www.rdbikes.co.uk [email protected]

[T1z3R]

its not mate it does it from cold as well. but like ive said it randomly fires on four throughout riding it and whilst idling.
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duck my sick!

[bish777]

try swapping the coils, if no change then its deffo carb related.
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GSXR1100J

[APNess]

Also try checking on the exup forum https://www.exup1000.co.uk/ and the american version https://exupbrotherhood1.com/ for info, might find some help there.

[APNess]

Ah - I checked up on the brotherhood forum - you did post there already - hope you get it sorted

[irving]

>>

[joe black]

when you fitted motor did it come from donor bike you bought , if so try fitting coils/cdi etc off donor bike its obviously problem with original electrics ??

[T1z3R]

from what i can tell the HT leads themselves cant be changed as they seem to be part of the coils? no ones been able to confirm that for me yet but its the way it looks.

i bought the engine only from ebay so got no other coils etc to swap around although i am keeping my eyes open for another set.

i havent touched it since thursday night when i replaced the plug caps and plugs but got a brand new mudguard delivered yesterday which i hope to fit before monday so ill try some of the other suggestions then as well.

cheers folks
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duck my sick!