Misfire on GPX750 - advice!

[Deutscher]

My GPX750 has developed an intermittent misfire:

- backfire on starting
- runs unevenly on low revs
- OK at higher revs

Reckon it's running OK on 3 cylinders with problem on 4th.
My thoughts are:

- spark plugs
- partial blockage in one carb

Any advice?

Thanks.

[brummiewedge]

Tight valve clearances would also give those symptoms, when is the last time you checked them?

[Conian]

If spark plugs are more than a year old change them 1st
Consider carb-balancing
Check condition of spark plug leads, especially the outer pair

Also... pull off one of the spark leads and fire it up ... engine off
replace lead, pull next one off and repeat
you can find out which cylander is misfiring that way

[Chilli]

How about taking it into your local Yamaha dealer.......and PX'ing it for a new R1??!!

[Deutscher]

Thanks for the input.

I'm going to change the spark plugs and drain the carbs this weekend and see whether that does the trick.

[Speed addicted]

I've got a GPZ600R which had a similar problem, turned out to be one of the HT leads was fecked.
3 quid later it was fixed. you can change the leads without changing the ends (they just screw off).
The GPZ is also quite sensitive to carb balancing.