My mate has a 3tj that runs like a dog - he had it to the bike shop and they concluded that it has weak spark on 1 and 4 as it works its way tup the rev range - how they have have worked out it is a weak spark I do not know. Is this common and if so what is the cure?

Thanks

That sounds like a coil to me.

4 cylinder engines have 2 coils which feed 2 cylinders each - these are 1+4 and 2+3 .... they fire twice in a four stroke cycle at Top Dead Center but because one of the cylinders doesn't have any mixture in and the exhuast vales will be open there's no bang ... it's called a Wasted Spark

Anyway you can get another 400 coil (make sure it's a 1 + 4 because the HT leads are numberred and different lengths) or fit a pair from a 600 750 etc I think these put out a higher pulse but not sure

Evil
Thanks for quick response - I omitted to mention that the coils have been replaced, sorry. Is there a wiring problem that could cause such an issue. On my 750 turbo bad earths cause all sorts of problems. just wondered if there is a known fault other than the coils (sorry again for not mentioning).

Mang

Nothing known or common - check the low tension wires going to the coils and see if the loom has rubbed against the frame at all .... looms aren't silly prices secondhand so you might end up swapping that out. I'm assuming you used different HT leads and plug caps when you swapped the coils.
Have a look for an earthing cable attached to the engine, make sure it's on ok and look at the wiring from the alternator - open the block connectors and make sure they're clean.

This goes on the garage being right about the spark of course, you could look for a second opinion.

I suspect if the bike shop that my mate took the bike to is correct then it's a low tension problem. One of the things I found with the turbo which is bit older than than the Yam is that age caused the loom to embrittle and the resistance was raised on certain wires where some of the cores had broken down. The result was that some things simply did not perform well. So it's a multi meter weekend for me. I quite enjoy electrics.

Will check that he replaced the HTs and sups.

apparently mechanic said they were OK can't find out how he tested them though!

He probably measured they're restistance with a multimeter

Measuring the resistance is all very well but that doesn't test for insulation breakdown when under load or warm. That's a common failing with a lot of garages.

The cost is buttons in the scheme of things to change the HTs and supressors so do it as a matter of course if you're going to the expense of changing the coils.