Chaps, I'd like some help please.

My bike ran like a real dog at the weekend. At 1/4 to 1/2 throttle (especially in the lower gears) it was mis firing and rough as helll, at 3/4 to full throttle, espcially in the high gears, it ran like a train, pulling very strongly and fast, although still some signs of not clear running.

I changed the plug after the first lap (plug was mid to dark brown but I did not do a plug chop) and it made no difference.

Tonight I have looked at the power valve servo and it was alll over the place, even turnong clockwise and one of the cables going really slack, so I changed the sero for a spare I have off a mk2. Without running it on the road, it does appear to be better.

The float bowl had some gunge in it so a possible cause. The jets I'm running (and these have been fine in the past are a 158 main (seems a bit big) and a 45 pilot jet.

Before I change the spark plug cap, HT lead and coil for a spare, is there anything obvious I need to check?

Yours, Mr frustrated

I can't think of anything else except the obvious if the PV servo still misbehaves - ECU.

The main is a bit big though and could probaly be changed.

Remember the golden rule:

Only change one thing at a time or you will never know what was wrong.

If jetting was ok before, it is ok now.

helisupp wrote:Remember the golden rule:

Only change one thing at a time or you will never know what was wrong.

If jetting was ok before, it is ok now.

Agree on both counts. If its the ECU I'm stuffed as I don't have a spare Mk3, only one off a mk2

What we could do with is some kind of parts library, where main things like ecu's, servos etc could be 'lent out' for testing purposes. Just so a known working part is available to eliminate failed ones.

This idea is fraught with problems, but its worth a mention

Good idea, but you don't want to try known working components on a faulty bike without knowing the cause of damage. In particular ECUs. If a bike has a faulty rec/reg for example you wouldn't want to plug in a decent ECU. Best done the other way round.

It's a nice idea Twisted but there is a matter of trust involved that includes damaging a working part and receiving a dodgy part that you are told is OK etc etc....

Though I think that we all have forum members that we can trust and I know this is happening to an extent already.

I have already offered that service for AR models, but you have to carry out the work at my house.

Everything for fault diagnosis including complete looms and every engin part.

Still get messages asking to borrow an ECU by post..............no chance.

Mudfacetony, check your voltage regulator voltages, you may be about to blow an ECU, but its strange its only happened since you reassembled the bike, check your earth wire (on servo bracket bolt) has a good ground by removing some paint from hole area.

if your ecu is on the way out theres twostrokeracelab on ebay that repairs them had mine done by him when i brought my crm

a year a half later its still working perfect check all wires for splits interference from a wire contacting any steel will cause a issue

Well, I know it is not the power valve sero or the jets. Both changed and the problem is still there so coil next, then maybe the stator (as it is easy to change) then onto the wiring as suggested and last the ECU. LT said £428 for a new one so a repair might be the answer especially as its a tad less than a new one

So MFT:

Everything was fine before refurb.

1.Voltage is correct
2. Earth wire is earthed.
3. Fuel is fresh[]
4. Carb is removed and cleaned, properly cleaned
5. Undo all electrical connections, clean with contact cleaner, reconnect, before thinking changing the ecu.
6. Coil would play up at high rpm as well as low rpm.


It sounds like fuelling to me.

If, as I understand it, the PV is still acting up then it is an electrical problem. Most likely false earth/short in wiring, reg/rec or ECU.

Mine was a broken female pin wire inside the ECU plug causing my PV to play up. Funnily enough tested OK with multimeter first time, must have been hanging on by a thread, then finally gave way when re-tested.

It's always a process of elimination.........

if i remember right a faulty regulator can causes a ecu failure? that could be worth a check over.
on my old fireblade and crm ive had to replace the regulator on both bikes the tell tale sign is very dim lights and a weak horn.
each time on inspection there were heat bubbles on the back from overheating, on the blade this fried part of the battery loom, i regularly check my electrics nowadays and its a good reason to strip and clean it

Kev (Stumpy) at LT has suggested the reeds. The one's I have are the origonal one's and are 18 years old. Me thinks inspect them and see if they are worn at the edges.