Pilot jet size?

[ralf]

Having failed to eliminate the Skorpion backfire on overrun over the last 15 years of ownership and having learnt a little bit about carburetion on other bikes in that period I decieded to get adventurous (for me at least) and change the pilot jet. This was based on the common advice that backfire through the exhaust on overrun is due to weak mixture at small throttle openings. Secondly, I noticed that the Skorpion parts book lists an alternative pilot jet of size 54 whereas the stock is 48 which gave me confidence to experiment. Of course changing the pilot jet requires removal of the carbs . Having replaced the 48 pilot with a 55 ( the nearest I could buy costing a trivial £4.50) and setting the mixture screw at the OEM recommended 2.5 turns out I set off for a test ride. The bike is transformed. Despite my best efforts to make it backfire it didn't. It is also smoother on the pick up and at steady small throttle openings. Apart from changing the jet all I have done is to reset the tickover.
If you're suffering the backfire it's worth trying a bigger pilot jet.
Best wishes,
Ralf.

[Blurredman]

This is great news..

You were riding around on it for 15 years before doing this?


Congrats

[Puffs]

Following a biased and flawed moderator choice (someone posting aggressive personal attacks is rewarded, while my technical posts are removed - behind my back & without any justification!), I have withdrawn from this forum.

[ralf]

The technicalities of burnt, unburnt and partially burnt mixture escape me. What I do know is that most forum threads you read it says backfiring is a sign of a weak mixture, what is also invariably said is that it's caused by an air leak on the induction side or an exhaust leak, rarely does it mention changing the pilot jet and only occasionally is there mention of opening the fuel mixture screw. Only when I saw a comment on general tuning that if your fuel mixture screw is more than three turns you need a bigger pilot jet and by pure coincidence I noticed the Skorpion parts book lists an alternative, bigger pilot jet (without any explanation why) did I put the clues together and get = try a bigger pilot jet. I know it sounds rather bizarre that I should have this problem for so long but in my defence there are lots of threads on Skorpions backfiring making it seem commonplace, even "oh they all do that sir", and you wouldn't expect Mr Yamaha to fit an undersized pilot would you? Anyway, another test ride completed and all's well.

[jimmuz]

Could you say where you bought the pilot jet from, Ralf?
I'm quite wary of doing anything to carburetors but my MZ backfire frightens the horses.
Jim

[ralf]

I got my jets from UK eBay, item number 183987699319, Carbpart UK. I don't think they are OEM but they are nicely made. They are listed as being for a Yamaha TY250/R or TYZ250 but whatever, they fit the TK primary carb on my Skorpion.
Hope this helps.

[jimmuz]

I've found it, thanks very much for the post.
Now for the rear suspension. The Bilstein gas shock absorber unscrewed itself some time back which doesn't inspire confidence. Apparently the fzr 600 3he shock fits but is too soft.