single cylinder 250/260cc Yamaha/Linhai

[ukhozi]

Hi all,
I have a fleet of 15 Buggys (GS Moon XYKD260 with Linhai 260cc engines, (Yamaha Majesty clone) and I am fitting Microsquirt to one as a test and if successful I will fit it to all the rest and do away with the 30mm CV carbs that suffer from dust problems, we get through about 30 replacements a year.

So far the leaning curve as resembled the shuttle taking off but I am getting there despite manuals assuming I know what the heck they are talking about or what all those strange acronyms mean. I can tune a bank of Mikuni or Keihin carbs on a bike to perfection but this stuff is all new to me.

Up to date I have got the engine running which may not seem much but as it has CDi and therefore no ignition advance details are published, that is pretty good, (the ignition is fixed, why do you need to know?) I took a spare flywheel and alternator in a side cover and using a protractor figured out it had 12 degrees advance and a trailing edge trigger.

Removed the CDi unit and attached the wires into the harness connector using 'tabs' into ignition, ground, +trigger, coil +. Set the fuel flow at 16.5 according to on-line calculator for a 250cc 17hp single and fired it up, great it worked but the rest has been a two week nightmare trying to adjust the map so it runs strongly through out the rev range and of course that will change when under load on the road.

The supplier on Alibarba, Hobbytubor, has been very helpfully and the hardware kit was a straight fit replacing the 30mm CV carb and the combined fuel pump/relief valve works well, I only had to open up the cable fitting hole from 8mm to 10mm to accept the standard cable and punch a hole in the top of the tank for the return. Omitting to mention the need for changing the vacuum 'petcock' for a manual tap wasted a few hours though as the pump wasn't getting any fuel until the engine turned over.

Also fitted a free flowing, but reasonably quite, exhaust which improved breathing no end.

I have since tweaked the fuel flow to over 25 to stop starvation at high RPM but trimmed down for tick over etc and can now rev to over 6,000 but it is still very ragged and the cursor jumps about a lot on the grid. I need to smooth it out so it pulls solid through the rev range up to 7,500rpm, then I can take it on the road and fine tune under load. Problem is, I haven't a clue what I am doing and Hobbytubor says I have done very well to get this far on my own in such a short time, unfortunately the boss think otherwise and wants it on the road NOW or it 's all that time and effort wasted and back to the CV carbs.

So, anyone "been there, done that" on a 250cc OHC single? Care to share please?

Peter

Marbella Buggys
Estepona
Spain.
www.marbellabuggys.com

[Matt Cramer]

Can you post a copy of the tune file and a data log of what's going on? This will give everybody more information to use to get this figured out.