2001 250 smoother running

[copemech]

15 hours ago, konrad said:

My first trials bike was a TXT 321.  It took 8+ kicks to start from dead cold.  As I'm sure you know, the enrichener (choke) jet is replaceable in the Dellorto.  Mine came with a #60 (0.023" orifice).  I drilled mine to a #70 (0.028") and it started much better from cold. 

an option, indeed!

[cgunzelman]

I had similar problems with my 2000 2.9 and replaced the reeds with Boyesen 2 stage fiberglass. Other posters here and on ADVrider suggested the Pro series reed set with 1 carbon and 1 fiberglass wasn't as smooth in the throttle as the regular fiberglass 2 stage. New reeds completely solved the vagueness in the idle mix screw. Now I never think about adjusting it during a ride. BTW, the Boyesen #665 reeds for these old Shercos are the same as a KTM 65 and KTM 50. I'm in the US and use non-ethanol gasoline with 36 and 118 jets that were in it when I acquired the bike recently.

I originally thought the problem was caused by the carburetor not being clean enough or a leaky O ring somewhere so I purchased a basic rebuild kit for the dellorto to replace every single rubber piece and gasket. Couldn't be happier with how it runs now.

Edited February 27, 2019 by cgunzelman

[knobbly]

Update - especially for anyone looking at this thread in the future.

I have now altered the ignition timing from how I found it,  which was + 1/2mm advanced(due to my less then fussy alignment of the stator during the rebuild!) to - 2 1/2 mm retarded giving a total change of -3mm from the my starting point.

The result is indeed a smoother pick-up without a lurch and the engine is now more controllable at low revs and no need to clutch it during initial pick-up.  There may be a little less power, but it appears to be not a lot less. The tick over is noticeably smoother without playing with the idle circuit too much as I will be making further changes, and the engine sounds to be generally happier and smoother.  It still feels a little rich at the top of the rev range, so I will be fitting a 122(from the 143!!!) main jet and the boyesen reeds this week. There is still the odd kick back during starting, but less severe and a less firm kick still manages to get the engine started.

I did use the trick of laying the bike over to raise the fuel level in the carb and it did indeed start more easily, but the day was a little warmer too.  I use this last trick on my XR400 and it has always helped when starting from cold in the winter.

Thank you for all the advice, it has been very useful in plotting a way forward and I will report back after the next changes.

[konrad]

It's great you reported back on your progress.  (And very considerate of you to think about the next guy with a similar problem.)

I'm assuming your ignition timing measurements relate to an arc at the perimeter of the stator plate (which is obviously a relative measurement).   I hope this does not come off as "preachy", but a more meaningful way is to specify an absolute measurement in degrees BTDC with a timing light, or mm BTDC with a dial indicator against the crown of the piston.  But, ultimately, it does not matter because trials is all about feel.  So, I would encourage you to experiment with other ignition timings so long as you have some way to reference each one and are able to return to it.

[knobbly]

In reference to the comments of Konrad(and I do appreciate him trying to clarify) I do of course refer to the mark on the periphery of the stator plate relative to the right hand edge of the crankcase casting mount.  Virtually all advice(and there is much, if both incomplete and inconclusive, hence my wish to leave a trail for future owners) inc diagrams on here relates to timing positions in this area.  I am not aware of severe problems because of ignition not being exactly as per the manufacturer (unlike say points fired Yamaha twins where holed pistons were a real risk) so we are looking at the best compromise of power vs controllability vs smooth running vs kick back vs starting and the possibility of an influence of other mods like jetting and reed valves.  For quick and easy alterations, it would appear that this is sufficient, especially as I might well be trying different ign settings after the reed and main jet changes. For now, to be clear there seems to be a better overall compromise which I hope to improve on yet again with changes that I will report on shortly.

[knobbly]

The bike now has the 122 main jet fitted from what was on closer inspection a standard badly stamped 126 jet(which makes sense!) and the Boyeson 2 stage reed valves. These and the previous changes of 35 from 33 pilot and retarding the ignition to minus 2.5mm on the stator/case mark with the fuel screw set at exactly 3 turns out,(changes now giving indication of lean/rich within 1/2 a turn) have given an engine that is far happier with a smoother and initially higher tickover that transits without a lurch as the throttle is opened under load. The engine picks up with the mixture sounding far better, with none of the previous knocks and bangs as the revs rise. The fuel is BP ultimate mixed at 80:1 The only drawback I have found is the now audible sound of the piston slapping, previously masked by the incomplete combustion resonating in the exhaust!

My experience suggests that the options should be done in this order until happy with the engine performance.  The jetting first as it is the cheapest and the ethanol content has definitely changed since earlier bikes were made, next the reed valves(065) as they should not lose but perhaps gain power and cost more(incidentally I got mine from the USA at almost 1/2 the cost of buying them here). Lastly the ignition timing, even if it is free if you have a puller, as you might be setting it for some loss of power and the previous changes might make it as smooth as you require

I will be riding it in a trial shortly and later in the year when I am more at tune with the bike will try the bike with the timing set back to standard to see if there is any more power to be had.  If any other comments are required, they will be posted on here no fail.

Given what I found I am amazed to think that the previous owner(s) rode it as delivered.  The changes have really made a big difference, so if yours is standard, have a go, one stage at a time and you should get a far nicer bike to ride.  I have not turned it into a new bike, but it is one that reacts smoothly to the throttle and gives me more much needed brain time to concentrate on my riding! 

Thank you for all that have helped with advice it was appreciated and I got where I wanted to in the end.

Nigel

 

 

[knobbly]

Having now ridden a few trials since the alterations as outlined above, there is quite a bit less, but far smoother power. Previously when in 4th and riding flat out the front wheel would come up as the power kicked in. The power is now far more controlled, but there is most definitely less of it(think TY 250 mono). There is never a shortage of power in sections when 1st is usually the gear of choice as it is quite highly geared. For the odd long climb in third the reduction of power can be noticed, but better than standard when the rear wheel might start to spin up as the revs picked up.

It overall makes life far easier for me, but a better rider might advance the ignition slightly if they could use extra power. Me, I can only wish for being good enough to use it!