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2001 250 smoother running


knobbly
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I have just rebuilt a 2001 250  with new mains / seals / ring / repacked silencer and running pretty well considering its age, with lots of compression.  There are a few queries about improvements that I hope to make. Any comments are most welcome.

Firstly, it it has a 35 idle jet and is a little blubbery on tickover, it is not responding that much to the mixture screw until it is turned approx 300 degrees either way with no real sweet area and it at times seems both too lean and rich as the throttle is opened. It is sometimes hard to start from cold, as perhaps the idle jet is set too lean.  This may be a reed issue which currently do not close 100%, as when it does pick up a few revs it flies!.  I am going to fit Boyeson 2 stage reeds so I hope that will sort the tickover and transit as it once did for a TXT270.

It can at times kick back whilst starting so am going to retard the ignition by 4mm at the stator, which should also help transit too.

It is fitted with a 143 main jet!  Now considering that Splat Shop recommend a 122 for the UK it seems far to large! The bike does rev quite high, but then seems to run rich at the very top.  I am a little wary of dropping the main jet by that much as I think that if it runs fairly well, albeit not perfectly on a 143 jet, a 122 will be far too small.  Were Shercos ever delivered with running in jets as some MX bikes were?

It already has a slow throttle, so in which order would be best to make the changes? I do not want to have too many changes in to deal with in one go. 

Thank you for all that have helped with issues with this and other bikes before, feel free to do so again.

Nigel

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Change the ignition timing first (which will affect the perfect mixture strength).  Retarding the ignition timing will make the engine more willing to rev.  Make one change at a time to understand its affects.
 
Hard to start from cold is likely the choke (enrichener) circuit.  (But retarding the ignition timing may improve this.)
 
You may be able to just flip the reeds over to make them seal.  (My Shercos seem fairly tolerant to imperfect reeds.)
 
Based on your statement regarding "running in" jets, I assume you are familiar with 2T MX bikes (which have an air screw for the mixture strength).  Despite begin a 2T, the Dellorto has a fuel screw, which I find has a smaller effect on generally rich 2T engines than an air screw.
 
Regarding feeling both rich and lean at the same time, it's possible the "transition hole" (engine side of carb from bore into idle circuit) is clogged.   Make sure every passage inside the carb is scrupulously clean by spraying carb cleaner and compressed air thorough all the orifices.  Don't be afraid to gently use a fine wire as well.
 
I rarely make a jet change of more than 1 size at a time.  The main jet only affects fueling from 3/4 to full throttle.  Since you're using a slow throttle tube, this may not be much of an issue.
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At one point i run a 118 in my shercos, so 143 is definitely too large, pilot i think 33. Are you definitely sure its 143? I didn't think dellorto did a 143, would be either a 142 or 145. Definitely a no regarding running in jets

Agree with konrad, the dellortos are rather insensitive to fuel screw adjustments in comparison to mikunis and keihins. Usually needs at least a half turn to notice a change

The odd kick back was standard back in the day. The 250s more so than the 290s. Retarding the timing would definitely help this. 

You've already done the most important thing regarding those era of shercos, repack the silencer, did you do front or back section, or both? Those sealed silencers were a PITA

I think 2001 was the year of the dodgy main bearings

Edited by faussy
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2 minutes ago, lineaway said:

 Faussy brings up a great point. The mains could be bad and the previous owner tried to jet the carb to hide the air leaks. Pull the flywheel cover off and grab the flywheel and tug up and down. If you feel movement the mains are gone.

He says he's done the mains. I was referring back to the day, (I think it was 2001) that sherco thought it a good idea to put C2 main bearings in. 6 months old and every bike needed their main bearings done. I hear what you're saying though, i doubt the bike should run as well as he describes with a jet so far out installed

Edited by faussy
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I'll add that trying the standard jetting is usually a good place to start with a stock bike.  I don't think there's any shortcut to jetting.  A lot of test-riding is needed to judge the changes.  Jets are cheap, and having an assortment on hand is a big help.    Some of it depends on the local fuel, and how picky you are.  Luckily, with trials bikes it's usually pretty easy to get at the carb.   I often use a 0.5mm shim under the jet needle clip to get things to my liking.

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Thank you for your replies coming so soon.  A little more info to clarify the situation.

The silencer, sealed triangular type, had the side cut out, cleaned, repacked and rewelded. The current oil is Silkolene MX type, label dropped off, but certainly no cheapo scooter stuff at 80/1

The carb was cleaned during the rebuild when I went to the larger 35 idle jet from a 33 as suggested by Splat Shop(Our USA Guru Copemech suggests a 36, but perhaps different amounts of Ethanol in the USA) as fuel has altered since the bike was produced. 

I use BP ultimate and there is no pinging under load and the jetting in the middle seems very good indeed.. The bike ticks over fairly well, if sounding a little rich, if trying to lean out the mixture(screwing in the adjuster, as I am aware that it is a fuel regulating screw)  There seems to be approx a 600 degree sweetish spot from centered around 2 turns out.  It hunts a little around that area, but never running really badly until outside of that area.  It picks up with slight hesitation/ 4 stroking - suggesting lean, but sounds a little wooly, which suggests rich!  I hope you get my drift on that.  Perhaps I am expecting too much, but having just ridden a friends GG200 pro where the jetting was such that it ran off the bottom so smoothly, almost like an electric motor, I have felt how good it can be.  There could of course still be dirt in the carb somewhere, which I will clean out again when fitting the new main jet + reeds which I already have. 

There are jets available up to 180, some listed as Idle/Main, are they one and the same, albeit with differing holes? I cannot confirms 143 until stripping the carb, but remember double checking as it was so far away from standard (126)

I hope I have now made it a little clearer.

Thank you for all the help so far.

 

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37 minutes ago, knobbly said:

There are jets available up to 180, some listed as Idle/Main, are they one and the same, albeit with differing holes?

No. Idle jets are an M5 thread, main jets are M6.

P.S. A rich mixture does not cause 4-stroking, but it can exacerbate it.

Edited by konrad
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the bikie will never run like the little gas gas because the cylinder porting is more aggressive. You may retard the ignition timing by 2-3 mm.

the dellorto fuel screw is not sensitive. run your 35 pilot and 118-122 main, turn the mix 3 turns out and try it. they are reliable carbs, just not precise carbs!

if really cold they can be a pig to start. lay the bike over on the left handlebar for a few seconds to allow it to flood the carb a bit.

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8 hours ago, copemech said:

if really cold they can be a pig to start. lay the bike over on the left handlebar for a few seconds to allow it to flood the carb a bit.

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. 

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