Mikuni 32mm Vm on tm250

[Freakshow]

Hey all im sure i have asked this in the old forums but i want to revisit it. 

MY 74 TM250 has a Muk with a 33mm throat so im assumning its a 32mm, i thought it had a main jet that says 270, but evey time i run it on a big track it dies in the arse like i kicked the choke on until you let it get back on the pipe, worst on long straight tracks, maybe i misread the jet as all my other 250cc (reed valves) run a 270+ i thought this was the same, where the Number is written its been moved by a burr, so maybe it says 210 ? can someone tell me what the standard main is ? and are they or why are the piston port motors on the same cc using less jet ?

got me fcuked, but its all i can think of right now.  bloody suzuki will be the death of me - mentally 

[cyclegod]

From what I've seen mains on TM250 run #180-#220 with a step of 10 in between, maybe you have an air leak? Are you using the standard airbox? Have a look http://alpha-sports.com/suzuki_parts.htm

[Lozza]

Your problem will almost certainly be fuel related.Best way to sort the main is to start with a jet you know will be to big. Say 320 and work your way down making plug chops along the way.If you do a chop when it dies what does the plug look like?? What sort of rpm is this happening at?

[Freakshow]

Surely someone can tell me the standard 32mm main jet on a TM250 ?

yes lozz i did the chops but track time is limited i know 300 and 270 is tan, but it only happens at 3/4 throttle and under load, if you free rev its like a chain saw i know that cause when i jet down at the track to the 270 the throttle got stuck wide open, my ears bled.

i think it could be a 210, but i dont want to go that low if its supposed to be around the 270.  BUt i have one more chance this weekend to get it right so id like to start close to the target, not 10 chops in.  The closer i look the old one looks like the number 210 not  270 .   My kindom for a straight answer.   what is the original 32mm main jet !   

[Freakshow]

From what I've seen mains on TM250 run 180-220 with a step of 10 in between, maybe you have an air leak? Are you using the standard airbox? Have a look http://alpha-sports.com/suzuki_parts.htm

are yes your right they list all 4 and have the 180-190 as the stock. IM assuming that the standard VM its shipped with was indeed a 32mm Mikuni >

Which would mean im not going totally mental, that would mean it is indead a 210 thats written on it ( mental note buy a jet tool for xmas and not an old spanner) 

So ill drop a 22o in and work back from there, thats saved about 3 hours of screwing with jets at the track.

So how come the piston ports dont use the same volume of charge ? i thought the reeds would have been more economical ? spo to speak what am i missing here that its about time i learnt about, ( my first piston port)

[Freakshow]

Ok the cut away was on 3 and it still wouldnt come on the top end but when i put a 1.5 it went off its face, but then struggled in the bottom end ( also stalled on deceleration) can someone therefore post what the standard carb settings for the 74/75 TM 250 are please :

Main Jet =180-190
Pilot = 35
Cut away = book says 3. ?
Needle = 6Dp17

These piston ports seem to be tempermental balance, so i think stock is on the cards to get this new bike sorted and then ill work from there.  Plug was a good colour, just ran like a pig depending on the cut away and it swung both was dramatically from top to bottom.
cheers . we are getting closer !

[halo1963]

This is straight out of the owners manual TM250L

main jet 210
jet needle 6DP1-2nd
needle jet 0-6
cut-away 1.5
pilot jet 35
pilot air adjusting screw 1 and a half turns back open
float level 25 mm

[oldfart]

Parts catallogue
                       lists pilot jet at # 40
                       jet options #230-210-220-240-250-260-270

    The rest is the same as Halo1963  post

[Freakshow]

i am giving up on parts books and manuals they all seem to say somthing differant.  i was trying to get it stock so i can get it right from there, but stock seems to be many variations:

my testing so far seems to point to this thing only running like it should on a 200 main, ( 210 flat track) <35 pilot and a <2.5 cut away, any other combo seems to either bog down on the main or bog down on the bottom i just cant seem to get the balance it seems to go left or right and not balance out, im going to get a new inlet rubber incase some air is leaning it out at the wrong times.

[oldfart]

Got the same thing happening with mine , seems to load up as if it's flooding .
Checked float level all good there -main jet is 200 with a 35 pilot 
Freaky did you manage to sort yours out ?

[Freakshow]

Yep.  I think the problem is the piston port set up, under throttle the charge is too rich and cant spark it when its open wide and loaded up on short tracks it kills itself in a cyclical manner.  In the end i found the 180 Jet and a 25 pilot fixed the problem but thats only on the short track i ran it on the long track with a 190 and it was ok too, seems the cycle of flooding bogs it dwon and kills off the combustion, charge and its all down hill from there till you throttle off, it clears out and then you force it again with a poor mix.  I only got a TM this year and now 3 bikes later it seems they all suffer from the same problem, i have no idea how the piston port works but with out the reeds it seems a balanceing act to set it up right, err on the lean and its fine.  the sympton was very muck like it was running out of petrol which was what confused me all the time, and you could rev it standing no probs at all, it was only when ridden did the problem show itself.

Im not sure of the track you on but if it has small straight and 3/4 throttle cut the pilot down to 25 and then jet your main around the 180-190 from plug chops. test that and if it runs off its face then you now your onto it, but do short runs so you dont run it too lean until you get your mains right, you might find with a 25-30 pilot you can support up to the 200 main, its just getting the transition right but dropping the pilot and main will tell you instantly if your heading in the right direction or its another issue.

Why ? well I found runniing lean on the pilot got it into a good balance so when the main came on it was still n the pipe and not hit with a charge it couldnt handle.

I firmly believe the 74/75 TM250 Should be on A 180 Jet unless you on 100mtr plus flat out straights then you can use the 190-200 sparingly

I got all 3 bikes as they are and told they ran, they did but not propally for racing my tracks, they all bogged down, so im guessing they where jetted over the years for trail riding and the kids etc. and maybe just there is some change in the older Super fuels to new unleaded in these piston ports ,that has a knock on effect in perforance/fuel characteristics for combustion as a full charge hit. 

Im not sure though if you have always had the bike why something has changed now if thats the case im not sure this is your problem , but if its a new purchase or an old problem for you i would put my money on you pilot and main being too rich and maybe an averge spark from either the plug ( change it) and maybe a coil or winding thats getting a little weak or tired.

[oldfart]

Thanks for the reply , Freaky I test ride my machines at home    as I live on acreage
This sarvo I went out and bought jets ranging from 180 to 290 in steps of 10
Also bought pilots 15 20 25 30 35 40 plus 4 plugs  so it's out to me mates farm (  Flat with a gully - ) which would come close to most of the QLD courses .
I also have a NOS  RH 250 barrel that I might drop on and see if there is any performance gains  just need to find a manifold intake , as they are 3 bolt

[GD66]

 Freakster, if it's happening at three-quarter throttle and under load, shouldn't you be leaning off the needle a notch or maybe two ? If your mainjet is in the ballpark, your needle setting is much more critical from quarter to three-quarter throttle really, worth a try before you go nuts with a pocket full of main jets. 

[oldfart]

Its me with the pocket full of jets  spent the last hour changing jets and needle positions
Just checked the parts book and yes it has the right CDI unit  Part # 31900- 30120
The thing that's spinning me out is , it idles like a dream -give it a hand full of throttle and she revs over the moon .Put it in gear and it loads up big time   
Might try a different carb    ( pe 250 )

[Freakshow]

Its me with the pocket full of jets  spent the last hour changing jets and needle positions
Just checked the parts book and yes it has the right CDI unit  Part # 31900- 30120
The thing that's spinning me out is , it idles like a dream -give it a hand full of throttle and she revs over the moon .Put it in gear and it loads up big time   
Might try a different carb    ( pe 250 ) 

testing a carb you would need the same one, size etc 32mm anything else is futile.   

put the 25 and the 180 in the bitch and thats the only test youll need to do to prove this is your problem. you need not read nay further till you have done that.  put the 180 in the bike and dont come back to you have. 

Mine sounded the same reved on the stand, blah blah blah. put it on the track and it filled up. lean it out on the pilot and then the main as discussed 25/180 and see if it make a differance under load, if it dont make any obvious change you looking in the wrong spot. full stop.

if it cant get on the pipe with those jets i would move onto the stator, coil and cdi, get all 3 tested as a set to see if they work under load, if you cant get an immediate change on the jets you need to suss your electrics, pull off the lot and post it to steve ( betta bikes) or similar even lozza was taking about building a bench tester up.  you need to know the spark is working under load, big fuel will kill a little spark dead - its a 2 pronged death.

Step one - do the quick and dirty jet test
step two - get the complete electrics bench raced especial the CDI (warm) they go mental after a while.
step 3 - tighten down you barrel nuts again
step 4 - do leak down test (visually  check all gaskets and seals with access to the crank)
step 5 - buy another one and come back to it after 2 month ( that worked for me) i had gotten to close to the problem ans it was staring me in the face. although yours might be differant it sounds related to one of the above, from living the problem first hand :O)