Hey guys, I recently picked up an MC18 track bike to muck around with, after using my 4 stroke street bike for a track toy for a year or two and as this is my first two stroke I have some (probably very simple) questions:

1) I think I have a stock airbox (and probably stock jetting too - 138/140 mains if I recall correctly). What is the stock jetting and which cylinder is meant to run the leaner setting and why?

2) I have a foam airfilter that looks pretty new and clean - again I'll assume its a stock item. Are these meant to run dry or wet? If its wet then I'll have to go and get some air filter oil.

3) All the oil pump and ancillary equipment has been removed. Are there any real modifications to the plumbing I should check to see if this conversion was done correctly? Is there a recommended pre-mix ratio (I read 30:1 somewhere) or should I just go off whatever's on the premix oil label?

4) On the topic of premix - some people have told me that running premix
might lead to engine siezure at the end of the straights - ie. back off the throttle at high RPM and flick down 3 or 4 gears for a slow corner, meanwhile the carbs are closed meaning no fuel, thus no oil for the engine right when it needs lubricating the most! Should I be worried about this? Is there anything I should do differently to compensate? (eg hold the clutch in and let revs drop while braking rather than flicking it out through each gear change like I do normally?).

5) After pulling the bike apart on the weekend, it looked like there was a sensor or valve or solenoid that was connected to the computer, and a couple of air hoses near the carbs. To me it looked like a pressure sensor that gives you relative manifold pressure. What does this item actually do?

6) The bike apparently hasn't been run for a few years, but after a bit of a clean out it started pretty easily and I rode it aroudn the block for about 2 or 3 minutes and everything seemed fine. Should I pull the barrels off anyway and inspect the pistons / rings / cylinders anyway given that I don't knwo the history of the bike, or if it seems to be running ok shoudl I not worry?

Thanks heaps people!

I think you'll find the answers to most your questions in the tech section of this site. Also check this site: http://www.dreamgate.ne.jp/NSR/

Then you can refine your questions better.
_________________
james
Its not how fast you go...Its how you go fast.
MC21 Rothmans
RS250
RC31
CRF250X
748R
Porsche (many)
http://members.rennlist.com/j911brick/rennlist3_002.htm

Yeah, have a good nose round the tech side of the site. Lots of usefull stuff there.

The 'end of striaght siezure' is an issue, but I don't honestly know of anyone who's suffered. I guess the safe thing to do is blib the throttle but quite how you achieve that whilst hard on the brakes I'm not sure! I kept the oil pump on when I was racing.

The rear cylinder runs hotter than the front, so is generally richer.

The oil pump should simply have a plate covering the hole in the crankcase where it would have been fitted. The input lines to the carb manifolds should either be blocked, or have a tube connecting the two with a ball bearing in it to stop air flow between the two.

The air solenoid is to alter the carb fuelling according to revs.

As for pulling it apart, I guess it depends how lucky you feel! If you have no idea of the history, and don't want to face being flung off when it siezes, then I'd remove the barrels and check - you can leave the head attached and only use one new gasket on refit. It would also let you check the state of the cooling system - they can and do crud up over the years, and overheating will lead to siezure too.

At the very least I'd have the exhausts off and have a good nose up the port with a good torch to look for anything obvious.

Best of luck with the track career!
_________________
MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Thanks alot guys, I'm slowly finding my way around the site - some parts are a bit hard to navigate through, but I'm learning!

There's a "members section" to get access to the wiring diagrams - how do you become a member?

Another concern of running no oil pump is if you run out of fuel you will instantly be running your crank dry. With the pump there you'd jst be robbing it of fuel.........
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread

On the subject of premix. When you slam the throttle closed at the end of the chute you do indeed cut off the majority of the fueling, but you do still have the pilot jet in there to bleed fuel in. Blipping the throttle certainly helps, but if the fueling is correct and the cooling system efficient you shouldn't have any problems.

How many of the 125/250's seized at the GP last weekend? Maybe a couple over the duration of testing qualifying etc, but it certainly isn't a common problem these days.

As for jetting. Think of the motor as 2 125cc engines and jet each one according to it's requirements. Don't ever think that because #1 got this size jet, #2 must have it as well. If you're a couple of main jet sizes different it's not unusual (as Tom Jones would say).

Or even to have different needle positions, eh Matt?!
_________________
Andy.
NSR-WORLD.COM

Please keep all responses to Forum posts on the Forum so that others may benefit.

Please DO NOT PM me for technical advice. My time is precious, and you will probably receive a faster response on the Forum anyway.

Check us out on Facebook! Please "like" our page!

Very true brother Andy.

Half a clip here and there never hurt anyone

Thanks heaps for the help!

If I'm to adjust the jetting on a per-cylinder basis, how do I tell which is running rich and which is lean? I used to struggle as it was to tell if my cbr250 as a whole was running rich or lean unless it was really obvious. Is there any easier methods other than plug-chops? ( eg. by "feel" methods, like when its bogging down with slow response it tends to mean the bike as a whole is running rich).

I'm only mucking around and not getting to serious, so I might just set it up on the local dyno and get them to do it (but getting it re-dyno'd after every change is goign to get expensive! - not that I think they'd know the first thing about 2-strokes either!).

Carb chops are the only way I can think of unless you want to buy a bunch of equipment.
_________________
james
Its not how fast you go...Its how you go fast.
MC21 Rothmans
RS250
RC31
CRF250X
748R
Porsche (many)
http://members.rennlist.com/j911brick/rennlist3_002.htm