Need help. Just bought it and it runs on both cylinders when revving it up in the garage, but under load down the road it does not run well in that it's "garbling" and unable to attain a clean crisp engine sound or powerband. I am a bike mechanic but new to the NSR250. Anything to look for? Overhauled the carbs but no change. Do the jet needles wear out? Could not see any sign of wear on the needles. Thinking of replacing the nozzles (emulsion tubes) but want to consult here first.
Thanks
John

Hi John,

I have been restoring MC16 for years now and to begin with it did run rough, very rough.

My steps to success were

1. Clean powervalve assemblies on both cylinders as best you can.
2. Replace float valves as although they may look ok this made no end of difference. Whilst apart set float heights details can be found here for the height.
3. Get some decent spark plugs B9EGS is what im currently using.

I also have recently replaced a CDI unit that was causing me to lose one cylinder once the bike was warm but this is definately not your issue.
Also take a good look in the fuel tank if its rusty as hell your carb bowls probably are full of rust too. I used flowliner to seal my tank before i replaced the float valves and all has been sweet since.

Jimmy

Thanks for the info. The carbs are clean, float height is good and floats do not leak fuel, will put stock plugs rather than the B9ES that are here. There is an inline fuel filter too.
I read about CDI's that the tach goes funny if a CDI unit is at at fault. Mine does not so I assume its OK.
I will investigate the power valves as the servo motor does rotate the valves but perhaps it is dirty inside. The piston and rings were replaced recently so not a problem.
Any other ideas?
Thanks
John

Hi again John,

Without a doubt i'd say the inline fuel filter could be the issue. I had one fitted for a while also and teh bike was dramatically under fuelling because of restricted flow due to the filter.

If you look at the diameter of the pipe its actually huge in comparison to something with a fuel pump and as its gravity fed must be a crucial link in the chain.
I removed my filter and used a piece of brass linkage to join the pipe but it still played up so i replaced with a similar diameter ployurethane pipe and havent looked back.
Do you find that one cylinder stops firing at all? Also does the engine seem to bog down when you shut the throttle and return to tickover? Those were both symptoms I had.

Regards

Inline filter eh? The one that is there is HUGE, about 3 times the size of the fuel line. I am going to try a flow test out of the filter to satisfy me that it's not the problem.
Hard to tell if a cylinder shuts down during the ride. While parked, it revs up quite nicely. But under load above about 4000 RPM it bogs and runs really rough..... Will still look into the power valve issue too...
Thanks