hi all my mc18 is up and running but left/right exhaust notes on idle [2000rpm,] do sound different,they are standard pipes/silencers ,as you sit on bike left side pops/puts like an old lambretta and right side sounds well like you would expect it to sound, do carbs need adjusting any advise, _________________ NSR250 MC18
Don't mean to worry you, but when my 21 had a dead crank seal (ignition side), the left side exhaust note on tickover was like a wet fart.
But that was a pretty extreme problem. Your carbs could be out of sync, power valve on rear cylinder not adjusted correctly, exhaust port / valve coked up??? _________________ Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30
Mate, I'm having the same thing on mine. It seems like one cylinder is idling faster, as if one pipe is pulsing faster.
I thought I might have got the CDi's around the wrong way, but it's still the same.
I'm gonna borrow some vaccum gauges and properly balance my carbs.
I wonder which CDi goes to what cylinder? I've got myself all confused now
The carb syncronisation adjustment balances the lift of each throttle slide in the carb. Its very fine adjustment indeed and a vacuum gauge is needed to measure the pull through each carb. A visual check will get you 9 / 10ths of the way. On tickover, the throttle is closed and both slides should be fully down. When you open the throttle, check both slides rise in sync.
For vacuum testing, you ideally want to run the engine with the tank off, so you can get to the carbs. A test tank such as this one (the 2 stroke lawnmower!!) http://www.nsr250.net/forums/viewtopic.php?t=6775&highlight=plug. Or simply suspend a bottle of fuel above the bike, like on a Dyno.
Get hold of a single vacuum gauge with a rubber boot, which fits into the carb bellmouth. On tickover, you can then see the pull on each cylinder, which in turn equates to the strength of pulse on the exhaust side. Think of the engine as an air pump, what goes in the carb must come out of the exhaust, unless there is an air leak.
If the carb slides are in sync, but one cylinder is pulling stonger than the other, then you need to check the piston compression + sealing integrity of the crankcases (vacuum & compression). These bikes are old, and dead elastomer crankseals are not uncommon.
If the engine does actually run on both cylinders, its unlikely to be a ignition / coil / cdi problem.
Its also worth checking that both powervalves are adjusted and working correctly.
hi,well after syncing carbs last night and ive just set up power valves [they were loose] the bike does run better,it revs up crisper and down faster,BUT the left is deffently still slower, do each carb have there own air screws and were should they be,i read on hear 1.5 turns out any more ideas guys, _________________ NSR250 MC18
Have you checked the float heights on the carbs? I had similar problem to you, one carb was overflowing and making that cylinder run rich and both float heights were slightly out, I ajusted them synced the carbs set the air screw's and it's now fine. It's worth a look.
RichG wrote:The carb syncronisation adjustment balances the lift of each throttle slide in the carb. Its very fine adjustment indeed and a vacuum gauge is needed to measure the pull through each carb.
Update to my previous comments on 'very fine adjustment':
I have just syncronised the carbs on my 21R track bike. Wow, they were a huge way out. One carb was pulling about 25% more than the other at 2000 rpm (constant). It took a few turns on the balance screw to sync them, made a huge difference to how the engine runs.
Checking float heights is also a good point. The last 2 sets of carbs I have stripped had floats way too low in the bowls (high measured float height), so low level of fuel maintained in the bowls. But more likely to impact on higher RPM fuelling than at tickover. _________________ Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30
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any more ideas guys, 
