Hi fellow enthusiasts.
I hope someone might have some knowledge on these symptoms which they would be willing to share. All suggestions appreciated.

My 1989 NSR 250. Racebike. Seems to be a cross between MC18, and MC18II. (PGM II)

I’ve spent a lot of time solving a lot of little issues and generally getting the bike up to standard.

First ride yesterday (If you don’t count a few runs up the “test track” - My street!)

The good news. I love the feel of the bike, handling is great, brakes are good. (I prefer it compared to my “sorted” Ohlins Equipped Ninja 300 SS300 race bike.

Motor feels “occasionally” “sweet”, but there lies my problem.

The bike starts first kick, idles nicely, and once warmed up, revs cleanly. (Probably revs cleanly cold as well - but I love engines.)

It might be coincidental, but it seems to run nicely through the range just once, then “hesitates” after the gear change. From then on there is frequent random hesitation/missing/rough running.
This seems to occur at random revs, and intermittently (but its affecting the engine most of the time). The bike will run up to redline even with the intermittent rough running. I stayed out and rode 10? Laps. It doesn’t get better or worse.

Background:
I’ve disassembled and cleaned the carbs.
Airbox. No filter.
I’ve tested the coils. (When cold)
New plugs and good looking caps.
Confirmed the exhaust valves test correctly.
The plugs “Read” well.
The bike seems to run around 70 deg.
The fuel is fresh BP “98” at 40:1 Motul 800 (full synthetic) - Is this a bit “lean on this type of bike? This would be pleanty of oil in a 125cc MX bike, but I’m new to pre mix road racers.

The bike feels very well tuned and pulls well at all RPM except for the issues described which seem to randomly occur between about 5k and redline. This leads me to think its not anything like crank seals. Something intermittent, probably electrical.

The wiring harness is as you would expect on a 30yo modified race bike.

I’m running out of ideas and need help from someone with more experience. I want to race the bike next weekend. (“Big Chill” historic meeting, Stanthorpe QLD)

Thanks for reading.

Worth reading the model guide for mc18-I & II
I would make sure the carbs and sensors match the pgm
No air filter might make it run leaner. If it's like a flat spot, running in on a dyno might show the issue. My 2J used to be about 75 degrees
_________________
Why do things simply when you can complicate them

Thanks for the suggestions.

I may have solved it (only possible to have a brief test ride. The big test will be the weekend.)
I removed every electrical plug I could find, and sprayed it with Lectra Clean before reassembly.
Nothing looked bad, but it seems to run better now. Fingers crossed.

It’s driving me mental.

Some of the symptoms are contradictory. It seems to be perfect on the first run through the rev range. It always revs cleanly in neutral (before and after the race). Coming onto the straight (after heavy braking) it always runs badly initially (tried coming on between 5000 and 9000 rpm)
It would run badly regardless of the revs (maybe one cylinder?) then come good for a bit.

I’d almost convinced myself that it might have been related to the “G” involved with the braking.
The “intermittent” nature is making diagnoses difficult. (Too difficult for me.)

It can’t be fuel delivery (blocked breather etc) because it comes good and revs cleanly to redline.
For the same reason it can’t be anything “permanent” like jetting.

I can leave the fuel on all day, and it starts first kick, so the float needles are working. (At least basically.) Do these carbs have an “overflow”?

I’ve tried 2 sets of new plugs.

Suggestions?

Hi Skypig.

A couple of initial thoughts:
1. Check the plug caps for cracking which may effect spark performance.
2. Check your float heights. If one is out, it could be empyting the bowl quicker than it can refill it.
3. Have a squiz at the woodruff key in your flywheel. If its damage, the timing could be shifting.

Is the bike running a PGM1 or a PMG2?

Whereabouts in Aus are you? Im in Brisbane and have a lot of mc18 spares (PGM #1 and RC control boxes) which could also potentially be an issue.

Good luck with it! It will be all worth the hassle once you sort it out!

Hi Duggs,

Sorry for the delay - just back from work over West.

Some good ideas there.

1. Plug caps seem good. (Doesn’t mean they are.)
2. I’m going to go through the carbs again. The symptoms are starting to steer me that way.
3. Never would have thought to check that - thanks.

It has a PGM2 system.
I’m on the Sunny Coast. (So close enough)

I’ve entered the Lord of the Lake Historic meeting. Second weekend in September. I hope I find the issue before then.

Oh nice. I am going to out Lord of the Lake on the Sunday for a look so ill keep an eye out for you. I'll be on a Red/White 2J MC18 so if you see me, sing out.

Something else to check if your bike is a hybrid (like mine)... I believe the flywheels are a different size on the PGM1 bikes vs PGM2 bikes. It might be worth doing some research on what the differences are and if you are running the correct one as that may well affect your timing too. It does sound a lot like a carby issue though as you mentioned.

Have you got an air compressor to blow out the galleries in your carbs.

My 2J runs a PGM2 too so worst case I may be able to help you troubleshoot by substituting some parts (carbs/pgm etc). I have a bunch of spare coils etc too we can try potentially.

I'll PM you my number and let me know what you want to do. Woo, teamwork!

Cheers,

Andrew.