NSR 250 R MC18, 1989

Only had this bike a couple of months. It was starting and ticking over fine. Then I took it for a ride and it cut out! (Pushed it a mile home!)

So, got a spark on the horizontal cylinder but not the vertical one. Tried swapping the coils over but still no spark on the vertical one, although the vert coil worked on the horizontal terminals. Cleaned all the terminals on both coils.

Arrrh! What next? Any ideas please?!

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Start from the beginning - fully charged healthy battery?
(If I read right you swapped the coils over onto the "good" cylinder and the suspect coil worked fine?)

Did you try swapping over spark plugs to see if you can "move" the problem... or better still just start with a nice new fresh set. Fresh battery, fresh plugs, coils that you know work, check all connections (not just the obvious HT/LT coil connections) but all the blocks leading to/from the PGM all the way down to the HT caps. Also any broken or suspect wires.
You can visually check, looking for corrosion and a good fit within the terminals (but do not be tempted to sandpaper them! If you find any crud use a proprietary electrical contact cleaner and a soft cloth/brush to clean them out)

Or use a multimeter and work back from the suspect cylinder until you get to a "good" reading. Then you know the issue is between the good reading and the cylinder.

After you know the electrickery is all 100% then you can start to think about fuel or air, it needs a ready supply of all three!
Let us know how you get on...from your description my money is on a fouled plug being the culprit.

Good luck 👍

Cheers Max! Ordered plugs last night so they coming. When I swapped the coils I left the plug in the cap, in just in case. Yes, 'suspect' coil produced a spark on the 'good' horizontal cylinder.(Thanks for advice on not to sandpaper terminals - too late! Ha! That's first thing I did! Why shouldn't you sandpaper them?) But only had spark on the one cylinder. I've done a continuity test from suspect coil wires back to connector box and they ok. Then wires go into PGM. Can I test through the PGM? Is there a wiring diagram any where?

I'm not a mechanic by any stretch of the imagination, just keen and frustrated amateur. I like things like seized bolts that I can hit with something! Ha! Anyone got one of those machines they have on Star Trek that you just wave over the offending item and all is fine and dandy?!

Daveyboy wrote:Cheers Max! Ordered plugs last night so they coming. When I swapped the coils I left the plug in the cap, in just in case. Yes, 'suspect' coil produced a spark on the 'good' horizontal cylinder.(Thanks for advice on not to sandpaper terminals - too late! Ha! That's first thing I did! Why shouldn't you sandpaper them?) But only had spark on the one cylinder. I've done a continuity test from suspect coil wires back to connector box and they ok. Then wires go into PGM. Can I test through the PGM? Is there a wiring diagram any where?

I'm not a mechanic by any stretch of the imagination, just keen and frustrated amateur. I like things like seized bolts that I can hit with something! Ha! Anyone got one of those machines they have on Star Trek that you just wave over the offending item and all is fine and dandy?!

Most of the pins and pegs within terminals are coated in a super thin coating which improves conductivity and resists corrosion...sandpapering or using emery is the default reaction of everyone, we have all done it one time or another.

OK, so you are confident that the coil, plugs and HT caps are all good for both sides? The HT side is complete.
So working backwards the problem is on the LT side of the supply to that particular cylinder...check back thoroughly again, all the connectors and blocks between the PGM and the coil.
Check continuity but also see if you can measure resistance values and compare each side.

As a thought have you checked for plain old supply voltage to each side when running/kicking over?

Maybe worth going through the Electrex fault finding guide in the workshop section of the main site.

A more and more common fault these days seems to be the small harness to the pulse coils. The wiring is quite exposed, and subject to constant heat cycles, and can break down.
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So, new plugs came and I tried them. Now I'm sure I saw a spark on the dodgy cylinder, so rigged up my dummy tank and tried to fire her up. But after 10 mins of kicking I decided to call it a draw. No joy.

So I thought I'd take the L/H engine cover off and take a look at the stator/flywheel. Took flywheel and have discovered the woodruff key is broken. That may account for intermittent spark. Where can I get a new woodruff key from ? Is there a parts catalogue?

Daveyboy wrote:So, new plugs came and I tried them. Now I'm sure I saw a spark on the dodgy cylinder, so rigged up my dummy tank and tried to fire her up. But after 10 mins of kicking I decided to call it a draw. No joy.

So I thought I'd take the L/H engine cover off and take a look at the stator/flywheel. Took flywheel and have discovered the woodruff key is broken. That may account for intermittent spark. Where can I get a new woodruff key from ? Is there a parts catalogue?

Fowlers £3.79
https://www.fowlersparts.co.uk/parts/view/13331-360-000

Daveyboy wrote:So, new plugs came and I tried them. Now I'm sure I saw a spark on the dodgy cylinder, so rigged up my dummy tank and tried to fire her up. But after 10 mins of kicking I decided to call it a draw. No joy.

So I thought I'd take the L/H engine cover off and take a look at the stator/flywheel. Took flywheel and have discovered the woodruff key is broken. That may account for intermittent spark. Where can I get a new woodruff key from ? Is there a parts catalogue?

Surprised that it's firing at all with the flywheel hanging off 😂
Still odd that it was running on one cylinder but not the other...anyway you will need to get the flywheel sorted first and then see what happens 👍

Thanks for the Fowler link Max. How do you know the woodruff key is the same size? I've ordered one anyway and hopefully that will sort the problem.

On the coils, even if they are working (hopefully!), they do look like original equipment. Can I replace with a new 12v coil off another bike model? Is there a well used replacement or upgrade?

Anyway, front brake refurb kit came and I've changed the pistons and seals a given 'em a good clean. All bled up, new pads and they lookin good. Although I'm going nowhere at the mo, I'm ready to go somewhere! Ha! Stropping is not a problem. going is! 😁

Thanks again for your help mate.

Daveyboy wrote:Thanks for the Fowler link Max. How do you know the woodruff key is the same size? I've ordered one anyway and hopefully that will sort the problem.

On the coils, even if they are working (hopefully!), they do look like original equipment. Can I replace with a new 12v coil off another bike model? Is there a well used replacement or upgrade?

Anyway, front brake refurb kit came and I've changed the pistons and seals a given 'em a good clean. All bled up, new pads and they lookin good. Although I'm going nowhere at the mo, I'm ready to go somewhere! Ha! Stropping is not a problem. going is! 😁

Thanks again for your help mate.

One woodruff key fitted all the 18's, 21's & 28's (and probably another zillion models) so you should be good to go with that.

Coils are easy enough to test with a multimeter, I do not want to lead you down a wrong avenue but if it is that lower cylinder it might be HT cap as it gets all the road crud and weather on it...but wait until you have the flywheel fitted on and spinning...then check the supply side to both coils for equal readings.

Max wrote:

Daveyboy wrote:Thanks for the Fowler link Max. How do you know the woodruff key is the same size? I've ordered one anyway and hopefully that will sort the problem.

On the coils, even if they are working (hopefully!), they do look like original equipment. Can I replace with a new 12v coil off another bike model? Is there a well used replacement or upgrade?

Anyway, front brake refurb kit came and I've changed the pistons and seals a given 'em a good clean. All bled up, new pads and they lookin good. Although I'm going nowhere at the mo, I'm ready to go somewhere! Ha! Stropping is not a problem. going is! 😁

Thanks again for your help mate.

One woodruff key fitted all the 18's, 21's & 28's (and probably another zillion models) so you should be good to go with that.

Coils are easy enough to test with a multimeter, I do not want to lead you down a wrong avenue but if it is that lower cylinder it might be HT cap as it gets all the road crud and weather on it...but wait until you have the flywheel fitted on and spinning...then check the supply side to both coils for equal readings.

Anyway you tried swapping coils to move the problem so...the fault has to be either in power supply to that coil or after the coil at the cylinder...