NSR with idle/low RPM issues

**Mon Aug 21, 2023 8:14 am » Post: #1 » Download Post**

Hello from sunny Brisbane, Australia. I've got a MC21 SP that I've had sitting in the garage a few years. It's an absolute hoot to own and ride!

It has developed a bit of a quirk recently. At low revs/idle the bottom cylinder is dropping out. I've just serviced the carbs thinking that was the issue and it has made no difference at all.

I've replaced the plugs on both cylinders.

It has aftermarket coils that the previous owner put on there, I'll test those later in the week.

Everything else is pretty stock.

Keen to find out if anyone else has had similar issues and what you've found to be the issue/solution?

**Mon Aug 21, 2023 3:22 pm » Post: #2 » Download Post**

Swap the coils over to see if the problem follows it to the other cylinder. I’d also find yourself a pair of genuine coils , I had aftermarket ones fitted by a previous owner and when one failed it took the PGM unit out (several times) it tested fine as well.
Other than that check your wiring and connectors to the coils.

**Tue Aug 22, 2023 3:15 am » Post: #3 » Download Post**

Thanks for the tip on swapping! Will do, where did you source your OEM coils from?

**Tue Aug 22, 2023 6:07 pm » Post: #4 » Download Post**

Coils are still available new but I managed to get a good used pair.

**Wed Aug 23, 2023 9:19 pm » Post: #5 » Download Post**

Hello MotoNinja, and welcome to NSR-WORLD.
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Andy.
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**Thu Aug 24, 2023 12:07 am » Post: #6 » Download Post**

Thanks for the welcome Andy 😊

I've tested the coils but still the same result. I'm thinking of just replacing them anyway just to eliminate them entirely.

Here's a video of the bike running at the moment.

https://youtube.com/shorts/fSWV27k1evE?si=25kJJAVSJMGTyzNq

**Sat Aug 26, 2023 10:48 pm » Post: #7 » Download Post**

What happens when you wind the throttle on slightly. When does that cylinder become stable?
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**Thu Sep 14, 2023 1:08 am » Post: #8 » Download Post**

Andy wrote:What happens when you wind the throttle on slightly. When does that cylinder become stable?

Hi Andy, sorry about the delay.

It stabilises at 3000RPM. Anything below that and it's weak and dropping in and out.

Thinking about pulling the carbs again to redo. Not sure where else the problem could sit. 😭

**Fri Sep 15, 2023 4:40 pm » Post: #9 » Download Post**

Hate to pee on your parade, but it could be worn crank seals, especially if it's been sitting a long time unused, causing the seals to go dry.
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**Sun Sep 17, 2023 12:01 am » Post: #10 » Download Post**

Yeah, I'm with Les on this one. Almost certainly the centre crank seal.

I think Les knew where I was going with the question! Confused

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Andy.
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