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'21 RR 300 Misfiring at RPM, dieing. Suggestions why?


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'21 RR 300 with 180 hours.
A few days ago it started missing and spluttering. I figured maybe oiled up from a fair bit of low RPM riding. Took it for a hard Km or two up forest firebreaks and all seemed well.
Rode it yesterday for nearly an hour, mostly slow, but a couple Km on a dirt road, but not fast. All good.

Today I go to an event and it started missing at RPM while I was warming up.

  • Figured maybe plug, so replaced that. No difference. Hmmm.
  • Checked tank vent and fuel flow out of tap - all good.
  • Carby out and check low speed jet - seemed clean, but gave it a blow out and visual check. Float level seemed a bit high according to the GG dealer on hand so tweaked that a little. Rode OK for 1 lap of 10 sections, right up until I got back to section 1 where it died.
  • Back to the car and carby off again. Connected tank to see if float needle was opening properly, seemed like it might not be. Removed needle, cleaned it and the seat (although it all looked good). Flowing nicely.
  • Reassembled, started & stopped, then lay the bike on the side to check fuel was flowing out the overflow, hence indicating the float needle was opening up OK.
  • Lap 2 OK, until I got back to section 1 where it died once again. At least it lasted until I was close to the car!
  • Wiggled every electrical connection I could find and it started for a while. Dug in deeper and disconnected the kill switch. Started and ran. Reconnected and seemed OK. Got the next two laps in, then as I was mucking about on some rocks it started missing at higher revs, then died again. Wiggled all electrics I could lay my hands on and it started, but I packed it in and came home. It could as easily be something to do with the time spent wiggling things as the actual wiggling.

I've heard & read the CDI can display this sort of erratic missing?
Possibly the coil failing? But I haven't heard anything about coils failing on these bikes. ?
Maybe the kill switch isn't opening properly? Seems an unlikely culprit to me as I wouldn't expect that to cause missing at RPM, rather just erratic death at any rpm.
Maybe still carby jetting problem?

My plan is to open up every electrical connector and check they're clean. I'll also add dielectric grease while I'm at it (I greased the CDI connector as a precaution a few weeks back - maybe that's contributing?).
Also meter then probably dismantle the Leonelli magnet kill switch and check that's all clean and functional. Lanyard kill switch is mandatory for comps.
I'll pull the carby and give it a thorough going over. I always run a Mann inline fuel filter so would be a bit surprised to see the jets blocked.

Not too sure what else to check, so looking for suggestions.


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Mine is the more complex electric start model, way more hours then yours and any time it acted up it was the kill switch (most often) spark plug (less often) or I did a poor job of servicing the air filter.  Your engine should not be loading up at low revs unless you fail to use the recommended 98 fuel octane @ 1% 


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Have not experience it myself but service manuals indicate spark plug caps and wires as being something you should suspect or replace fairly frequently.  Spark plugs are cheap and available because they are the same ones commonly used in chainsaws, I'd change it again just incase you bought a bad one (it happens)

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Thanks @lemur , & for the DM info.

I'll proceed progressively simple -> complex & see how it goes.

Possibly quicker long term to dive right in & check everything, but I'm always optimistic for the quick/easy/cheap fix.

In Australia 98 octane is awful in trials bikes. 94 or 91 are much better, along with a 4 plug instead of stock 6.

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5 hours ago, bikerpet said:

Thanks @lemur , & for the DM info.

I'll proceed progressively simple -> complex & see how it goes.

Possibly quicker long term to dive right in & check everything, but I'm always optimistic for the quick/easy/cheap fix.

In Australia 98 octane is awful in trials bikes. 94 or 91 are much better, along with a 4 plug instead of stock 6.

I had a problem once with fuel boiling and percolation , fuel evaporates in the float bowl ,  caused by a coolant hose touching against an aluminium fuel tank on a GasGas Pro the fuel got very warm and the bike ran erratically not as bad as yours sounds but it is just one for the pot and runs alongside a weakened mixture causing poor running? 

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  • 2 weeks later...

I pulled apart every electrical connector on the loom, cleaned with contact cleaner, added dielectric grease and reassembled. Including dismantling the magnet kill switch.
I left the map switch, light and light switch disconnected initially, then reconnected them once it showed that it was running consistently. Nothing changed.

About 5 hours riding now and it's run faultlessly the whole time.

Seems a somewhat unlikely solution, but so far so good.
If it reverts to bad behaviour I'll report back, otherwise read it as problem solved.

I'll just say that working on the electrics is the first time I've sworn at the designers of this bike. Whoever designed the electrical layout could not have been the same person as designed the mechanical structure!

Edited by bikerpet
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  • 4 weeks later...

OK, I'm back looking for more ideas. Today after about 15-20 hours of faultless running the gremlin came back.

Missing and spluttering, stalling at idle. Seems OK at higher RPM, but I wanted to get back home ASAP once it started to carry on - didn't like the idea of a long push if things got worse.

Pulled the plug (BPMR6A at the moment, although I've used 4's in the past at the Ozzie dealer's recommendation) - looked nice, light tan going slightly toward white. I've been making sure to give it a good blast up some long steep hills every few hours to keep the engine and pipe a bit cleaner.


I've found the primary coil resistance value (0.255 - 0.275 Ω) so I'll check that tomorrow.
The woodruf key has been pointed out as a possibility, although seems unlikely to me in this case.
Also the carb needle as a potential issue, again, I don't think so as it has run perfectly for several weeks and the needle was fine when I had the carby apart for the initial problem.

I'm reluctantly heading toward buying a replacement CDI.
I've read through various other TRS CDI threads and while mine isn't quite the same symptoms, it's close enough that I reckon that's probably it.
It was a warmer day today, so perhaps that contributed? If it happens again tomorrow I might try dousing the CDI with water to cool it and see what happens. I used to have some freeze spray, but it seems to have gone AWOL.
It's probably a silly little capacitor buried in the CDI potting that's gone bad, worth 5c. But I know from experience that digging out the circuit board and finding that faulty cap can take a very long time!

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An inexpensive thing to try is replacing the spark plug cap (which do go bad).  My favorite 2T spark plug cap is the NGK TB05EMA. It is a nice waterproof cap with a 5K-ohm resistance.

Although your spark plug has an internal resistor, I think it's a good idea to use a resistor cap as well.  Resistance in the high-tension circuit lengthens the rise-time of the spark event which helps minimize radiated electromagnetic interference that can upset the workings of the CDI.

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  • coil resistance - my meter isn't good that low but it gave more or less sensible readings.
  • plug cap resistance and trimmed the HT cable.
  • grounds.
  • Flywheel, woodruff and windings (only visually)

I then put the CDI in a cool oven for 10 minutes. About 50-60C.
Put it back in the pre-warmed bike and bingo, started spluttering and carrying on at higher RPM.
Cooled the CDI down again, replaced it and bike ran fine.

New CDI ordered.

I'll keep this one for winter riding only. 😆

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