RD125LC 1983 MODEL - A BOG OR MISFIRE BETWEEN 3900-5000RPM

MARTIN RD125LC · Derestricted Danger Joined: 05 Jan 2011 Karma :

HI MY RD125 RIDES FINE UP TILL AROUND 3900 THEN IT SOUNDS LIKE IT MISFIRES THEN AT AROUND 5000RPM IT SHIFTS AND FLYS.. ITS GOT A ALL SPEED EXHAUST AND A K&N AIR FILTER AND WAS SET UP BY A SHOP WHICH I HAVE PHONED AND THEY SAY THIS IS NORMAL FOR THIS MODEL OF BIKE... BUT SURELY NOT AS ITS A PAIN IN THE ARSE AS WHEN IN TRAFFIC THESE REVS ARE USED CONSTANTLY.. ANY ADVICE OR OPINIONS WOULD BE GREATLY APPRECIATED!
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RIDE : 1983 - Yamaha Rd125lc

0l0dom0l0 · World Chat Champion Joined: 21 Oct 2009 Karma :

I THINK YOU SHOULD GET A POWERBANDS FITTED TO SORT OUT THE PROBLEM. IT WILL MAKE THE BIKE GO FASTER TOO.

On a serious note, a couple of things.

Firstly, the 'Caps Lock' button is there to add Capital Letters to words after full stops etc, and not for writing full posts with. Just so you know its located on the Mid, left hand side of the keyboard... So you can turn it off for your next post

Secondly, it sounds like its where it goes from the idle jet, to the main jet, and there is sometimes a cross over. Iirc its common on 2 strokes. Is it just on part throttle openings?

You could try a new plug, as the old one might be knackered. Thumbs Up

Syris the Indomitable · Crazy Courier Joined: 22 Oct 2010 Karma :

Just read your post from the 5th Jan. You had the carb up-jetted.
Is this the same prob as from then?
Or is it unrelated?

If the carb has been up-jetted for a re-bore, has the two stroke mix been been amended accordingly.
I am guessing you add the two stroke manually into the tank, if so the pre mix will be the same regardless. If you don't get this right you will struggle with power and your exhaust port plus power valve(If you have one), will get gunged and coked up in no time, affecting power.

Just asking because mid rev spluttering could be power valve or exhaust port needing de-cokeing, because you are struggling to pass the first power band, (dunno if yours is pre power valve or not)
I also reckon your two stroke feed may be way too rich.
If so it will take no time at all to get gunged and coked up in there again seriously effecting power.
You must get the two stroke mix right!

All a power band is, is when the piston revs up to a certain speed, more petrol and air mixture is prevented from escaping through the exhaust port, thus more is burned than before.
Your bike is struggling to pass the first power band (Some of the old two strokes have two power bands)

Anyway either carb/petrol flow or two stroke mixture issue.

Hope it's not the CDI!
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Those that mind, don't matter...Those that matter, don't mind. Do you agree with this?

MARTIN RD125LC · Derestricted Danger Joined: 05 Jan 2011 Karma :

0l0dom0l0 · World Chat Champion Joined: 21 Oct 2009 Karma :

The K&N is probably causing the problem then. It does however sound like its to rich, rather than to lean. The Airbox's on 2T's are often shaped in a specific way to maximize power throughout the rev range, so putting the standard one back on won't be a bad idea.

Not to sure on the restriction side of it though. It will either be, in the exhaust, or in the inlet somewhere.

Someone will know for certain though!

Kickstart · The Oracle Joined: 04 Feb 2002 Karma :