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Thread: Advice needed regarding Rs125 lack of power

  1. #1
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    Advice needed regarding Rs125 lack of power

    Hi hope somebody could shed some light into why my sons Rs 125 is so slow and lacks power. Its a 2009 rs 125 11kw arrow race exhaust its had a cdi mod and a snorkel mod or so we was told

    Basically my son got this bike about 6 week ago. The bike was sold as rideable but low in compression it was very boggy had no power and hard to start so we took a risk and purchased it for him.

    On inspection of the cylinder using a endoscope i could see scoring on the barrel so i ordered a cylinder, piston and ring top end baring. The rs doesnt have power valve but it has a plate and dummy valve inside thats been shaped to just sit flush with the exhaust port in a open position i guess.

    Long story short we are running the bike in and not going over 6k rev but it wouldn't even get there if we wanted too. The bikes power seems too low the max speed possible is 30 mph and you go slightly up hill the bike slows to 15 mph. When the revs hit 4k the bike just judders and dont seem pull any more. I have also noticed in neutral if i rev the engine to 5 1/2 k the N light flickers and the bike will almost cut out yet the battery has good charge. Honestly i had a rs 50 when i was younger and that had far more power.

    Im kind of gutted because i advised him to buy the bike knowing i could fit the cylinder and do the rebuild for him and I believed thats all it needed to put it right everything else i checked looked fine ie: reeds / carb condition the plug is br10s i believe and looked like its running a little rich but not that rich that the bike looses 60 % of its power.

    Any advice welcome.
    Last edited by Ziplob; 10-30-2017 at 04:07 AM.

  2. #2
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    Just a little more info about the rs. My son went out on it tonight to go to the gym he said he started it and it backfired and the bike got very loud towards the engine he drove home which is about 1 mile and showed me. I looked and i believe the back fire unseated the exhaust off the springs so i wiggled it and it clipped back in.
    While he drove it home he said the bike had so much more power and even accelerated up hills no problem.
    Now im just confused even more because i would of thought the exhaust leak would make it have worse power rather than better power.
    I really dont know where to start, also as anyone here ever known of or think its possible for a backfire to push the exhaust ajar?
    Last edited by Ziplob; 10-30-2017 at 06:58 PM.

  3. #3
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    Without the power valve solenoid it's not going to be full power.hence being 11kw.(restricted).so you need to remove the pinned blade from cylinder.replace with a working solenoid,cable,blade and housing.your exhaust will be fine as long as it's the full arrow system and not just the end can.on restricted models the airbox snorkel was restricted.ie one hole blocked.so check that too.The rest is down to the carb jetting/setup.

  4. #4
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    Thanks for your comment, and yes i understand that model bike is restricted. My question is now are they restricted that much that it can only make 15 mph on slight hills and not even able to make it threw 4th to 5th due to lack of power.
    I have spoken to a tuning shop round where i live and he said to me that model has a cat in and sometimes it can get very gummed up. He also said he can remove the cat for me for 100 pounds which seems fair but would that cause even more problems not having a cat in and would it pass a Mot?

  5. #5
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    Yes they do have a cat in the restricted exhausts.100 to remove the cat..hes pulling your pants down.its a 20 job.anyway go and buy an arrow expansion chamber and just junk the restricted one.yes it would pass an mot.

  6. #6
    apriliaforum Member Moraxxis's Avatar
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    From the sounds of it the bike has the PV in the open position all the time, this is fine i your going racing, as it boosts power over around 6-7k RPM, hence the poor power under the 6rpm.
    installing a PV kit with solenoid should keep the power a bit more linear, keeping the PV closed under 6k, and open over that. Alternative, get a closed PV plate, this should keep it learner legal.
    as for the exhaust, not sure if a backfire would move the exhaust... but 2 strokes actually need it to give decent power. the expanstion chamber creates a bit of back pressure while also allowing the gas to expand. this stops too much of the fresh air-fuel mix being spat out.
    removing the cat will give you a better flowing exhaust, any youll make the bike much lighter, so easy gains if you remove it, however its necessary to fix you issues.
    hope this helps
    Warming the world two strokes at a time

  7. #7
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    Ok so i had the cat removed by a friend and we found it had colapsed on its self and got blocked. We removed it completely and now the bike runs fantastic. Thought I'd give you guys a heads up on what the problem was just incase anyone gets the same symptoms. Thanks.

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