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Losing power after powerband!?! Please help!

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My bikes a 2009 ktm 150,

I've just had the engine completely rebuilt apart from the gearbox..

It's had:-

Cylinder replated

Piston

Conrod

Bottom end / big end bearings

clutch plates

Clutch basket

I've been running the bike in for 3 hours now, I Havnt hit powerband until I hit the 3 hours.. I then started going through the gears at about 3/4 throttle, and all seemed great.. Pulled like a train.. Until I slowed down, went to accelerate again and it was completely flat and the engine note completely changed.. At first I thought the exhaust had come away at the joint between the header and the pipe.. But that wasn't the case, I then rode again, hit the power, all was fine, pulling like a train again, I slowed down again, went to accelerate and again it was totally flat, no power at all and the engine note had changed again.. So I then thought the power valve was sticking wide open.. But I've just taken the barrel off, checked the power valve and cleaned it all and all seems perfect so I don't think it's the power valve, I just put it back together took it for another ride and its still the same..

Does anyone have any advice? I'm at my whits end.. 2 months and £900 later and I still can't hit the tracks!

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Nope not bogging at all..

Dimension is set to 0

Plug seems great

Reeds where fine last time I checked.

I don't know if this is relevant, but After the engine got rebuild, I did my first heat cycle and noticed a small leak at the centre gasket, so I taken the whole bike to the mechanic and he stripped it again and replaced the gasket and rebuilt it, since it's came back, it won't idle properly, it idled beautifully before I sent him the whole bike, I have to turn the idle screw the whole way in for it to idle.. The Choke doesn't seem to work either.. I asked him had he changed anything on the carb and he hadn't.. I was planning to take the carb out and inspect it this weekend after running it in.

Edited by Xmathewx

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so it was running good and then it wasn't?  Sounds like your powervalve has an issue and is not opening.  Check it by pulling little cover on left side of cylinder.  Rev motor and see if the lines move.

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Power valve seems to be working perfectly.. I've taken the barrel off and cleaned it all, and when inspecting the covers it is open and closing properly.. I'm going to ride it down the road tonight with the inspection cover off, hit the power then quickly stop and inspect the power valve.. Will report my findings later after work, cheers

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I don't think its the powervalve.  Reeds would be my first guess, then the carb.  I had a similar issue with my 08 125sx and after troubleshooting everything else it ended up being the reeds.  I couldn't get the bike to idle quite right and it would cut out sometimes at mid to full throttle.  during the down stroke, bad reeds let a portion of the fresh charge flow back thru the carb, picking up a 2nd shot of fuel.  Take the reeds out and hold them up into the sun, you shouldn't be able to see any light getting through.  A very small amount is ok but not any more then that.  Check the edges for any fraying.  Also try the water test.  Fill them with water, it should take 15-20 seconds for the water to drain out.  If its draining out any quicker that might be your problem.  Pull the carb and clean everything, also check all your vent lines and make sure they're clean. 

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Thanks for the reply, my Reeds do have alittle damage on one of the petals in the corner, it's nothing major though and only let's through the slightest bit of light.. My mechanic said don't worry about it just yet, plus the bike was running perfect pre rebuild, didn't actually even need the rebuild tbh, the crank shaft seal went so I thought I'd just have the lot done for piece at mind more than anything.. Only thing I did change was, the damaged petal was on the top of the reed cage, and I swapped it with the bottom one which wasn't damaged, I'll inspect them tonight or tomorrow and swap back if all seems well and go from there, will give the carb a good clean while I'm at it too.

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Well I've just taken the carb and the Reeds out, cleaned the carb, inspected the Reeds, cleaned them.. And it's still doing it?.. The power just goes flat after I let the thottle off after hitting power band and the engine note changes and sounds like the exhaust is blowing.. It will still hit power band, just takes ages to get there.. Which is making me think it is the power valve.. But I've Checked and cleaned it, and when looking through the inspection cover it seems to be working perfectly.. Could it be a fouling spark plug?.. I'm totally lost for ideas now and I have a race on a Sunday! :(

Edited by Xmathewx

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might be no spark advance.  You should be able to take the flywheel cover off and make a mark on the flywheel.  Then hook up a timing light and see if the mark "moves" as you rev the bike.

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Well I thought I better update this seeing as I've just found the problem and solved it.. I went for a blast without power valve inspection covers on for a second time.. As soon as I noticed the drop in power I quickly stopped, checked the power valve and this time noticed it was stuck wide open.. Took the barrel off, cleaned everything, still sticking wide up.. Scratching my head I was about to pull the inner clutch cover and check the centrifugal timer.. But I thought I'd adjust the power valve quickly and see if that made and difference, and sure as hell, it stopped sticking open, I did have it set to the Langston setting.. But half a turn either way and the Sticking stopped.. No idea what was causing it, but it's running sweet now! Happy days!

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