underpowered 2 stroke, rekon it needs a de coke?

[buddy]

hey

i have a honda nsr125 (a lil 2 stroke) and its a little down on power. ive removed ALL restrictions and it only does about 85mph

it has done 22,000 miles and is only 20psi down on compression so i think it has had a rebuild at some point. the jetting seems ok.


so its 10-15mph down on performance. i rekon it might just be coked up. ive cleaned the plug

so how do i go about de coking the exhaust and powervalve?


any help is appreciated. thanks
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When this baby hits 88 miles per hour... you're gonna see some serious shit.

[MaybeGuy]

i dont think an NSR can go faster than that.
cagiva mitos and RS125s top out at around 95, and an NSR is in a lower state of tune.

20psi is a pretty big whack. id suggest a rebuild.
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Blue_SV650S wrote: it was a sh1te wheelie, but it proves that he can get it up in 3rd and can do angles. In summery, mattsprattuk is a gobby little sh1tebag, dopehead tw4t, but sadly for all of us, he probably isn't THAT full of sh1te!!
Kickstart wrote: Hi I tend to agree with Matt. All the best Keith

[MarJay]

Well jetting and coking aren't going to affect compression. I would guess that it hasn't had a rebuild, or if it has it has a worn barrel and needs a rebore.

To de coke you can take the exhaust off and carefully fill it with caustic soda solution. Get a bucket of water and put some caustic soda crystals in it. Stir it with a stick you don't want and throw away the stick afterwards. Wear rubber gloves, long sleeves and wear goggles. put a potato in one end of the exhaust and fill the other end with caustic soda solution CAREFULLY.

Leave it for an hour or so then pour it down an outside drain. Its used as a drain cleaner anyway so it should be OK.

To de coke the powervalve you'll need to dismantle it. if your going to do that you might as well do a top end rebuild and a rebore and hone if it needs it.
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[chillpill]

rebore on a nicasil coated barrel , tut tut marjay i thought you knew better than that

matt hell be quoting clockspeed , nsrs should get at least 100 on the clocks

[buddy]

well the compression is 120psi when it should be 140(also i forgot to hold the throttle wide open during the compression test,,, DOH)

. thats really not too bad only 20psi down and it feels like it has pretty good compression when turned over by hand

the main thing i was asking is. do you think a coked up exhaust system could knock 15mph off its top speed? it sounds pretty plausible. ive de restricted EVERYTHING (exhaust, inlet restrictor, jets and it has de restricted electrics tv78 and ci626 and standard gearing)

one question about the caustic soda.
the exhaust on the nsr is one giant pipe. the muffler, powerbomb and manifold are all one piece. could the caustic soda damage the muffler? or the wadding inside the muffler(if it has any, im not sure what kind of muffler it is)


also to de coke it properly im gona have to take the cylinder off right? can this be done in the bike? this would make it a hell of a lot easier to clean the powervalve aswell
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When this baby hits 88 miles per hour... you're gonna see some serious shit.

[the grim reaper]

Do NOT use caustic soda to de-coke your exhaust, if anything in there is made of aluminium it will dissolve before your eyes. Use a 50/50 mix of paraffin and petrol, it will do exactly the same job but safer, just don't smoke at the time

FWIW, every 2-stroke I've ever worked on that was running poorly had an exhaust full of coke and oil residue, especially the typicall thin part between the spannies and the end can.

Cheers

Grim
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[Kickstart]

Hi

Compression is within spec. Honda list it as 143psi give or take 28psi. While you reading is at the bottom end of that I suspect it would be a fair bit higher if you had done the test with the throttle open.

To be honest I would doubt a coked up exhaust would knock that much off the top speed. Through much of the exhaust a couple of mm build up of coke is not going to be critical.

You do not need to take the cylinder off to decoke the exhaust. You could take the head off to decoke that (DO NOT use caustic soda on the alloy head), but a build up in the head would have increased compression.

Check the power valve is correctly adjusted and turning. If you take the exhaust off and look up the exhaust port you should be able to see it operate when you turn the ignition on.

Also check normal service items such as the air filter and plug.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[buddy]

hey, thanks for your replys

kickstart you doubt that it is a coked up exhaust?
well ive checked the plug and cleaned it up and gapped it. ive also checked the air filter, it honestly couldnt be cleaner

so you think it may be the powervalve. well how do i check the operation of the powervalve with the exhaust off. the powervalve moves when it gets to 2000 and 7000 rpm. how on earth do i check that its working properly when i have no exhaust on it? surely you dont mean start it up and look at it lol (that reply sounds abit snydey but its not ment to lol)

hi grim
youre saying about your 2 strokes running poorly and needing a de coke. well i have to be honest its really running pretty well. its just abit down on performance
and thanks for the tip about the caustic soda, that could have turned ugly

also i contacted the previous owner and he owned it for several years and used SCOOTER OIL IN IT!!!
so that probably hasnt helped the situation.
im currently using the last of that rechid stuff(i only recently bought the bike and am using the last of it in the tank) and im going to get castrol tts, ive herd its the best

im prob gona de coke it anyway. ive also herd from an old-school guy that he used to fill his coked up exhausts with petrol and set them on fire lol apparently the extreme heat made the coke discintegrate. i dont think i will be doing that lol
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When this baby hits 88 miles per hour... you're gonna see some serious shit.

[Kickstart]

[ben350]

Be worth checking that you have both the full power CDI and power valve controller fitted. I dont know what you need but I do know that you need to change both for full power if it is a UK 12bhp model. Italian models will be full power from the factory (where they are made).
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Ducati 996 Biposto
Suzuki Goose 350
DT125R Supermoto (Project)

[MarJay]

[MarJay]

[the grim reaper]

[chillpill]

[buddy]

hey all

ok so ive had the exhaust off today and the exhaust is hardly coked up at all

the powervalve opens and closes very very well. infact the powervalve isnt even black

so this puts me back to square 1
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When this baby hits 88 miles per hour... you're gonna see some serious shit.

[user]

Are you sure 85mph is an accurate figure? Ride next to a car or a bigger bike to check.

Does the engine rev to the redline at 85mph? Gearing could have been changed at some point...

TBH as mattsprattuk said, 85mph seems a fairly reasonable speed, don't belive everyone who claims their 125 does 120mph and regularly embarasses 'busa riders

[buddy]

lol i know what ya mean about people lying about their bikes but there are even vids on youtube of "standard" nsrs doing 95-100mph (indicated)

and it was at about 10,000rpm

but ive herd that the yamaha yzf125 can hit 85 and that is a bloody 4 stroke so i think mine is prob a little underpowered


also i nearly hit 90 today

also im gona buy a new plug. iridium

i think maybe my powervalve isnt quite opening fully. the powervalve cable is pretty loose and the powervalve itself has 1-2mm of play so im gona take a look at that


also a big lol to my mate who claimed 130mph out of his mito125 and 10-60 in 3 sec
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When this baby hits 88 miles per hour... you're gonna see some serious shit.

[iooi]

Daft question time.....

Are you a large person, as weight of rider is going to make a diffrence. Or do you have anything like a box or paniers on as well. Tyre pressures OK ?


Personally, just take the head off and clean it all up.
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Just because my bike was A DIVVY, does not mean i am......

[Vin]

[buddy]

lol im 13 stone and about 6 ft 1 thats not too big

nope no box or anything just me

i havent checked the tyre pressures since i bought the bike lol


also i discovered that the exhaust has been blowing where the manifold connects to the engine but to a missing bolt. ive cleaned it right up chucked loads of "liquid gasget" all in and around it and before you ask no its not repairable, there is a piece of casting missing aswell as the bolt so its irrepairable.

and the second question that is no doubt gona be asked. "what kind of liquid gasket did you use" the answer is im not sure, i used some of my dads stuff. apparently its like £30 for a lil tube of it so its good gear. stays rubbery and ya can get a blowtorch on it and it dosent burn or melt, WIKED STUFF!!

anyway that should sort out the blowing of the exhaust which should give me a few mph right? correct tyre pressures. another few mph? iridium plug, another few mph?

i think its just a combination of things rather than 1 simple to fix problem. so what lil things can add a few mph?
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When this baby hits 88 miles per hour... you're gonna see some serious shit.

[alains]

you should dismount your barrel , inspect the piston .
if you need a piston change , look under the barrel near the exhaust port and order the same size piston . rings are standart , change also piston pin,clips,small end bearing . polish top piston & cyl. head squish as mirrors (this avoid coke to stock on) , clean exhaust port and valves . you need also base cyl. seal, oil seal (valves shaft) , cyl. head gasket
Note : if you barrel is marked "E" you may fit a "D" piston because no way to spend £100 for a "E" piston
look where the barrel is marked https://www.geneva-racing.ch/~alains/lettre.jpg (it's an E) . the sign 238 D is the cast serial
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the worst day fishing is better than the best day working

[buddy]

well ive just put it all back togeather earlier

but its really not coked up at all the exhaust port is just a bit black not what i would call coked up and the compression is pretty good

when i get some gaskets and a half term or sumthing i will have the whole top end off. im at college all week so i have to do stuff to it at the weekend and have it back in working order by monday to get to college

anyway i may have allready mentioned that the exhaust was blowing around the manifold because it wasnt getting a seal against the cylinder due to a broken bit of casting. i may have now fixed this. and im gona get that irridium plug(the one in there now looks very very old) and check the tyre pressures lol (29psi front and 33rear?)

im also gona do the airbox mod in alains manual. what sorta jets am i gona need once i do that? i presume more than 138

and im gona see what sort of performance i get by doing all that before i start taking it all to bits again
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When this baby hits 88 miles per hour... you're gonna see some serious shit.

[alains]

the exhaust-intake exchange is the key point on 2 strokes . you will loose power if you have exhaust leaks or clogged exhaust
for the same reason carbon reeds allowed the engine resonance at higher revs
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the worst day fishing is better than the best day working

[G]

[Kickstart]