Low Compression,,both primary and senrodacy.Mostly Primary compression blow by.Not only does it interfere with scavenging,it has TIME at low speeds to lose substantial pressure And charge volume which cheats the transfer ports out of a BUNCH of bulk flow they need to do their job.And it happens at an rpm range when it’s pumping efficiency is lowest,,,and it needs as much bulk airflow as it can get during transfer period to overcome the extended TIME the exhaust port is open.You may/may not be able to see it on a compression gauge,,,,but when top end is apart you can see a virtual diagram stained on the piston skirts.How long has it been since you put a fresh piston rings in it? ..…OK,Alex ..Think about WHEN this began occuring.Before or After the De-restrictions?And at whatever time it began,,did it just seem to Happen suddenly then not get much better or worse.Meaning,,,it’s more or less stabilized in it’s maners.Or has it gradually gotten worse over time?IF:If it seems to have been a progressive sorta thing,,getting worse over time -my #1 suspect would still be Rings/Compression.That judgement is not as clear as it sounds.Because All Engines are always Gradually deteriorating from normal wear tear.We usually just sorta Notice one day ,,,,actually at different stages.Till it gets really bad.Engines wear/decline in pretty in a steady curve,,We perceive that as more like bouncing down the stairs cuz it’s too subtle until the power loss builds up to a noticeable amount.As little as 4~5,000 miles can have a Big effect,,especially on 50 s~80 s,100 s.Needing rings,,or even a piston is Bad News.Lots better stuff to spend Time Money on,,always.But,,,it’s pretty much Normal .If it just seems to have all happened very suddenly,,PARTICULARLY after doing the de-restriction ,,,that’s very POSSIBLY a tuning Prob.Ignition Timing doesnt change,,,and it would act lots different USUALLYYamaha over the years has used some odd carb versions on some of their 2 strokes.Occasionally they will HATE to have any air filter mods.Some are Mind Boggling BAD about it.I mean ,,even just removing the Lid off the stock airbox ,,,,and the Throttle suddenly acts like an ON/OFF switch.They’ll run Wide Open ,,,but below that they make NO PowerYou can Try putting the Air Filter assembly back the way it was originally ,,,it thats possible.That will make a BIG Change,,,or it wont make much difference at all.IF it does help,,,,You can Try going Up 2 Size Numbers on the PILOT JET in the carb.I cant get any info on those bikes cuz I’m in the States.Look at your little pilot jet,,and get one 2 sizes Up.Also the Needle in Carb Slide can be raised,,,lower clip position.Those 2 actions will kake it a LOT Richer at low/mid RPM range.The Adjustment screw air screw will probly need to be closed some,,down to 1/2~3/4 turn out . ..Another possibility,,,if it starts and Idles OK,,,but has severe low speed power loss,,,It MIGHT have a leaky reed valve allowing backflow thru the carb.It could be blowing out most the fuel mixture it’s sucking in.At High RPM’s the sheer inertia and bulk volume of airflow overcomes a lot of leaky reed problems.But at Low Speeds,,leaky reeds make them run really weak.Normally ya would Not suspect reed Probs on such a new engine,,,,but it can also happen at any time. .Exhaust De-restriction.The Usual De-restricion on 2 Strokes is limited to removing a Reducer at the Exhaust Flange,,,between Pipe Engine .Fiddling with the OUTLET end can be very tricky.Years ago when factory Mufflers were just mufflers ,,,Removing the Baffles or Spark Aresstors helped a little.RARELY did it HURT.Because the pipes were NOT very fine tuned.For the last many years,,,MFGRS have taken advantage of the Free Power available by Exhaust Tuning.Most 2 strike exhaust these days are very Expansion chamber-like in their design.SEMI-Race type exhaust.Some actually are Fully Tuned pipes.Thats Great -but it also makes them Extremely Sensitive to Changes.On certain bikes simply removing the Baffle/Silencer really upsets things,,and power drops thru the floor.So again,,If Possible,,,You can TRY putting the Exhaust back the way it was.A Front Restriction ,,,I wouldnt worry about.But anything done at the OUTLET end of the pipe can make a Big Difference. Not saying,, Ya Blew Up Your Bike,Dude! 2 Strokes are SO Simple,,they are Complicated.Going Wrong when Modifying them is Not Screwing Up ,,,,it’s just moving in the wrong direction.It’s Always an experimental thing to some degree.The things You’ve changed may have NOTHING to do with the probs you’re having.But they ARE VARIABLES that have been added to the equation.Troubleshooting stuff ,it’s always best eliminate as many variables as possible.The Air Filter deal is ALWAYS a suspect.You can even just cover the air inlet with a Towel to choke the air a bit before it goes into the actual filter.That’ll give a quick/easy idea of what sorta Influence the Freer-Breathing filter is having.If Bike DOES run better with restricted air-inlet ,,,Get busy increasing Pilot Jet size foolin with the air screw Thats about all I can think of.Good luck
me podrias dar el precio de las estriberas de la montesa de 75? si la tientes
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