What causes a hot 2 stroke engine to perform S****y in low rpm?

Question

I have a DT50R 2007 Moped, Its an Yamaha Enduro style bike, try not to worry about the engine size too much, its the same basic components as in any other 2 stroke only smaller.

The bike uses a carburetor and I installed a 104 jet in it, I have tried re-jetting it from 95-115, seems like I have the right jet.

My problem is that whenever the engine is running hot (about 5/10 on the meter and hotter), it almost completely loses power in low rpm, to get it going after stopping I have to use the clutch rev it up and release resulting in well nothing really but irritation(little power and low gear ratio keeps me from wheeling into the next wall).

My question is what can cause this (both in technical details and in physical, that is thermodynamics and all that ****), I have tried tampering with the carburetor settings, with little luck.

Thanks.

Answer 1

HOT !! but if so its not in its power band at low rpm

Wind that bad boy up

Answer 2

Low Compression,,both primary and secondary.

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?

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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

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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--USUALLY

Yamaha 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 Power

You 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 s***w "air s***w" 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.

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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 "s******g 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 s***w"

Thats about all I can think of.

Good luck

Answer 3

Please don't use foul language.  Thanks for the deuce.

Answer 4

Sounds like the air s***w on the carbureter needs to be turned in to richen the idle mixture.Timing may also be a little off.If you have a battery that is old that also can cause the problem because the charging system will not put out much charge at idle.