Re: Scoot: NO rpms past 5,000

 
|>|>> |>> > > They told me it might be the valves bcuz when they
tested the
|>|>> |>> > > compression was only @ 110psi and they said the norm was
between
|>|>> |>> > > 130-150psi and when the blew air in the cylinder it was
definitely
|>|>> |>> > > leaking air through the valves. The test was the advise
of Yamaha.
 > (|>|>> rear pulley that drives the vbelt, it seems it doesn't want
to go

Did the $tealer$hip do a valve job or not? If the compression is that
low, the engine barely has enough power to run, so it cannot move the
scooter fast enough
to cause the CVT to "upshift".

 ¿ wrote:

I looked up the compression for the Suzuki AN400, which is a competitor
to the Yamaha, the manual says 122-128 psi when hot, lower limit 88 psi.
  I'm guessing those numbers might have to with the Suzuki having a
compression release.  If the Yamaha is similar, then perhaps 110psi is
not unusual, or maybe they did the test cold.  I presume to do a
leak-down test, they would need to disable the release, assuming the
Yamaha has one.  The ones I've see always seem to take a couple of
seconds to fire on a cold start, which suggests to me that they probably
have a compression release that doesn't close until a certain rpm is
reached.  Just guessing, since the problem arose suddenly, maybe it does
have to do with the transmission, clutch, belt and maybe worn rollers in
the variator too.


BTW, I think the original post at
http://www.pashnit.com/forum/showthread.php?p '4264


was:

(quote)
Somethings Wrong.
My scooter has been in the dealer since last week. They can't seem to
find out what the problem is.
Here's what happened.
I was going into the freeway when all of the sudden the scooter wouldn't
give me more speed. I was at 4200 rpm and I had to ride side of the
freeway till the scoot built up some speed and eventually it did and I
was only able to top at 55mph. I tried giving it more gas but it would
respond anymore. I took it to the Yamaha dealer and told them what happened.
They told me it might be the valves bcuz when they tested the
compression was only @ 110psi and they said the norm was between
130-150psi and when the blew air in the cylinder it was definitely
leaking air through the valves. The test was the advise of Yamaha. When
it was due for pick-up they said there was not much difference they were
only able to produce 5000rpm and no more. When the bike is on its stand
it was fine they can rev it up to normal rpm but when weight is put on
they can't get the same output/outcome.
After a few hours wait goodnews Yamaha said the problem might be the
rear pulley that drives the vbelt, it seems it doesn't want to go
up/squeeze 2gether to bring the belt up all the way and so the speed is
minimized and also the clutch got carmelized bcuz of the dust that built
up inside the belt housing and the repair is covered w/YES.
This saturday past I called and asked what time I can pick up the
scooter and they said it still had the same problem.
A mechanic friend suggested that it might be the cat/converter clogged
and its restricting air flow/chocking the exhaust and affecting the
drive but a muffler shop I called said that it wouldn't be the
problem............ So what's wrong???? Help!!!
james
__________________
www.watchtower.org
(end quote)

 

Good point. The Yamaha Majesty does have an automatic compression
release�which can be seen on the CAM CHAIN drawing here:

http://www.powersportspro.com/pages/parts/viewbybrand/parts.aspx

 
|>
|>> I presume to do a leak-down test, they would need to disable the release, >
assuming the Yamaha has one.
|>
|>Good point. The Yamaha Majesty does have an automatic compression
|>release?which can be seen on the CAM CHAIN drawing here:
|>
|>http://www.powersportspro.com/pages/parts/viewbybrand/parts.aspx


    That's a Cam Chain TENSIONER, as in
(http://www.powersportspro.com/pages/parts/viewbybrand/parts.aspx )
right....

    What we want is a valve cylinder head Compression Release, of which
I haven't found yet or haven't looked hard enough......

    I could figure it isn't tooo necessary, just like the "oil gallery"
bolt which is used mainly for tough problems with the oil pressure
situations, which don't usually occur from the cylinder head.


    Ok you may be right about the Cam Chain, being Paul's intent on the
Decompression Lever, which is found on **page 5-12 of the YP400T? Shop
Manual**, but this a Cam Chain de/compression lever found on the camshaft
called the "decompressor cam"........

    How does a "decompressor cam" have any influence on the
piston/cylinder compression ratio, or are we saying that the Cam Tensioner
will have that kind of effect on the 5,000rpm limit???

    The  CAM compression ratio is chronological, and the problem would
persist throughout, and since I haven't personally seen the problem on the
bike I can speculate, since it goes past the 5,000rpm  without a load, the
Camshaft Timing is not the real issue.....

    I wish the guy with the problematic Yamy Majesty 400 would add some
input to this.........

    It seems like  a piston ring blow-by problem, but what we really
see here is a w"warranted" bike not being corrected by Yamaha....

 

http://webservices.motorsportdealers.com/parts/partImages/YAM/2/RVA/2007/YP=
400W/IMAGES/CAMSHAFT_CHAIN.Gif

5 DECOMPRESSION ASSY

7 CAM, DECOMPRESSION

8 LEVER, DECOMPRESSION SPRING

Though I can't see from this view, the #5 decompression assembly
probably has spring loaded, crescent-shaped flyweights on it that
cause the other parts to change positions.

If the flyweights are stuck, or the other parts are stuck, the exhaust
valve will be slightly open at 1500 RPM and up, leaking compression
and reducing power substantially at 5000 RPM.