RM250 compression problem
Rob
die when he let off of throttle. No power loss, nothing seemed wrong other
than not idling. shut it off and when he tried to restart... very little
compression. I checked the plug to make sure it was tight. Before I tear
down and replace rings/piston what else could cause low compression on a 2
stroke?
--
Rob
Wudsracer
***********************************
It's only got one ring, so they don't last that long.
Other than that, I'd check for problems with the reeds, and maybe an
air leak, if you don't find anything else wrong (including blockage in
the carburetor passages or pilot (slow) jet.
Wudsracer/Jim Cook
Smackover Racing
'06 Gas Gas DE300
'82 Husqvarna XC250
Team LAGNAF
Rob
I hope it blew it out the exhaust. You recommend OEM or aftermarket
piston/ring set?
"Wudsracer" <babb...@Lucy.com> wrote in message
news:hu7nd514ji9j63ti9...@4ax.com...
Wudsracer
*******************************************
>On Thu, 22 Oct 2009 19:14:05 -0400, "Rob" <bucf...@SPAMtampabay.rr.com> wrote:
>Tore it down...small chip out of the front of piston, no damage to cylinder.
>I hope it blew it out the exhaust. You recommend OEM or aftermarket
>piston/ring set?
>
>
>
******************************************
Stock piston are always good quality, but most mx bikes use only one
piston ring. I much prefer a two ring piston and it's longer service
interval.
My favorite aftermarket piston kit, available in the US, is sold
here as the Athena brand kit. They are quality pistons manufactured
by Wossner in Germany, and have a non-stick coating on the sides of
the piston. All the 200cc and larger Athena piston kits that I've seen
have two rings; which I prefer for longevity of service.
(The Wossner pistons that were supplied as OEM GasGas pistons had no
coatings on the sides.)
I like Vertex pistons, but the replacement rings cost almost twice
what the Wossner and Wiseco rings cost.
Wiseco pistons have given me excellent service.
Use new gaskets in your reassembly.
The piston might not have been your problem. I've seen two stroke
engines run fine with a piece of the skirt broken off. It will depend
on what part of the porting the broken piece covered. So... while
you have the top end down, check and clean the powervalve assembly and
check the reeds. Clean the carb's passages, while you have it off to
check the reeds.
Good riding and wrenching to you!
Jim
Rob
concern is the exhaust valve. It looks like the ring may have caught on it
as it looks like one side of it sticks into the cylinder maybe 1/2 mm. When
I tear it all down, I'll see if there is an adjustment.
Rob
"Wudsracer" <babb...@Lucy.com> wrote in message
news:ef53e5p814r4oagi6...@4ax.com...
HardWorkingDog
Wudsracer <babb...@Lucy.com> wrote:
> *******************************************
>
> >On Thu, 22 Oct 2009 19:14:05 -0400, "Rob" <bucf...@SPAMtampabay.rr.com>
> >wrote:
>
> >Tore it down...small chip out of the front of piston, no damage to cylinder.
> >I hope it blew it out the exhaust. You recommend OEM or aftermarket
> >piston/ring set?
> >
> >
> >
> ******************************************
> Stock piston are always good quality, but most mx bikes use only one
> piston ring. I much prefer a two ring piston and it's longer service
> interval.
I hate to butt in here :) but I know YZ's have 2 rings on their OEM
pistons at 250cc; I looked at ronayers.com's Suzuki parts fiche, and
looks like the RM is the same. 125 & below are single rings.
(Checked, and the YZXXXF's have *3* rings on their 250 & 450 pistons!)
...snip good recommmendations...
> The piston might not have been your problem. I've seen two stroke
> engines run fine with a piece of the skirt broken off. It will depend
> on what part of the porting the broken piece covered. So... while
> you have the top end down, check and clean the powervalve assembly and
> check the reeds. Clean the carb's passages, while you have it off to
> check the reeds.
I assumed the chip was in the top of the piston, where it was
preventing the ring seal from working to build compression. I agree
that it's worth looking for what caused that piston to break.
My only similar experience was when the Pup's YZ125 "died" during a
hare scrambles. He finally decided it was a fouled plug, replaced it
and it started right up. He did well in the race, but afterwards I was
looking at the fouled plug from his camelback. (Dirt racers NEVER
litter.)
The ground electrode was touching the center electrode.
Weird.
The next race, his crankshaft grenaded about 1/4 mile into the race,
just as he was passing into the lead. I'm guessing that a crankshaft
bearing chunked a bit in the previous race and a piece was propelled
into the spark plug, shorting the tip together. Would've been better
to not just let that "weird" thought to go unexamined as I did.
--
Charles
'99 YZ250
Wudsracer
>
>I hate to butt in here :) but I know YZ's have 2 rings on their OEM
>pistons at 250cc; I looked at ronayers.com's Suzuki parts fiche, and
>looks like the RM is the same. 125 & below are single rings.
>
>(Checked, and the YZXXXF's have *3* rings on their 250 & 450 pistons!)
>
>preventing the ring seal from working to build compression. I agree
>that it's worth looking for what caused that piston to break.
>
>My only similar experience was when the Pup's YZ125 "died" during a
>hare scrambles. He finally decided it was a fouled plug, replaced it
>and it started right up. He did well in the race, but afterwards I was
>looking at the fouled plug from his camelback. (Dirt racers NEVER
>litter.)
>
>The ground electrode was touching the center electrode.
>
>Weird.
>
>The next race, his crankshaft grenaded about 1/4 mile into the race,
>just as he was passing into the lead. I'm guessing that a crankshaft
>bearing chunked a bit in the previous race and a piece was propelled
>into the spark plug, shorting the tip together. Would've been better
>to not just let that "weird" thought to go unexamined as I did.
************************************************************
Thanks for the clarification, Charles! I guess that was confused, by
a previous search for 125cc aftermarket pistons.
Rob
obvious signs of wear or scrapeing. Piston...well, along 2/3rds of the edge
it is pitted, scrape marks down the front. It looks to me like it got hot
although the bike showed no signs of overheating. Spark plug looks good, not
ash colored. I'm stumped as to what it could be, maybe just a freak thing.
Rob
"HardWorkingDog" <har...@mush.man> wrote in message
news:harvey-70282A....@individual.net...
Wudsracer
**********************************************************
>On Wed, 28 Oct 2009 20:17:27 -0400, "Rob" <bucf...@SPAMtampabay.rr.com> wrote:
>
>Pulled the piston out today. Both rings are stuck. Cylinder looks great, no
>obvious signs of wear or scrapeing. Piston...well, along 2/3rds of the edge
>it is pitted, scrape marks down the front. It looks to me like it got hot
>although the bike showed no signs of overheating. Spark plug looks good, not
>ash colored. I'm stumped as to what it could be, maybe just a freak thing.
>
>Rob
>
*******************************************************
Rob,
You describe a piston condition that could be caused by a dirty air
filter, but considering the stuck rings, I'd say that your bike
experienced a heat related seizure, caused by a prolonged (read that
as "long enough to cause damage") lean condition. You describe a
piston that was on the verge of melting in places.
(A "four corner seizure", caused by improper engine warm-up looks much
different at the piston that what you are describing.)
A two stroke needs to be jetted for intended use, as well as a
compromise for intended conditions.
For example, a properly jetted mx bike can grenade if ridden down a
gravel road at wfo throttle for a couple of minutes solid. If those
conditions are in a bike's possible usage, then a main jet large
enough to protect the engine in that usage needs to be installed.
(Then a compromise setting can be done to the rest of the carb
circuits to allow the bike to still be fun to ride when not wfo. This
isn't rocket science, but rather a time consuming "trial and testing"
procedure. It is worth the time and effort. Trust me; I learned my
initial jetting basics by melting holes int the pistons of big bore
IT Yamahas. I learned what happens just before damage is done, and how
to recognize it.)
Rob
>>
>
> *******************************************************
> Rob,
> You describe a piston condition that could be caused by a dirty air
> filter, but considering the stuck rings, I'd say that your bike
> experienced a heat related seizure, caused by a prolonged (read that
> as "long enough to cause damage") lean condition. You describe a
> piston that was on the verge of melting in places.
> (A "four corner seizure", caused by improper engine warm-up looks much
> different at the piston that what you are describing.)
>
> A two stroke needs to be jetted for intended use, as well as a
> compromise for intended conditions.
>
> For example, a properly jetted mx bike can grenade if ridden down a
> gravel road at wfo throttle for a couple of minutes solid. If those
> conditions are in a bike's possible usage, then a main jet large
> enough to protect the engine in that usage needs to be installed.
> (Then a compromise setting can be done to the rest of the carb
> circuits to allow the bike to still be fun to ride when not wfo. This
> isn't rocket science, but rather a time consuming "trial and testing"
> procedure. It is worth the time and effort. Trust me; I learned my
> initial jetting basics by melting holes int the pistons of big bore
> IT Yamahas. I learned what happens just before damage is done, and how
> to recognize it.)
>
Well, My like new, barley rode RM250, only ridden to church on Sunday, has
been re-jetted... A LOT! I now know what caused the piston to chip.
Needle-two notches lean, Power jet- from a 55 to a 45, Main jet from a 172
to a 168, Slow jet from a 50 to a 48. I've ordered all the stock ones.
Rob
Wudsracer
********************************************
>On Thu, 29 Oct 2009 22:40:54 -0400, "Rob" wrote:
>
>
>
> Well, My like new, barley rode RM250, only ridden to church on Sunday, has
>been re-jetted... A LOT! I now know what caused the piston to chip.
>Needle-two notches lean, Power jet- from a 55 to a 45, Main jet from a 172
>to a 168, Slow jet from a 50 to a 48. I've ordered all the stock ones.
>
>Rob
************************************************
This bike may have been jetted for higher altitude than where you
ride.
Rob
everything back together and it seems fine. Just waiting on the stock jets
now.
Thanks Again!
Rob
"Wudsracer" <babb...@Lucy.com> wrote in message
news:9ftke5hvrcmakspk2...@4ax.com...
Wudsracer
*************************************************************
>On Fri, 30 Oct 2009 17:55:00 -0400, "Rob" <bucf...@SPAMtampabay.rr.com> wrote:
>No, It has been a local bike sense new. Thanks for your help. I've put
>everything back together and it seems fine. Just waiting on the stock jets
>now.
>Thanks Again!
>Rob
>
>
>
***********************************************
You're welcome.
Remember, the stock jets still won't let one run the bike down a road
at full throttle for extended periods. You will have to go larger on
the main jet for that.
You may (will probably) need to fine tune the jetting to make the
bike run just as you want it to run. (idle, smooth transition to
mid-range, and a strong progressive run out to full rpms.)
It's really worth the efforts.
If you need specific jets, and use the standard Keihin jets, I can
probably help you with those you need. I still keep a pretty full
selection.
Jim