Is it time for a new top end ?

[helio]

Yesterday I have done a compression test.
I have drained the carb and after 5 kicks with the throttle opened I have read 150 psi without any gas and oil in combustion chamber.

As a matter of fact I have 2 question:
- Is it ok to read the compression without fuel and oil in combustion chamber ? I do not think so because with oil I can feel more compression under my peg when kicking...
- I have now around 95 hours with this Vertex/ring. I am not an aggressive rider(Woods), never overheated with this top end, always Castrol TTS under 32:1, always air filter very cleaned, no rattling or strange noises, start after 1 or 2 kicks, idling perfect, jetting very close, unlead gas 98 octanes.
I know that this compression is very acceptable for a 125, should I replace the piston and ring ? or just the ring ? Can the piston blow up at any time ?

Any though should be very appreciated.

 

[IrishEKU]

You readings will be higher with the case sealed. The correct way to do this is removing the pipe and carb an sealing the openings. 160 isn't bad for 95 hrs. If you want to just replace the rings they arn't that expensive.

 

[marcusgunby]

I would replaced the piston as well-my friend just had a vertex break up due to excessive hours -it made such a mess-cylinder needs welding and replating, cases need to be split and probably new bearings all through.95hours is more then you shoudl expect and pistons really arnt very expensive.

 

[Durt Cycler]

Even tho the topend shows it still has OK compression, 95hrs on a piston can cause wear to the piston skirt and ujpper rod bearing. The pisotn can cause slap with the cylinder bore and shatter the piston skirt into your bottomend which very much sucks! I think it would the right thing to do is I would replace the piston, ring, gaskets, and upper rod bearing. That is better then having a piston shatter!!!

Just for laughs I put over 150hrs on my old KX topend and it still had enough compression to start! (I replaced the topend tho :confused:)

 

[helio]

Just more one point...
After the new top end should I rejet ? I think so.
I think with more compression the engine will needs less gas. I am right with this assumption ?

Thanks

 

[jmics19067]

100 hrs is a bit excessive. pistons do shatter. if you do get a significant higher reading by having oil in the cylinder the pistons and rings are showing their wear.

on the jetting I don't worry about it. If the airflow of the engine changes the jetting recquirements is going to keep on changing until the rings are finally seated. That and if your jetting was close to begin with , while you are taking it easy breaking in the bike, you really shouldnt have any problem. But by all means as you you begin to more and more of a load on the engine and the rings are seating you notice that you do recquire different jetting correct it

 

[Jaybird]

Helio,
My opinion is that assumtion is the mother of all screw-ups.
Remember that jetting can need tweeking with the change of the weather, so try not to look into your crystal ball for jetting.
Look at the temp, humidity, fuel mix, and what sort of riding you are doing during those conditions to make jetting decisions.
I doubt that you will gain very much compression from what you are stating. That seems to be an incredibly good reading at those hours.

 

[Rooster]

I am at a similar situation, my compression is still within spec, the engine seems pretty tight, but I have over 100 hours on the top end. I don't sleep to well at night, thinking about what it would be like to get to Dirt Week and grenade my piston the first day.

In my opinion, and several others too, doing a full top end isn't that expensive (as compared to a busted piston skirt shredding up your engine). I only trail ride my bike, so somewhere around the 80 to 100 hour mark I do it up. I don't usually notice too much of a difference, but then again, I don't ride well enough to make that call.

The parts are in and I'm tearing it down tonight. Break in tomorrow afternoon. I can't wait. :) I just love the feeling of having it fresh.

 

[DEANSFASTWAY]

Yeah you should put in a new top end and Id clean the P/Vs real good too . Maybe lookat the reeds . Then your bike will be fresh and youll have peice of mind . Ive had VERTEX pistons break also . I like forged only because they never shatter. If Wiseco would put some thicker rings in their pistons I would think that would be grand , Or maybe make like a dura piston for 125s with 2 rings . Keep the jetting the way it is , if youve got @100hrs on the top end witth no problems you should rejoice to the mothergod of topend dursabiity and shoulnt mess with her MOJO. On those 98s wew were fitting them with a CR250 airboot for mure flow of air to the motor . And the bikes ran really strong . If you lookie at your air intake boot between the carb and airbox youll notice a part that has a bump or its convoluted blocking off the path of air . Alot of people changed them in 98 . GOOD LUCK DEAN

 

[helio]

Finally my bike is running again with the new Vertex piston.

I have some doubts and I need you help.

1) I have the feeling that the rings did not seat yet after 2 hours...(I feel less compression with the new piston) Could it be true ?
2) Now the bike seems running rich on pilot after this top end(the bike was rejetted to myself to use a BR8ES but the mechanic decided to put a B9ES to break in this piston. Should I rejet with the B9ES or replace for the BR8ES and verify what happens ? or 2 hours is not good enough to change anything ?

Thanks alot.

 

[Senior KX Rider]

The rings should be broken in after 2 hours there are other ways to lose compression. I would check compression to confirm that it is low then start looking for the reason, there are many

 

[helio]

Here I am again after 4 hours of riding...
Now I can read around 168, 170 PSI...Is it acceptable(normal) this compression after a new Vertex piston ?

About the jetting I can feel the bike rich on the pilot.

Any thought woul be be very appreciated.

 

[marcusgunby]

Sounds good to me.have you tried adjusting the airscrew for best responce?

 

[helio]

Yes Marcus I have tried more 1/25 out on AS(now I have 1-1/25).
As a matter of fact the carb was jetted by myself with a BR8ES and I have break up the beast with a B9ES according to the mechanic.
I think the best way should be back again to BR8ES and try the AS again till 2 turns out or replace the pilot by a size lower.
Personally I prefer run with a lower pilot and AS set around 1 out, I think the PJ does not like to much turns out concerning AS...just my thought.
Pls give your opinions.

BTW The experience telling me when we change a piston generally we will have a rich condition(perhaps on main and others on pilot...)

 

[marcusgunby]

It sounds like you have the correct method-use more turns on the airscrew as a test to see if responce improves.I would go back to the 8 as thats what im running nowdays after advice from steve125.

i would expect it to run leaner with a new piston, but i dont know enough to say why.My 1985 CR125 always ran rich with a old piston, but that may have been due to weather changes-it only needed a piston once a season as i was alot slower.