YZ250 2002 top end.

[robertaccio]

This is lesson post. From me, the aviation guy who solves problems on very very expensive high tech machines.
Big Max's bike (Massimo di Veneto) a well set up YZ250 for him. Big Max because even larger than me, Taller and Heavier (6"3" 240).
All the yea-hoos are telling him to install a 300 kit....its down on power you are too big.....etc etc
My take is NO slow down....lets do exploratory surgery ( top end disassembly). We know the previous owner, Cody from OZ and he kept up the maintenance very well.
Anyway, we pull the cylinder , piston shows wear and evidence of ring blow by- its brown all over the sides...down on power of course it is.
Then I notice a lot of rattling under the PV control cover. I open it an voila (WA LA in English!) both screws that hold the secondary port control slug control arms to the control shaft are backed out and floating around inside the chamber----so no secondary port PV function, down on power of course.
Not effecting PV function but one of the support fixture screws was also floating around inside the chamber.
The final screw loose issue was the main PV slide screw on the control shaft and it was backed out quite a bit allowing for so much movement and chafing on the PV that we need a new PV. So for a little under our 500 buck budget....the bike will be a new machine....lets hope the bottom end continues to behave.
We will also rebase line the carb install a BR8EIX Iridium plug and continue the use of 40 to 1 mixture even though Yamaha spec is for 32:1 mixture, its a novice Int offroad trail bike.
I am trying to convince him to do some bling as well ie; a cheap but effective graphics set and some other clean up stuff- the Polisport plastic remodel upgraded look is cool but at $150 he is reluctant to go there.
Bike has a 9oz FWW, FMF Gnarley and FMF turbine core, nice pegs, tall seat, raised bars to match tall seat. and some other bits as well.

 

[robertaccio]

Lesson don't get all wrapped around the axle...slow down and exercise logical procedures. Start from basics. Don't try to fix whats not there. Baseline before throwing stuff at your problem. Don't play with jetting when the rings are shot....or when the PV is not correct etc etc etc

 

[Miezie666]

I do have 05 YZ250 and some 125s - the "powervalve parts coming loose" problems are common for older bikes, someone did not tighten the bolts up with enough torque or somebody overtighten them...
i ve seen it very often- and i also would not recommend to bigbore a bike in the first place- u will always have to buy much more expensive pistons, the engine (in most cases) will not rev like it has done before, exhaust, carb etc are all set up for stock bore...
you will run in more "problems" with bigbore..
just keep your bike maintained is always the best money spent!

Btw: i have a friend, he is about 225kg heavy and riding a YZ125- we changed the front sproket to 12tooth and rear for a 51 and the bike is climbing hills with him without so much problems.
But- his bike is always spot on- piston/rings are checked/replaced on regular basis, jetting is spot on.

 

[robertaccio]

Here's my tech report (all for fun of course). For your viewing please , also note all loose hardware was recovered inside PV control cavity so no FOD.
BTW I will need to completely disassemble to install new main PV, I had reattached the one side secondary to test the actuation.

 

[robertaccio]

Bike will simply be a top end rebuild, (and remain as a OEM 250cc YAM!!) re baseline jetting, a new FMF Gnarly pipe and cool blue bling hoses.
What's already there- looks good upon initial inspection V Force 3 assy , bottom end is tight, Steahly FWW 9oz, FMF Turbine core muffler, tall seat, renthal bars and some etc etc. Max will be a happy Big Northern Italian Dude riding with FahQ MC Italia.

 

[robertaccio]

shes well on the way back to fitness, the bike will re-rip soon

 

[fran...k.]

Is that a genuine Yamaha piston?

I believe Yamaha is the only two stroke "dirt bike" that has not gone to case reed. That piston is real long and trimmed differently at the bottom on the carb side than the case reed ones I have seen. One kind of round hole on the intake side instead of two oval holes like the Sweedish ones I have. That anti-friction coating just where they want it, Presumably for break in.

 

[Zomby woof]

Lesson don't get all wrapped around the axle...slow down and exercise logical procedures. Start from basics. Don't try to fix whats not there. Baseline before throwing stuff at your problem. Don't play with jetting when the rings are shot....or when the PV is not correct etc etc etc

I've said exactly the same thing so many times.

What separates a professional mechanic from a backyard mechanic is the ability to troubleshoot, and having a baseline is critical to solving a problem.

Your post reminded me of something that happened some time ago on a local forum and it's the same thing that happens in shops and tracks every day.

Local guy asked for help jetting his bike. It starts ok but after a while doesn't run right and shoots flames out the exhaust.
No matter how he jets it, it always does the same thing.
The responses were every thing from it's low compression, valve adjustment, worn guides, stator, too rich, they went on for two pages.
I told him that jetting does not change on it's own, and it's probably ignition. Start it, turn the lights out in your shop and spray the plug wire with water and see what happens. It will only take 30 seconds.

Three days and two more pages of jetting and he takes the bike in because he's so frustrated. Shop mechanic trims the plug wire back and bike runs fine.

Back to basics.

 

[robertaccio]

Wiseco 2 ring piston with wiseco's armorglide coating for fran k and what the man from Canada said. There are also single ring set ups but this is a trail/enduro bike for an ammie , we want long term life and the little bit heavier piston is fine along with the 9oz FFW. Plus its a 2002 model!!

 

[ptkatoomer]

I told him that jetting does not change on it's own, and it's probably ignition. Start it, turn the lights out in your shop and spray the plug wire with water and see what happens. It will only take 30 seconds.



Back to basics.

What are you looking for when you do that ? Sparks, arcing or what? And how does that tell you it's an ignition problem?
Thanks.

 

[Zomby woof]

Arcing from the plug wire.

Those wires don't last forever, especially when they've been exposed to water on the regular basis, and the electricity will find the easiest path to ground.

 

[robertaccio]

Insulation breakdown, that is the term we use. heat, moisture, chemical processes, age are all factors and others that can contribute to insulation breakdown, and when you have @ 50K Volts...it loves to jump to a ground at any opportunity less than the spark plug gap.

 

[robertaccio]

waiting to install serviced rear shock all else is pretty much done. bike will rip.

 

[Huskynoobee]

Nice work Robert.

 

[robertaccio]

bike rips once again. I raised the needle to be safe. but with the oem 50 pilot and the oem needle raised shes a little fat in and spoogey. we will drop the needle and play with the mix screw. all else is great, put on a new gnarley and she has v force 3 reed setup that looked good as is. novice rider max followed and completed one of local gnarliest trails not very long but tough