yz 125 '08 cylinder head - detonation ?

[Stigy]

3 minutes ago, mlatour said:

Since bikes are designed manufactured in Japan or Europe (KTM, GasGas, Beta, Sherco etc.) where the RON method is used,

most north american (Canada/USA/Mexico) owner's manuals will show the RON method rather than the R+M/2 method used here.

 

Other manuals will only say 'use the highest pump octane number available) which is anywhere from 91 to 94 depending on region.

Ok. But I'm still wondering if I'll need to re-jet after changing from 95 to 100 octane fuel... I think it's likely it won't be necessary, but I might be surprised

[mlatour]

In my opinion, the difference in jettings specs for north american (NA) and euro models could be attributed to:

 

-the euro YZ's have traditionally had a longer silencer (quieter) to meet euro sound specs.

Since 2011 the NA models were switched over to this longer silencer, they now have similar jetting specs as the euro bikes.

 

-MX tracks in europe are likely to be more on natural terrain with significant elevation changes (putting more load on the engine)

In NA, MX tracks are often built on relatively flat land and all the obstacles man-made, more 'supercross' than traditional motocross.

 

-soil condition, I think apart from the south of France and Italy, most northern MX tracks in Europe tend to be deep soft soil or sand,

again putting more load on the engine.  Factor in rainy weather and muddy condition as well.

 

Hence the richer settings on the euro YZ's.

 

Myself run a #35 pilot, #43-74 needle (2nd clip) and a #410 or #420 main jet, 

(that's 2 sizes leaner on the pilot, a half-clip leaner on the needle, and a stock main jet)

quite lean on the bottom but that's how my engine responds the best. (on relatively short & tight corner MX tracks)

Edited April 20, 2018 by mlatour

[Tahoe Gator]

YZ125 ..Top good fuel.. no Ethanol.
Bottom.. crap fuel.. pump crap...
(previous Owner for sure..)

Is the one at top an RK Tek head? Looks like my YZ250 head from RK.

[Dodging_roost]

19 hours ago, Stigy said:

Ok. But I'm still wondering if I'll need to re-jet after changing from 95 to 100 octane fuel... I think it's likely it won't be necessary, but I might be surprised

No rejetting should not not be required based on an Octane change, I would check the plug after a few hard pulls however just to make sure it isn't gray but a nice light chocolate brown

[mrdsee]

Higher octane fuel will tolerate a slightly leaner jetting if your jetting is on the leaner side currently. I have to run straight VP110 in my YZ144 since I have a high compression head. I also run a forged piston, so make sure you really warm the engine up prior to jumping on it with forged pistons as their expansion rates are different than cast pistons and will seize if not properly warmed up.

Edited April 20, 2018 by mrdsee

[Stigy]

37 minutes ago, mrdsee said:

I also run a forged piston, so make sure you really warm the engine up prior to jumping on it with forged pistons as their expansion rates are different than cast pistons and will seize if not properly warmed up.

Yep, as far as I know the forged pistons are prone to cold seize. Just wondering: is it because they expand before the cylinder expands enough so the clearances are to tight for this moment (I think that's not the case here), or because they may rattle for a heating period, because of bigger clearances and expanding slower because of bigger mass (forged pistons materials are denser, that means more mass = longer warm-up time) and rattle may cause seizing ?

Edited April 20, 2018 by Stigy

[mrdsee]

"To better understand why forged pistons are more seizure prone, you need to understand what makes a forged piston different than a cast piston.

A cast piston is manufactured by pouring molten metal into a mold. The final shape is machined to it's final exacting tolerances.

A forged piston is made by taking a chunk of metal, and beating it into shape with a die-press under enormous pressure. Like casting, the final shape is achieved through precise machine work.

The main difference between a cast and forged piston is the grain structure. A forged piston is beaten into shape, and as a result the metal stretches and compresses as the piston takes shape. The varied, elongated grain structure is like fiber reinforcing, and it makes for a very strong piston. Microscopic cracks don't readily propogate through the structure of a forged piston due to the high density and the irregularly spaced and sized grains. A cast piston, on the other hand, is made up of grains that are all the same size, because it starts out as a liquid that, after being poured into a mold, undergoes a controlled cool-down process that allows the metal to reach a near-perfect equilibrium right out of the mold. The highly regular grain size and distribution makes them more prone to crack propogation and failure.

The break-in of the two types is very different, because the metal properties are very different. The forging process produces a lot of internal stress from beating the metal into the intended shape. The stress is trapped in the metal of the finished part. A cast piston has lower internal stress, because it was able to seek it’s own internal equilibrium as the liquid metal flowed around inside the mold and then underwent a controlled cool-down. Since a cast piston has lower internal stress, it won’t distort nearly as much as a forged piston will when heated to a high temperature. The forged piston's propensity to distort when heated is the reason they require an elaborate break-in procedure.

To relieve the internal stress, and maintain it's correct shape, the forged piston has to go through a series of heating and cooling cycles. As it heats up, the grain structure will re-distribute itself into alignment to relieve any trapped stress. As it cools, the cylinder will contain and restrain the piston, maintaining it's shape. After a few heat/cool cycles, the internal stress reaches equilibrium, and the piston will no longer distort when heated in the engine. It will maintain it's shape for the rest of it's life.

Forging produces a higher grain density than casting, making the part much more durable under high-heat, high-load conditions. As long as you are patient enough to break a forged piston in correctly, you will have a piston that is more durable under extreme conditions."

 

[mrdsee]

"Problems with Wiseco pistons are almost always caused by one of these three issues:

1. Improper break-in-This can be a death sentence for a forged piston.

2. Improper warm-up-A great way to cold-seize the engine. This also kills crank and rod bearings, not just forged pistons.

3. Cylinder-to-piston tolerances too tight-You can't just slap a Wiseco piston into a brand new engine with no time on it. Forged pistons require about .002" more cold clearance than a cast piston. Once it's time for the first top-end job, a cylinder will usually have loosened up enough for the forged piston to have enough clearance. Most Japanese two-strokes spec .002"-.004" clearance for the OEM pistons. Wiseco specs .003"-.005" clearance.

Another less common cause of problems with all pistons is when the person doing the top-end work doesn't drill the lube holes in the piston for an engine with a central exhaust bridge. A forged piston will actually tolerate this better than a cast piston.

Anytime you rebuild the top-end, you have to measure the ring end-gap, and file the ring ends if neccessary. I rarely have a set of Wiseco rings that don't need to be relieved slightly. If the end-gap is too tight, the ring ends will hammer the locator pin right out of the piston, with devastating results. Wiseco specs .004" per inch of bore size for their ring end-gap clearance.

All of these problems are caused by the mechanic that does the work, or the rider that's too lazy to perform a proper break-in or proper warm-up. You can't blame the product for your own incompetence (although that's usually what people do)."

[Stigy]

OK. But:

1. none of above explains what "cold seize" is.

2. breaking procedure... hmm... there are so many "correct break in procedure" nearly every rider, mechanic, machinist, producer, piston manufacturer got his own, unique one I've read nearly every option:

a)Jeff Slavens and Moto Man: use mineral oil and ride it under load from first seconds

b ) be careful, heat cycle it, cool it down and ride it for with light load through all gears than ride it like you stole it

c) heat cycle it a million times, than ride carefully and when there's one hour left on your brand new piston - now You can ride it hard (for this remaining time)

3. Warm up: Just curious about opinions: You warm up it by left idling or ride cautiously for a few minutes ?

[mrdsee]

"caused when the piston expands faster than the cylinder and the clearance between the piston and cylinder is reduced."

[turbo dan]

Looks like foreign object damage to me. 

[mlatour]

13 hours ago, Stigy said:

3. Warm up: Just curious about opinions: You warm up it by left idling or ride cautiously for a few minutes ?

Don't idle the engine more than necessary. (especially a newly rebuilt / not broken in yet piston & ring)

 

For the initial cold start of the day,

pull the choke*, start engine, push the choke off right away (engines usually rev too high with the choke on)

keep the cold engine from stalling with small blips of the throttle, no more than 1/8 throttle or so.

 

Using you hand, when you feel the cylinder & radiators are getting a bit of heat in them gently ride off.

Putt around the pits/parking area in 1st gear, get on the track 

and ride using very light throttle, up-shift before the engine gets on the pipe.

 

Push the rpms higher the next lap, the engine may load up a bit and stutter as it gets on the pipe

but don't be tempted to clear it out too soon. When you feel the engine has reached normal operating temps,

only then push it to it's maximum rpms.

 

In cooler temps, this might take 5+ minutes to reach riding at a moderate pace.

In between practice sessions the engine will cool down but won't be stone cold like the initial start, 

by experience you'll feel how much warm up is required by how it responds to throttle input as you get back on the track.

 

May sound excessive but, a gradual warm-up before working an engine hard goes a long way in increasing piston/ring/cylinder bore life.

 

Choke*, actually an enrichment circuit rather than a butterfly 'choke'

Edited April 21, 2018 by mlatour