YZ125 Compression question

[fredrader]

I checked the compression on my 95YZ125 today after riding it for a while..I rode it for about 20min and before I shut it down I made a long full throttle run and killed it at the top of the gear and pulled in the clutch so I could check the plug..Anyhow I was curious what the compression is supposed to be on a fresh 95 YZ125 ??? Mine was 170lbs and the plug looked like it is jetted about rite since I dropped the clip from the 2nd from the top (Where it was sputtering at WOT when it was 30-40deg out) to the 3rd slot fron the top where it opens all the way up without sputtering now..My question is can anyone tell me what the compression is supposed to be on my bike if it had a fresh top end ? I took a couple pics of the plug for the heck of it..Oh and I am running a a BR8ES plug instead of the BR9EG..The bike had the BR8ES in it when I bought it and it runs great so I didn't change it..I don't know what the difference is in the plugs either so I don't know why the previous owner ran this plug instead of the recommended plug for the bike..Maby it has something to do with the Phathead Racing head he installed on the bike but who knows..Sorry for the long post but I get long winded :nod:

 

[Oldman125]

170# on a stock engine sounds good to me.

 

[fredrader]

Thanks,Though it's not exactly stock since the previous owner installed a Phathead racing head and the next size up from stock dome..It is supposed to raise the compression a little higher than the stock head and the head itself is supposed to allow for better cooling..I guess I will have to stop in my local Yamaha shop and ask the mechanic what the compression should be and how do I know when it is time for a new top end..I checked the compression befor and it was only 150# but I did it cold and without opening the throttle..This time I rode id first and opened the throttle wide open while I kicked it..It gave me a 20# higher reading but I don't know if it wad because it was warm or if it was because I opened the throttle...Hey thanks for the help.. :cool:

 

[MX86]

how long ago did ya put that plug in? it looks lean to me.

 

[adam728]

There's no set-in-stone number on what your compression number should come in at, too many variables. 170 psi is quite good, so I wouldn't worry about it.

If it were my bike I'd go up 1 main jet size to cure the lean WOT, leave the needle for the 1/4-3/4 throttle jetting.

 

[fredrader]

how long ago did ya put that plug in? it looks lean to me.

That plug has been in it from back when the previous owner was riding it (Over 2 Yrs ago) and I have been riding it on and off for about 2 months..I heard that to take a good plug reading I had to run it for a while hard and wind it up to WOT in gear and kill it so that is what I did..I checked the plug and compression immediately after I shut it down and rolled it into the garage..

There's no set-in-stone number on what your compression number should come in at, too many variables. 170 psi is quite good, so I wouldn't worry about it.

If it were my bike I'd go up 1 main jet size to cure the lean WOT, leave the needle for the 1/4-3/4 throttle jetting.

I will get a jet kit for it..The previous owner said he thought it had the stock jets in it when I asked him..I didn't have any problem when the weather was warmer but it started missing at WOT when it got cold here so I dropped one clip lower on the needle and it cured it..I'll raise the clip back up to the 2nd one and get one size larger jet and see what happens..Thanks guys..

 

[Jaybird]

#170 reflects the high side of compression for that engine, IMO.
(just curious if you installed the rebuilt and set the ring end gap yourself?)

Read up on jetting technique and you will find out how to read that plug after a proper plug chop. Lots of variables involved, and we really can't (or shouldn't) comment on jet size suggestions by simply seeing the threads and the end of the plug.
You are close and on the right track though...I think I can say that.

And I would suggest to NOT buy a "jet kit". They have gotten many riders much closer than they were before, but it would be shear luck to buy one that would give you what you needed for top performance all of the time.
Study up on what you think you need to do to get closer to perfect (or degree of perfect that will work out for you), and simply buy a range of jets that will give you things to try. And remember that a simple change of atmosphere and temperature can cause what you had running well yesturday, to need changing today.

Experimenting and taking things to the edge requires that you stay on top of things all the time, or you risk damage to your engine...while being on the fat side of things allows room for error without much risk of a lean condition problem, even with big changes in the weather.

You can also read what folks post as their "optimum" jetting...typically refered to as "my bike rocks" type of specifications, but I would tend to take those sort of recommendations with a grain of salt..or sand as it were.
If a rider improves the running of the engine just a little bit, they often will feel that is what needed to be done and can't wait to proclaim to the world what the magic jetting number really is. When in reality they may well have found a whole new world, had they approached the jetting thing with a little more precision. After all, they probably got the specs they spout off from a generic jetting chart to begin with.

After saying that...I will say that from my experience, yz125's seem to run well with lots of oil in the fuel. I find it easy to jet around a mix of 20:1, and it provides plenty of parts protection as well as being an easy mix (1qt to 5gal).
Even getting my main jet straightened out, I will have a bit of spooge. I leaned the slow jet out and it helps loads, but there is still a bit of spooge...mainly because I'm fat and slow, and spooge never hurt anyone.

 

[fredrader]

(just curious if you installed the rebuilt and set the ring end gap yourself?)

The only thing I have done to it is change the needle position..I run it 32-1 Bel-Ray synthetic but the PO ran it 32-1 Klotz.I run the 110 octane racing gas and so did the PO.He is the one that did the top end on it as well as changing the head to the phathead..He said he did the top end pretty often like every 5 - 10hrs but he ran it very hard all the time..I will get the needle put back where it was and try and find some jets and play with it and see what happens..I don't have any spooge at all so maby I should use a little more oil since I am 240lbs and I wind it up pretty tight some times..I do a lot of putting around though riding with my grandkids so I really don't want to start fowling plugs so I don't think I should go too heavy on the oil..Thanks for the help I really appreciate it,I am very new to the dirtbike thing especially the 2 stroke's..Thanks again :cool:

 

[Jaybird]

I do a lot of putting around though riding with my grandkids so I really don't want to start fowling plugs so I don't think I should go too heavy on the oil..

Adding oil leans things out, contrary to what you might think.
Only engines using lots of oil that are not jetted properly will cause plugs to foul...and so will engines running 60:1 that are not jetted properly.
It's more about the proper metering of the fuel ratio that you are using...no matter what that ratio may be.

 

[fredrader]

Cool,Then maby I will start running more oil and maby I will be able to go longer between top end rebuilds.. :cool:

 

[braapbraap]

i have a 2000 yz 125 and i kept fouling br9eg plugs so i ran a br8es and it ran great. i think the br8es burns hotter so it burns off ann access oil instead of it wetting the plug.

 

[fredrader]

I searched but couldn't find out which plug was hotter The BR9EG or the BR8ES..I am still searching..I couldn't figure out why the previous owner ran the BR8 instead of the BR9..I know the 8 is like $2.25 and the 9 is around $4.50ea.I just bought a box of 10 BR8ES off Eba* for $18 shipped.I couldn't find any deals on the BR9EG plugs on there though..

 

[adam728]

Lower number = hotter plug.