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View Full Version : YZ 125 Help!!! Fouling plugs!!!



neptuneman
04-11-2006, 07:07 PM
I recently bought a 2002 YZ125 from some one who used to race the bike. I will be using the bike to trail ride and mess around. I have a few problems: The bike "spits" alot of black junk from the silencer. Also, if I run it slow for a few mintues it fouls plugs!!! What fuel/oil ratio should I run, what oil and what octane? I know little about tuning but with some help I can follow directions!

RoostRider
04-11-2006, 10:26 PM
Sounds like a classic jetting problem. Is your elevation difference considerable from where it was raced? Is the weather considerably colder than it was when it was raced? Have you cleaned/ reoiled the air filter? (it could simply be that)

If not, it is likely because you are riding it much less severely than the last owner. (it was not jetted properly in the first place, but he might have never known, because he kept the R's up enough that it never fouled a plug)

You are running too rich on your fuel : air mix. The likely culprit is the idle circuit/needle clip position, but you should jet it from the bottom (idle jet) to the top (main jet) so that it is running optimum in all ranges.

As to fuel : oil ratio. No matter what people are telling you, it has very little to do with whether or not the bike will spooge residue out the exhaust. That is the result of improper jetting (which is marginally affected by fuel : oil ratios). Also, despite what others might tell you, you should run what the manufacturers recommend for a fuel : oil ratio. In your case 32:1. Unless you have special requirements, or are particularily easy or hard on the motor (extremely fast guys sometimes go as high as 28:1, real casual riders can get by with 50:1 sometimes)

The truth is that, if your bike is jetted properly, you will not spooge, or foul plugs no matter how you ride or what ratio you are running, but it is better to have too much protection (32:1 for most riders) than not enough (50:1 or less), because if it is jetted right, you will never notice the difference.

You should also clean out your exhaust and repack your silencer before jetting, as with the spooge build up it may affect jetting. Also, check your powervalve for excess build up from being ridden too rich. (always clean your air filter)

This site (http://justkdx.dirtrider.net/) has a great article on proper jetting under "tech tips" "Engine" and "carb tuning". It is specific to the KDX, so don't use any jet suggestions in it, but it gives you the why and hows of jetting principles that will adapt to any modern 2t dirtbike.

neptuneman
04-12-2006, 09:58 AM
Thankyou for all of the great info! The bike was bought locally so the temp and alt are the same. The air filter is new and clean so that is not the issue. I am running 32:1 synthetic oil as directed by the manual- my problem mostly happens if I am riding very slow in first gear not so much at higher RMP's. Should I just follow the jetting gide that Yamaha has in their manual or is there other links? I assume this problem can be fixed by jetting-leaning the bike out?

Thank you for your help.

JP

jmd
04-12-2006, 10:08 AM
Try raising the needle's clip 1 position and see if that helps. If it still runs blubbery on the main jet (full throttle), change the main jet to one size smaller. If that all makes an improvement but not enough, do it again, one step at a time.

RoostRider
04-13-2006, 01:21 PM
sounds more like it is in the idle circuit to me... (but the needle clip would be my next bet)

Yes, use the jetting guide in your manual to get a starting point....

it may just be the air screw is wrong (should be near 1 1/2 turns out)

wrenduro450
04-14-2006, 07:07 AM
I ran my yz 125 at 50 to 1 . Also i would go down one on the pilot. Needle also. Are you losing tranny oil maybe crankseal? Just my 2 cents worth.

HiG4s
04-14-2006, 12:46 PM
I recently bought a 2002 YZ125 from some one who used to race the bike. I will be using the bike to trail ride and mess around. I have a few problems: The bike "spits" alot of black junk from the silencer. Also, if I run it slow for a few mintues it fouls plugs!!! What fuel/oil ratio should I run, what oil and what octane? I know little about tuning but with some help I can follow directions!

Has this bike been jetted before or does it still have factory jets in it? Generally Yamaha YZs come jetted rich. Seeing as the person raced, he may have changed the main but never changed the pilot, not much going slow on an MX track.

On my son's 2003 85 and his 1996 125 we changed from stock to one size smaller on both pilot and main, and dropped the needle (raised the clip) on both.. Both ran much better and stopped fouling plugs. But the 85 still spit spooge out the pipe at times until I changed to aftermarket reeds.

Matter of fact, I did the same thing on my 99 Husqvarna 125.. Try one size smaller on the pilot jet. For the non racer, that is usually the reason for fouled plugs.

As far as the spitting out the pipe, the packing in the silencer is probably soaked with oil and needs to be replaced.

neptuneman
04-16-2006, 05:21 AM
I took the carb apart and made sure all was clean and working correctly. I raised the clip 2 positions (1 up from stock) and ran the bike all day yesterday and only 1 fouled plug- big improvment. I will try to change the pilot jet to one size smaller as well. As long as I keep the RPM's high and don't let it bog down like my bigger 4 stroke 600 then I have no problems. Learning as I go!

Thanks for all the help.


JP

detonator
04-16-2006, 02:27 PM
Now that you're closer on the jetting, definitely take the time to repack your silencer ($10) and verify that your air filter is neither crusted over nor oversaturated with oil. Also, if you suspect your premix ratio is weird, or the gas is old, drain it and mix up a fresh batch.

Also, when I had my YZ 125, I ran a 1 range hotter spark plug, and it responded very well to it in off-road (non-track) situations, particularly this time of year.

FWIW, although oils are often a matter of preference, I can testify that Golden Spectro premix and Gear Saver in the gearbox ran really well in mine.

Don't forget to post a follow up to help out future plug foulers.

neptuneman
05-10-2006, 02:08 PM
Well, I have riden the bike about 5 times over the past month and still fouled a few plugs but not nearly a many as before. I have raised the clip one from the top which made the biggest diffrence. I have a smaller main jet but haven't put it in yet. I went to 32:1 Yamaha syn. oil with premium gas. I figured out as long as I keep the RPM's up and not "bog" the engine in the wrong gear I don't foul plugs...... "Its the Indian, not the bow-n-arrow" theroy! Thanks for tall the help.

RoostRider
05-10-2006, 11:02 PM
Sounds like you need to go after that idle circuit. Lean it out one at a time and see what works.

How does the bike respond off idle? With a proper idle jet it should require the airscrew to be 1-2 turns out to get good throttle response off idle.(it should SNAP off idle when it is right) If you are further out than that, you can be relatively certain you are too rich on the idle jet.