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Mikuni Tmx 38

the Eel

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#1
One or two quick ones here - Mikuni TMX 38 on a 1999 Husky WR125 ....

I opened up the bottom of the carb and screwed out the main jet, which appears to be a 430. The top end on this bike seems pretty flat but the throttle response seems OK. Can I improve the top end with a slightly larger or smaller main ? Which way ?

Next, after taking out the main, the next in line is another jet which is longer. Is this the Pilot ? (the main was screwed into the bottom of this longer jet) This seems to be a 60. The bike seems to bog when I roll on the throttle and then gets back to normal after the revs build a bit. I've read several articles on jetting but am still a little unclear as to whether this is the result of the pilot being too lean or too rich. My impression is that it is too lean, hence I ordered two larger jets - a 65 and a 70. Am I on the right track here ?

The mid seems fine. The only things on my wish list would be to get rid of the bog (or miss) down low and maybe get some more top end out of the bike.

Any thoughts ?
 
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#2
Oh dear, oh well the main screws into the needle jet. The needle pokes into it right. These can’t have been cheap.

The pilot is the little biddy one down the hole to the side of the main. Remember jets are responsible for their relative THROTTLE position, not the revs. The pilot will affect from closed throttle -cracking it on. After that you are into the realms of the slide then the needle & needle jet. Full open is the main’s domain.

Chances are they set them up rich as too lean can fry your engine so try rich first then lean. Make one change at a time. Chant that bit about throttle position for a while & it will all make sense.
 
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#3
Eel, welcome to the 2stroke world. The long jet is the pilot and the short one is the main.
 
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#4
Eel, welcome to the bad information world. The long jet you had removed was indeed the needle jet.

Go to Eric Gorr's site and read up on carb tuning. You seem a little lost.
 

the Eel

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#5
Thanks Gyrros - I confirmed this with a Husky dealer. They suggested a 390 main and a 30 pilot. I'm gonna try a 410 main and a 45 pilot and then go from there. By the way, when are we gonna do Gorman again ? That was a a fun day last time.

Marcad - are you sure ? I have studied a .gif file of this carb ... what I'm talking about is definitely not the jet needle / needle jet ... or is there a difference ? The Mikuni site I looked at leads me to believe it is indeed the pilot. There is no taper on this at all. FYI - Eric Gorr's site is great and very helpful. I had already scoured it for info but still had a few specific questions.

Here is the Mikuni link - I think it's a good one.
http://www.carbparts.com
 
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#6
I didn't see a needle jet in that gif. The needle jet is what the needle slides into and main jet screws into from the bottom. That is pretty much what you desribed. The pilot (showed in the gif) is off to the side.

The last dealership I went to didn't even know what a needle jet is, where it is located or if it ever existed. I'll try to find a better link for you.
 

the Eel

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#8
Marcad - you got it !!!! Thanks - that looks exactly like what I revmoved - the needle jet ! Now what carb is that - is it flat slide ? Barrel ? TMX ? What size ?
 
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#9
Hey Eel, I have this week end off. I was planning on going sat or sunday. if you want to hook up just let me know.
 

the Eel

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#10
Gyrros - check "Places to Ride / Organized Rides" .. look for "Phelon 9-1"

I'll be there Sat at 6 or 6:30 AM for the 7 AM start - it's Saturday morning. The directions are in the post. Hopefully I'll see you there !
 
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#11
The carb in the pic is from a 98 YZ125, a TMX36. Many of the parts should interchange with yours. The Mikuni jets are cheaper than the OEM ones if you can find them.
 

HiG4s

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#12
Factory stock is 400/35 Which is what I'm still running on my 99 CR125, but I do have a PC pipe. I run 40 to 1 with Honda synthetic oil and it runs pretty well. 390/30 would be one step leaner than stock on both jets, mainly for hot temps at sea level like here in FL. Higher altitudes may need to be leaner. I have a 390 but the 30 is back ordered and I want to change that first as it seems to run good up top but hesatates a little down low. If you have a fax machine I can fax you the jetting chart and carb parts pics from my shop manual.
 
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the Eel

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#13
Al - you want to go to 30 on the pilot - is that why the bike hesitates down low ? 'Cause the pilot is too rich ?

The reason I ask is because that is the only complaint I have - there is a hesitation when I roll on the throttle - not what I want when I'm trying to get over an obstacle on a 125 !!!! It also smokes quite a bit when I start it up. Is that another sign of the pilot being too rich ?

The specs you mention - 390 main, 30 pilot - were exactly what was suggested to me by a Husky dealer in Michigan. However, there was a 2001 125 test in DirtRider executed at tracks local to me here in CA. DirtRider kept the 35 pilot (stock), went to a 410 or 400 main, and varied between 2 or 3 notches down on the clip (stock is 2 per the article).

I changed the main to a 410 today and kept the pilot at 35. Unfortunately, since I've never fiddled with a carb before, I could not get to the needle. I couldn't get the darn throttle cable out of the slide housing !! Oh well. I also have a 390, a 400 and a 430 main. The 430 was in the bike when I bought it. It worked fine it seemed but I wanted to try something different to see what effect it had. I'm riding tomorrow - I'll report back on the results.

Will changing the main affect the top end in any way ? It seems pretty flat on top.

Also, is the choke off when it is all the way down ? Or all the way up ? I'm thinking all the way down is off. I hope that's right.

I'll email you regarding the fax.

Thanks !
 
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#14
Holy cow!!!! You are lost aren't you?

I suggest you find someone with some experience dealing with carbs and ask for some help.
 

HiG4s

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#15
Yes, down is off on the choke. Main jet only affects from 1/2 to full throttle so if it runs good hard on the throttle the main is probably OK. from off to 1/4 throttle the things that affect it are the air screw and the pilot jet. Inbetween the two is the needle, the needle valve and the throttle slide cutaway. I personally don't mess with replacing the slide or the needle valve. For midrange I dropped my needle one clip position, and yes getting the throttle cable off, even if you have done it before, is a pain.

Husky's stock settings of 400 main and 35 pilot are suggested for 70 degree temperature at sea level. They suggest for about every 15 degrees hotter go down (leaner) one main size and for every 15 cooler go up one size. Pilot recommendations aren't as straight forward but still leaner for hotter and richer for colder. They never recommend leaner than a 30 pilot for any condition.

They also recommend for higher altitudes to go leaner, about one main jet size every 1200 feet. Remember running it too lean can damage the engine, too rich will not. Knowing that also realize most manufactures stock settings are a little rich to protect the engine. If you don't plan on changing jetting with the seasons you should jet for the coolest temp and lowest altitude you plan to ride at. I personally am running 400/35 with the needle on postion #2 This seems a little rich but still runs good on top but heasitates a little down low. Which is why I plan to try a 30 pilot jet. I may also change the main to see how it runs, but once it cools off here, I would change it back anyway.

Another thing to consider, most 125s don't have a lot of power down low to start with and if the power valves are gummed up and need cleaning it will have even less than normal. What oil ratio do you run and what type of oil. I run 40:1 with Honda HP2 full synthetic. I tried a synthetic blend and my bike would foul plugs and drip spooge and hesitate even more. With a full synthetic it is much better even with out changing the jetting.
 
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