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DT175 to 125MX change


Captain Triple
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I have recently converted my 1981 DT175MX to a 125. Seems silly I know, but it was a nice cheap way of getting my lad mobile for his 17th Birthday, and I made him do the work by way of some education. It runs ok using all the 175 carburation set up, but its not quite as sharp as it could be ( Its hard to tell as Im used to it as a 175). Its pretty well run in now and the plug colour is a bit on the dark side. Im thinking I ought to try changing the jet or perhaps the clip position. Anyone out there with a view on this. I think Im on a 160 main and the clip position is number 3, standard exhaust and air box.

Nb the long term plan is to put it back to a 175 but tests have to be passed before this happens, so I just want it running as best is reasonable without too much change.

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I was a bit confused by the info in the Hains manual it says 110 and 130 dependent on country, most of what i read points at 130 ? the 160 is standard DT175 so Im guessing your right its too big, Im just a little wary of droping a minimum of 3 sizes, never really messed with carbs before always played safe and standard. I decided to stay safe at 160 while it was running in and it goes smoothly enough riden steady eddie, probably hasn't been at WOT yet. I was going to give it a bit of a thrash this weekend ,spose that will make it stutter a bit.

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yeah I was thinking 130 will be more like it, try it WOT when you can and see what its like, buy a few genuine mikuni jets from allens they arent expensive, start with 140 if you like and go down in 5's from there watching plug colour and feel / temperature 130 is actually 6 sizes smaller...they increment in 5's at this size

as you know, you should fit the biggest one that it will run sweet with at WOT ;)

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The data I have for the DT125MX say's

CarbType = Mikuni VM22SS

Main Jet = #130

Jet Needle / Clip Position = 4H16 - 2

Needle Jet = 0.2

Slide Cutaway = 2.5

Pilot Jet = #22.5

Airscrew turns out = 1 and 1/4

Float Height = 21mm

Idle Speed = 1,300 rpm

Hope this helps

Tony

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Thats a big help, thanks for that. I think 130 is probably where Ill end up but Im going to go a little at a time and thanks for the detail Ill match it all up to what I've got.

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The data I have for the DT125MX say's

CarbType = Mikuni VM22SS

Main Jet = #130

Jet Needle / Clip Position = 4H16 - 2

Needle Jet = 0.2

Slide Cutaway = 2.5

Pilot Jet = #22.5

Airscrew turns out = 1 and 1/4

Float Height = 21mm

Idle Speed = 1,300 rpm

Hope this helps

Tony

I would just do the main jet for starters, this info is for a 22mm carb and you have a 24mm, .......

is this fir a mk2 airbox Tony?

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I know this was the carb spec / jetting that was on my 1979 V reg DT125MX that had the silver box section swinging arm and had the airbox on the left hand side (as you're sat on the bike) with the two stroke oil tank on the right hand side.

I know this is a 175 but this was the colour and graphics that was on my DT125MX

http://www.bikepics.com/pictures/1214879/

Tony

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Its clicking into place - looks like the Mk2 is a 22mm carb and the Mk1 125MX is 24mm with a right hand airbox . My manual which only goes to Mk1confirms 110 /115 or 130 dependent on country on a 24mm carb. Im sticking to Airheads advice and taking it down 1 size at a time and checking how it runs at WOT. Effectivly as a 125 Ive got a Mk2 airbox with a Mk1 carb size and Ive got to find the sweetest run between these two . Thanks all for your help.

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You have the best of it. The early airbox is great if your up to the gearbox in mud but it is very restrictive. As can be seen from the jetting and carb size.

You effectively have an upgraded inlet assembly. With a pipe she will be quite the rocket ship.(for a 35year old design).

On that basis i would expect her to be happy at around 135 maybe a 140.

Compared to the 175 you really do need to rev the nuts off the thing but aside from that the performance is pretty close.

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Sounds like im sorted once Ive found the right jet size. My lads ridding it mostly and he's just started learning, so its probably not getting much revs yet. Im going out on it at the weekend to get the feel of it and will be packing on the revs doing a WOT check. Thanks.

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