VM28 Jets
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Hello, I've got 1988 bike, and I'd like to try different main jet.
Carb is VM28 I assume... that round slide... It has 125 main jet now, and I suppose it's original... many have changed up to 250 main jet, but thats seems to bee too big for my bike... where should I start? I have changed DT230 snorkel rest is original so far...
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VM26ss. Should be.
What does it actually run like first?
Not sure many of us know much about the early 88' DTR carburettor.
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R euro models should have VM28SS carb, as far as I know... I think before 1988 it was VM26... But there are different variations for sure...
First (before I've done nothing) it felt quite fine... I have no experience from 125 2t bikes before, so I can't really say is it good or bad
It revs abt 8000 rpm and stuck there. After updating main jet, it revs less and sound weird... It's not running clear as 2t should. Maybe I should try 200 main jet next?What I was wondering is that why later models can use 250 main jet, even with quite original stock bike? And for my bike it is way too big? Is it due to different carb (VM -> TM change)?
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@jgr72 I'm pretty sure the early 3DB1, DTR were VM26. Have another check on your bike.
There is a difference between Numbers in the VM -TM series and they are not correspondent to each other.
Sounds like the bike is running rich...
A leaner main jet perhaps, but do try to adjust the needle position to, its important to Mid to full throttle running.
The standard jet size for a 3DB1 - 88' DTR is a 125 Main jet
You have to trial jets, and it's very individual to each engine and the environment..
Can only suggest you try smaller jets than 200, I know a TZR125 from the same period, with the VM26 uses a 180 main,, have a go with it, best to try some different sizes and see how it feels.
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My bike is 3BN, if it will make any difference... I will have to take another look of the carb
Last time I took some measurements: engine side diameter is 35mm, air filter diameter is 43mm and its abt 84mm long, but didn't look the exact modelIsn't jet numbers all the same? It's a hundredth of 1mm? But the carb body effect that, there have to be different jetting, even the size is the same... for example 28mm.
By the way, isn't 3DB an DT125LC'88 model? And 3BN an DT125R'88?
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@jgr72 Jet Numbers are not the same.
I haven't any experience with the 26mm round slide carbs, but you would do well to grab the later Mikuni Flat slide carb.
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This site says its an LC model:
https://www.motorcyclespareparts.eu/yamaha-parts/yamaha-model-prefix-motorcycles-1970-1994.aspx
try to fin 3DB and it says: "DT125LC'88"
Anyway that site doesn't say 3BN to be an R model... confusing...
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@jgr72 https://www.fowlersparts.co.uk/browser/manufacturer/yamaha/bike/dt125r/year/1988/model/dt125r-3db1-1988-999-a
The Haynes Manual will tell you more about the production runs.
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Okey
So my bike seems to be an early '88 !? -
Where to find mikuni identification? I tried to look both sides but didn't see nothing...
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Today I took the airbox out, and blowed dirt away, I also changed main jet to 200, and that feels much better than 250. I also measured the carb size, and it is 28mm:
I couldn't find any information about the carb size or model, it has some Yamaha stamps but nothing else...
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@jgr72 sorry to hijack the thread but does anyone know if those late year round slides are a direct fit to the 1990
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Measure both ends and length... This VM28 is 35 at the engine side, and 43mm at the air filter side and total length is abt 84mm.
