Hi, my name is Markus kemper and I live in Lisbon/Portugal :-)

I own a 1981 DT175MX with:  Engine Number:     2K4-062720
Frame:             2K4062720
Date:             31.07.1981The bike has a damaged Mikuni 24mm model 3U5 carburettor installed, and I need a model 2H5 or 2K4.Can anyone (please) help?

Thanks a lot,
Markus

Greetings from Tennessee USA  

 from the UK.

There appear to be some on ebay, located in Germany.

Yes, I looked at them, but neither are the right model

The model code list shows the 2H5 as an "Oceania" model...  the 2K4 as England and Germany...  and the 3U5 as a 1980 Oceania model.

As far as I know, the 1978-81 DT175 engines were nearly identical.  Here in the US, we got 3 different carbs...  1978, 1979, and then 80-81.  Changes to carb were made here to meet newer emissions requirements which included "anti-tamper" provisions such as the idle air screw cap and the loss of multiple slide needle clip slots. 

It looks like "2H4" and "2K5" look to be earlier (1978) models.  If your bike is a 1981 model, it's possible the 3U5 (1980 model) carb was original on your 1981 bike?

In the USA, the 1978-79 had the airbox on the right side and the oil tank on the left.  1980-81 had a better flowing airbox on the left side and the oil tank on the right. 

Bottom line is if you can't find a 2K4, a carb off another model in that 78-81 range will likely be fine...  if I was concerned, I would look for a model that had the same airbox design.  (That said, I have run the 1978 US carb on a 1980 model that originally had the newer "anti-tamper" restricted emissions carb and newer left side airbox design and it ran perfectly.)

Uau, thank you SO MUCH for your very detailed answer. It certainly helps. I sent the carb to the UK fro servicing and the reply I got was:

My Yamaha information lists the bike as a 2K4 model 1978 DT175 MX UK or Germany spec.
The carburettor for this model should be marked 2H5 or 2K4
davesbikes.weebly.com/uploads/4/8/4/5/48...x78_2k4partslist.pdf

The carburettor you have sent me is marked 3U5.
My Yamaha information list this carb as originally fitted to Oceania region (Asia) DT175 G and DT175 H 1980-1981

The 3U5 carburettor you have sent to me has differences to the 2H5 carburettor, main jet, pilot jet, needle jet, needle and throttle slide.

3U5 carburettor
main jet 90
pilot jet 20
needle jet O-8
needle 4J6
throttle slide cutaway 1.5

2H5 carburettor
main jet 130
pilot jet 25
needle jet O-2
needle 4J13
throttle slide cutaway 2.0

The needle jet has a different fitting for both carburettors. The needle jet in an O-2 size will not fit the 3U5 carburettor.
You should look for a replacement carburettor for your model which is correct for the model and is fitted with the correct parts.

Hence my doubts

Markus

It looks like the "Date 31/07/81" is causing the confusion here - what does it relate to and what document is it on? It could simply be a late registration of an earlier bike, or maybe even the date it was imported to and first registered in Portugal. In this regard, looking at pages 43 to 46 of the parts manual that you posted a link to, the DT175MX was never officially sold in Portugal.

yes, you are right. It cerainly is the registration date of the bike...

Whilst you were typing, I added another sentence to my reply.

Do you know how it ended up in Portugal - eg old stock sold there as new, or a used bike imported from elsewhere in Europe?