1978 DT175 restoration in progress. 

I came upon a NOS Mikuni carb made for the newer (1980’s-1990’s) international model DT175’s. It’s essentially a VM26 Mikuni made to spec for this model. When it arrived I compared it to my OEM Mikuni VM24ss and was surprised how it’s nearly identical. I took out my calipers to measure the spigot mounting locations (air filter side/ reed valve side) inside diameter and outside diameter, overall length, height, etc and it’s all exactly the same except for the venturi ID being 26mm on this new one vs the 24mm on the OEM one. The jetting situation on the inside was the main difference. 

My DT175 is the right hand side airbox model which I understand influenced factory jetting setup. 


OEM:
VM24ss
140 main
22.5 pilot
needle 3rd clip
cutaway 2.5

new carb:
VM26
150 main
22.5 pilot
needle 3rd clip
cutaway 2.0

I’m wondering if the new to me VM26 would be a better setup as is? Another factor is that I fitted a Fresco “big one” expansion chamber (unrestricted, no baffles) to the bike with the stock muffler, possibly requiring richer jetting.

anyone have experience with a similar setup? I’ve read conflicting info on people trying the VM26. Some say they can never get the jetting dialed as well as they can with the stock VM24ss. 

Thanks!

Dan

 

The needle and needle jet specs are extremely important to the main and cutaway requirements, you can't compare just "some" of the jets. 

That said, I've used the VM28 (RD350) carbs a couple of times on my DT175's and it worked fine with minimal jetting. 

The danger with the later models is Yamaha used a "boost bottle" set up on the 1980's and 1990's on some models which can significantly affect the mid range jetting.  Not a problem if the needle jet series have different sizes available.  But could be an issue if, like RT325 posted on another thread, you are stuck with the needle jet it came with.  They tended to use needle jets a few sizes smaller on the "boost bottle" models from what I remember.

Thanks for the info. Yeah I left out the needle jet specs on my original post. I have a 4J13 needle in the 24ss and a 5GN38 in the 26. I haven’t looked at the needle jet specs yet but all the jets are removable if needed. If this carb does indeed have a needle jet sized down to compensate for a boost bottle system, this would give me a lean condition in the mid range correct?   

My last order of parts should arrive in a few days, so I can finally put the bike back together and fire it up for a test run. It’s getting a new top end and left side crank seal. 

Okay so I just checked the needle jet situation and found something. In the NOS vm26 I found it has a 332 O-6 needle jet which is interesting because per the internet, for the vm26 series, the only needle jet option is a 332 O-0. So the O-6 might be a Yamaha specific jet for the boost bottle. 

That's why I mentioned the possibility of not being able to get a needle jet if you need to change it. 

I can't guess what you'll need, but there are a lot of needles available...  the 5GN needle should be longer and is probably going to be a lot leaner than the 4J so that might be something to try if it seems lean.

Roger that. I’ll report back after some test runs. 

Just make damn sure the 4J is long enough that it doesn't come all the way up out of the needle jet at full throttle!  (if it does, it can stick the slide wide open!)