I'll be doing most of my riding at 5,000 ft. and above, so I plan to drop the main jet down at least one size. Both the Yamaha service manual and the Clymer manual state the stock main jet for a 1978 IT175E is #360. However, I pulled the original jet out and after some devarnishing I can almost read what I think says 230.
 
I have owned the bike since it was new and don't remember ever changing the main jet. Does Mikuni have more than one numbering system, or are the manuals just wrong?

I ordered a 300 to 390 jet kit which hasn't arrived yet, thinking that would give me some good options around the stock size. Did I order the wrong jets?

Thanks,
Jeff

If you still have the original 34mm high top carb and original needle and needle jet the main was a 360. 

(Mikuni does have different numbering systems but the jets were different designs.  That looks like the "small hex" style which is original design.)

Thanks, MARKT. It is indeed the original 34mm Mikuni and the whole bike is 100% original, so I'm very perplexed about this jet size discrepancy. And it runs just fine, so I know the jetting is not way off. Am I wrong that the number stamped on the face of the jet is the jet size?

Yes, it should be.  I have to say that your jet looks like it's literally beat to hell so it's not like nobody ever touched it.  It wasn't uncommon to lose a jet and drill a smaller one out so numbers may not mean anything. 

I have some that soldered up and re-drilled smaller too.  We did what we had to and parts were not nearly as easy to come by like they are today. 

I remember in the 1970's racers where I lived used to convert Honda's to Mikuni because Keihin jets were literally impossible to find. 

Okay, mystery solved!

It turns out my father commandeered my IT175 in the mid-90s to go on a three-day trail ride and camping trip with some buddies. They would be riding on mostly national forest trails, and the other guys rode like "old women" in his words, so he installed the off-road detuning kit that came with the motorcycle. This was to ensure he didn't have any problems with the forest rangers regarding noise level or spark arrester, and also kept him from scaring his friends on Honda Trail 90s.

The detuning kit included a #230 main jet, among other things, which he installed. I just spoke with him and by some miracle under 40 years of garage accumulation, he found all the original parts that he swapped with the detuning kit. He is sending them to me so I can restore the thing back to the original tune.

Thanks for the jetting advice and I'm actually glad to have dug into this or I would never have known about the original parts.

Wow.  Glad your Dad remembered!  Jogged my memory and I found the kit instructions....  I think the head gasket was thicker and they restricted air box and muffler. 

 
 

Yes, he installed everything in the kit except for the head gasket, thankfully, so I can return it to stock relatively easily. The thing actually runs pretty good now, so I'm excited to see how it does when I get the airflow unleashed. I expect on the top end is where I'll really notice it.

I love that model.  10,000 rpm screamer that will give many 250's of the era a run for their money and lighter/more fun to ride to boot.