If you were in UK have several 22 carbs removed from TY175s you could have.

To address your question of "Is VM22 too small", it's definitely not.  When these bikes were new, I remember we used to look up the specs and bench race about how Bike A had a bigger carb than Bike B so it must be faster...  I don't know if this mentality came from magazines or just a general "bigger must be better" attitude?  Anyway, Yamaha tended to use carbs on the large side...  too large in some cases.  In the mid 70's one of the ways to make your brand new YZ125 faster was use a smaller carb than it came with. 

Anyway, I could go on about real life racing experiences where the "bench racing" BS of the guy running the big carb he just spent a lot of money on and the claims of how much faster he is now and how much better the engine runs are quickly disproved when you blow past him at the start and pull away on the straights during the race. 

Mikuni published data on carb size recommendations...  it's in their carb tuning manual.  The CT1 with about 16 HP on a good day, should have about a 19mm to 23mm carb...  Yamaha used a 24.  Venturi size chart below. 



And a chart showing circuit overlap by throttle position which might help with jetting:




 

CT2 carburetors like to stick wide open. Don’t push start your bike.

asco

MarkT, I wonder have you ever actually tried fitting a bigger carb, and improving the very restrictive intake system on one of these bikes?

About 5-6 years ago, I fitted a bigger carb, and a much more effective air-box, to an otherwise completely stock TY175.

The bike ran so much better, that the rider was able to win a British twin-shock trials championship round on it, which is usually dominated by things like highly modified Fantics.

They will certainly run fine, with a carefully rebuilt OE carb fitted, but Japanese manufacturers tend to design in restrictions within the intake systems, these being related to longevity, improved fuel economy, and helping make the bikes more accessible to less experienced riders.

Just wondering if you have ever carried out similar changes, to those I carried out on that TY175, and if so what were the results?

TY175's in standard trim í 'think' are restricted by the exhaust where top end isn't the main focus but designed to make the bike narrow & quiet.
I had one for the kids set up with std bars & forward footrests etc & was perfect for the job.
But in recent years i fitted a TY175 cylinder to my AT1 which goes like stink & revs off the clock plus it pulls from zero.
Carburetor--dare i say Chinese--was meant to be a 28mm but turned out semi finished on the engine side to about 26mm.
Head is CT2 type AG175 farm bike head.
So--performance must be coming from the AT1 trail pipe which i pretty much gutted about 40 years ago.
I seem to recall the piston has quite big windows but google imaging they all seem that way.
So what i'm saying is--not sure what i'm saying--except sometimes the strangest combinations work out.

Here in the UK the majority of TY's have had the stock exhausts changed already, but they are certainly very restrictive (and heavy!).

Porting on the TY175 is pretty much identical to a DT175, with the only real difference being the fin area on the lower part of the TY cylinder is reduced, and they use the smaller DT100 type reed block.

Pipe probably does make a difference, as does running tighter squish clearance.

Yamfan asked:  MarkT, I wonder have you ever actually tried fitting a bigger carb, and improving the very restrictive intake system on one of these bikes?

More times than I can count. 

Absolutely yes on the carb.  Most recently on a monoshock DT175.  I have a VM28 and a (very rare) small body VM30.  Both ran very well on the bike after jetting tweaks.  Both reduced the the top speed attainable which is an indication of LESS horsepower.  (Someday I plan to build a dyno)

On the airbox, the earlier right-side airbox DT175 monoshock was fairly restrictive.  I obtained a spare cover and case which I proceeded to liberally ventilate.  I also tried a K&N filter that one of my bikes came with.  Rejetting was necessary.  Top speed increased slightly with stock carb.  Larger carbs still reduced top speed.

Oh, and these tests were done under carefully controlled conditions.  Same section of private road.  (slightly uphill)  I used a relative air density gauge to make sure the weather did not affect the results even though most test was done back to back.  Tedious as hell. 

On the "Red Rocket" landspeed CT3 project bike that had extensive threads on here...  DVM and Meis's extensively modified engine reached nearly 100 mph at El Mirage with the stock size 24 carb.  Extremely impressive for an old air cooled 175.  Larger carbs were tried early on and reduced the top speed...  eventually they did get a larger carb to work and along with other mods got a few more MPH.

The above testing was done along the lines of something you may have heard of in school.  The Scientific Method. Tedious but it works.

(Most people use only the first part of the Scientific Method...  the "hypothesis".  They just never follow through with actual testing or have changed so many things at once they don't know what worked)

For the first time I can remember you posted an example.  Trials bike.  I'm pretty sure maximum HP does not win trials?  It's not a race in the conventional sense. I'm not disputing that bike didn't run well.  (I am curious if this bike had a Nibbi carb you continually recommend?)  Anyway, I would like to point out that you also changed the airbox.  I'm not familiar with a TY airbox. (Perhaps that made the biggest difference?)  And while you said "otherwise completely stock", later you mentioned "Here in the UK the majority of TY's have had the stock exhausts changed already" so I wonder if this bike had the exhaust changed?  And maybe squish?  And porting?  Reeds?

I spent years racing and testing...  I learned early on to change one thing at a time and test, test, test. 

Decades ago I read a funny story about Colin Campbell or Colin Chapman or some other famous English racing legend..  The details of exactly what prank he pulled with a "fake modification" escape me.  I think a length of hose was glued to the hood or something like that. 

My friend Dan and I enjoyed similar "celebrity" at the off road track at Glen Helen.  We were racing Honda FLR350's with about 40 other guys almost every weekend.  Early on there were "stock class" rules.  Following the lead of the story mentioned above, I convinced Dan to glue a section of radiator hose to his airbox, as did I.  (There was no hole added to the airbox, just a hose that went nowhere)  We showed up with towels covering the "modification" removing them only right before going out onto the track, and covering them up immediately upon return to the pits.  Feigning great secrecy.

We did very well finishing 1st and 3rd or something like that...  there was an uproar about out "illegal airbox mods".  After the races and before the awards the race officials inspected our "illegal modification".  I had already been telling everyone it was just a hose glued to the box and did nothing.  Nobody believed me.  We passed inspection of course. 

The next race a couple of racers showed up with sections of hose glued to their airbox. 

There is a point to my story.  My FLR350 had a lot of time and effort put into it...  lots of little tweaks.  All legal.  And it was definitely faster than it was off the showroom floor.  The hose was an obvious "tweak", but it did nothing for performance. 

By your own admission other other modifications were made to the TY175...I'm not sure you know for sure if the larger carb helped or hurt performance. I'm not disputing your claim "it ran well".  I am suggesting that the "larger carb" may have been about important to the performance as a radiator hose glued to the airbox. 

In any case the "type" of performance for trials is different than for an Enduro.  It's a completely different bike and purpose.   It can be important if a 175 Enduro can get up to 65 mph on the highway rather than 60 mph.  I doubt that's at all important in trials. 

Most people here just want to get their bike to run as good as it did stock.  There is no doubt that performance can be improved with a series compatible modifications that work together...  especially if done by experts like member RacerClam (Rich's Taylor'd Porting) or Klemm Research.  Porting, pipe, squish, airbox, reeds, carb, etc.

In my opinion and experience, recommending someone install a larger carb (or even a different carb) on an otherwise stock 175 Enduro is sending them down a rabbit hole that will easily cost a lot of time and money with no benefit and likely a detriment.

Probably the biggest factor I've experienced personally and with human nature in general is "ego". After I've spent a lot of time and money on a performance mod, I'm very reluctant to admit it was a waste.  For example I cut apart a US monoshock DT175 main exhaust pipe to remove the internal baffling.  My inspiration was posts online where others had done it.  Took hours of careful work.  Bike got louder. Sounded much better!  I just KNEW it was better and faster on my first ride! 

Then I tested.  I tried different mufflers and swapped between the gutted pipe and the stock one. Checked and adjusted jetting if needed with each change.  Best measured performance was with the stock pipe and muffler.  

But then I also have a CT "sleeper pipe".  It was modified by DVM and Meis.  It not only sounds better (slightly louder with a sharper "bark") but there is a real measurable performance increase.  They spent a ton of time with actual testing to get those results.  Someday I plan to modify the gutted DT175 monoshock pipe to see if I can accomplish an improvement over stock. 



 

I have personally "experimented" with bigger carbs. Back in the day (many moons ago) Dave Meis and I had our CT's (his CT2 my CT3) very equally matched. We tried changing to various larger carbs to check the effects and we found that the original carbs (24mm) made our CT's run the best. Our CT's were basically stock at the time.A larger carb can certainly help if the engine has been well modified, porting, etc. Even with the engine mods that I have made on my CT3 I kept the original carb .
Mark T, actually on the "red rocket" CT we used a larger carb

I believe MarkT nailed it… most of us folks on here are just getting the bikes to run as best as it can… STOCK…
Sneezles61

DVM, when I spoke to Dave a few years ago about the challenges I was facing with my plans to attempt a 175 stock production record he told me that RR got over 95mph with the stock size carb in the early attempts. And yes, later a larger carb was used.   If that is not correct, thanks for the info. 

P.S.  Glad to see you still visit here...  you had the best running stock appearing CT3 I ever had a chance to ride!  It really impressed me with how hard it pulled.