The Hodaka Club Forum

While my Reeded CW Motor is out for vapor blasting (FINALLY!!!) I wanted to ask if I should run a bigger than 28mm carb on it, since it is Reeded. I do have a 30mm which should bolt right on. I was thinking this carb might be a good choice? Yes? No? I know the stock version for the Combat Wombat was a 28mm. So I’m thinking I will at least try the 30mm first. Or would folks think a 32mm like on the Super Combat would be a better choice?

I figured the difference between a 28, 30, or 32mm carb can all be adjusted in jetting anyway, right? So if I run the 30mm and jet it for optimal performance on the reeded Combat Wombat motor, with a Model 97 piston assembly, I should be able to tune it in.

Let me know what your thoughts are. I got this 30mm pretty inexpensively, so I’m hoping I can make it both run well with it and make it perform well with the reed valve system for this motor! Thanks in advance for any/all input! Victor

Bump....

The old rule of thumb was smaller carb, better low end power and throttle response. Bigger carb for long full throttle running at max rpms. Has to do with the velocity of the air being pulled through the carbs bore. Putting vanes in a bigger carb is one way to trick the airflow to increase air speed at variable throttle positions.

Tuning high performance engines with changed carburetors can tax the expert. I would contact Rich Taylor ============Clarence

Hmmm. So is there really a huge difference going from a 28mm to a 30mm and just adding bigger jets for the reed intake? Victor

While the jetting differences due simply to the switch from 28mm to 30mm may not be "huge" - there will be differences (which can sometimes tax an expert in tuning). There is no assurance that the 30mm carb you get will be even close for your cylinder/porting/air cleaner/compression/pipe/elevation in terms of Pilot jet/slide/needle jet/needle/main jet.

Note that while performance will likely be improved (after all required tuning of jetting is completed), the performance increase would probably likely be somewhat "wasted" if I (<--- repeat "I") were to be riding the machine. The CW when properly operating is already quicker/faster than I am. (Your mileage may vary.)

Also, there is no doubt that going to a larger carb WILL increase the "pickiness" of the carb in terms of delivery of the proper fuel air mix as you trail ride to higher or lower elevations and as general atmospheric conditions change (air temp/pressure/humidity). While the sensitivity to changes is OK for someone wanting/needing maximum performance and who is ready to "re-tune" frequently to match current conditions, that increased sensitivity can be a bit of a pain in the backside for general trail riding.

Finally, if you stick with the 28mm - you will likely need a richer pilot jet, probably/possibly a richer slide, possibly a different "notch" for the needle clip . . . and certainly a richer main jet. Be sure to follow standard Mikuni tuning procedures - tune the pilot jet FIRST.
Ed

OK, well maybe I should back up a bit more.

The motor was given to me by someone ridding himself of his remaining Hodaka inventory, along with 5 frames and 6 boxes of parts. The motor was disassembled and did not include a carburetor at all. It did, however, come with a reed valve intake assembly.

Ed Chestnut confirmed, via photos on this forum, that the cylinder had been ported for reeding. So I have no starting point from which to record changes in performance.

Given this information, the fact that this motor has been ported for reed valve, has a reed valve assembly & a Model 97 piston, where would you likely start? I know it can depend on exactly how the porting is done.

Looking at the intake ports, I’m guessing it was an owner porting job. Here’s the same photo again, which was the photo show when Ed confirmed it was ported for reed valve. And this picture was before having the motor both soda blasted and vapor blasted, which will happen soon.

Basically, I’m just looking for a starting point (hmmm...there’s that TERM again. Don’t shoot, I’m just unarmed).

I looked at the Model 97 std specs for a reference. As a reminder the Model 97 came stock with a 32mm carb with a 350 std main jet, a 35 pilot jet, a 6F9 needle and R 9 main nozzle & what appears to be either a 1.0 or 1.5 carb slide #973407 (X.Y size not identified on the HP parts fiche).

This 30mm carb has a 3.5 carb slide, what may be a 240 main jet(not clear cause it’s worn) a 60 pilot jet and no slide needle at all with what I’m assuming to be a 0-6 main nozzle.

So I have a bunch of needles, some main jets and I think a few pilots which I can try. But I’m not sure where to start. Victor

Diagnostic information needed. What is the measurement from the head gasket surface to the top of the exhaust port? Note: The stock measurement for the CW is 28mm. Getting this info will provide a hint about whether or not the porting job is mild or wild.
Ed

Can’t measure it right now. Parts are out bring vapor blasted. Victor

OK, we'll hang loose.
Ed

Modern carbs do not lose nearly as much low-end in larger sizes.

I am running 1970's vintage 38mm Mikuni's on my 1970's vintage 250s and have enough low-end for trails.

Today, people run 38mm carbs on 90's or later 125s and get just enough low-end due to their power-valve exhaust. All of their intake/port/exhaust passages ARE much larger than yours and they do peak at 12k RPM or more. According to Gordon Jennings, maximum safe piston speeds on a wombat are probably around 10,700. If even possible or reliable, that kind of tuning would overpower everything else on the bike.

Still, a 30mm Keihin with power-jet seems to work well for me and would be a reasonable compromise for you. I do not know your altitude but am at 6500 feet. Anything to get even a little more of that thin air into my engine seemed like a good idea.

Well, the motor parts are back in the mail. Here’s a photo from Alan, showing the intake ports after vapor blast “clean up!” I had to adjust the photo to make the top port visible, so excuse the brightness. As soon as the box arrives, I will give the measurement Ed requested. Victor

So did I mention the carb I acquired was a 30mm from a Hodaka SL250? Just adding the info for discussion. Here’s what I was thinking, right or wrong.....

The carb was missing the original needle, a 5DP7, so I ordered this needle because I believe the carb still has the stock needle jet.

The 28mm flange mount was stock for the Model 95 motor. However the stock motor was not ported for reed function. IF my understanding is correct, the reeding allowed for greater fuel intake. Both larger main jets for fuel and a larger diameter carb intake allows for more air to accompany the additional fuel (stop me at any point if my understanding is wrong and advise what is correct and what is not).

It’s also been discussed here that the larger the carb gets, the harder it is to tune. So I figured the 95 used a 28mm carb, and the 97 used a 32mm carb. So, my inexperienced brain decided that a 30mm carb wouldn’t be too big to cause tuning issues compared to a much larger carb. I also figured somewhere between what the 95 carb is at stock tune and a Model 97 is tuned at stock tune, might be a good starting point. Like I said, just a guess.

I’ve already changed the pilot jet to a 40. The original main jet in the 30mm carb, as bought, was just a 140 main. So I ordered a set of 12 main jets running from 210 to 320 in size increments of 10. I’m hoping that I can start somewhere around 300 and work my way back down to fine tune it in.

I also have the following pilot jets if it seems too lean in the idle circuit. 40, 45, 50 & 60. Hopefully one of those will get me where I need to go.

So, this is my current game plan for carbureted aspects and work through it from there. Let me know what your thoughts are, & I will get Ed the measurement requested, as soon as the parts arrive back. Victor

OK. Correction time! The carb I was inspecting/working on was not a 30mm it was a 28mm spigot intake! In my defense, they aren’t marked. So the sizes quoted above are in the 28mm carb.

So here’s what’s in the 30mm carb. Main jet, 300. Pilot jet, 35. Slide is a 2.5, and the needle is a 6F9. Looks like the stock needle jet, which, according to the micro-fiche, is a 0-6.

So I have the option of a 28mm or a 30mm. I thought the 28mm was a Road Toad carb. So since it didn’t measure 26mm, I had to regroup. Victor