Model 98 carb on Combat Wombat with reed

[bchappy]

I am converting a Combat Wombat to a reed and trying to decide on the 28mm stock flange mount carb or the 28mm carb from a model 98 Super Rat. The model 98 would be easier to mount to the reed. If anyone has used this setup (98 carb) can you give me a starting point for all the jetting, slide, needle, etc.? I have both carbs and just don't want to purchase a new one.

[Zyx]

Just as a matter of convenience, I would use the spigot mount carb. I have a 28mm used on a model 97 which worked fine. I will pull it apart tomorrow and list the jetting. Probably not much different than the SR.

[racerclam]

The stock Rat seting should be fine except for necessary altitude changes. That carb will also save you about 20lbs overe the combat carb.

Rich

[hodakamax]

Bill, I bought a new VM-28-49 with spigot for my 94A. It's reeded with Combat head and slotted Wiseco piston. The main is 190 now which is slightly rich. It works great. I suppose you could look that up and find the other specs.

Max

PS--It looks like the stock main jet is a 250 for a 98. Seems like quite a gap compared to what I'm running.

[Zyx]

The jetting in my 28mm Mikuni is:

Main 350
Pilot 35
Needle Jet 169 0-4
Needle 5F3
Slide 2.5

I have used this on a stock ported 97 cylinder and it worked just fine in the woods. Took out most of the mid range blah in this cylinder and was rideable, jetting seemed normal, not rich or lean. I like the spigot mount because it is easy to make jet changes and needle changes on the trail with minimal tools and time.

[hodakamax]

Wow, so much for AZ and Max's main jet report! Somewhere between 190 and 350.

Max

[bchappy]

OK thanks guys (especially AZ for taking his carb apart - above and beyond the call of duty.)
The spigot mount I have was an eBay purchase a couple years ago. It was said to have been run on a Super Combat and it has:

Main 390
pilot 40
needle jet 277 P-6
Needle 4D3
Slide 2.0

I think I will start with it the way it is and if I can't smooth it out in all areas I may try AZ's as it sounds like it is working.

[Bullfrog]

Do a thorough visual check to assure yourself there won't be problems with that #4 needle. If the tip of the needle comes up out of the hole at full throttle, it COULD move off-center and jam the throttle open -- which could be wayeeee worse than just un-handy.

I believe #5 length needles are correct for the 28mm carbs.

Ed

[bchappy]

Will do. Thanks Ed

[Zyx]

Someone may have simply stuck a needle into the carb in order to sell it, as opposed to having run it that way. I would think the 4 needle would be wrong from the git go.

The jetting I have was provided with the carb by Stictly Hodaka, but whether Paul changed the jetting before sending it to match my intended use, or had acquired the carb as jetted for a 98, I don't know. The reason I got the 28mm was for my 125 Webco/Hodaka, but I ran it that way only a couple of times before pulling the engine out of service. It ran fine on the Webco, which is pretty mild by most standards. I then bought the 98 spigot to mount the carb on the 97 to see if it worked better on local trails than the stock 32. It did. It was MUCH better than the stock 32. But then I had the 32 modified, and while it was just a bit softer in the midrange than the 28, it was better mid range up than the 28. So I am using the modified 32, and will use the 28 on the Webco once it is rebuilt. I will see which of the two carbs works best on my new 03 cylinder.

But I have found that jetting is not a precise science. I had jetted the 32 for use here at 6300 feet, and was concerned that when I got to Oregon this past June, it would not run well because of the mile or more difference in altitude. But it did run, and actually ran better there than here, all without changing the jetting. I moved the needle a clip position, and that's all. I think the jetting is more flexible than we think it is. No matter where you live, your pressure altitude can change by thousands of feet over the course of a few days, but unless you are racing and are adept at jetting, mostly we go with what we have without problems. Once I have a combination that works, I rarely change jetting at all.

[bchappy]

Greg, Another thing to think about is the high temp at Athena this year effected jetting similar to your higher altitude. If it had been cooler you may have been too rich. Am I thinking about this correctly? I just remember "back in the day" running great on a cool Sat night and the bike being too rich the next day on a hotter Sun afternoon race.

[Zyx]

I understand density altitude. I have a pilots license although I haven't flown for years now. Still, nearly 6,000 feet of altitude change attributable to temperature would be a lot of shift, and the difference in temperature between here and there was less than 20 degrees. I was just expecting it to be way off, but it wasn't. It was fine all week, including running up into the Blues. Perhaps if you tune right at the edge, you notice changes from day to day. I generally don't notice them although I suppose they are there.

[dirty_rat]

You might want to verify the carb is a Mikuni VM roundslide carb, as Mikuni don't list any of the VM's with a 277 series needle jet. The proper needle jet series for a 28mm VM carb is a 169. There are some Mikuni roundslide carbs out there that are actually made for a 4 stroke and I have seen people list them as VM carbs. Might be worth a check.

[bchappy]

Good idea. It was an eBay purchase. How would I verify is it was for a 4 stroke?

[hodakamax]

Just curious why I'm running a 190 main jet on my 94A with a VM-28-49 when you guys are using 350-390 mains. Mine is a new production carb rated for both 2 and 4 stroke.
Pilot 60
needle jet N-8
jet needle 5F21
throttle valve 2.5
needle valve 2.5
slide type VM-28/56
Comments?

Maxie

[Bullfrog]

Maxie, do you have a completely stock 94A air cleaner? If so, the snorkel is choking down the air supply and your leaner jetting matches the amount of air which can be delivered to the engine through the air cleaner system. Not to mention that a 94A exhaust system is primarily intended to A) look sort of nice and B) carry exhaust products to the rear of the machine. Whereas the Combat Wombat pipe and Super Rat(98) pipes are genuine, for real "expansion chamber" designs which actually improve the performance of the engine . . . and which have some effect on required jetting.

bchappy, I'd recommend looking up the stock Super Rat(98) carb specs and using those as your starting point - - I too am having trouble with the concept of 350-390 main jets in a 28mm carb on a reeded Combat Wombat. My 40 year old recollections of riding reeded Combat Wombats has me thinking of something more like 240-250. Still, only jetting runs will confirm what you need on your bike with your particular collection of modifications/porting/air cleaner/exhaust/etc. (Note that carb jetting sensitivity to atmospheric changes, rises steeply as the diameter of the carb throat gets larger than about 50% of the cylinder bore diameter. 24's, 26's and 28's aren't all that finicky on 56mm bore Hodies -- but 32's, 34's and 36's can be a real PITA! (don't spit or sweat when near a Hodie with a 36mm carb - the humidity change will affect the jetting! )

Ed

[admin]

Hi Bill sorry just catching up on your post. Needle Jet 277 series ?? could it be 247 series ??
Not showing a 277 series available from Sudco?
247 series NJ was the same as the 169 series but longer. This was used with the deeper float bowl drain nut. We used these with the Wolverine and some others as we were running out of fuel. That darn Taylor........
Paul

[hodakamax]

Hey Ed, thanks for the info! I am running the stock air cleaner but I do have a Circle F pipe. The engine is quite peppy and has a wide power band but I think I'll take your advice and look into changing or modifying the air filter and re-jetting.

Maxie

[Zyx]

I don't find a 277, either, but do find 270, which I believe was replaced with 169. Also available are needle jet extensions which drop the main by, I think, 8mm. It's just a screw in adapter.

Just reading through what some suppliers offer as "standard" main jetting, the mains all look small to me. A 28 Mikuni in a VM series shows a standard main of 200, but I don't find any background to go with that, ie, is that for a four stroke or two stroke. Clearly, jetting is predicated on the entire intake capacity from end to end, and not just what a 28mm bore will pass.

As for the jetting called for by Hodaka, sources and results vary. The Super Combat Owners Manual provided by Strictly Hodaka shows a 32mm VM SC main jet as 410 standard, but the Clymers manual circa 1980 shows the 32mm VM SC main jet as 350. I am currently running a 395, and I believe it is too rich generally, yet it worked well in Athena except for the downhill sections of the Bad Rock, where jetting was so rich it fouled a plug. I had to drop the needle to the top slot and go one heat range hotter to get off the mountain. For the Sunday scrambles, it was okay but the difference, aside from altitude, was on Sunday I was running largely wide open, whereas coming down the trail, I was backed off the throttle in third gear a lot of the time, which just loaded up the crankcase. I have a 350 main that came in the 28mm Mikuni which I will try in the 32, just to see what difference it makes.

Eds comments about the larger carbs being more touchy appears to be correct.

[bchappy]

Looks like 277 P-6 to me. (Click on the pictures to make them larger.)

Left Side

Right Side

Rear

Front - Looks like #278 A1

Anyone recognize this carb. Maybe I should sell some stuff and buy a new one with the money instead of playing with it.
(I will give it a good cleaning if I keep it.)

[Bullfrog]

Bill:

I went to the basement Hodaka parts mine (the ore is rich there, but sometimes things hide from me) to find the pristine SR98 carb (28mm) in my possession.

It is different from your photos . . . but not by much that really counts.

1. My carb is black
2. My carb is die cast aluminum (I think) - yours looks like zinc/pot metal???
3. There is NO stamping of any kind where yours has the 278 A1
4. The fuel inlet on mine slants slightly toward the engine
5. The pilot air screw on mine is on the other side (compared to yours)
6. I don't have that rubber bulb/cover on the side below the starter circuit plunger
7. We do both have a chrome plated brass slide.

Comparing your photos to mine . . . if I were you, I'd work with the carb you have on hand. Except for the location of the pilot air screw and possible difference in body material, they are very much equivalent. The pilot air screw will work the same regardless of its location . . . and the body material difference causes only a weight difference (no big deal). I'll see if I can shoot some photos when I fire up the camera for my Resonator Revisited article which is coming dangerously close to the submission deadline.

[bchappy]

In searching for "Mikuni 278" I found one link that takes me to a picture but no info. Apparently there was a 278 A-1 made for something "back in the day." Maybe I will give it a shot. I am sure it will end up with different jets than any of you have described unless I change all the innards to the ones in the model 98 carb (and that could get expensive.)

Thanks to all of you for you comments.

http://vintagevillagecycles.com/product ... kuni-14-3/

[Bullfrog]

With any luck at all, you'll see photos of a stock Hodaka Super Rat (98) Mikuni 28mm carb below.











Bill, the only thing I'd worry about with carb you have is that #4 needle, it is not correct for a 28mm carb. Be sure to get and use a #5 needle. I wouldn't worry about the different "style" of your needle jet, it is "correct" for the carb you have -- the "size" of it is important, but I wouldn't worry about the "style" (unless, of course, you have to find another "size", then the rarity of that item may become a problem).

Ed

[bchappy]

Just like you described. Thanks Ed. I think I am going to clean up the one I have and give it a shot.

[admin]

Nice photos Capt. !!