Weird Combat Wombat Jetting

[DesertRat54]

Greetings Collective,
I dug into my carburetor box yesterday looking for a 28mm Combat Wombat carb for a project I'm working on. This particular carburetor came with a Combat Wombat engine that I'm sure had been raced back in the day. The motor belonged to our local Hodaka/Penton hero here in Roanoke, Virginia back in the 1970s. He had a shop, and raced both motocross and cross country, built his own motors for both, so knew what he was doing. While cleaning up this particular carb, I saw that he used a 130 main with a 5CN3 needle. Obviously, both are way off the plantation from the needles and jets referred to in either the owner's booklet or the shop manual. I'd be interested in hearing from anyone who might have an idea as to what he was up to with this configuration. All of the other carburetor internals are standard. We're at about 1200' above sea level here, and summers are as humid as they get here in the mid-Atlantic. I'm not sure if the CW this was on was reeded or not.
Thanks,
Bill in VA

[hodakamax]

It does sound way lean, maybe the previous owner was a "jet driller." It happens, check it visually. My reeded 94A with a 28mm usually requires a 190. Life is full of mysteries!

Max

[Bullfrog]

I don't think I'd lose a lot of sleep trying to figure that one out. Maybe he had a really restrictive air cleaner inlet, custom designed to keep water out when the creek crossings were as deep as the seat?

At any rate, it doesn't matter what jets, etc. he ran then . . . what matters is getting the carb jetted properly for the fuel, compression, porting, pipe and air cleaner you have now.

My own Combat Wombat is jetted with a 190 in a stock 28mm flange mount Mikuni (about 1200 feet) but in the dry inter-mountain west.

Ed

[DesertRat54]

Yeah, Max, it seemed to me, too, that the carb was set up to run lean. For what it's worth, I found that particular needle was used with the Suzuki TS250, which also ran a 28mm carburetor. But a 130 main? I don't get that. I checked to see if it was drilled, and no, it is stock. The only thing I can come up with was that he had a pretty good idea how long the motor would last running that lean, and jetted it accordingly. If he's doing a complete rebuild between races anyway, who cares how lean it runs, just short of seizing?

Haven't lost any sleep yet, Ed. You make a good point about a restrictive air cleaner. This was really an academic exercise, hoping I could learn a thing or two about the nuances of the Mikuni carbs. I'll be restoring it to the stock CW jetting before using it.

Thanks gentlemen,
Bill in VA

[DesertRat54]

Update: Today, I ran my Combat Wombat project bike with with the needle and main jet from the carburetor I referenced in my first post. In short, it absolutely flew. I've been running the same stock motor with stock carburetor settings in my race bike (that I haven't raced in many moons) for who-knows-how-long. It always ran well enough, but not like this one did this afternoon: clean and crisp, with outstanding throttle response. I mean, this was a noticeably different animal. I still can't fully explain why this needle and jet combination works so well, but I'll be doing the same to my race bike.

So for those of you who are also running a stock CW motor and carb..... Or you can just buy this one. I'll be posting it on the board within in the next few days.

Bill in Virginia

[Bullfrog]

It would be really good to confirm what needle jet ("main nozzle") that carb has -- it may not be stock.

. . . and while you are at it - what pilot jet and slide cut-away?

Ed

[hodakamax]

Hey Bill, a note of warning here. Most lean engines do run "crisp and clean." I personally would try a much larger main and work back down. You have nothing to lose in the old tried and true method of starting too rich but you might be on the verge of piston seizure with that set-up. Lean engines do seem to have more power but may fail on a sustained demand. Proceed with caution! (Max has experienced this very situation with catastrophic results.) We sometimes forget that most cooling of that type of engine comes through the carb with the atomization of fuel and that all of the lubrication comes through the main jet at full throttle.

Only my opinion, just trying to help!

Maxie

PS--Quite a discussion on this a while back under "The Seizing of a Piston" in both the Forum and the Resonator newsletter of the Hodaka Club.

[DesertRat54]

Max, I'm with you on that. As I wrote above, my first impression of this setup was that it might have been built to run lean for maximum output, and to last just long enough for one or two heats. The guy was the only Hodaka dealer/racer in this part of the state, so had access to any/all parts he needed. For all I know, he could've gone racing with a motor for each heat. I didn't ride it long for this very reason, and checked the plug afterwards. It was a tan color, which was a great relief.

As Ed suggested above, I'm going to take it apart again to be sure that all other jets and the slide are stock. As I recall, my first inspection of the carb showed that only the needle and main were different. With any luck, I'll get to that later today.

Thanks guys,
Bill in Virginia

[Bullfrog]

I'm having real trouble accepting the "philosophy" of "jetting lean for power" or "jetting for power with rebuilds expected after every race meet". Jetting lean does not produce more power . . . it produces DNFs (Did Not Finish).

There is a "happy middle" between jetted lean and jetted rich . . . that's where the power is AND where getting to the finish line is. In some respects, I don't care what the specifications are on the components in the carb as long is the mixture is in that "happy middle" at ALL throttle positions. HOWEVER, I am worried about that 130 main jet because NO OTHER Combat Wombat in the world will run right with that main jet -- UNLESS (<-- that is a BIG unless) there is something decidedly not normal about the Combat Wombat engine or carb. So be careful with that jetting. Lean kills engines, rich doesn't run so well but doesn't kill engines . . . and the "happy middle"? Well, life is happy there.

Ed

[hodakamax]

I felt I had to warn you because if you are just putting around with only short bursts of throttle everything will seem normal. It's when you open the throttle wide open (such as a long straight) that you will have only one place (the main jet) for lubrication and fuel to be entering the engine. If that jet is too small bad things happen. Not only is the engine severely damaged but the rear wheel can lock at a bad time. I don't mean to lecture anybody of course but this happens quite often in air cooled two-stroke engines when failing to jet properly. When road racing two-strokes Max always kept a couple of fingers on the clutch lever when going down long straights. (After proper jetting of course!)

Maxie

[michael_perrett]

Ed, remember a couple of years ago, I installed a reed kit on Rick Mott's Combat, ended up running a main jet smaller that "normal". It is still running, not sure why it called for a strange size.
On another note, at last years BSA gathering at Toronto, Ohio, they have dinner rides, usually riders with English iron. I rode my vintage Ducati. Anyway, there was a rider with a vintage street legal Ossa. Must not have done any real street riding, 'cause a few miles to the restaurant, his bike started to spew out pieces of carbon, then seized. As you know,
there is a major difference in tuning for sustained highway running vs. trail riding. His Ossa no doubt would have ran very well on the dinner ride, if it had been jetting properly. If I had know what was going to happen, I would have let him use my 175SL.
Mike Perrett

[taber hodaka]

I have never heard of a difference in tuning for highway or trail riding. I treat tuning one the same.-----------Clarence

[michael_perrett]

Tuning for highway use, maximizes the heat output of the engine, especially for long uphill sections at high RPM. This is where the greatest chance of seizure will happen. As you know, the ability for cooling on an air cooled engine is very important. That is why I use a low front fender on my 175SL and developed a Suzuki TM 250 head for my 250SL. I want to make sure that I have the best chance to get where I am travelling to.

Mike Perrett

[Bullfrog]

A couple of points about jetting:

1. Any "error" on jetting MIGHT have its effects magnified on the long stretches of street/highway simply because the throttle might be held in that "one" spot which is slightly lean for a long time -- and the results can be serious. By comparison, the same small "error" in jetting may not cause any problem on the track or trail simply because the throttle may never be held in that position for more than a few seconds.
2. It is pretty common for people to unconsciously select the quietest throttle position they come across when cruising the highway or street. And guess what? That quietest throttle position is quite likely to be the spot where the small "error" in jetting (lean) is located. Oweee.

So, when on the highway - be aware of that "quiet" throttle position, and don't keep the throttle at that spot. Change things up. Run the throttle up and down - or find a "rich" spot (a noisier cruise throttle position) until you KNOW your jetting is right for the street/highway.

Ed
PS: Mike, restrictive air cleaners are prime causes of unusually small main jets on Combat Wombats. There just isn't much room in the air box to allow air to all parts of the air cleaner if the element is on the thick side or if it stands too tall and gets compressed a lot as the inner and outer covers are installed.

[RichardMott]

Mike,

You are in error about the main jet on my reeded CW. We installed a 280 jet at first try and it is still in there after jetting tests.
Here is a photo of the plug after the jet test....

[RichardMott]

I went back and checked the invoice for the jets I bought from Paul. It is a 240. Unless we used a larger one from Mike's supply. I really don't remember.

[Zyx]

You could always take it out and look.

[RichardMott]

Yeah, I was going to, but it is in my trailer and it was raining yesterday and before that the sun was in my eyes!
It has a banjo bolt so it is easy to pull and look at. I will pull it tonight if I remember and post my findings.

[DesertRat54]

Well, I'm back after a second disassembling and final accounting of the mystery carb from "back in the day." As I said in my first post, the 5CN3 needle (on second slot) and 130 main jet are all that were different from the stock settings; everything else is stock, and I don't see any obvious changes to the carburetor body. And yes, I was mindful of staying away from long straightaways. After thinking on everyone's comments, I've decided to put the stock carb on the bike when I put it up for sale. Since the stock one seems a bit too rich even with a 190 main and the needle all the way down. I'm going to bump the main down to a 180, move the needle all the way up, and see what plug color I get; right now, it's a tad too dark, and is a little blubbery on pipe. It's funny though, that I got a clean tan color with that "lean" mystery carb. Still, with a rebuilt engine, I'm not going to take any chances.

BTW Mike, I read that you modified a TS250 head for your 250SL. The 5CN3 needle is stock on the TS250, which I think also runs a 28mm Mikuni. Every time I see a TS250, it reminds me of the 250SL, and have always wondered just how similar those two motor are.
Bill in Virginia

[RichardMott]

I checked my CW reeded main jet. It is a 270.