Wombat dead

[Bill2001]

Well, temporarily. First major road trip and the piston seized. About 10 miles of mixed rural-city driving at 35-50 mph. Engine had 4-5 heat cycles by the PO and 6-7 two-three mile pasture runs by me. New carb (old one was flakey).

Onto the lift today, then off with the head and jug. Any advice on separating the piston and cylinder appreciated.

I hate old bikes. But wouldn't have it any other way.

-- Bill

[Bullfrog]

Has the engine been pressure tested to assure there are no air leaks? I would HIGHLY recommend pressure testing BEFORE taking the top end off - if you find a leak, then you have a very strong indication as to why it seized. In the absence of the pressure test, identifying the cause of the seizure may turn into a guessing game.

Were there any hints before it got quiet?

Finally, unless the seizure is massive - removing the cylinder should be pretty much the same as always. Usually, mild seizures do not lock piston and cylinder together permanently.
Ed

[Dale]

Fresh engine and new carburetor. Thinking some about the new carburetor, did you note any dull spots anywhere in the throttle range? Anything that sounded like "Whaaaaaaaa" accompanied with a noticeable lack of power? Do you know what jetting your new carburetor has and did you have to make any changes to it? Main, pilot, needle, needle jet, slide?

[Bill2001]

Let me get Dale's post now and the other's later. I did note a "sag" on clutch engagement, almost like the engine didn't catch. I was thinking that I gave it too much throttle at too low an RPM, which slide valve carbs don't like. I chalked it up as my inexperience with small displacement 2-strokes.

Could there have been a mixture problem?

Jetting: main 160 needle-jet ?? needle 4F15 pos=3rd slide2.0 pilot 25. (new carb, diff than old).

--Bill

[Dale]

Bill,
I am not sure what was going on with your "sag" description? Are you saying that it bogs coming off of idle? Was this one time or consistent?

I don't want to focus too much on the new carburetor as a potential culprit. It is just one of many possibilities... Tell us about your fuel, oil and mix ratio.

As for the carburetor, I am running the same replacement on a 94 Wombat and it has a 30 pilot, 160 main, 4D20 on 4th notch. The slide is stamped 2.0, but has been modified to a 2.5. I have tried the 4F15 needle and did not have success with it. Each individual carburetor is going to be slightly different (not to mention differences in elevation, humidity, temperature) and I can not emphasize enough how important it is to run through a jetting sequence, with a reference indicator to throttle position, after any changes are made to a bike. With this new carburetor, is your air screw between 1 and 2 turns out for the best idle? Did you confirm that the 160 main is one step below 4-stroking at WOT? Once those 2 are set, then if you are crisp and clean with the needle on the 3rd notch, does going to the 4th notch make it too rich?

Too many questions at one time...
Dale

[Bill2001]

I don't think it was the carb (entirely)-- it ran too well. The carb was replaced because the old carb had water-corrosion inside and was erratic. Although the jetting-- needle and slide-- were different, there is an envelope of needle-slide-jet combos will work. The new carb also had differences in the venturi.

At any rate, a carb for that model should work out of the box, but may need "fine tuning".

The bottom end/case was s'posedly built by George Cone with the top end by the PO. I don't suspect leaks, the Alt and sprocket seals are good & new and no gearbox oil was lost. I'll check all that as I proceed.

All this is "IMO", advice is always welcome.

--Bill

[Bill2001]

Rolling update: pulled the sparkplug this PM-- the color and appearance are a textbook _perfect_ mixture. This after a 1-1/2 mile hiway run at 45mph. The "plug cut" was the piston seizure.

Oil is Yamalube 2R, nominally at 1:32 but mixed at 1:28 for this first tank.

My machinist/guru gets back Tues, I'm holding off doing anything pending.

--Bill

[---]

I don't think it was the carb (entirely)-- it ran too well......

Although the jetting-- needle and slide-- were different, there is an envelope of needle-slide-jet combos will work......

The new carb also had differences in the venturi..........

At any rate, a carb for that model should work out of the box, but may need "fine tuning.".......

I don't suspect leaks......

--Bill

Bill,

You know, of course, that each of these statements (except one) is an assumption. Running too well is actually a potential symptom of a mistuned carb, depending on what it is you interpreted as "running well." Running too well could also be interpreted as borderline lean, a condition that apparently you didn't verify. If it were too rich, you would think it was a bit rough perhaps, but you probably wouldn't conclude that it was running too well.

Yes, there is an envelope of differences within which any bike will work, but in the end, only one combination is correct for any given bike on a given day. Without verifying where in the envelope you are, it is only guess work whether where you are is also good enough, or will lead to a seizure. Sometimes the difference between running well and seizing is the time spent at wide open throttle, or at whichever throttle position was too lean for conditions.

Not sure what you mean by differences in the venturi, since any 24mm Mikuni is going to have the same basic design as far as the venturi itself is concerned. Differences in body casting shouldn't be all that apparent. What did you see that was different?

A carb "for that model" usually has whatever jetting the manufacturer thinks should go in it, since it could mount to anything from a moped to a go kart. Even if you get one "pre-tuned" by a Hodaka supplier, it is still just a ball park estimate of what will work, so no, I would not agree that one offered for that model will necessarily "work" out of the box. It might run out of the box, and it might even run pretty good out of the box, but unless or until you actually jet it, it isn't anything but assumed to be okay. No one knows reliably what jetting your bike requires, so no one can pre-jet a carb for you. Stock or manufacturers jetting is only roughly in the range of where it will end up based on assumption about displacement of an engine. New carbs often need more than a bit of "fine tuning."

I don't suspect leaks generally either, but as Mark Twain said, "Supposing is good, but finding out is better."

Bottom line here is that if you didn't verify tuning or leaks, you don't know if it was tuned correctly or if there was an air leak. What you do know is that it seized, so something wasn't right. There are so many possibilities, it is not really possible to point to the cause without a detailed tear down, and even then, you may not find the exact cause. Still, I would expect the consensus guess to be that it was too lean for one reason or another.

Not all seizures result in damage beyond use. Once you take it apart you will know more. If it won't kick over there is a good chance that it will need at least piston and rings. About the only thing I don't think was the cause is your fuel mix. Yamalube at 28:1 is fine.

[Bill2001]

Yep, it's subjective based on limited time on this 2-stroke engine type, but ran well enough based on years of work with other 2-strokes, and with 4-cycle engines. But when it gets up & running we'll check the jetting.

--Bill

[Bill2001]

I consulted with my "guru" and agree with y'all, we'll check for leaks, jetting, etc, as we do the repair. This shouldn't have happened since this is a new engine with just a few miles, but it did and there won't be a repeat.

What pistons do we prefer?

--Bill

[---]

Personally, stock Hodaka.

[Bill2001]

Looking at the carb slide: the old carb had a 2.5 slide which was spec'd. The new carb slide is marked "2.0" but the cutout is milled and it may be a 2.0 modded to a 2.5. The amount of the cutout is the same in both slides. What determines the "Slide #" ?

--Bill

[Bullfrog]

The height of the cut-away defines the "#" of the slide. So it sounds like you do indeed have a new slide which has been modified to 2.5 specs. I would recommend re-marking the modified slide (vibratory engraver?) to eliminate future confusion.
Ed

[Bill2001]

Already planned to be done.

--Bill

[bchappy]

Might be good to mention that more cut-away is leaner. Most higher numbers in carbs are richer, but not so with the slide. So a 2.0 slide that has been cut to a 2.5 will run leaner at about 1/4 throttle. I always thought they should have numbered them backwards but they didn't ask.