Need Some Diagnostic Logic

[KitV]

After experiencing a mild piston seizure on my '73 Wombat as the result of failing to rejet the carb after moving from Bend, OR to Medford, OR (i.e. 3,800 ft. to 1,400 ft.), I cleaned up the piston,checked the compression & of course put in a larger main jet. All seemed good. (I only provide this for background information).

The problem: When running the bike for the first time since the seizure, it ran fine for about 10 miles on city streets at about 3/4 throttle. However, when pulling up a modest hill, the bike seemed to bog down slightly. So I dropped down a gear. Still seemed sluggish. Once I crested the hill, the bike ran for about 100 yards and without warning the engine died. All attempts to start couldn't get it to fire.

In the garage, I confirmed a healthy spark. The points gap was a tad tight but likely within spec. Nevertheless, I regapped the points. Compression was about 150 psi after squirting a bit of engine oil in the spark plug hole.

Out of curiousity, I pushed down on the kick starter & the engine lit off immediately. I was a bit dumbfounded since the only thing I did was to open the points a tad.

Assuming that the cause of the engine failure was either gas or electrical related, I would appreciate your thoughts as to the possible culprit. BTW, I removed, disassembled, & thoroughly cleaned the carb. Nothing seemed stuck or plugged.

Thank you or any guidance you can provide.

Kit Vercella

[matt glascock]

You may have failing condenser. As the ignition system heats up during operation, the condenser fails and the bike won't fire no matter how heroically you kick it over. Then the bike cools off and...presto! The bike runs again. This annoying cycle repeats itself until you change the condenser. Also, make sure the vent hole in the gas cap and/or gasket is not obstructed. If there is a blockage, a vacuum will be created as the engine pulls fuel from the tank ultimately shutting off fuel flow and starving the engine. Then, pressure will slowly equalize to atmospheric and fuel flow will resume and the engine will run again. During the gradual decrease in fuel flow as the vacuum is developing a lean condition will exist in the engine - never a groovy situation in a two-stroke plant.

[Dale]

Two thoughts Kit. It is possible that you have gone too rich. I recently went too rich on a Wombat after a rebuild and it would bog and die under load. Change the plug and it was good to go for a while and then repeat. What method have you used to select your main jet?
Another possibility is a faulty condenser. They can fail once hot and be okay after cooling off. If it continues to die when hot then the condenser could be the culprit.

Edit: Matt and I must have been typing at the same time... it is unanimous, it must be the condenser!

[matt glascock]

We're goin condenser, Dale.

[KitV]

Matt & Dale,
Thank you so much for your thoughts. I have a spare condenser so I can eliminate that possible cause. Will check the gas cap vent as well. (Of course that is way too obvious, which explains why I overlooked it. .

As far as the possibility of a rich mixture is concerned, I must admit that it was tedious trying to dial in the correct mixture at the lower elevation here in Medford. I used the "Ed Chesnut method" of carb tuning which entailed systematic trial runs with the bike starting with a #240 main. From there I decreased the size of the main jet incrementally until the bike indicated the symptoms of a lean mixture. At this point, going one size large I settled on a #220. Admittedly, the bike ran a bit rich at wide-open throttle. But I knew from experience that the piston would seize when running a #210.

So, let me replace the condenser & check the gas cap vent and I'll report back. Thank you again for your thoughts. Of course, if anyone else has any suggestions, I would truly appreciate them. Leaving the house on the bike & subsequently calling my wife to rescue me is getting to be way too humbling.

Kit

[Bullfrog]

Um, that is not exactly the method I recommended. I kind of wince at the thought of finding lean then going one step richer.

Find rich. Then go down one size of main jet until "rich" just goes away.

Fuel cap gasket plugged - or failing condenser.

Gotta run.
Ed

[Dale]

Kit, Is your Wombat stock? If so, 220 should be way too rich. If you can find a nice quiet country road with a stretch of a slight incline, you can test in 4th or 5th and find the correct main jet that is just one step under too rich at WOT as Ed described.

Do you recall which main jet you used before you moved to a lower elevation?

[Bullfrog]

Its an 03 engine with a 28mm carb?? (am I remembering that right?)

Hey! I just noticed that I ASSUMED you had spark with the same plug that was in the engine when it died. If that assumption is correct - then I'm sticking with my previous comment. But if you had to use a new plug to get spark, then I want to see the dead plug.

Ed

[Dale]

My statement on main jet size was based on the assumption that you have a chrome tank Wombat (model 94 or 94a). You did state that it was a '73. Either way, the theory is still the same but if you are working with the later Wombat (model 03), the jet sizing would be larger.

[KitV]

Kit, Is your Wombat stock? If so, 220 should be way too rich. If you can find a nice quiet country road with a stretch of a slight incline, you can test in 4th or 5th and find the correct main jet that is just one step under too rich at WOT as Ed described.

Do you recall which main jet you used before you moved to a lower elevation?

Dale: The Wombat is stock except for the ported cylinder & reed valve. The bike ran excellent when I lived in Bend (elevation 3,800 ft.) with a #210 main jet. When I moved to Medford (elevation about 1,600 ft.), I spent a few hours on a desolate paved road with slight incline running the bike with various main jets starting with a #240. Ironically, the bike exhibited a rich mixture until I ran it with a #210 (the same size jet that ran perfectly at 3,800 ft.). Being somewhat dumbfounded, I consulted with the Oracle of Milton-Freewater (Ed Chesnut) who suggested that the high humidity level in Medford might account for the #210 jet being correct at the two different elevations.

Unfortunately, on the first run after rejetting, the piston seized on a long uphill stretch. The seizure was mild so I was able to limp home under power. Thus, I installed a #220.

[KitV]

Its an 03 engine with a 28mm carb?? (am I remembering that right?)

Hey! I just noticed that I ASSUMED you had spark with the same plug that was in the engine when it died. If that assumption is correct - then I'm sticking with my previous comment. But if you had to use a new plug to get spark, then I want to see the dead plug.

Ed

Ed: It's a Model 95 with a 28mm carb. Not sure how to identify if the engine is an 03.

Yes, I did have spark with the same plug when the engine died.

Kit

[KitV]

Ed, Dale & Matt: Thought I'd give you an update on the Wombat issue that I originally posted. I replaced the condenser & checked the gas cap vent hole. With a certain amount of trepidation (actually, a lot!), I ran the li'l bugger for about 45 minutes straight up & down a continuous ribbon of hills. Ran great. So, maybe we resolved the issue. But considering that I ended up dead on the side of the road at least 3 times in the past year, it will take a bit of time to reinstill full confidence. Anyway, thank you guys for your help. It was truly appreciated. BTW, while installing the new condenser, I had a wiring question. But I'll ask that in a separate post. Thanks again!

Kit

[Bill2001]

Good discussion. I'm going through the same trepidation getting my W'bat back running after a piston seizure.

--Bill

[matt glascock]

Great news, Kit! I seized my Road Toad about ten miles from home last month and had to engage my mother to provide the "tow of shame" (which, ironically, is exactly how I got back to the pits on my last hare scramble on my modern bike less the mom part). I feel your pain, but now that you have it schooled, you're going to have a blast!! I never fail to have a big smile on my face every time I fire up one of my Hodies. They are riotously fun to ride and race.

[KitV]

Well, Matt, undoubtedly I feel your pain. Not being a Harley rider, I am unaccustomed to bringing my bike home in the back of my truck. (That was a cheap shot I couldn't pass up ).

Just for a bit of background, when I built the Wombat from several boxes of parts I acquired from Chuck Swanson, the bike seized on its inaugural run (going about 50mph in a right-hand sweeper). Thanks to the trained eye of Ed Chesnut, we noticed that the cylinder head had been shaved apparently to increase compression. After some cylinder work, a new piston, ring & cylinder head, the li'l bugger ran great. That is until a friend of mine on the OR coast asked me to come down for a ride. Don't even ask me how I overlooked going from 3,800 feet in elevation to sea level. But I did. Without rejetting, the bike ran for about 1/4 mile before another piston seizure. Two seizures in 2 weeks.

Kit

[dirtsquirt74]

Your post had good information. I am one of those people that live at the 1,000 feet in Eastern Washington State. Over the years I have listened to racers going to other locals and talking about rejetting their carburetors. Seemed a little silly to me at the time but so true. We have the inverse problem. As an example, the Hodaka Days trail ride takes us to higher elevations. Now I understand why my 03 Wombat runs so rich and loads up on that ride. Thanks for the post.

[KitV]

You're welcome.

Kit

[matt glascock]

Hey Kit, This brings up an interesting thread. Jetting is really crucial for optimizing the performance on these small-displacement 2-stroke motors. Here in Iowa where we have a very wide range of temperatures and humidity as the seasons roll on jetting becomes a constant, dynamic process. I race a reeded '71 Super Rat that absolutely rips. The temp and humidity can change to the extent that I have actually rejetted between races and on occasion, between motos. There is an exceptional piece on this site written by tuner-extraordinaire Harry Taylor on carb tuning that is well worth the read. On many Hodie models, there is a quick change banjo bolt/jet set-up that allows for easy main jet exchange. On others (Aces, Road Toad, Dirt Squirt eg) you can only approach the main jet through removal of the float bowl. I'm wondering if any of our advisors here on the forum are aware of a float bowl mod that allows access to the main jet without removal of the float bowl.

[KitV]

I believe that I read Harry's piece on carb tuning. Nevertheless, I will read it again.

The Mikuni on my '73 CZ 250MX has a threaded plug on the bottom of the float bowl sump. Remove the plug & swap out the main jet. That would be a nice option on the 28mm Mikuni on my Wombat.

Kit