94 Wombat Exhaust Leak

[Simonkav]

Model 94 Wombat that I'm still trying to get sorted. I'm close but I think my problem is my exhaust....

Two part question: 1) Does my diagnosis sound reasonable, 2) how do I get a good seal on a model 94 exhaust

I'm not a 2 stroke expert at all....trying to learn
Recently rebuilt motor - new seals, bearings etc.
Pressure test after rebuild was good
Carb is fresh - new one from Strictly Hodaka, jetted to the stock 94A specs
New points, condensor, plug, all the other electrics are stock (I'm assuming)
Strobe timed
Starts easily, idles.
Bogs when I blip the trottle, ultimately recovers and then anything above 1/4 throttle feels sharp
I upped the pilot to a 30 from the 25
A little better, still not right
Air mixture screw all the way closed, engine runs best
Tells me air is coming from somewhere.
Intake stack is all new, new gaskets, etc.
Exhaust I can't get a good seal on the pipe to the exhuast port/manifold
Blip the the trottle, I can see small bursts of smoke coming out of the joint.
Once I'm up in 1/2 to full throttle, pulling nice, feels good, plug looks good. This is all with the air screw closed.
So, I'm thinking my vacuum is good in the cases, seals and intake side but I'm pulling too much air in through the exhaust, worst just coming off idle because there's not a lot of gas in the mix. Sound reasonable?

So, again, model 94. Has that threaded exchaust mount. I refreshed that crush gasket and the collar snugs down, used a hook spanner on it, but I can still feel play in the pipe. Any tips for getting that to seal up?

Thanks,
Simon

[TheBevman]

Hi Simon, I have an M94 as well and I take it that you haven’t made much progress since nothings been said. You’ve given a wealth of info but these things are tough to diagnose in this format. I’d like to know a few things that You didn’t state:

Have you vacuum tested the engine? Since you’re getting air it might not be a bad idea. There’s info available online, same specs too, max vac 6 lbs for at least a min (I usually aim for 3 min.) Obviously, it’s tougher to find a vac leak, I use gear oil and grease by dribbling/smearing it over the joints/seals. It’s thick enough to temporarily hold the vac as it gets sucked into the leak.

Did you clean out the exhaust since the rebuild?

When you say “intake stack is all new” what do you mean? Does your bike have a reed conversion or is it stock (24mm carb)?

More questions will probably follow too. Best advise I can give you is check everything systematically.

As far as the exhaust gasket... did you tighten it down all the way and crush it? I ask because I didn’t my first few rides and had the same prob.

Bev

[Al Harpster]

I don't know that exhaust leak is part of your problem. Unless it's belching I'd GUESS not.

It SOUNDS LIKE your pilot jet is too lean.

Screwing in the pilot adjust all the way in helps, but I'd guess the jet's just too small.

I'd try a 40, and have a 35 on hand too.

These mikuni pilot jet's (and main jet's) are $2 each at mfgsupply.com.

If you spend $20 on an assortment of pilots and mains you can tune to your own satisfaction.

That said, go richer first.

Later try leaner.

Don't forget the needle clip position when tuning. But I don't suspect the needle position has anything to do with your initial complaint here.

Yes, it's a pain to change pilot jet's. Bright side is it gives a chance to be sure they're clean.

It don't take hardly nothin' to partially clog them. I've had a partial pilot jet clog even with an in line fuel filter.

[Simonkav]

Thanks for the suggestions

1) Regarding the exhaust, I have not cleaned it with any solvents, just visual inspection and it honestly didn't look that bad. I should probably do a cleaning. If you have any suggestions on how, I'd welcome them.

2) By intake, I just mean the gaskets, block and o-ring are all new between the carb and manifold so I don't suspect they'd leak. It does not have a reed valve.

3) I've not been able to get a good crush on the gasket. I think it's because the flared end of the pipe is not perfect. I can try cleaning that up, not really sure how. Any tips for that are also welcome? Or alternatives for a larger crush gasket or some other type of sealant would help? glass rope, putty, something that might fit under the collar and really help push the pipe up against the cylinder? I'm probably over thinking this and just need to make the existing collar/pipe/cylinder mate well but again, I'm open to ideas.

4) I'll keep going larger on the pilot. Direction I had from Paul at SH was the jetting was generally pretty good. I had the 30 laying around so went up to it, don't have a 35 or 40 but I'll acquire them. Is it typical to go that many sizes over the stock jetting? FWIW, I'm in bay area, pretty much at sea level, maybe related.

[matt glascock]

You could try shimming the exhaust joint with a washer between the pipe and the crush washer so that you can properly crush the washer against the exhaust spigot. That would eliminate the problem with the misshapen flared end of the pipe.

[viclioce]

Sounds like you should do a pipe BBQ! We like to set the pipe on an open fire and let it burn up the oil & carbon inside the pipe. I’ve purchased several used pipes and they all had build ups in them. Some of them even became flame throwers while on the fire pit. Good way to clean them out, as long as the pipe isn’t chrome! Then clean the exterior after BBQ’ing and spray the pipe down with gloss or semi-gloss black engine paint! Victor

[Al Harpster]

The idle mixture screw closed says the pilot's small or clogged.

Saying this based on all the new parts and work you've reported.

Pilot jet size & condition makes pretty big changes in initial acceleration and overall run.

I'd go there first. It takes time, but the parts are cheap.

"Standard" jetting: good guideline. If you've bought a modern strictly Hodaka 24 mm, like I use, don't fear going a little richer.



The red high temp silicone available at auto parts stores may give you at least temporary relief on your exhaust gasket concern. Put some on the clean gasket & reassemble.

Exhaust baffels can be cleaned with a heat gun & time if they're chrome. Take the baffle out & look it over if you already haven't.

[Bullfrog]

Lots of "ifs" and questions here:
1. 25 pilot is "right" is for stock Wombat carb. I'd expect it to be "right" with a new round slide Mikuni unless . . .
2. Is this still a stock piston port Wombat top end? If it has had a reed valve (and appropriate porting) added, then it WILL NEED a richer than stock pilot jet - perhaps a 40???? (note - IF reeded, it will also need a richer main jet . . . and slide cut-away and needle jet/needle may need attention too)
3. Achieving best idle with pilot air screw all the way "in" and 30 pilot jet MAY indicate a partial blockage of one of the pilot circuit passages in the carb body (IF we are talking about a stock piston port Wombat top end).
4. I tend to think that an air leak is probably not the cause of your problems because there has been no mention of "run on" (engine rpms don't return to idle like they are supposed to). That being said, a pressure test of the engine is always to be recommended. Pump up to 6psi, leaking down to 5psi in one minute or less is a FAIL. Find the problem and repair it.
5. It is HIGHLY unlikely that the exhaust manifold attachment has anything to do with the running problems. However, exhaust system problems (plugged/partially plugged) are HIGHLY likely to cause engine running problems. Pipe barbecue is recommended (or thorough inspection/clean out by other method).

Hang in there - you'll find the problem and have some fun!
Ed

[taber hodaka]

As far as I am concerned you cannot tune a engine with a leaky exhaust, they can run so lean, you can ruin the engine.----Clarence

[TheBevman]

Simon, not knowing how “bad” sealing surface/ lip of the exhaust pipe is, i figured I’d share a trick I’d picked up. Old Triumph motorcycles are susceptible to having their pushrod tube ends distorted if you don’t premeasure for the correct seals. Their ends are flanged just like your Wombat’s exhaust. Years ago I made this mistake and had to correct my mistake so they’d seal again, with the correct seals, of course.

Here’s what I found if you have a wrench big enough to fit around the circumference of the pipe just below the sealing flange. You can use it as a dolly (i.e. support) to back your work as you straighten the flange with a hammer.

Let me know if that description isn’t clear.
Hope it helps.
Bev

[Dale]

A minor detail but make sure the pipe is loose at the two mounting points on the side of the bike when you tighten the head nut. The pipe should be free to move a bit as you attach the head nut. Once it is tight, then tighten down the the side mounts.