Kawi2strokes.com Forum

I recently purchased a Mit-T Vac and did the crank seal test. It failed miserably!

----PSI - VAC
LH - 5 - 5.5 for 2 min
CTR 0 - 0
RH - 3 - 5 for 30 sec

So it looks like I'll be getting that crankshaft out for a rebuild this fall!
Currently the engine has quite a but of resonance and surging at light throttle. Should I use this opportunity to swap out those pistons with the cut skirts or do you guys think the crank seals are the main cause for the surging? My plan is to run stock carbs and mufflers so the current modifications are porting, cut piston skirts and squish bands.

In general, surging is caused by a lean mixture and not related to crank seals. I found that If I richen it enough to eliminate surging my fuel economy is lower than I'll tolerate. My cure is... (see signature below) I average about 30 mph with my H2, with H2R porting, 34mm carbs, and chambers.

I'm confused by your center cylinder reading of essentially no sealing. If that's true, the cylinder next to it can't be sealing either, yet both outer cylinders are not terrible.

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If it surges, that's normal, upshift.

It wasn't totally zero Jim.
I could pull it down or push it up but the needle just returned to zero in very little time.
The resource site quotes healthy seals as holding 6 psi for six minutes.
You say the outer cylinders aren't terrible, what would be the minimum you would accept?

I suspect you had a leak in one of your plugs for the center cylinder when you did the test. As I said above, if one of its seals is bad, the cylinder next to it must also be just as bad, but your numbers disagree.

I don't have acceptable numbers for you. If I tested my crankshaft and one cylinder held 6 psi, both vacuum and pressure, for 5 minutes instead of 6 I would not get the crank rebuilt, but I can't tell you from experience when slight leaks become a big enough problem to make a rebuild necessary. Things like the RPM not returning to idle speed when you let off the throttle are signs it's a problem.

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If it surges, that's normal, upshift.

Thanks for that.
Just to confirm that the RH cylinder would not pump up past 3 PSI, and in 30 secs it was back to zero.
The most vacuum I could pull was 5 lbs and again it was back to zero in about 30 sec.

Have you measured the big end connecting rod radial bearing play? My crankshaft would have needed rebuilding for that reason alone, and its seals were shot on top of that.

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If it surges, that's normal, upshift.

I'll just post this once again, for those whom missed it all.

Testing is not only vacuum, but also pressure, as the seals must seal both directions.

The specification for our engines is 6 for 6. 6 in/hg of vacuum for 6 minutes, 6 psi pressure for 6 minutes.

The only issue is, earlier triples used an open labyrinth seal, one that was changing pressure/vacuum loaded onto the crank race, everything else uses a positive pressure seal, one that keeps seal lip pressure on the crank race all the time

BUT, testing all our triples should be all 3 cylinders tested, both ways, at once, not individually

Haven't got that far into it yet Jim.
There was much that went on with the engine when the original owner had it. The bike shows a little more then 19000 miles currently. It came with a Clymer manual with the following hand written notes:
15620 miles -1977 - Engine was ported. Raised Exh to 35 mm from top and removed 3.5 mm from intake piston skirts.
17130 miles - 1978 - Re-sleeved the ported cylinders using (74/75) sleeves, New rings and pistons. NOS Crank installed.
17200 miles - 1978 - Milled heads .030, cut squish bands.
Then it looks like he rode it for 1800 miles and it got parked.
The 2nd owner just stored it for 10 years and then I got it.

I think the engine had been dormant for 30 years or more until I fired it up in the driveway this Spring.
So it will be interesting to see what the mechanical condition of the crankshaft is. The Rubber seals are probably suffering from 3 decades of dry rot.

Interesting.
I did my cylinders individually as the resource site says it can be done that way with rubber seals.