72 GT550 Compression Question

[lymbic]

Hello everyone, this is my first post. I got a GT550 a few months back and have had more spare time recently to dig into it.

A little backstory, it sat in my grandparents barn in Iowa since 1985 with oil in the tank and gearbox and no gas. It’s not pretty at the moment thanks to humidity and pests, but everything seems to be workable - most importantly the motor kicks over. I’ve got a couple of 70s Hondas, but this is my first two stroke.

I’m in the process of sorting out the carbs, airbox, spark, and compression to see what I’m getting myself into. The first problem I’ve run into is compression.

The bike doesn’t currently run so the best I can do is a cold compression test with the kickstart. I had the carbs and headers off while running the compression test on each cylinder since those needed to come off for cleaning. I figured a good reading for this bike would be 150-160 psi. After 10 kicks on each cylinder I got: Left: 200 psi, Middle 125 psi, Right: 200 psi. After the tests, I noticed the L&R exhaust ports leaking oil, but the middle was almost bone dry. I don’t think any oil was left in the top of the cylinders when the bike was put up. I think it was just oil pulled in from the pump. Like I said, I’m new to 2 strokes and am not sure what’s normal here. I understand the oil helped create a better seal for L&R - giving a higher compression reading, but I don’t get why the middle is different. Maybe a blocked line? See photos in link. https://imgur.com/a/wB9E0Jc

This is my first noob issue of many, but I’m looking forward to sorting it out. I’m not planning to keep the wind screen on when all is said and done, but keeping it on the bike for now is my best option for storage. I’m not sure if there’s a market for those, but let me know if you’re interested.

[karl pa]

Welcome, I am sure someone will be along with better idea of what might be going on with it, but I can tell you 200 is way to high. I think 120-130 is more in line for being normal. It could be excess oil being trapped between piston and head causing a hydro lock situation increasing pressures. If you remove the sris valves at the bottom front of crankcase you can drain any excess oil from crankcase. The check valves feeding oil from pump to cylinders can leak by, allowing oil to fill the crankcase, which in turn is pulled to the top of cylinder when spun over.
The main concern of 2 stroke sitting for long period is crank seals drying out. you can test this by doing a leak down test, or a quick test is to hold your hand over carb boots while turning engine over feeling for suction on hand, (this will show a big air leak), you will get good compression even with bad crank seals. To help soften seals I have taken cylinders off and filled crank with diesel fuel and blue devil seal repair, turning crank occasionally, sometimes it works sometimes it don't.

[lymbic]

karl, I didn’t know those valves existed. Makes sense though. I will try draining excess oil and a suction test tomorrow. And this is would 2 stoke oil building up in the bottom of the crankcase right, not the gearbox oil some how getting by?

[karl pa]

Yes 2 stroke oil.

[lymbic]

Pulled the sris valves on all three and sure enough oil drained from left and right for 5 minutes. Measured it out to about 250 mL from EACH. Hardly any came out of the middle.

Retested compression and I’m at: L - 137, M - 122, R - 122.

If it's leaking by at the pump, why isn't there excess oil in all three cylinders? And does this call for pulling the oil pump to check it out?

I did the quick and dirty leak check with my hand on the intake of the carb body at WOT and no headers on. Did not feel any real amount of suction on my hand at all.

[karl pa]

The check valves are in the unions bolted to the cylinder, each cylinder has 2. The pump doesn't really control the bleed through. If you have oil in left and right, one or both valves are leaking and center is sealing. But after sitting that many years it might be a small leak through, some of mine bleed through a little but not enough that I worry.
Compression should be within 10 percent on all cylinders, so the 137 is getting on the unbalanced side, but I wonder if some oil might be affecting that pressure. or the center and right might have stuck rings keeping there pressures down.

[lymbic]

I bet you're right on both fronts. A little excess oil in left cylinder and the rings still need to loosen up a bit all around.

Unless you guys think there's any red flags here, I'm going to continue on in the resto and perform compression and leak down tests again once I get it running.

[karl pa]

One thing I would suggest is, if you haven't done is drain as much 2 stroke oil, and replace with new, they say not to mix old type oil with the new, it gels and clogs the feed.

[lymbic]

One thing I would suggest is, if you haven't done is drain as much 2 stroke oil, and replace with new, they say not to mix old type oil with the new, it gels and clogs the feed.

got it on the list now