'77 Sting troubles

[greginncia]

Hi guys!  I'm a newbie here and am just getting started in vintage sleds.  I've got a good friend who is a diehard John Deere guy but when I was growing up my father and grandfather sold Scorpions, so that was what I was familiar with back in the day.  I bought a '77 Scorpion Sting that is a twin to the last new sled that we had from the family dealership.  They sold the dealership in 1978 before Scorpion went under.  It ran, but leaked around the crankcase so I tore it completely down and re-sealed it with all new gaskets and crank seals and used Yamabond on the case halves, and re- ringed it.  I tried to time it according to the service manual directions but didn't have much luck.  I tried using multiple digital ohm meters and I even had the old analog one that was used at the dealership, but I couldn't get them to behave how I thought they should when the dial indicator was in the appropriate place.   I marked the stator before I removed it so I replaced it exactly as it was before I tore it apart.  It started well and ran well on the stand but there has been little snow in north central Iowa this winter so I had to wait till we went to Wauconia to give it a good test ride.  At Wauconia I ran it around the trailer a little while as a test run and it ran great.  As we started to ride around the lake, I made it about half way around when it essentially seized up.  It was running great and as I slowed to round a corner it slowly died, and when I tried to start it, the rope wouldn't pull.  I could just about hold my hand on the fan side cylinder, but the PTO side was sizzling hot.  We towed it back to the trailer and let it cool down and I rode one of my buddies spare Deere sleds.  Later in the afternoon when it had completely cooled down, the motor would turn over like normal but wouldn't start.  I've got spark and compression in both cylinders but it will fire a bit but just not enough to take off. 


I rode the spare sled the rest of the weekend because I wouldn't have trusted it even if I had gotten it started again.  I know the Cuyuna's are noted for burning pistons on the PTO side.  I swapped pistons when I re-rung it because the PTO piston had a slight burn mark in it.  I farm for a living and work on diesel engines regularly.  This two stroke stuff is a whole new thing in my world!  Am I looking at a timing issue?  I'm running 91 octane gas with Klotz oil mixed at 40:1.  I need to go through the carb yet, but it ran well for the 8-10 miles that it ran.  It seems to flood very easily, but when it ran, it ran very well.    I bleed John Deere green when it comes to farm equipment, but I was hoping to be able to make the old Scorpions run again due to my families history with them.  Hopefully I don't have to start shopping for John Deere sleds too!  Anybody got any advice?

Greg

[shreddagnar]

What was your ring gap set at when you replaced the pistons? Did you pressure test the crankcase? It could be center seal, timing, air leak? Check compression again.

[shreddagnar]

More than likely just needs timing

[greginncia]

I did not check the ring gap. There is no mention of this in the service manual that I used from 1976. The center seal looked good but I did not pressure test it. I've since found out how to set the ring gap and pressure test it in late Cuyuna Ultralight manuals that I found online. I think I need to get a timing beeper to have a bit more confidence in how to set the timing. With my digital ohm meters and even the analog one I couldn't get them to change consistently in the same place according to the dial indicator.

[shreddagnar]

The seizing up during your ride and starting after cooling down leads me to believe your ring gap is to small and closing when the rings heat up. That is only my guess and others may chime in with more experience. I'm new to these engines myself. The flooding and the temperature difference could be caused by a timing issue. You may be running on one cylinder. I'm having the same problems with my 77. I ran on one cylinder and could tell because full throttle was only 35-40 mph. I melted the PTO piston. Not good but lesson learned. I would remove your cylinders and set your ring gap to .007-.031 in. I think mine were .013. Then time correctly.

[weverat]

For timing, I disconnect the wire harness from the engine. The connector from the engine has a yellow, yellow w/green stripe, 2 black wires, and a green or brown wire.

The 2 black wires come from the points and go to the kill switches. 1 is for each cylinder, when you touch these to each other it kills the engine.

Hook your buzzer devise to 1 of these black wires to check one cylinder, and the other black is for the other cylinder.

If you have an analog multi meter, it will have a buzzer on it. The digital multi meters have a buzzer also, but they are not sensitive enough, you see there is always continuity to the condenser, so your buzzer is only going to change notes, it will not be on or off. You could use a light bulb and a battery for this, and it would get bright and dim.

Now for the best part, you get to stick your finger in the flywheel in order to hold the little lever into its fully advanced position, as you rotate the flywheel and read the dial indicator to see when it is firing.

1 thousands of point gap will change the timing 5 thousands. Set both points at .014 to .016 to start with. You can move the stator plate to get 1 cylinder spot on.

In order to adjust the other cylinder, change the point gap by a thou or 2 as needed.

It is really nice if both sets of points are around .015 when done. It is normal to have one set at .016 and the other at .014

Your timing can be way retarded and it will still run good, way advanced and you will burn a piston. I use NGK B9 plugs just to be safe.

[weverat]

Couple more things I have found, wipe your feeler gages clean with contact cleaner and a paper towel as not to get the points dirty.

Cut a slice of white business card paper about the same size as a feeler gage, dampen the end of it with contact cleaner and drag it or snap the points on to it until perfectly clean.

When, and if you get that dam flywheel off, file and sand the key, and the key way so there are no burs or snags. The flywheel goes on first, then tuck the key in there.

The key is only there to locate the flywheel into position upon assembly. The big nut and tapered shaft hold that bitch from turning.

[greginncia]

Thanks for the info weverat. I may try to set up a battery and a light bulb to determine when the points break. I think with your explanation I can hopefully make sense of it now. I tried multiple digital ohm meters but was looking for no continuity when the points opened. That's obviously not going to happen with the condensor. The old analog one that I remember Dad using in the shop doesn't appear to work consistently like it should. I can't imagine why, it's only 45 years old!

I've had the flywheel off and filed and cleaned the points. I'm currently running NGK B8 plugs. Should I try a B9 on the PTO side? Do you think the fact that I didn't check the ring gap could be causing it to sieze? Hopefully I get a chance to work on it again soon. It sounds like we might get enough snow to ride this week. My daughter certainly hopes so. We may just have to ride the modern sleds, but I'd sure like to take the Sting for a short run!

[weverat]

Usually with the modern aftermarket pistons, if your piston skirt to cylinder wall clearance is .005 to .007 you will have more than enough ring gap.

It is always good to check to make sure you have at least.005 ring gap per inch of bore. Our 440 pistons are 67mm or about 2 1/4" so we need at least .011 ring end gap. I usually wind up with .025 to .035 which sounds like a lot, but with 2 rings it is perfectly ok and normal.

Back in the day, the piston manufactures sent rings that fit with a minimum of clearance, with the thought that we would file to fit. Modern day, here is your rings, and by the way, they have a giant gap!

You could use rings from a piston 1 size over, and then file these to fit more snuggly, but then you would have rings with a lot of tension. I have no problem running a large ring gap.

With the single ring racing pistons, it helps to fit them much tighter.

Yes, I always use B9 plugs in the Cuyunas, only switch to the B8 if you have a problem with fouling.

Try to keep that engine cool, air vented with duct work into the fan, and vented out from under the hood.

[greginncia]

One more dumb question. How do you set up the battery and light bulb to take the place of an ohm meter? Positive from the battery to positive on the light, then negative from the light to the black wire on the engine harness and negative on the battery to the engine case? Every time I think I've got my head wrapped around this I start thinking about AC voltage in the house and come up with a different way to hook it up. Thanks for any help you can offer.

[weverat]

Ah shoot your making me think now, and I have already had a couple beers ( good ones) and am into my first screw driver!

You know what to do.

I am pretty sure one of those 20 dollar analog multi meters fro fleet farm will have a good buzz on