Sudden G1 engine failure

DirtDiggler

After running perfectly for several months since rebuild, my G1 suddenly died this weekend. While starting up, it made a light bump noise, died and then would spin over and almost start but not all the way. After checking all the basics, a quick compression test showed only 30psi, 55 psi wet. When I tore it down, there are major groves on the piston with ,of course, matching grooves on the cylinder wall. Both are ruined. I thought a ring had broke- but there is NONE of the ring missing though they are worn in two at the groove. I cant figure out what happened- good air cleaner system, no missing screws in carb, no debris in the crankcase. When I assembled the engine, everything was cleaned very well and I had checked the aftermarket jug for for slag or loose casting pieces. There is no evidence of impact on top of the piston or under the head. Any ideas what happened? Also, any recommendations for jug kits or are they all pretty much the same?

Attached Images

piston pic-1.jpg (55.3 KB, 27 views)

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chuddly

if you look at the pistion (at least in the pic) it looks like you were getting some blow by for quite a little bit (see the burnt part of the piston around where the scar is) and it looks like it had been doin that for a while. Is the scar at the end of one of the piston rings? Some times if a ring breaks/ chips off then the debris goes on through it makes it look like a wear but im still suspect of the rings in this case.

DirtDiggler

The ends of the rings are on the other side. There is a matching dark colored area on the other side over the wrist pin. I have not seen very many 2strokes apart so I wasnt sure if that was a problem. I have looked at it - a lot- and I just cant see any of the ring missing. But, I agree the scar looks like a piece of ring caused it. Amazing thing is, this thing ran really, really good. Made several 4 mile trips at wide open throttle, used it daily on my farm. I recently clocked it at 26mph with my GPS. Obviously, it took a while to do this, I just cant believe this problem has been there.

BigBlockMan

Look in the ring groove, Is the pin still there that keeps rings from spinning?

DirtDiggler

Yes! They are both still there.

GMB74

Something got caught in there. Is this damage in the area of the transfer ports? Did something come up from the crankcase and get jammed in there? Check your main bearings carefully. Do you have a bearing cage coming apart? Maybe a piece of intake reed valve?
Another possibility is ring clipping. If you have disconnected your governor, you can over-rev your engine. Under compression, the rings expand to press against the cylinder. As pressure drops, they contract into the grooves and ride over the intake/exhaust ports. When the engine is over-revved the rings don't have time to pop back into the grooves and can get caught or "clipped" by the edge of the ports, causing a piece of the ring or edge of the port to break off.
Just some thoughts.

DirtDiggler

I am not sure what the transfer ports are- if they are the oval shaped ports in line with the exhaust and intake then yes- the damage is in the center of the largest one. The port edge has slight damage but I think it was rubbed off, not broken off. I just removed a ring and found that it matches the width of the groove pretty close. You might be onto something here- the governor is still connected but I had a lot of issues with clutching when I first got the cart running and I bet I overrevved it while it was setting on the jackstands and it just now showed up. Thanks- any recommendations on jug kits?

rokcrlr

Yamaha bravo snowmobile engine. Read a thread on hear and it sounds like a great swap to me. ignition parts are similar , you can run the engine backwards for reverse still and have about 3 times more power. Just an idea.

sho305

Most 2 strokes get lean and burn, and always at the exhaust port. They will melt the top edge of the piston and put lines down it in front of exhaust port. Since that is near the pin it is the side of the piston, maybe transfer port. That looks like a piece of something got in there to me, my best guess. I would also check the reeds, crank bearings, etc, make sure nothing chipped off the piston skirt. Usually if they are steel bore they don't mess up the cylinder that much, you get another piston and hone it some, slap it back together. Does the head have any marks in it? Dump the exhaust pipe out see if anything is in there. Even check the inside of the air box/filter did something break off/etc. You can swap a bravo engine (250cc) in it they are faster, in particular if you keep the pipe on it. You have to mount it up and install the G1 ignition pickup at the crank. If the bottom end is ok you can flush it and put new/repair the jug really they are not that pricey. I recall many years back I paid 180 for pistons and rings for a twin yami sled, from weisco, and still had to get it bored.

GMB74

As far as I know, all aftermarket upper end kits are made in China, probably the same manufacturer. Buy a kit from one of the site sponsors- support the site and have peace of mind that you are buying from a reputable dealer.

Here's a link to the Ultimate Solution:

http://www.buggiesgonewild.com/big-b...lockin-g1.html