717 Top End Rebuild? Need Advice

[Papa Romano]

Hello, I have a 1997 GSI with the 717 engine, single carb. I have been having issues with it all summer after a carb and fuel system rebuild. I still can't get it to run correctly (starts immediately but takes a long time to accelerate, bogs, doesn't maintain top speed). After short rides, I've noticed the engine and air intake cover will be wet with oil, and I suspect it's coming from the engine cylinder base gasket because I see a dark spot of concentrated oil there. I have been focusing all my attention on the carb and fuel system, but now I'm wondering if I need to do a top end rebuild to fix my acceleration/bogging issues. When I checked compression at the beginning of summer, it was 140-150 psi. The last time I rode it, I had the high/low speed screws at factory settings and it would not want to run above 4500 rpm. I feathered the choke and it took off, so it was running lean but I could not seem to tune-out the bogging issues making it more rich in 1/8 increments. I have been thru 2 mikuni carb kits and 2 fuel system rebuilds with little prolonged success.

- Is it possible my issues are coming from the cylinder base gasket leak?
- Can I get away with just replacing the gaskets?
- If not, how will I know to replace the cylinders?
- Is there anything that absolutely can't be reused after it's removed?

I have never worked on small engines before. I want to just crack it open and see what the condition is, but want to know if there's anything important to know before diving into it so I don't screw it up. I appreciate all opinions and advice. Thanks in advance!

 

[burtshaver2021]

Is it possible my issues are coming from the cylinder base gasket leak?
- Can I get away with just replacing the gaskets?
- If not, how will I know to replace the cylinders?
- Is there anything that absolutely can't be reused after it's removed?

Yes,,it’s very possible that the issues are coming from the cylinder base gasket leak, I don’t think you can tune out an air leak. Yes, you can simply remove the jugs, replace the gasket with the same gasket that came out and put it back together as per the service manual
When will you know when it’s time to replace the cylinders? I’m thinking you mean when is it time to rebuild the top end with new pistons and rings and a bore? When your compression gets low, to around 120 pounds, your compression is fine at 140, 150
There’s not much else I can think of, you won’t be removing the wrist pin so you won’t need to worry about the careless needle pin bearings falling into the crankcase, I guess the only advice I could offer is follow the manual for torque values and thread lockers,

 

[Papa Romano]

Yes,,it’s very possible that the issues are coming from the cylinder base gasket leak, I don’t think you can tune out an air leak. Yes, you can simply remove the jugs, replace the gasket with the same gasket that came out and put it back together as per the service manual
When will you know when it’s time to replace the cylinders? I’m thinking you mean when is it time to rebuild the top end with new pistons and rings and a bore? When your compression gets low, to around 120 pounds, your compression is fine at 140, 150
There’s not much else I can think of, you won’t be removing the wrist pin so you won’t need to worry about the careless needle pin bearings falling into the crankcase, I guess the only advice I could offer is follow the manual for torque values and thread lockers,

So i opened up my engine just now, and I see the pistons looking like this. I know I ran lean the last time I rode it, but this looks much worse than I was anticipating. Wondering what your thoughts are, if the conditions of these pistons are okay, and that they are so different. Thanks

 

[burtshaver2021]

I don’t know how to read piston wash at all, if I had to guess I would say the clean piston has had water in there but that’s just a guess. Do you see or feel any scratches with your finger nail? I would measure the bores to see where you stand, your going to be taking the cylinders off to change the base gasket right? At that time measure the ring end gap, if the ring end gap is still within spec and there is no scratching then I would say your fine to re use those pistons and cylinders, if the ring end gap is out of spec. And you don’t want to invest in a mic and bore gauge you could just order new rings and see what the ring end gap measures on them. I would do a leak down test to test the outer crank seals once you have it back together.

 

[Papa Romano]

I don’t know how to read piston wash at all, if I had to guess I would say the clean piston has had water in there but that’s just a guess. Do you see or feel any scratches with your finger nail? I would measure the bores to see where you stand, your going to be taking the cylinders off to change the base gasket right? At that time measure the ring end gap, if the ring end gap is still within spec and there is no scratching then I would say your fine to re use those pistons and cylinders, if the ring end gap is out of spec. And you don’t want to invest in a mic and bore gauge you could just order new rings and see what the ring end gap measures on them. I would do a leak down test to test the outer crank seals once you have it back together.

So I think I found the culprit, I had 2 very loose jug screws right next to each other, and all the gaskets are toast. I have the cylinders off and I don’t feel any scratches. Everything seems very smooth. There is just some side to side free play with the rings and the piston heads. Not sure how much is normal without trying to measure.

 

[burtshaver2021]

you could measure the ring end gap with a set of feeler gauges. If that checks out, you could put the same base gasket on that came off, squirt some 2 stroke oil on the rings and piston skirt, cylinder wall, torque everything to spec using whatever is called for in the manual for thread locker and see how it goes?

 

[Papa Romano]

you could measure the ring end gap with a set of feeler gauges. If that checks out, you could put the same base gasket on that came off, squirt some 2 stroke oil on the rings and piston skirt, cylinder wall, torque everything to spec using whatever is called for in the manual for thread locker and see how it goes?

The base gasket is in pieces so I can’t reuse it, but I bought a gasket kit from OSD, so I’ll be replacing all the gaskets anyway after cleaning up all the parts that came off. I have feeler gauges at work so I’ll be able to follow up on the rings in a few days.

Looked like there were some water droplets in the area surrounding the piston connecting rod sitting above the oil. I guess the 2 loose jug screws were not only letting in air at the base gasket, but they were also letting in water from the cooling system. So I would agree that your previous statement of water getting inside is accurate!

 

[mikidymac]

Front piston is rich or water ingestion, rear looks lean.
I would have the pistons and cylinders measured with a bore gauge just to make sure they are good since you have everything apart.

 

[burtshaver2021]

The base gasket is in pieces so I can’t reuse it, but I bought a gasket kit from OSD, so I’ll be replacing all the gaskets anyway after cleaning up all the parts that came off. I have feeler gauges at work so I’ll be able to follow up on the rings in a few days.

Looked like there were some water droplets in the area surrounding the piston connecting rod sitting above the oil. I guess the 2 loose jug screws were not only letting in air at the base gasket, but they were also letting in water from the cooling system. So I would agree that your previous statement of water getting inside is accurate!

Sorry, when I said the same base gasket, I mean a new one with the same amount of holes as the old one, if that was visible when you took it apart? I’ve had 3 of the 718’s apart and the base gasket has always been 5mm ( 5 holes) not telling you that yours was a 5mm but it’s a pretty safe gasket to go in, worst that could happen with it, is you may have slightly lower compression if the one you took out happened to be a 4mm

 

[Papa Romano]

Front piston is rich or water ingestion, rear looks lean.
I would have the pistons and cylinders measured with a bore gauge just to make sure they are good since you have everything apart.
View attachment 66434

Thanks, that’s a helpful visual. I agree I’ll take some measurements when I get ahold of some tools

 

[Papa Romano]

Sorry, when I said the same base gasket, I mean a new one with the same amount of holes as the old one, if that was visible when you took it apart? I’ve had 3 of the 718’s apart and the base gasket has always been 5mm ( 5 holes) not telling you that yours was a 5mm but it’s a pretty safe gasket to go in, worst that could happen with it, is you may have slightly lower compression if the one you took out happened to be a 4mm

Ah I understand now. Unfortunately it looks like the old gasket is too far gone to see the holes completely. I think I can see the edges of a couple holes, but not all of them. The kit came with a 0.5mm, so I will run with that.