Figured out why compression dropped on SPX

[One1]

My "new" ski had been rigged to start and show compression to be sold. It tested above 120 on both cylinders, started, ran, and sounded fine so I bought it. Came home, it wouldn't start.... Checked the compression again and it was 90. This was 45 mins later. Sold by a store as is. I have to find a stock front jug and piston and get this season out of it, then I'll build it from scratch. Now to find one.

Here's why......

(Click for larger)

 

[Digdog]

Check the piston clearance on the other cylinder also, too much slack and it can hook the port just like this one did. I would also rebuild the carbs before putting it back into service, look over the oil system also.

 

[etemplet]

I have a couple of pistons from an SPX 717 and the rings might be what you want. Cheap fix Piston size is 3.224" I can check the rings for you. I could't get the freakin head bolts off and broke 5. I'm doing a full top end and making it tight.

 

[racerxxx]

I have a couple of pistons from an SPX 717 and the rings might be what you want. Cheap fix Piston size is 3.224" I can check the rings for you. I could't get the freakin head bolts off and broke 5. I'm doing a full top end and making it tight.

That's not a 720 engine, it's a 657.

 

[etemplet]

They all look alike to me. Ha ha.

 

[One1]

Piston and ring i can handle. Hard to find a jug with stock bore that isn't worn out or brand new. I need to keep things close between the cylinders

 

[mikidymac]

You could do it correctly and have both cylinders bored to new oversized pistons.

 

[etemplet]

Yep... may as all do it right while you have it apart. I purchased 2 cylinders from Nick and Jess and will rebore this week.

 

[One1]

I don't see a need to quote my post.