Low compression

[rusty nail]

I am new to 2 stroke engines but love the idea of tackling a project. I am on this forum now what is probably a simple question but i do not know the volume of the answer. I have a 951 engine that i purchased this way and would like to "rebuild". So this is what i am not familiar with two stroke engines.......does this mean a full replacement of pistons and doing a complete oversized bore of the top end? Or more simply, honing the existing cylinders and installing new rings? I have not opened the engine yet to see the damaged that has happened so my question may be answered when i start the work, but is there a more simple option if the damage is not "severe" whatever that means.

Thanks,

 

[rusty nail]

is there no one that can answer my question.....any help is welcomed


thanks

 

[etemplet]

How low is the compression? What you do to the cylinders depends on their condition, size, out of round and taper measurements. You'll find varied opinions on what should be done. It's very difficult to give good advice long distance. My opinion... on an unknown engine... safest thing is doing the whole engine up right. Full rebuild.

 

[AKnarrowback]

What compression do you have in the cylinders right now? Is the motor in operating condition? If it runs does it run well without any problems?

If the crankshaft is still good then rebuilding the top end on a two stroke is not too complicated, but can be disasterous if corners are cut. How much work, bore vs hone vs new pistons, depends on how much damage is done.

Give us some insight as to the current condition of the motor.

 

[mikidymac]

I can tell you the 951 is a highly stressed engine and a used 951 crank will not likely survive a new top end.

You will be well advised to do a full rebuild including crank, balancer, reeds, pistons rings, bore and carbs

 

[etemplet]

I'm doing one right now.... what you wanna know?

 

[etemplet]

Here are a few pics of the starter job that went awry. It's been my experience that anything below 140psi has a reason. This one of course was 113psi. Caught this ski just before it blew up. Lots of people think the rings are just worn or the cylinders got a little scouring. Well..... I noticed snap rings were used to hole in the piston pin on the PTO Cylinder. Of one was missing so that accounts for the scoured cylinder on the side. As for the top.... I think we need to look at the carbs. Is that lean or....

 

[drjack]

Miki, you say it's best to do a full rebuild. Roughly what will it cost to purchase all those parts you suggest?
Or is it more advantageous to seek a motor rebuild exchange with a reputable company.....or is there a reputable company that's trust worthy?
New engine purchases are out of the question since there are none available, I believe?

 

[Sportster-2001-951C-Stock]

My carbed 951 blows ~120psi last time I checked it, still runs fine. The injected version of 951 runs higher compression than the carbed version.

120psi makes me a little nervous. I don't think you'll ever see 140psi on a carbed 951, that's like a brand new motor and even then very good. 132~135psi is more like a healthy number for carbed 951.

ALWAYS confirm your compression gauge is correct if there's any doubt at all, new one's aren't to be trusted out of the box.

 

[Sportster-2001-951C-Stock]

Here are a few pics of the starter job that went awry. It's been my experience that anything below 140psi has a reason. This one of course was 113psi. Caught this ski just before it blew up. Lots of people think the rings are just worn or the cylinders got a little scouring. Well..... I noticed snap rings were used to hole in the piston pin on the PTO Cylinder. Of one was missing so that accounts for the scoured cylinder on the side. As for the top.... I think we need to look at the carbs. Is that lean or....

Looking at those pics, I'm surprised it was blowing 113psi, pretty crusty looking! Yep, lean I'd say, looks like the piston crown was running hot. I've seen water leaks do a good job of washing piston patina as well but it's much more common for carbs to be compromised on the lean side.

If it hesitates, better find out why (usually carb issues or fuel line air leaks creating a lean condition).

 

[mikidymac]

Miki, you say it's best to do a full rebuild. Roughly what will it cost to purchase all those parts you suggest?
Or is it more advantageous to seek a motor rebuild exchange with a reputable company.....or is there a reputable company that's trust worthy?
New engine purchases are out of the question since there are none available, I believe?

Honestly from what I have seen I don't trust any shop or place like SBT for engines. But that's just me as I have the skills to do a better job than the shop and use only OEM parts not chinese parts. For me to rebuild an engine using OEM parts is about $1,000 and no warranty and I only have Group K do my cylinder boring. When I am done it is the exact quality that it left the Seadoo factory in so I expect to get another 20 years out of them.

 

[etemplet]

Looking at those pics, I'm surprised it was blowing 113psi, pretty crusty looking!

I've been surprised at the compression numbers these engines put up even when damaged. but... If that top ring is in place.... it will give you a decent number. I've learned not to bank on compression readings. LOL The grooves in the cylinder walls were pretty deep. I'd be .020" at least. I ordered the full monty Carb kit.

 

[Sportster-2001-951C-Stock]

How do the piston skirts look, any signs of cracking?

 

[etemplet]

How do the piston skirts look, any signs of cracking?

Skirts and pistons actually look really good except for the tops. The MAG cylinder had good compression.

 

[mikidymac]

Skirts and pistons actually look really good except for the tops. The MAG cylinder had good compression.

Let's keep this on track for the OP please.

 

[etemplet]

Sorry I thought I was helping a guy that doesn't know much about these engines by showing a few pics of what I found.

 

[mikidymac]

Sorry I thought I was helping a guy that doesn't know much about these engines by showing a few pics of what I found.

No problem when it got into your pistons and how they are it starts derailing a little.

 

[etemplet]

No problemo my brother.