MTech305

OK. To make a long story short.
I bought a 2002 xl700 with a gp1200 engine swapped into it. I got it for $1300 with double trailer.

Checked compression today, and it was 95 in the rear cylinder, and 120 in the other 2.

Looks to be a lot more than 10%, so is a rebuild due??

What could of been the cause of this??

Do you think I could get away with a top end rebuild?? I think it has an SBT top end in it already, it has painted head bolts, and 2 chrome head bolts. I gues they're from the Yamaha top end. Plus it had a SBT sticker on it.
The bottom end is still Yamaha though.(crank and rods and case). Since it still has the Yamaha sticker on it.

It still runs OK. I took it out and it rode fine. 62mph once.
If I hurry up a rebuild it before it really blows, will I save some cash?? Maybe just a simple honing to the cylinders??

How can I check if the rear seal is OK? And the rod? What are the chances of a rod being damaged??

Ernest T

With 95 psi I doubt you have any major damage like a bent rod. You would need to pull the cylinder and take a look. You might get by with just new piston and rings for the low cylinder for now. I wouldn't mess with the other 2. 120 psi is where you want to be. Look at the piston when you get it apart and its condition may give you some indications as to cause. The #3 cylinder is the most common to fail. Frequently due to oil line coming off if you are still using it, but that usually results in melted piston and much more damage (you would see no compression). Try squirting a little oil in the bad cylinder and then repeat the compression test. If it goes up with the oil, most likely leaking around the piston rings. Also if that engine is a powervalve versioin, its recommended to disconnect the PV servo motor when doing the compression test.

Only way to check the rear seal is pressure test it. There are kits available, and it is not hard. Not a bad thing to do any time you have the engine apart. I doubt you have a leak or you would be seeing some running problems. Sounds like overall its in pretty good shape.

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2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100

MTech305

Thanks.

MIght I add that I did the test after it was running for about 5 minutes. I forgot to check it while it was cool. I also think I may have not gave the 3rd cylinder enough to give a proper reading.

So I am going to do the test again with the engine cold and see what happens.

I still have the oil pump. The engine was supposidly istalled 6-7 months ago. Along with a carb rebuild.

If the compression is still low, ill put some oil in it to see if it comes up. What if it doesnt come up? What else could cause it?

Also, how much of the exhuast needs to be removed to take the cylinder off? Im pretty sure its a non PV engine, so it would have the one piece cylinder, and sleeves correct??

No nikisil plate to worry about right?

MTech305

OK. Just re checked compression while engine was cold and same results.

I put some oil in the cylinder and it shot right up to 120psi.

Ernest T

That confirms piston/ring/cylinder problem versus valve.

On non-PV GP1200 it is steel not plated, so can be bored. (I think only the "r" version from 2000-2002 had Nikasil plating).

Most guys do not recommend running a different sized piston in one cylinder.....it messes up the balance of the engine and may lead to early failure. Options are to do a top end exchange, or bore all 3 and go with new pistons. Wiesco, or ProX are often recommended. I think the boring will be cheaper and give you more power (if it hasn't already been done on your engine).

Just go ahead and pull the engine. That will be easier. SBT has a removal/installation guide on their site: http://www.shopsbt.com/pdfs/y11001200npv.pdf

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2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100

MTech305

So they arent even sleeved? Just a cast cylinder? That would be great.

I think I am going with the SBT top end kit that comes with all 3 pistons and rings.
I think Ill get the 1.5mm over bore.

I might as well rebuild the carbs while im at it, and install a primer kit.

Im going to start tearing the engine down Monday. Hopefully, this is a stock engine that hasnt been bored yet. What should I look for on the pistons as far as markings to tell what it is?? Yamaha or SBT?

Ernest T

Oops. I'm guessing it is sleeved since SBT offers a sleeve replacement for the non-PV engines. The Yamaha pistons usually have the engine number stamped on them (think it would be 65u for the original piston on a non-PV motor). Don't know about SBT.

__________________
2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100

MTech305

Do you think a loose headbolt can cause this??

What is the specs for the headbolts in FT LBs?

Ernest T

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__________________
2008 Yamaha FX HO Cruiser
2010 Hurricane Sundeck 2100

MTech305

OK.

Got the tear down the engine today.

Found out it is in fact an original engine with the original 65u pistons. Head bolts had loose washers.

Took the head off and piston #3 is Fd up. It has a bunch of nicks on the top. The top of the head has a bunch of nicks as well on the #3 part.

Looks like the port on the sleeve cracked!!!!
And bit of the ringland are missing.

So how am I going to fix that crack? Does that mean a whole new cylinder set??

Its a pretty small crack on the left side port.. The cylinder walls are fine, not one scratch in it. Nice and clean. Just the port has a crack...


It looks like the sleeves are welded in??

Any help is appreciated.

Crank/rod is fine. No blueish color.

What couldve cause this?
I noticed when taking the carb plate off, it has a gasket, and number 3 carb's gasket ripped off. Probably letting air in. The other 2 are intact..