Top End Decisions

[RiverBandit]

Hello! New to the forum ... I started riding jet skis in - cough - 1980

Back in the day, I was supposed to ride with Chisty Carlson but she never returned my calls ... and security didn't believe I knew her so we never got to hook up

With introductions out of the way, yes I can still ride, although I don't recall this much pain.

I purchased a 1987 Kawasaki 650 SX from the original owner.. can't make this stuff up --- He's 84, and I watched him ride it. I hope this experience was the universe allowing me a glimpse of my future.

In any event, this is the Jet Ski I could not afford in 1987. It's basically original except for the carburetor. The ski actually runs, and it's rideable, albeit slow. I literally took possession of this old girl on the owner's dock -- the first time in my life I've ever purchased and kicked a ski off the dock and ridden it home. This guy told me he's never touched the engine ... Compression 90/90

Would you - drum role - A. hone and slap -- B. Hone and replace piston and rings -- C. Perform a complete rebuild

I'm thinking based on a compression of 90/90 and the fact this thing still runs and throws a jet that this engine has worn evenly. I am in Georgia and can ride through October/November. I'd like to get some tray time before committing to a full rebuild. At this stage of life - 37 years of use - am I risking grenading this thing with a new top end? Does anyone have an experience to share?

My local machine shop is closing their doors -- retiring -- and so it looks like rehabing these old 2-cycle engines will require UPS. I can manage a simple top end rebuild.

Thanks in advance

 

[Quinc]

I would ride it the rest of the year then bore the cylinders 1 over with new pro-x pistons. If you want a little more power it would be worth having it ported too. Would also upgrade the carb and intake to a mikuni sbn 44 carb. Impeller is probably pretty beat after 40 years too. Would upgrade it to a hooker 9/15.

Edit: For a little extra cheap power you can do a dry pipe mod to the exhuast:

650/X-2 - modding the stock 650sx exhaust

Just wondering if anyone has modded their stock exhaust to run drier, I've been reading up a bit on it, just wondering the pros/cons and how much it actually helps. I have a welder so I've been thinking about tryin it...

www.x-h2o.com

 

[RiverBandit]

I would ride it the rest of the year then bore the cylinders 1 over with new pro-x pistons. If you want a little more power it would be worth having it ported too. Would also upgrade the carb and intake to a mikuni sbn 44 carb. Impeller is probably pretty beat after 40 years too. Would upgrade it to a hooker 9/15.

Thanks for replying!

Carb is currently the SBN 44 and it has a nice shiny, polished intake. I'm debating now if this exhaust is OE ... I ordered a rebuild kit for the carb, new fuel lines, filter, and new reeds.

Performance right now: Finiky starting but it always starts, lacks bottom-end pull and you really have to maintain momentum and stay on the throttle in turns. Once you get it wound up it keeps going. I was clocked at 35mph GPS wide open ... I'm sure once I feel more comfortable I can get a few more MPH out of it ... like at least 2

Do you think with a carb rebuild and reeds I can rock this for the rest of the season? In my younger days, compression under 100 was considered no-fly

 

[Quinc]

Thanks for replying!

Carb is currently the SBN 44 and it has a nice shiny, polished intake. I'm debating now if this exhaust is OE ... I ordered a rebuild kit for the carb, new fuel lines, filter, and new reeds.

Performance right now: Finiky starting but it always starts, lacks bottom-end pull and you really have to maintain momentum and stay on the throttle in turns. Once you get it wound up it keeps going. I was clocked at 35mph GPS wide open ... I'm sure once I feel more comfortable I can get a few more MPH out of it ... like at least 2

Do you think with a carb rebuild and reeds I can rock this for the rest of the season? In my younger days, compression under 100 was considered no-fly

You will be fine. Make sure your mikuni rebuild kit is a real Mikuni brand kit and not some china knock off. Or you will be fighting issues next season. Post a picture of the engine bay and people here will let you know what you have.

 

[RiverBandit]

 

[mn_reeds]

If the engine really hasn't been touched in 37 years it's going to need a full rebuild. The finicky starting is also likely from an air leak.

Since you mentioned rebuilding the carb and reeds just order an exhaust gasket too and do a pressure test to confirm. Depending on how bad it looses pressure you can decide how much risk you take with a lean seize.

After that it comes down to expectations and budget. The 650sx can be a fun ski when upgraded (I have 2) but isn't winning awards for buoys or freestyle. You could 750 swap it with a cheap donor ski as well. If you are going to keep it long term it doesn't matter as much but you can get way over market value in parts in a hurry too.

 

[mn_reeds]

Just saw the pic - that is a stock headpipe with maybe an upgraded tailcone. You should do the dry-pipe mod for sure. It's virtually free outside of time and yields a nice improvement. FWIW I didn't even add a nipple on my tailcone and just routed all water overboard.

When you mod the exhaust make sure to check for a diverted ex manifold and upgrade if you don't have one. They didn't come on 87s and are a key piece to the overall system.

Lastly - you'll want to replace that red head as they have been known to warp.

 

[RiverBandit]

If the engine really hasn't been touched in 37 years it's going to need a full rebuild. The finicky starting is also likely from an air leak.

Since you mentioned rebuilding the carb and reeds just order an exhaust gasket too and do a pressure test to confirm. Depending on how bad it looses pressure you can decide how much risk you take with a lean seize.

After that it comes down to expectations and budget. The 650sx can be a fun ski when upgraded (I have 2) but isn't winning awards for buoys or freestyle. You could 750 swap it with a cheap donor ski as well. If you are going to keep it long term it doesn't matter as much but you can get way over market value in parts in a hurry too.

I'm tracking with you ... I ordered gaskets for the exhaust, carb base, and reed block/intake, along with hoses.

A 750sx will drop into this hull? I think I am oriented more toward freestyle. I'm on a nice, relatively large, and quiet lake. No waves to speak of. I will spend most of my time hanging coves entertaining myself. I appreciate bottom end...

Thanks!

I really believe this ski is untouched ...

 

[RiverBandit]

SBT a valid option for a rebuilt powerplant?

 

[mn_reeds]

Hull looks mostly original outside of the nicely updated chinpad.

Since you are already diving into the carbs I'd recommend doing the rebuild yourself and even a little home porting as Quinc mentioned. I've run SBT cranks purchased through JetManiac but wouldn't go complete motor if you are going to modify for performance.

 

[Quinc]

SBT a valid option for a rebuilt powerplant?

Will be cheaper and you will know what you have doing the rebuild yourself vs buying a rebuilt engine from sbt. For freestyle you are going to want to shave the head for some more compression as well. check out groupk.com for info on mods etc.

 

[Sparkplug]

I'd bet your compression gauge is wrong. A 650sx hitting the mid 30s is about as fast as they go stock. Check those readings with another gauge. 90 psi would barely run and definitely wouldn't hit 30s. If it pops your finger off the spark plug hole. It's not 90.

Throttle wide open. Known good well charged battery.

 

[kevbo]

The 87 crank and case may not have a lock ring on the bearing, I know mine didn't. I would NOT put a fresh top end on that case if all it has is a pin. The crank tends to walk fore and aft and can push the front seal out. The 87 exhaust chamber was bolted to the head pipe and would break. Find an 89-95 case and crankshaft, they were known as the "full ring" type. . The newer stock exhaust were mounted in rubber and seem to last forever. The 650s are not that bad if it has a good pipe and impeller, and if the rider isn't very big.. The stock bars may work for kids, most adults will benefit from a flatter angle. 750s are an easy swap, the 750 standup E-box fits perfect over the battery. A simple mount must be made. Also 750 impellers can be used with a 750SS driveshaft.

 

[RiverBandit]

I'd bet your compression gauge is wrong. A 650sx hitting the mid 30s is about as fast as they go stock. Check those readings with another gauge. 90 psi would barely run and definitely wouldn't hit 30s. If it pops your finger off the spark plug hole. It's not 90.

Throttle wide open. Known good well charged battery.

Thanks for replying ... I was thinking the same with respect to the gauge. I checked again last night ... cold engine, carb removed - open intake - and I used the screw-in adapter. Essentially the same result. The battery is new although I did not actually check the charge. The engine seemed to turn over quickly and I cranked to the count of 5/1000

I don't suppose prayer will help my case?

 

[Jr.]

Where are you in Georgia? Im able to help with all machine work. Im just north of Atlanta.
older K650 parts are getting tough to come by. Just need to know where to look
like others here are saying, you should consider a full rebuild considering the skiis age
in a couple weeks there is a freeride on Lake Allatoona. Southern Fried Freeride

2023 Southern Fried Freeride Sept 21-24th

Sept 22-24th More info coming. Same rules. You book site P11 and i will ban you! Lol

www.x-h2o.com

this area has a fairly large Vintage Kawasaki following, especially this ride. Consider attending.

ski ya, Paul

 

[WFO Speedracer]

I wouldn't worry about the pin in the crankshaft , that issue is easily fixed by coating the bearings with a thin film of red locktite after that they will not move .

Ryde it the rest of the season and rebuild it this winter, I wouldn't bother Pressure testing at this point, pull it completely apart, evaluate what you have and go from there, that way you can spin all the bearings, replace the crankshaft seals and take the pistons and cylinders to a machine shop to have them measured, they will advise you where you need to go with the top end.

Most don't realize this but 650 carbs and exhaust manifolds are a matched set , the skis with the 28mm carbs have a smaller opening on the exhaust manifold , you want to at least go to the later model manifold with the diverter built into it , do the pipe mod, it's cheap and it works well , you can either stick with the 38mm carb or go with a 44mm carb and an adaptor on the stock manifold, you will have to sand or grind the manifold out for the bigger carb. but they work just fine on the stock manifolds.

Truefully all this will still need to be done if you do a 750 swap as a stock 750 without modded exhaust will not be a huge gain in performance.

I know of what I speak I have modded many an 650 as well as done 750 swaps on 650SX's. , I run modded 650s in most of my personal skis as well.

 

[RiverBandit]

Where are you in Georgia? Im able to help with all machine work. Im just north of Atlanta.
older K650 parts are getting tough to come by. Just need to know where to look
like others here are saying, you should consider a full rebuild considering the skiis age
in a couple weeks there is a freeride on Lake Allatoona. Southern Fried Freeride

2023 Southern Fried Freeride Sept 21-24th

Sept 22-24th More info coming. Same rules. You book site P11 and i will ban you! Lol

www.x-h2o.com

this area has a fairly large Vintage Kawasaki following, especially this ride. Consider attending.

ski ya, Paul

I am in Gainesville ... north side of Lake Lanier by the Olympic Center

 

[Sparkplug]

Thanks for replying ... I was thinking the same with respect to the gauge. I checked again last night ... cold engine, carb removed - open intake - and I used the screw-in adapter. Essentially the same result. The battery is new although I did not actually check the charge. The engine seemed to turn over quickly and I cranked to the count of 5/1000

I don't suppose prayer will help my case?

A top end that wore from 145 psi to 90 psi. Evenly. Is very unlikely. I've never seen a 650sx with les than 120 psi. Is it possible. Sure. It's much more possible your gauge is wrong. That's why mentioned using a different gauge. It should spin over fast. It's a tiny little engine. You can also very clearly hear a low compression engine with a little experience.

Does it pop your finger off the plug hole when you try and hold your finger on the hole. And when it does. Can you feel the air rushing last your finger, get very hot.

A ski that had 90 psi would be incredibly hard to start. As not just because someone slapped the 44 on it like every joe blow did. Totally possible it is worn to that point. Unlikely tho


The amount of cut up superjets. Pipe dream hull builds. Blown up engines. And sunk skis from lack of experience. Is on alot of the guys on this site that recommend newbs tear into stiff willynilly

 

[RiverBandit]

A top end that wore from 145 psi to 90 psi. Evenly. Is very unlikely. I've never seen a 650sx with les than 120 psi. Is it possible. Sure. It's much more possible your gauge is wrong. That's why mentioned using a different gauge. It should spin over fast. It's a tiny little engine. You can also very clearly hear a low compression engine with a little experience.

Does it pop your finger off the plug hole when you try and hold your finger on the hole. And when it does. Can you feel the air rushing last your finger, get very hot.

A ski that had 90 psi would be incredibly hard to start. As not just because someone slapped the 44 on it like every joe blow did. Totally possible it is worn to that point. Unlikely tho


The amount of cut up superjets. Pipe dream hull builds. Blown up engines. And sunk skis from lack of experience. Is on alot of the guys on this site that recommend newbs tear into stiff willynilly

This is why I need people like y'all in my life ...

Yes, the compression absolutely blew my finger off. I checked again with the same gauge and this time used the rubber-tipped fitting instead of the hose and screw-in adapter. I checked 3 times and achieved 145-150. Thank you!

 

[Sparkplug]

Yeh. I figured