1975 Kawasaki S1 Rebuild: Installment Ten

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It’s down to the heart of the matter – the engine.

I separated the crankcase halves – which is tougher than it sounds when you are dealing with an almost 40-year-old machine that has been sitting idle for at least the past 20. The first thing you’ve got to do, after removing incidentals such as the clutch drum,sprockets, etc., is get the ignition rotor off the crank. This is a big ol’ magnet that press fits onto one end of the crank – and it’s something that rhymes with itch to get off.

You need an abnormally small puller to get the claw ends on the back of the thing, which is nestled snug up against the case side. It took more than a week of intermittent fiddling to get that little monster off.

But still, the cases would not come apart.

I double checked and yep – all the bolts were indeed removed (sometimes one will hide amid the grease). And – according to the manual – the case halves are only held together by a “light cement” and should come apart easily. Not. Worse, you must resist at all costs the temptation to pry them apart. Two-stroke engines depend on the integrity of the seal between chambers (and cases) and if you start jamming screwdrivers and pry bars between those soft aluminum cases … well, just don’t do it. Be patient. If the cases won’t come apart, get a rubber mallet and tap – gently – on the big bosses (mounting points). Remember: Aluminum. Cast aluminum. Don’t hit it hard – much less with anything actually hard, like a regular hammer.

Eventually, the cases will crack open. And once they are apart, you’ll have access to the guts – the crank, transmission and kick-start mechanism. At this point, you’ll be faced with at least three jobs:

* Deal with the crank. Probably it will need to be rebuilt  – and almost certainly, it won’t be you doing the rebuilding because it requires machine shop tools to get the pressed-together Kawasaki triple crank apart and expertise plus special equipment to press it (and the new crank seals it will surely require, along with everything else) back together again the right way. I found it’s no easy thing to find a shop that really knows old Kawasaki triple cranks.  Newer two-stroke singles (motocross bikes), ok. But not three-throws. Not in America, anyhow. But in the UK, I did. The Brits still love triples and I found several outfits that could help. I went with Kawasaktripleparts.com, which I’ve bought a lot of other stuff from in the past (including the piston kit for this rebuild). They sell a complete rebuilt crank assembly – new seals, con rods/bearings, the whole works – ready to drop into your engine. This route struck me as the quickest, probably best route to go, so that’s the way I went. I also ordered a new set of outer oil seals for the tranny and kicker, etc.

* Inspect the transmission. Obvious problems will be obvious – chipped teeth and so on. But also check all specs. according to the shop manual and if anything’s not right, you’ll have to scrounge replacement parts or buy a new (to you) “parts” transmission off eBay or some such. I was lucky here in that my particular bike is very low miles and I have an even lower-miles complete parts bike (’74 S3 400) on hand for just-in-case.

This is also a good time to check your clutch pack and if you need new plates, order them now. Put the transmission and all related parts aside in a clean place where you can keep track of everything. Because I am absent-minded (and even though I have a shop manual with lots of disassembly pictures) I like to lay everything out – on a clean towel – in the order it was taken apart (showing how it all goes back together) and also take lots of my own pictures with a hi-res camera, for reference.

* Clean the cases. Easier said than done, too.  There are lots of nooks and crannies – and 40-year-old congealed and baked-on grime does not submit readily to either solvent or the toothbrush or hot water and Brillo pads and elbow grease. It will take some combination of all these things, applied over several hours, to get them really clean. And as you scrounge, you may discover things, too. I discovered, for instance, a nasty gouge made by the chain (chain whip) at some point long ago in the bike’s former life. Luckily, the damage was merely cosmetic and I decided to leave it alone rather than braze or otherwise fill it. Gives the bike some historicity, is my thinking.

Once degreased you’ll have to decide whether to go farther – as in, whether to glass-bead the cases to return them to that factory cast aluminum look. Since this resto is pretty involved I decided to do this but it’s not necessary to the function of the rebuilt engine. Note: Be careful not to expose the machined undersides of the cases to the abrasive media. Only glass bead the exterior/outer sections! I also decided to not remove the studs (for the cylinder barrels) as I feared snapping one or several off in the process and did not want to risk that.

With the cases apart (and cleaned) and the transmission/clutch/kicker assemblies all ready to go, all that remains is to await the arrival of my new/rebuilt crank and oil seals.

So now it’s time to dig into the carbs – which were hideously gunked up and (at first) impossible to separate from the cables for individual disassembly because the throttle slides were all stuck.

To get the throttle/start (choke) cables off, you have to unscrew the top of each carb, then slide the throttle slide out of the carb body, in order to access the little clip that secures the cables in place. But if the slides are stuck, you’re out of luck. Here again, avoid the temptation to get mad and start manhandling things. What I did instead was to soak the carbs for several days in a bath of carb cleaner. This eventually made it possible to work the slides out and get the clips off and get each carb separated from the mass of cables and so ready to work on individually.

These carbs were a mess. The kind of mess that happens when you leave them sitting for 20 years with 20 year old gas in them, which turns to goo. Literally, goo. Each float bowl was filled with a dark gelatinous substance which I assumed to be the decomposing remains of circa 1989 unleaded regular. Very carefully, I took each carb apart and down to its constituent components: Upper body casting, lower float bowl, brass float, needle and seat, main jet, pilot screw, etc.

Very careful – and very thorough – cleaning ensued. This is detail work and requires patience as well as good eyes. You must be absolutely certain that all those little passageways are in fact clear. Do not use wire or anything that could gouge (or enlarge) these orifices and passageways. I use little sections of Brillo, which I weave into a thread of the right diameter, and work that through the passageways. Blow everything out with air or pressurized solvent (carb cleaner). Be sure – 100 percent sure – that every passageway/air bleed is open. If not, the engine won’t run right no matter how much you adjust the screws.

I was lucky about the floats (all brass) which were all undamaged and cleaned up to as-new. The jets and needle/seats were another matter. Each carb’s jets were more occluded than Bill Clinton’s arteries. I managed to get them cleaned up without damaging them (or enlarging the tiny jet orifice) using Brillo and lots of patience. The jets are brass, so they at least didn’t suffer any decomposition of the metal. It was just a matter of getting them de-crudded. One carb, however, had no needle in its seat. I wonder how the bike ran last time it ran – with wide-open fuel flow to one of the carbs.

Miraculously, I found a carb kit for my S1 on eBay that has a full set (all three carbs) of replacement factory needles and seats, plus jets. It’s on order and once it gets here, I will probably go ahead and replace all the factory jets as well as the factory needles/seats with the new parts and keep my originals for spares. I also have on order a new (NOS) throttle cable set and am looking for a new (NOS) choke cable set as my originals are shot.

I’ve also got to decide about the pipes. My originals are probably functionally usable. They don’t have any major dents and no rust that I can see. Just old, pitted chrome and very probably baffles that are coated with black ooze cooked to a ceramic-like hardness and which will take a couple of weeks of soaking in solvent just to get them loose enough to remove and clean further. But even once cleaned, they’re going to look rough compared with the rest of the bike. Should I send them out to get chromed? Or use an aftermarket set? I’ve found a place (also in the UK) that reproduces Denco-style chambered pipes for Kawi Triples. They’re mean-looking (check ’em out http://www.higgspeed.com/) and mean sounding, too – but of course, not “factory.” So my dilemma is: Stick with stock? Or sacrifice some show points and get some more snap, crackle n’ pop out of the bike?

I’ll tell you what I decided in the next installment!

Throw it in the Woods?










  1. I truly enjoy old bikes and presently have a 1977 Honda cb 5 spd 4 carbs -the last of the kick-electric start!- and an 82 Honda 900 10 spd 4 carb and my lastest an 84 Honda Magna 1100 6 spd 4 carb.All are very enjoyable and most pull any Harley twice the cube. Thanks for your enjoyable post.Ken

    • The S1 250 in stock form makes about 30 hp. It’s not a rocket, but 30 hp out of a 250 cc engine is impressive (it’s about twice the output of a same-size four stroke engine). With some tuning, expansion chambers, etc. I can probably boost it to 35 or so. If you want sick speed in a two-stroke, what you want is the S1’s bigger brother, the H2 750. With some tweaks, you can make 120-plus hp. Stock, they’d pull the front wheel up on the throttle, no problem – and run a 12 second quarter mile. A modern sport bike is quicker, but “back in the day,” nothing could touch an H2 in a straight line.

      Other vicious little two-stroked include the RDZ350 – which gave you 50-plus hp out of a 350 cc two-stroke!

  2. Hm???. Stick with stock or go for ‘Snap, crackle, pop’ with possibly a good bit more urge. Remember that back in those good ‘ole days nobody left their machines ‘standard”bikes reflected their owners own personal choice of bars, farkles, pipes and state of tune. (Does that help any?) So – big decision time – personal taste or rebuild for show? I know which way I would be heading.



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