The "pulled" clutch with 8 plates. The arrows point to the bolt that pulls them apart.

Ready to put in the frame.
The lowdown on the engine:
Over bored to 800cc, using Yamaha XV 750 "flat" pistons, diameter 83mm; compression ratio: 6.8 : 1 ; compression pressure: 8-8.5 bar.
Clutch with 8 plates, operated from the right hand side.
Oil cooler with thermostat (keeping the temperature down to 110 degrees Celsius).

The moment of truth ... SnakeII on the dynojet. A mobile dynojet in Heesch, Holland on a custom show. The operator was Ruud Fredriks, the importer of the dynojet in Holland. And he has forgotten more than most people know about bikes and testing them. Here he is setting up the bike. See the big fan in front of the bike, to provide the necessary cooling for the engine.

And here the real work. First warming up the bike, then accelerating hard through the gears. The faster the bike can accelerate, the more power you have. The dynojet computer can measure this and show a nice graph of it.

Big fan to keep the engine cool during the run.

This shows three runs in the same graph. Two runs in 4th gear (shown in black and blue), and an all gear run. That is starting in 2nd and shifting through to 5th. (shown in red)
I marked some lines (in gray) showing the rpm´s of the engine in 4th gear.
The engine only has 42 hp, but a lot of torque low down in the rev range. Resulting in a poor top speed (only something like 160 Km/h on the road) but a great acceleration.

Great for pulling wheelies, and making that poser on the GSX whatever looking like an ass at the red lights :) In fact I could just dump the clutch at 2000 rpm´s and the bike would shoot away with a screaming back tire :))))))) The all gear run shows that the engine picks up nicely when shifting through the gears. Ruud gave me a couple of tips on getting even more power, most important enlarging the main nozzle in the carbs because it was running slightly poor, not getting enough fuel that is. I did that when I got home and it really did help a lot, and keeps the engine from overheating.

The search for more power continues. I did some more things to the engine to gain power. Unfortunately I didn´t get any world shocking results.

I had some pistons lying around that gave an incredible 890cc. Again flat pistons. I tried it out. Resulting in using 1 liter of oil on 500 meter. The liners are as thin as cigarette paper. Due to the heat they go oval, resulting in gigantic oil consumption. I tore it apart very quickly!

The problem with the 800cc engine was the compression ratio that was much too low, resulting in poor results. This was caused by the "flat" pistons. I restarted from scratch. I had the engine over bored to 850cc, with specially adapted Suzuki pistons. With the standard cylinder head this gave a compression ratio of 11.5 : 1, I think this is a bit too high. I still had a cylinder head with bigger valves and an enlarged combustion dome, giving a nice 9.2 : 1.

On the left hand side an original valve for this cylinder head. On the right what you get after painstaking hours of work. The valves were polished to mirror finish (not the valve stem of course!) and where it seats I removed all sharp corners. I then grind them in on the seat with the minimum of area touching.

The old chromed steel exhaust I got from a mate. He had some stainless steel tubes bent for me later on. (Thanks Gert-Jan) I had to change the collector because it didn´t fit my bike.

On the left Mikuni BS38 carbs, originally for a 1977 model XS650. They work quite nice but lack the extra midrange power. On the right hand side the Mikuni VM34 carbs. They gave pretty good power and very good responsiveness to the throttle, but never got the bike started when cold. (No electric starter motor on my bike ... it only needs to a small battery and the crankshaft stays in line!)

The throttle slide of the Mikuni VM34. On the right the original one, on the left an adapted one. I changed the angle of the cutaway. Putting them on the cutaway on a flat surface allowed me to get the angle on both just right. This helped throttle response enormously.

never solved all problems with these carbs. I never continued beyond this point as I want to first finish the snake project and get it back on the road. Some people suggested these 2 stroke carbs are unpractical to set up for a four stroke, but it can be done. Below a reply from Dave Sentz to the Micapeak mailing list in regard to a thread about the Mikuni VM34 about his positive results with a set of 36 mm vm´s :


From:
Dave Sentz

The 36mm VM´s (they´re actually closer to 38mm now) I´m currently running on
my 762 engine I bought off of ebay this past winter. They were off of a Yam
TZ250 road racer, complete with powerjets. Very nice condition and they were
a steal at $40.00 for the pair. JB weld took care of the powerjet mounting
holes and extra gas circuits. They fit perfectly in late model 650 intake
manifolds. All of the jetting, with the exception of the pilot jets, was
changed to Bob B.´s specification. Cables are a problem with the VM´s but
Bob can fix that too. I´m running UNI filters since I couldn´t find any
K&N´s that would fit but they seem to flow fine and are much cheaper than
the K&N´s. They carburate as well as factory carburation except maybe at
constant speed, small throttle openings, say maybe 30MPH in 4th gear where I
get a little bit of surging but nothing I can´t live with. I couldn´t
imagine trying to run this engine or a stock engine for that matter with the
CV carbs. The difference they make on a stock engine is truly amazing, as
Bob said. I personally think they´re easier to work on than the CV´s, parts
are plentiful and much cheaper, and when you work on them, you actually get
results unlike the CV´s which can sometimes be very frustrating when you´re
trying to correct a problem.
Geez, these VM posts are getting to be like deja vu all over again. I´m a
believer. Wish everyone had a chance to try out a 650 with these carbs.
You´d all be believers too.

Best regards,

Dave Sentz

Alternative carbs, post by Dave Sentz (dcsentz A hotmail DOT com)

Hi Dan,

Your 36mm VM´s will fit perfectly in the late model (34mm CV) 650 intake
manifolds. And in addition to UJ, another source of carb parts, tuning
info, cables, throttles, etc. is Bob B. He straightened me out. : )
Good luck.

Dave Sentz

The lowdown on the engine:
Over bored to 850cc, using Suzuki pistons
Cylinder head with big valves, big ports, big combustion dome
fast camshaft
Mikuni VM34 carbs
K&N air filters
2 into 1 exhaust
Clutch with 8 plates, operated from the right hand side.
Oil cooler with thermostat (keeping the temperature down to 110 degrees Celsius).

The dynojet graph. In blue a test in 4th gear. Maximum power 47 hp. The mixture was too lean. After we changed the main jet from 190 to 200 it was right. And it boosted the power. In green a test with the new main jet in 3rd gear. Maximum power 52 hp.

I needed something to hang the headlight on and made up these neat alloy headlight-thingies. Cut out of 1.5 mm alloy plate, they weigh practically nothing compared with the original steel stuff. And they look much better!