This motor went back in tonight. I rode it around quite a bit figuring out where I left off with jetting. This time around it's breathing through my homemade airbox though so the previous settings wouldn't be very useful anyway. Ended up with a 155 main, 60 pilot and DEQ needle in the 39mm Keihin PWK. May even go leaner on the main, it's been better and better each step and the plug is still indicating a safe mixture.

My initial impressions are that it is vastly improved over the first go-around. Low end torque is better despite the 1mm increase in exhaust port height. It chugs along strongly from the bottom up, carburetion is actually quite good. The top end power is better than any version of the 360 I've tried so far, either the ported DT or the MX360 cylinder. I know there is more in there but I think unless I find another cylinder in good condition I would have a hard time altering the port timing in this one. Part of me wants to go ahead and raise the port all the way up to 35mm but I know it'll last a lot longer the way it is now. The power spread is wide enough as it is that the four speed gearbox is no problem, either banging gears wide open or short shifting discreetly it never feels like the next gear was too tall.

The reshaped shift star works great. The shift feel is good and I had zero issues with false neutrals. The original build would sometimes hang out between second and third and third and fourth, then jump all the way over third and into fourth. I made no changes to the gear stack, just the detent mechanism. I will absolutely be doing this with the gearbox in my 360 while the motor is out as it has also had a tendency to half-shift into some gears especially under load. I've never had one of these shift as well I would like so perhaps this is the magic bullet.

I will get some video soon.

Took the head off for a quick inspection. I've already put more time on this thing than I did the first time I swapped this engine out. It's running really well.




The bore looks perfect. The only sign of distress is the vertical streak on the intake side, which was left over from the initial test run many months ago. When the sleeve was installed, they did their best to replicate the original port widths but I think they were somewhat conservative, perhaps leaving room for a final port job to match things up properly. The way the boost port was set up relative to the ring peg on the piston had just the very tip of the piston ring end riding on the slightest edge of the port, which quickly began to irritate this edge of the port and perhaps generate enough heat to streak the cylinder wall. Since this was clearly not sustainable I decided to widen the boost port enough to set the ring ends entirely within the port. I would feel better about this if the ring pin was centered in the boost port but it is what it is. Seems to be fine.

The carbon pattern looks pretty good. They're never perfectly symmetrical but this is about as good as any of them. The rear transfer port hooks are evident and the flow through the main transfers appears well balanced.

It's been through a number of wide open pulls through the gears. A guy could really get in trouble with this kind of power in this kind of chassis. It's wild off road, when you catch traction on some hard packed soil and it just about yanks your arms off. It compares pretty well to a modern 250 two stroke in terms of brute force.

Feast your ears on this:



This is at about 5700 feet, so I'm down about 20% on power on account of the air density. Same effect as taking away 20% of my displacement, so all I get out of that SC500 is equivalent to about 397cc's. That said, I find it's performance to be quite good. Would be rowdy at sea level.

Awesome 

Dan, that portmap must be a typo on the A port width, I assume its 39mm.
my sim takes "flow widths" which are measured with an inside caliper at the approximate angle of the port. I checked my yz465 cylinder I
had sitting around and its about 4mm less for the "flow width" measurement. for the width of the flow width of sc500 A, B, C transfers I gave a good guess of 34, 24, and 20mm. 60mm ex and intake width.

I have inputted the specs into engmod with the radiuses and shape of the ports you have shown in your photos, with 190/126 timing and arond .5 cylinder deck height. I have guessed on the reed valve dimensions as 4x 30x45mm ports at 45 degrees with a reedblock measureing 50Wx68Hx50L 68x47mm wide dual pedals. and a 12mm reed stopper distance.




The balance of yer porting is OK overall, but the intake port should be opened up just 1-2mm, and the exhaust is the weak point of the engine. You can see it has only enough exhaust port to make 37hp at 6500rpm at this boresize, when you already have enough blowdown time for 8000rpm. Raising the exhaust port is the only way to increase the area once you hit max width you are comfortable width, but raising it makes the imbalance with the blowdown worse and results in a peaky engine.
Raising exhaust to 195+ adds power but results in too much blowdown and makes a peaky engine,
Raising transfers to shorten blowdown results in less low end power and high rpm engine, not good for big bore.
Best move to balance the cylinder is to widen the exhaust to 70% of bore at 63mm, and use a tighter 10mm corner radius like YZ490 port shape.

Old article in CycleWorld shows the port shape I would go with if it were me: magazine.cycleworld.com/article/1982/7/1...for-the-yamaha-yz490

Doin this without raising the height results in +5hp at 6500rpm, without narrowing the power. Ring life would be shorter, but its all a compromise! Looking at these ports I am not surprised the bike is running good, you already have a much better than stock.

You've got the timings around the sweet spot, 125/190 is my go-to for street bikes and enduros.

Thanks for taking the time to run that through the program. It is very difficult to pass up five easy horsepower. I hope to find another cylinder so I can try some silly shit with that one and keep this one as is. It really does run well all around, would be a shame to gamble with it. Ring and piston life is unfortunately a significant factor too. I really should have had the exhaust port bridged when they sleeved it. I didn't think it was necessary but man, that would have opened up some serious horsepower.

Interesting to see the software suggesting a 35mm carb. I do have one. Might try it to see what it does.

Blimey AKARA1that's way over my head but i know ya talking to TURBODAN.
I guess you're aware the 500 is rediculously over square at 'by memory '95x70 compared to a 490, or the 465 you were talking which is a couple of mm smaller 'than 490'.
Anyway--i don't know what i'm talking about haha, that's obvious.
Have mixed memories of the SC500 i raced new, good on fast rolling grassy tracks. Sorry--carry on.

The 35 carb size is just based off the 7000rpm input, it says 38mm carb for 8500rpm.

Widening the port is a definite improvement





It would be super interesting to see what a bridged port would do, when I enter a bridged port arond 80mm total width, if that is possible with the SC500 casting??? it shows a really balanced powerful cylinder. CR500 uses bridged exhaust port 70-75mm wide.

If the casting is too thin I'm sure it could be welded up as needed. Little more work but it seems doable. 55 hp would be insane.

I'm going to examine the thickness of the casting, maybe I can fit some small sub exhaust ports in on either side of the center port. That could potentially be done to this jug without resleeving and without compromising ring life or its usable spread of power.

I love this thread!  Good work and thanks for sharing!

I enjoyd the video, it sounds awesome, my 360 also feels really gnarly offroad for the old chassis
Youll notice you have a lot of transfer area, this is good but A port gets in the way of making the wide exaust port, If I made a new sleeve I would narrow the A transfer away from exhaust a few mm to try to maintain that distance from the exhaust port to prevent short circuiting, basically, converging towards the CR500s design.

the sc500 exhaust port is short on area for bulk flow, sub exaust ports would make more power but will increase blowdown and make it peaky. I've added sub ports to blasters before and it does make them peakier, something to be aware of if the ultimate goal is midrange or lowend