Turbo I talked to Joey he said there was a chamber do you want to see if he will sell it his dad’s health is really going down got dementia or worse so sad 

It would be very interesting, and I want to say yes, but I'll have to pass. I haven't worked since the end of June, went back to school in August. Trying to cut back on fun stuff.

For a drag bike or a hill climber, the 500 has great potential. For a dual sport that is just cruising around most of the time, it's like trying to get a gorilla to wear a tuxedo and dress shoes.

If he told me i could have it , first i would have made sure he wasnt drunkenly giving away things. Then i would have taken it and found a way to transport it in the vehicle i was in regardless if it was the 2500 or the miata.

Resurrecting this project for the hell of it. I have been searching and searching for a chassis I can permanently install this motor into with no luck yet. Until then I figured I could at least make sure the engine was fully sorted out.

I spent a lot of time on cylinder porting, which I left close to stock the first time around. This time I matched the transfer port heights, flattened the port roof angles and enlarged/hooked the rear transfers. I also raised the exhaust port about 1mm, closer to where Yamaha had it originally. It currently at about 190 degrees. Can always raise it further if I choose.

Since it's easy with the engine on the bench, I figured I would make an effort to improve the shift detent mechanism. The stock design uses a rounded star pattern with an unfortunately large radius across the tip of each point where the detent pin struggles to rotate the shift drum. In the event that the shift dogs were out of alignment during a gear change and the shift detent mechanism has to finish the shift on it's own once the dogs rotated into place, the mechanism sometimes struggles to get the job done. Rather than screw with the gear stack, which could be improved with shimming and machining, I'm going to try this modification to the shift star:





In conjunction with a modified detent pin, this gives the mechanism much more leverage after each peak, greatly reducing the chance of a false neutral between gears. Yamaha did something like this on their later transmissions, the XS750 for example:



I wanted to do this on my RD400 but I never had that motor apart. I will try the modified design on this 500 and probably install the XS750 star in my 360 while the motor is out.

DT250A is exact same as the 2 SC500's

cliff

4 speed trans may tell how much torque this thing has.
Never gets caught in too low of a gear!

Is that decompression hole size in the new sleeve big enough to let off much steam. Original hole looks way over done but might be pic angle. Be sure the drilling down to the exhaust port outlet is clear. Original ignition retards electronically to avoid kickback 'if ya lucky'. If ya running a modern ignition I guess they have retard built in anyway. They need a heavy-ish flywheel to make it nice being a rediculously short stroke for a 500. When I was riding them I don't really think mine was 'that torquee but sure revved good.

The original decomp passage was too large and my resleeve passage was in fact too small. When I was doing the porting revisions I enlarged it into more of a slot. We'll see how it does.











I hope to improve bottom end power with the porting changes. At the end of the day, this thing would really require a bridged exhaust port to get enough time area to make power without having to raise the port as high as they did from the factory.

With the porting changes I hope to pump up low-mid by improving scavenging and reducing charge loss through the exhaust. The main transfers are aimed to converge at the center of the bore and the rear transfers are hooked forward to help kill the momentum of the transfer streams as the enter the bore. This has worked well in my 360s.

@automan

The SC500 does make torque, just not where you would expect it.  I've been hoping to improve the low-mid with this thing but if all else fails I will raise that exhaust right back up to 196 degrees and let it make the power it seems stubbornly determined to produce. 

The factory configuration hits at about 4000 RPM and hangs on to 90 percent of peak power until 8000.  I'd like to move some of that down for longevity but it is set in it's ways so far.

 

your intake Port is looking really small, if you post up the port map I can run the areas through engmod2t to see how balanced it is if you like

I would for sure take you up on that but it is fully reassembled. Intake port width is close to stock in terms of width and I lowered the floor by approximately 5mm.

This is the stock port map from the service manual:



I'm 2mm lower on the exhaust port height and I have all of the transfers opening at the same time with a few degrees more duration than stock.