That is really weird. Did you ask the person you bought the engine from what it is? Why would you want a spark plug in a cylinder head? Maybe some sort of sensor is supposed to go in there?Got a closer look at it and it seems to be a homemade modification, it's a mini spark plug on the side of the head. No idea what it does.
That window was kinda pointless after you'd started the bike a few times. Plus they were prone to leak oil there.Note the clear window at the cam was replaced with a alum plug. The window was there to tell you when start to kick it thru.
My guess is that it was someone's attempt to twin plug the head - quite a common mod for 2 valve combustion chambers as you can't get a plug in the centre the same way you can with a 4 valve chamber.Got a closer look at it and it seems to be a homemade modification, it's a mini spark plug on the side of the head. No idea what it does.
I almost post that info too.That window was kinda pointless after you'd started the bike a few times. Plus they were prone to leak oil there.
I never looked at mine - just went by feel at the kicker.
Small lever next to the brake lever would fit with the decompressor theory...Long story short... I have on hand almost 40k worth of parts half new and half used mostly Harley parts, misc parts, exhausts, tanks, forks, seats, etc... including x2 evo96, extended fat boy frame, sr500 engine. Took 3 days to do inventory. That sr500 engine was off some racing crude cafe bike found the rest of it in other containers in pieces had custom homemade aluminum gas tank with boost gauges, Also has a small lever next to the brake lever, not sure what that is for. Almost every part was custom machined. Unfortunately there was no frame for it. The funny thing is Even though i have all these parts, it's not exactly the parts I need to build my cafe. Now in search of a xs650 or cb750.
The round machined thing (with two allen screws) in the middle of the cases, is a place for an oil line to an oil tank. The engine breather is at the back of the cases, as per stock.It is indeed a '78-'81 Yamaha SR500. The "2J2" stamping before the serial number gives it away. Interesting aftermarket carb mount and CC breather. Cannot help you on the red circled area, though.