650 Overheating/popping on decel

I just helped my friend rebuild his '93 650 (the street legal version). It had been ridden in the sand with an unoiled air filter.

We bored it out 40 over, new tensioner, cam chain/sprocket, etc. Valve seats cleaned up nice, did not replace the valve guides.

Anyway, I have fired it up and ran in up and down the street several times. It seems to be running very hot, to the point that the crankcase is too hot to touch withing 4-5 minutes of ridding. It also backfires on deceleration.

I peaked back under the valve cover to make sure I wasn't off a tooth on the cam, and it was fine. I pulled the intake manifold back off to check for cracking to see if I had an air leak. Checked the vaccuum lines that run down from the carb to whatever that pump is on the side. The manual said to turn the scew out 2 1/4, so I did that. Checked the exhaust for leaks. I'm going to check the valve clearances today.

It runs and sounds great. Very smooth all the way through the gears. Very powerful. I've done several XR 100's and a 200, nothing this big before this one. But, I took my time and followed the book, so I don't think the problem is with my work.

As far as I know, the jetting is stock. We're at about 5K feet altitude.

Thanks in advance for any ideas.

it still sounds like a vacuum leak, I know you looked for that, but try this: with the engine running, spray WD-40 all around your carb/manifold/hoses, etc ... if engine speeds up, you found your leak ... worth a try ... you might also inspect your jets, to see if they have varnished up from sitting during the overhaul ... :applause:

Do you have a Service Manual? And did you check to see if the oil is getting pumped back up to the frame?

Double-check the oil pump operation and about priming it whenever engine work has been done...

Do you have a Service Manual? And did you check to see if the oil is getting pumped back up to the frame?

Double-check the oil pump operation and about priming it whenever engine work has been done...

I do have the Clymer Manual on it.

Also, I cracked open the oil line going into the head right after I started it for the first time after the overhaul. Oil started to leak out of it. Also, when I opened the valver cover up to double check the timing, oil drained out. So, I know I am getting oil to the head at least.

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