New to the CB750 and have header question.

I plan on doing a compression test but I really think the engine is solid for the most part. I heard it run, and saw a video of the guy I bought it from recently ripping down the block on it.

Upon further inspection, it was missing exhaust gaskets also in the two kick stand side exhaust ports and that’s where most of the carbon and a little bit of oily deposits were left. Obviously if you run the bike on the kick stand and have no gaskets on those sides, you’re bound to have some crud left over. I also inspected the header ends and none of them had any oil in them. Just what looked like normal carbon deposits on a 43 year old bike. I’ll do the compression test and know more then though.
 
Loose Chain said:
You might be putting the cart before the horse.
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I see what you mean but not really if I have the money and know how to fix it if there is a major problem within the valve train or a compression issue. I’m already starting to get a better grasp on this engine so if there is a major problem somewhere I’m not too worried about having to dive into it. I’m an automotive mechanic by trade also so that helps a little.
 
So just to update on the compression test, I finally had a chance to do a dry test and every cylinder number was sub 100psi. So right away my mind automatically goes to the worst place and I’m thinking fuck, valve train issue. So I went and did a wet compression test and thank god, the compression numbers went through the roof. I then used my bore scope to check out the cylinder walls and everything looked good. So all in all, I think I’m just going to be into her for new rings and a little bit of time. 👍
 
I'm going to go out on a limb and suggest if your rings are worn, then you are likely due for a valve job. I would do that before removing the engine.
 
brettp said:
I'm going to go out on a limb and suggest if your rings are worn, then you are likely due for a valve job. I would do that before removing the engine.
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I mean after 43 years, I’m assuming a valve job wouldn’t be a bad idea. My only hesitation with doing it is, one, I don’t wanna do it lol, and two, I just want to get the compression back that I’m losing. I mean, like I said, a 43 year old bike could use a valve job for sure, it’s just something I’d rather tackle down the line when I know it’s time or I’m just bored and want to do it.

When I say my compression went up with the wet test, I mean it went from like anywhere from 50-90psi’s dry to +200psi wet and wanting to blow the gauge right out of my hand. That at least tells me the valves are sealing good enough to have that kind of jump in compression.
 
And speaking of taking the engine out, I’m almost down to just the frame with an engine sitting in it from my complete tear down. Now the xs650 engine is a heavy mother fucker and a pain to get out alone but I’m a pretty strong dude to where I’ve always been able to shimmy it out enough to where I can get a good hold of it, lift it up, and set it on my engine stand with almost, but not having a hernia. This engine…there’s no fucking way I’m picking it out of the frame bare handed.

I have read some ways of getting this beast of an engine out whether that’s with ratchet straps, two people, and a steel pipe to lift on each side and shimmy it out the right side, or I’ve heard the lay the engine/frame on its right side on a furniture dolly, and essentially remove the frame from the engine that way.

Is there any other ways people remove the engine from the frame by their selves?
 
I removed my engine by myself. I used a furniture dolly with 1/2" plywood fastened to the top of it. Bike on center stand, slid the dolly under the engine. Oil pan off, blocked between the dolly and engine front and back, removed bolts. Then added 1 1/2" more block at the front of the engine to tilt the engine to clear the frame, wheeled it out. Used the same blocks to put it back in.
 
mrtwowheel said:
I removed my engine by myself. I used a furniture dolly with 1/2" plywood fastened to the top of it. Bike on center stand, slid the dolly under the engine. Oil pan off, blocked between the dolly and engine front and back, removed bolts. Then added 1 1/2" more block at the front of the engine to tilt the engine to clear the frame, wheeled it out. Used the same blocks to put it back in.
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That’s a slick idea. Problem is is my center stand was removed from the bike before I bought it. 👎

As far as taking the oil pan off, do you have to drop the pan to get the engine out of the frame?

I am hardtailing this bike, so theoretically I could just cut the rear end of the frame away from the engine and then just slip the engine out the back side. My cuts will be made at the points in the photo so I’m really only keeping a little bit of the front portion of the frame anyways.
IMG_6182.jpeg
 
I like the idea of a DOHC hard tailed bobber, good frame size for a hardtail, I have a XS650 hardtail mocked up ready for final welding and paint, small frame, small bike, but takes good pictures.:laugh2: Start a new thread here when you start to change the frame or change the title to this thread and keep us up on your progress.
 
I would still do a valve job before tackling rings. My 750k wouldn't go above 6k under load (revved all the way up in neutral). I had compression numbers somewhere around 120 (I think). Bought rings, but decided to do a valve job first. Found a few loose and tight valves. Afterward, bike ran freaking awesome. Still have the rings sitting around. (If you want them, maybe we can work something out.)
 
mrtwowheel said:
I like the idea of a DOHC hard tailed bobber, good frame size for a hardtail, I have a XS650 hardtail mocked up ready for final welding and paint, small frame, small bike, but takes good pictures.:laugh2: Start a new thread here when you start to change the frame or change the title to this thread and keep us up on your progress.
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Will do for sure. Here’s a couple xs650’s of many I did. I figured it was finally time for me to step away from the xs650’s and sportsters and tackle the ever elusive cb750.
IMG_2022.jpeg

IMG_1320.jpeg
 
brettp said:
I would still do a valve job before tackling rings. My 750k wouldn't go above 6k under load (revved all the way up in neutral). I had compression numbers somewhere around 120 (I think). Bought rings, but decided to do a valve job first. Found a few loose and tight valves. Afterward, bike ran freaking awesome. Still have the rings sitting around. (If you want them, maybe we can work something out.)
Click to expand...
Yeah I’ll definitely inspect the valve train when I do the rings and if I find ANY problems (missing, broken, or worn parts…etc) then I’ll definitely just get the valve job outta the way.
 
brettp is referring to a "valve adjustment" calling it a "valve job", could change your compression numbers. Nice hardtails, I did a similar exhaust as your blue bike with stock chrome headers, same exhaust tip and a DIY muffler in between similar to a B&S muffler. Have no idea what it will sound like. Why the extra trans shaft on the blue bike? Chain clearance?
 
mrtwowheel said:
brettp is referring to a "valve adjustment" calling it a "valve job", could change your compression numbers. Nice hardtails, I did a similar exhaust as your blue bike with stock chrome headers, same exhaust tip and a DIY muffler in between similar to a B&S muffler. Have no idea what it will sound like. Why the extra trans shaft on the blue bike? Chain clearance?
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Yeah extra chain clearance. I wanted to build an xs650 with a 200mm wide tire setup so in order for that to work, I obviously had to widen the ass end of the frame substantially and then I built a jackshaft and figured out the 4 sprockets ratio numbers to have enough chain clearance for such a wide tire. That whole build was the monthly feature bike for xs650chopper.com. You can see the whole process here… https://xs650chopper.com/joshs-200mm-wide-tire/
 

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