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Top end rebuild question

jimminy

CB750 Enthusiast
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los angeles, ca
Hi, I have a '78 CB750 SOHC I recently bought that is blowing smoke like crazy and I've been considering doing the top end, though I've never done it before. I'm a little worried about getting it all dismantled and getting stuck, having my bike out of commission, and having to find someone to pick up the pieces.

So, I started wondering if there is any way to go about it another way. The idea would be to find replacement parts (the whole top end if needed) without taking my bike apart, fix what needs fixing if it needs it, then remove the engine and swap what needs swappin'. In theory :), this makes sense to someone who hasn't done it before.

I would love to know if there's any way to go about doing it this way or something similar. The main thing is to make all the mistakes while still flying around recklessly on my bike like normal.

If not, I would love to know if anyone knows someone around LA who can do the work. Believe it or not, I can't find anyone willing to work on it and the dealer wants a disturbing amount of my money to do it.

Cheers!
 
Hmm.. I would suggest you start by determining why it's blowing smoke and what needs to be fixed. Then you'll know if you need to pull the cylinders and how deep you're going to have to dig in. Whenever you pull the pistons out of their cylinders, you'll need at the minimum a hone job and new rings, but if there is wear, the cylinders will need to be bored and new pistons and rings installed. You might determine that your rings are fine and the oil is coming through the valves, then you could get a new head, rebuild that, then install it on your old engine to save time I suppose. You might find a hole in a piston. Who knows? Depending on what you have to do, you might end up having more into a rebuild than getting a complete good used engine to just swap out. Most of the time when people have an engine apart they replace a lot of things just because they are in there and want everything to be fresh. Have you done a leak down test? I guess if it's blowing smoke like crazy, you should probably stop riding it anyway to avoid any potential further damage to the engine you have. Good luck! Let us know what you find or what you end up doing.
 
Ah yes, I should have mentioned this. Blowing smoke like crazy is a little bit exaggerated, I have to admit.

I did the leak down test last week and the result was:
#1 cyl-30%
#2 cyl-17%
#3 cyl-30%
#4 cyl-23%

So only one cylinder is even close. This was at a dealer and they didn't really want to do the work. The tech there really knew his stuff though, remembered when my '78 was on the floor for sale. He said pretty much the same thing as you, Travis, about what I might find, but he thought it was probably no worse than the rings and maybe the cylinders. He didn't say anything about the valves but maybe he saw the look on my face when he gave me the quote and decided to spare me any additional apoplexy.

Even so, I would love to get in there and really get it flying, I just don't want to take on more than I can handle. I think I could deal with swapping the engine out with something in good shape, but I don't know where to find one yet. I'm going to spend some time today calling around and see what I can find though.
 
How much do you want to spend on a motor? Cycle X rebuilds stock motors on up to as big as you want. I would bet the farm that the valves leak too, if the cylinders are glazed to cause that much leak then the cylinder head cant be in much better shape. She is just old and needs some love.
 
There is a place up in Franklin new Hampshire called "coopers motorcycle salvage" the have sooooooooo much stuff and the owner reeeerally knows his stuff big time give em a call im posting via cell phone but ill try to post a link to them later on
 
I wish I knew how much an engine in good condition was worth, I'd pay about that much :). The thing is, if it's well over $1k to get it fixed by someone who knows their stuff, buying something ready to install looks pretty good if I can just find it. But if the idea is to replace the whole thing it would have to be local around SoCal, New Hampshire is a long way away to ship a 300 lb block. Unless it's possible to replace less than the whole thing and do the job.

That's a question for guys who know more than me. Everything else seems in good shape, like the charging system, ignition, and what isn't are things I want to replace part by part anyway (tank, seat, lights, controls, gauges, pipes, and I could go on) and that I know I can handle. Gettin' into the guts of that engine make me nervous!

Anyone know what I should have to pay for a stock (doesn't have to be pretty or perfect) engine? Or know what I should look for if I'm doing it in pieces that'll get this bugger going?
 
You can always get a cylinder machined for new pistons and have the head rebuilt. Or just take your cylinder and head of and send it off to have it rebuilt. Its fairly straight forward if you follow a shop manual. If you buy a "good" used engine then you have to take there word for it, unless you can see it run.
 
I know it would cost an arm and a leg to ship the block to Cali I meant dumb little shit like other stuff mentioned maybe even a head or valve cover etc. The last time I was at coops a lady came in looking for a mirror he asked her what side it was for she told him "can't Remember what side it was" but he said ok that side was reverse thread the man has incredible knowledge IMO. So big pieces maybe not that practice but smaller tid bits hell yea. Just trying to help a fellow rider/builder :thumbsup:
 
Hey bigcity, don't get me wrong, I appreciate the info. I checked the place online and it looks like a guy who has the know-how and the goods. And I ALWAYS buy from people I get a referral for.

So it seems like i can theoretically find a cylinder and head, get that in good shape, probably means new pistons too, and I should be able to pull the engine and swap out those parts with the new stuff, and as long as I do it right, be in good shape? Are there other things I should do while I have it out?

Sorry, I feel like this is the stupid questions thread of the year.
 
jimminy, yes you can have everything rebuilt ahead of time and just swap it on later. You wont know what the bottom end looks like but... Do you know what the mileage on the motor is? We have had several other threads on here when guys did motor rebuilds, I would do a search just so you get some more ideas on whats involved. You can message me and I can go into more detail if you want more info.
 
Cool, once I get the stones up to actually do this I'll take you up on that dirtdigger. I don't know the mileage on the engine unfortunately, so I just assume it's not a small number and I'll just do what makes sense for a high mileage engine and try to do it right.

I guess if I'm swapping in new pistons, cylinders, and head unit, maybe I should take it up a notch and do a large bore setup? Am I wrong that I can do that with all I'm changing out? This is getting interesting :D
 
You can do the 836cc kit or Cycle X 850cc kit without having to do much else. Any bigger then these kits and you need to do some aftermarket stronger connecting rods. You can run a stock cylinder head with these two kits but the performance increase will not be nearly as good. You really need a good ported head and a good camshaft to make these kits run good. When the time comes we can talk about tit more. Everybody always says they are going to do a simple rebuild then it turns into well I might as well add..........:laugh:
 
Well, my '81 CB750 Custom with only 39,000 Miles ran out of timing chain adjustment, so after looking at the cost and hassle of replacing the chains, I opted instead to buy a "new" engine on Ebay. Found a winner that was still in the frame of a Custom that had been stored in a garage since 1995 with only 12,000 miles on the clock. I asked the seller to check that he could turn the engine over with a wrench on the alternator nut and he went so far as to send me a video of that, so at only $125 plus $250 shipping, I bought the engine and it was perfect. I did a leak down test when it got here and checked the valve clearances. All good.

Swapped the "new" engine out and the rest is history. Engine runs like a new engine. Great compression. Plenty of adjustment left in the timing chains. I also got the alternator rotor and starter motor with the engine.
 
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The 81 750 custom uses a totally different cam chain and cam chain adjusting setup. The adjusting wheels are more likely to go bad in the sohc 750 then the chain itself, if you are using a stock camshaft and springs. If you are going aggressive with spring pressures and cam profile you need to upgrade the chain and really should upgrade the tensioner to the style that doesn't use the wheels like the one in my current motor....I think I have a picture of it in my gallery. But yes you can get good used motors but sometimes you just get lucky and sometimes you get screwed, thats why I wont buy a used motor, plus I like rebuilding them. But thats just me.
 
Yeah, that's the thing. If I bought one supposedly ready to go I would be going to a lot of expense buying and shipping it and wouldn't know a thing about the engine until I either put it in and cranked it up or took it apart and had it looked over. And then I might be right back where I started, but stuck now with 2 bad engines. At least if I buy it in parts I'll know the condition of the important parts before installing.

There are places all over the US who probably have the parts I need, I see some here on the forum, but I don't personally know which I can trust. Does anyone have a recommendations for the kind of things I need? I guess I'll just shoot for stock parts, in ready-to-install condition, but if it's a good deal, I don't mind buying it and having the machining and conditioning done here. Something in Southern California would be the best, but I'll go with quality over locality any time.

BTW, thanks to all for the tips so far, I'm starting to see the path I'm going to take. It's a big help :cheers:
 
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