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Offline Gpreciado

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So I aquired a CB750
« on: September 15, 2011, 10:34:18 PM »
Hi gang,

Just recently obtained a half finish cafe racer project from a family member that gave up on the bike. It is a 78 cb750 and I started gathering bits and pieces for it like a seat and other stuff. I have always lurked on this site but since I owned a yamaha really didn't have a GOOD reason for signing up.

With that said now that I own a CB I have alot of questions.

First the bike didn't have an engine and the PO bought one but it has the head removed. I don't know anything on te engine only that it looks like the cam chain roller guide looks pretty chewed up. So I am thinking the seller of the engine was hearing a slapping and tore it apart to find out what it was ad just didn't put it back together. The cases are powered coated and there is alot of chrome and polished parts also. So my question is can I to on the safe side split the case to check the bearings,chains,etc.. Without having to replace anything if it is ok? Or is there items I WILL have to replace one the case is split?

Second should I just replace the roller go one size up on the top end and put her back together? 

Finally can I use a cam from one set of cam carriers on another set of carriers?

Thanks guys
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Offline Retro Rocket

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Re: So I aquired a CB750
« Reply #1 on: September 16, 2011, 12:53:50 AM »
Welcome to our little community Gpreciado, don't forget to add pictures mate, we love pics.... ;)
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Offline MoMo

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Re: So I aquired a CB750
« Reply #2 on: September 16, 2011, 05:11:40 AM »
Do you know the mileage of the engine?

If you replace the guides, you also need to replace the cam chain. There are replacements available that use a peened master link instead of the endless chain.  Your other questions require splitting the cases-only way to tell for sure if boring is necessary.   I am not sure what the cam carrier question is-do you mean are the cams towers interchangeable or are they matched?

Download the manual from this site and welcome...Larry
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Offline Bob Wessner

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Re: So I aquired a CB750
« Reply #3 on: September 16, 2011, 06:04:42 AM »
Welcome. Ditto on the request for pictures.  ;)
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Offline Gpreciado

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Re: So I aquired a CB750
« Reply #4 on: September 16, 2011, 08:30:03 AM »
Thanks guys for the warm welcomes.

I don't know anything on the engine the PO doesn't talk to the family anymore and I can't get a hold of him to ask. I do know there was a lot of money put into the engine to get the cases powder coated and parts chromed so I can only assume that it was gone thru while it was disassembled to powder. The head and cylinders are currently off the engine and that's how I found it and saw that the roller guides were thrashed but, the stationary guide is actually in good shape. The cylinders look good and maybe I just need a bit of honing and a new set of rings.

Do you guys think I should split the case to double check everything? Or should I be fine just doing the top end? Please be frank with me.

On the cam I have two cams and two sets of towers in a box with some other stuff. One set has a good cam but gouged cam mounting surfaces, the other has a rusted cam that looks worn at the rocker locations but the towers are good. So can I use the good cam with the good towers even though they are not a matched set?

I will be posting pics later today ofthe new project.
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Offline MCRider

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Re: So I aquired a CB750
« Reply #5 on: September 16, 2011, 09:55:31 AM »
Last first. Mix and match the cams and bearings to suit. No problem, done all the time. Little choice really.

You can't assume anything, other than the worst. Still if the bottom end is bolted together, and it once was apart, I'd leave it and move on. If its not going to be a daily rider, primary transport sort of thing. I'd maybe pull the oil pan and see if the pump screen is clean. That would be an indication that it was more than cosmetic rebuild. And you'd know the screen was clean.

Personally, if you could get by honing and re-ringing that's what I'd do. Replace all the rubber, including the slide that looks good. Those 2 buttons on the roller axle can be used over.

I'd disassemble the head, check for valve leaks first with solvent in ports test, replace the valve stem seals, lap the valves in if they did leak, reassemble.

It'll be sweet.
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Offline madmtnmotors

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Re: So I aquired a CB750
« Reply #6 on: September 16, 2011, 10:58:01 AM »
If it were mine, I would put the top end back together (replacing whatever needs replacing) and hear it run. Even if you have to tear it back down, you're only out what, three gaskets? (Valve cover, head, and cylinder base). I would hate to tear the bottom end open and realize "wow, I wish I hadn't done that, everything looks fine" and then get overwhelmed trying to put it back together to the point that it doesn't get done.
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Offline Gpreciado

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Re: So I aquired a CB750
« Reply #7 on: September 16, 2011, 03:17:22 PM »
I was thinking on doing the exact same thing you stated, as long as the cylinders are good with a simple honing and a new set of rings I would only be 1-200.00 into it. Of o have to get the cylinders bored the next size up should I then just do the top end still? Thanks again guys

Oh I forgot should I change the engine studs since the head and cylinders are off?
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Offline Stev-o

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Re: So I aquired a CB750
« Reply #8 on: September 16, 2011, 03:51:55 PM »
First off welcome. No need to Change the studs if not damaged.

I agree w/mad mountain...
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Offline Gpreciado

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Re: So I aquired a CB750
« Reply #9 on: September 16, 2011, 05:38:27 PM »
Thanks guys I think that is what I am going to end up doing. One last question I know In order to check v8 engines if they need a honing or require a boring is to place a ring flat/level in the cylinder at different heights and record the measurements of the ring gap with a filler gauge and look at the min. In the service manual. Is this the same procedure for a motorcycle? Also what would the min be?

Thanks gang
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Offline MCRider

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Re: So I aquired a CB750
« Reply #10 on: September 16, 2011, 06:46:29 PM »
Quote from: Gpreciado on September 16, 2011, 05:38:27 PM
Thanks guys I think that is what I am going to end up doing. One last question I know In order to check v8 engines if they need a honing or require a boring is to place a ring flat/level in the cylinder at different heights and record the measurements of the ring gap with a filler gauge and look at the min. In the service manual. Is this the same procedure for a motorcycle? Also what would the min be?

Thanks gang
Yup same deal. Its a pretty wide range, generally 4-5 thou per inch bore. 61mm = 2.4 in x 4.5 = 11 thou in a perfect world. Actually that's tight. 13 thou is a little more relaxed.  Up to 25 thou is serviceable per my Clymers manual, though that may be a bit much.
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1988 NT650 HawkGT;  1978 CB400 Hawk;  1975 CB750F -Free Bird; 1968 CB77 Super Hawk -Ticker;  Phaedrus 1972 CB750K2- Build Thread
"Sometimes the light's all shining on me, other times I can barely see, lately it appears to me, what a long, strange trip its been."

Offline Gpreciado

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Re: So I aquired a CB750
« Reply #11 on: October 02, 2011, 09:35:45 PM »
So I had a chance to look at the engine and I measures the ring end gap and I am at the serviceable limit of .07mm so I guess I have to go up one size. Also my engine number is cb750e-2533803 so in looking at the database it is a 1976 F1 engine, can I use the .25 pistons from the 68-76 k-k6? The website show that I can use the rings but the engine is not mentioned in the piston kit. Finally what are the differences between the f1 and k? I know one is a supersport it that it?

Thanks again gang
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