78 cb750f Rebuild Recommendations

ChrisH

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Hey guys, I currently have a 78 cb750f motor completely torn apart.

I am building a cafe racer, and am looking to build a street reliable motor with some mild horsepower upgrades. Would like to keep stock carbs if at all possible.

I’ve been looking at Cycle X’s .50 to 1.00 over piston kits. I wouldn’t mind running a 836cc kit. But I don’t really know if the cost/horsepower gain would be worth it in the end (I know I’d probably be upgrading the connecting rods if I chose this route).

Also, while I have the motor completely disassembled, any other parts to upgrade besides the head studs and valve train that I should know about?

Any comments or recommendations would be much appreciated.
 

dirtdigger

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Try searching a bit on the sight, I have talked about it too many times to count. But you said you are looking for more HP so count out the stock overbore sizes....you wont notice any increase there. The 836 will give a noticeable increase. Stock rods can be used but I suggest upgraded bolts at a minimum. New heavy duty cam and primary chains, heavy duty cylinder studs a must. New timing chain and primary chain tensioners also a must. Cylinder head is where all the power is made in the sohc....the F model needs additional care as it has valve spring retainers that fail and more than likely the exhaust guides are worn out. At a minimum get the head rebuilt from cycle x with new iron guides and their valve spring and retainer upgrade. The F cylinder heads flow really well compared to the K heads so you wouldnt need to port them but wouldnt hurt if you have the money. At the same time a new camshaft will only benefit the big bore, cycle x has some good ones to choose from.
 

ChrisH

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Thanks for the reply. I appreciate your insight. I’ve poked around on the forums a bit. You seem to have the most expertise when it comes to these motors.

I spoke with Ken at Cycle X. For my application, he pointed me in the direction of the .50 to 1.00 kits. He also mentioned what you said regarding head work/valve guides/retainer upgrades.

Unless the cost difference isn’t that much more for a 836cc kit, Im leaning more towards just going with one of the stock overbore sizes, getting the head work done, upgrading rod bolts, and doing all the upgrades you and Ken have recommended. I’ll probably have the head ported too if cost is reasonable
 

dirtdigger

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The 836 kit only requires boring just as the std oversize so the only cost difference will be the difference between the piston kits. If you are fine with the stock power then stay with std oversizes and dont bother with porting as it is pricey. If you want some extra power then you will need to step up to the 836. The F head flows better than the K head so you would still have a good little motor with a rebuilt stock port head with either engine size. But if you have some extra money and want even more power then add the porting to the head. Also when looking at parts remember the F engines use there own piston design to work with the bigger combustion chamber as compared to the K engines.
 

Snottyvar

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Very Timely thread. I'm considering a Rickman build and 836 is the way to go. It's a tough choice all around. I'd end up with a machine to valuable to keep that would tap out the pension for a year. Damn it would be great though.
 

ChrisH

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Haha darn it. You guys are talking me in to the 836. The wife is already pissed with how much time I’m spending on it. Now I’m gonna have to tell it’s going to cost more too lol
 

dirtdigger

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As the saying goes....speed costs money how fast do you want to go. I like like my cb750's to be fast so my bank account is always empty.......
 

Snottyvar

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My missus has a horse. I highly recommend this as the horse costs more and is a bigger pain in the ass that I could ever be. Hence I have five bikes. It's a great system.
 

Old_Crow

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My missus has a horse. I highly recommend this as the horse costs more and is a bigger pain in the ass that I could ever be. Hence I have five bikes. It's a great system.
Funny how I never had a problem affording parts for my wife's bike, but holy cow, look out if mine should need something.
 

ChrisH

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I think so. I’m pretty sold on the 836 kit now. I’ve been compiling a parts list of what I’ll need. Just need to pull the trigger I guess….

Been working on getting the cases all cleaned up. I was fighting those stubborn dowel pins for a bit there. But got all those out. I noticed there’s a small 1/8” hole in the case where the rear mounting bolts goes through, along with some other minor imperfections. I’m thinking about taking the tig welder, using a junk motor cover as filler, and filing the holes/imperfections. Just not sure how that will go with all the impurities in the metal. Just trying to fix what I can before I put it all back together.
 

dirtdigger

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Does the hole and imperfections cause a problem? If not leave them.....this old aluminum can be temperamental to weld. The aluminum they used in the 60's and 70's wasnt the best and can be porous and grainy and not weld very well at all some times. Welding when not needed also causes un needed stresses and can cause warping.
 

ChrisH

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No. Just all cosmetic stuff. Yeah that’s my worry.

Any suggestions on how to clean the cases? Hot water and dish soap?
 

dirtdigger

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Depends on how dirty the cases are. I normally use my parts wash tank then follow up with brake clean and compressed air. If very dirty you could use a degreaser and pressure washer. If over all not bad use couple cans of brake clean and compressed air.
 

Snottyvar

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I wash them in hot with a good degreasing soap then bead blast at home then take them for hydro blasting. This brake plate was a pitted mess. You can get the level of shine you want.
 

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Bagger John

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Here's a good 836 F vs. K discussion point:

Assume the same exhaust setup on each bike - decently flowing aftermarket 4-1 or 4-2. Stock carbs, airbox. Electronic ignition and a good set of aftermarket coils and wires. Cams to match the state of tune and head design. 836cc kit with 10.25:1 compression ratio. Lower end reinforced accordingly.

What do the HP/TQ curves look like for each head design?

Should I find the right candidate 750SOHC I'd like to build another daily runabout rig in the manner of the one I used to ride. The finished project will have a Windjammer and Shoei bags but probably carry no passengers - my other bikes are better for people-toting. The setup with the higher torque curve is more desirable for this use case, and It'll be operated at normal highway traffic speeds. Yes, I have road missiles if I want to obtain the coveted 6-point-operator citation. :lmao:
 

dirtdigger

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The stock k and f are within a couple HP so you will only have a couple hp difference with the 836 kit. The f is a more top end hp motor with the better flowing head.
 
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