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Discussion Starter #1
ok so im new to the site, seems great and lots of info on how to do just about anything. However im looking for some advice from the guys that have been doing Cafe Racers and know whats up. I have a chance to take over a guys CB750 project that looks like a nightmare for pretty cheap. The bike is in pieces and needs freaking everything done to it. The dilemma...Its a 1975 CB750K but he doesnt have the OG motor, hes got a CB750F from a 77 supersport. So is this little engine swap here a No No in the scene? or whos gives a shit just build it and ride?! is the F motor even worth using at all? Just hoping to get the scoop from some guys who know, Id hate to dump a bunch of time and money into a project that has an unspoken rule i dont know about!

thanks guys, have at it!
 

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Welcome, sounds like a real hand full.
What's your background (in do you have the skills to build a bike from scratch with an engine swap thrown in?)
If this is your first build I would forget about something like this unless you want the wife to leave you (she will take the dog)
I don't have any info on swapping the cb750 motors from different years as most of my knowledge is currently surrounding brit bikes.
 

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I have no real knowledge on the 750, but I have CB500f and CB400f and there are lots similarities on all Honda four generation , I think there where changes on the 750 but more in terms of compression but mechanically see no reason why you owuld have an issues as the engine is similar. People will only care if you are doing a classic restoration and you show it, then some geek will tell you the bike has the wrong seat pattern for the year type thing, its a custom bike!!

However ignoring all that.. taking on an existing build already named a knightmare is time to run for the hills, I did this with a bike, in truth she is my baby now but took 2 years and lots of money to sort all the issues.. and I bought it thinking oh thats a cheap bike.. its not its a myth theres no such thing as a cheap project bike!
 

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Discussion Starter #4
My experience with rebuilding motorcycle specifically is low. I've worked on them a lil over the years but have rebuilt an old v8 car engine and have a great understanding of what goes on with taking apart a motor. I think I could handle the amount of work and learn the bike as I go. 4 carbs is the only thing that looks over my head so far. Thanks for the tip on taking over a half project. Seems maybe hold out for a more stock complete bike.
 

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Formula is easy, next time you buy an old motorcycle make sure it is low milage, complete and not previously mangled. If you find one like that which just happens to have interchangeable parts with what you have now, you've got it made.

BTW: a 1970's CB750 is a terrible motorcycle to learn to ride on, you might want to invest in a little dirt bike and figure that part out first ;) some crashes will be involved.
 

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Have you found out what the actual difference between a 75K and 77F engine?
After you figure that out then you have to figure out if that difference means anything to you.

I say buying and have at it.
Far better idea then the thought of more clean stock bikes getting hacked up for no good reason.
Fuck cafe racers.
 

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Discussion Starter #7
Not new to riding. Won't be building this to learn to ride.

Differences in the motor seem to be mostly top end parts. Interchangeable parts here and there. F motor parts seems to have a little less aftermarket availability.

I do like a challenge though. Guess the purists could really only get pissy if you motor swapped a complete restoration build and tried to play it off.

Just don't wanna be the guy that puts a bunch of time into a build, then everyone says "Who the F would do that?"
 

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If that is the model of bike you "really" want and can get it DIRT cheap maybe. But look at it as buying a bunch of parts for the other running bike you'll need to buy. Buying a few "boxes of motorcycle" does not a motorcycle make. :) Only a good idea if you see allot of quality parts you might need. JMHO Cheers.
 

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ok so im new to the site, seems great and lots of info on how to do just about anything. However im looking for some advice from the guys that have been doing Cafe Racers and know whats up. I have a chance to take over a guys CB750 project that looks like a nightmare for pretty cheap.
The cheaper the bike is, the more expensive a bike is going to be to make it a motorcycle again. Free bikes are almost always the most expensive projects. BTW, what's cheap? To me a cheap cb750 project that needs everything is $300 or less (unless it is a special model which a 1975 cb750K is not).

The bike is in pieces and needs freaking everything done to it.
everything is a lot of things. There is (was?) a member here (sabercafe or something like that) who is a professional mechanic and he brought home a cb750 in boxes. Take a look how long it took him to put that bike together. I want to say it was over a year and he already had tools and experience. How much of those things do you have and how far are you thinking you are going to get?


The dilemma...Its a 1975 CB750K but he doesnt have the OG motor, hes got a CB750F from a 77 supersport. So is this little engine swap here a No No in the scene? or whos gives a shit just build it and ride?!
I have no idea what you mean by this? I suspect you are trying to talk about value of the completed bike, in which case why? it's a motorcycle, if it works what makes it a "no-no"? I mean it will never be worth anything close to a stock original, un-molested, prime example will be worth but so what, doesn't sound like you want that anyway. It's a cb750 and nice running bitsa ones are worth about $2K (after you have dumped $4K into them to make them work).

is the F motor even worth using at all? Just hoping to get the scoop from some guys who know,
Well this is where you need to do some research, and I mean a lot of research, not just asking a forum questions and hoping people spoon feed crap to you. Here is what I will tell you - if the engine is already out and the mileage is unknown you are going to need to take the head off (don't cut the timing chain) and have valve guides done. The 1977-78 F engines have more power than the previous generations, except for the 69-70 K0 bikes, because of a big valve head which tends to wear guides out at around 30K miles.

The 77-78 bikes also have a wider chain line which means you need the sprocket drive and rear wheel spacers from a 77-78 bike. The engine won't just slot into the frame and the chain line up, you have to adjust the rear wheel as well. It's not hard but you will have to search for the parts.

The carbs are different too and if you have the 77-78 carbs you are better off already.


Id hate to dump a bunch of time and money into a project that has an unspoken rule i dont know about!
This is just stupid. I would rather you worry about dumping a bunch of money and time into a "project" that you don't have the skills or resources to complete. The sad fact about project bikes is no mater how much money you sink into them they are generally worth the same as what you paid unless you finish them. So if you genuinely don't think you have enough time/money/experience etc...to finish it then don't start it.
 

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I think you'll find very few "purists" here, just a bunch of motorcycle fanatics that value functional, ridable motorcycles and nicely restored or modified machines over mismatched bitsa chopper bobbers that were created out of hot parts just to look at while parked.

Previous riding or racing experience goes a long way towards being able to relate to what you are trying to accomplish. Buying an old relic and sticking a bunch of ultra modern bits at each end really is not well received here, having a functional motorcycle and making it truly better is generally considered impressive. If they like you, technical assistance where needed shall be forthcoming. If you post up some incomplete piece of shite and say 'look at what a wonderful cafe racer I built for resale' you'll probably get shot down pretty bad.

lol welcome to the site.
 

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Differences in the motor seem to be mostly top end parts. Interchangeable parts here and there. F motor parts seems to have a little less aftermarket availability.
eh...you are not doing enough research. If by top end you mean everything that is not the engine cases....well...you know what even the engine cases have slight differences. You can make other older stuff work sort of but really the only thing that makes the 77-78 bikes interchangeable is they use the same valve cover and the mounting points and lower engine covers are the same. That's about it.

Guess the purists could really only get pissy if you motor swapped a complete restoration build and tried to play it off.
Purists? what purists. The purists generally piss and shit all over anything that isn't a Sandcast or a K0 so.....
Here is something else, honda engine and frame numbers for SOHCs never matched. not from the factory. Nobody looks at them unless you are talking about a stock restored bike and even then it is to make sure the engine is in the "Range" of correct year. It's a bitsa bike - it will never be worth with a stock original bike is worth, which when it comes to 75Ks or 77F's isn't that much more money anyway.


Just don't wanna be the guy that puts a bunch of time into a build, then everyone says "Who the F would do that?"
It's a custom bike, people are going to say that about your modification choices anyway. who is this bike for? you? or them? If you are holding on to the illusion of resale value I can tell you that it will basically be tied to how cosmetically clean you make everything (paint, polish and chrome) up to a point and that point isn't really worth very much.

you wanna know what people really will give you shit about? a cut up and hacked frame. thinking you are going to take a sawzall to the part of the frame past the shocks and put a hoop on there? yeah get ready for people to verbally stomp a mudhole in your ass and walk it dry.
 

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Discussion Starter #12
Great. Lots of direction. Quick read on my phone at work so I will give it another pass through when I have time at home tonight and go through some of the points made here.
 

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I should add here that if this chassis has no title, and you don't know how to get a title in your state, you might as well forget the whole thing. Ownership papers are the difference between a bike and a part out.

The advantage of a SOHC cb750 in general is:
- pretty decent aftermarket
- lots of used parts
- good communities supporting them
- still raced competitively in vintage racing

The advantage of a 1975 cb750 chassis
- interchange of chassis parts from 1972 to 1976 (with minor changes) cb750Ks
- better looking stock than the later 1977-78 K bikes
- most aftermarket support in terms of other parts.

Advantages of a 1977 CB750F engine
- more power
- wider chain line to accommodate a wider rear tire (with appropriate rim a 1975 chain line can support a 140 size tire, a 1977-78 chain line can support up to a 180 tire provided you have changed the swing-arm and use the correct size rim).

Disadvantages to this project
- likely lots of pieces missing
- not cheap
- 77 F engine will need a going through at at least valve guides, timing chain adjuster, gaskets, an F compatible exhaust or conversion parts to the earlier spigots (F bikes use studs and a copper gasket for the exhaust flange, 72-76 K bike exhaust slip over a pressed in spigot and use a clamp), and the rear sprocket carrier from a 1977-78 K bike (because spoke rim vs comstar) and rear wheel spacers.

post some pics of this pile, let's see what you are working with.
 

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Discussion Starter #14
ok so takeaways here are taking performance and function into main consideration. Matching a motor to frame year isn't important so that answers that question. Money spent on a project will most likely be more than resale value, i am not aiming for a profit sale, just building for me. As far as the bike in pieces goes its good advice to keep looking i think. This bike came across me (sorry don't have pics) and i can pass and keep looking for something slightly more complete than someone else's failed project. I would skip any frame with out a title, dealing with DOL is not for me. Definitely gotta dig deeper into research. I pretty much looked up differences in the motors and didnt even think about what other things could be different between the years. Ill keep reading about the CBs and keep looking for the starter project that fits my budget and go from there. Hopefully my garage has the beginnings of a bike build very soon! Ill share as soon as I get it home. Thanks guys!
 

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Geeto is wrong!!!
First thing to do is get your rolling chassis together. I mean just bolted up. Don't spend any money on anything yet, just get it together to make sure you can and figure out how much stuff you will need to buy. Next, do a leak down test on the motor and see what you have. A leakdown test can also pinpoint whether any compression loss is the valves or the rings. If you have acceptable leakage then I would stick the engine in the frame and run it seeing how it does. You are going to have to take it all down to the bare frame again anyhow, so I would run the engine and see what you've got. So now you have a complete running bike that you can ride and see what it needs. What if you had done a fresh top end and then found out that it needs a main shaft, a handfull of gears, a primary chain and shift forks. That is way easier to diagnose if you have a running engine in a frame.
Go ahead and get it, just don't plan on having anything other than a slapped together bitsa for a while.
 

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I have a chance to take over a guys CB750 project that looks like a nightmare for pretty cheap. The bike is in pieces and needs freaking everything done to it.
Me personally.....without seeing it yet......these two sentences tell me all I need to know.

I wouldn't buy a nightmare. In fact I would run away from this "nightmare" and treat it like every part had the Ebola virus on it.

You could probably get one running on CL for $2000....titled and currently registered. Unless this was "Pops" old bike or J-Lo had her ass on it....I would be tempted to pass.
 

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It's a giuliari seat from Italy. Really great seat if you can find a 1970's original. There is a place called classic cycle city that reproduces them since the original company is out of business. Last I checked they are in the $250 range....but, they are completely bolt on, work with all the stock hinges and latches, and even have a small glove box in the tail.
 
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