Looking for my next project.

[Pat_Beattie]

Hello everyone. I am new here and dont actually own a cb750 yet but I am looking at buying one for my next project. I am currently looking at a k3 that had a burt cam journal. The bike comes with a spare head. I have a fair amount of mechanical experience so I am not afraid of mechanical work. I'm not super familiar with these old honada though. I've attached a few pics of the bike that im looking at. What do you guys think of it?

 

[Pat_Beattie]

A few little things that I've noticed so far is that its got a later style fuel tank, with the petcocknon the left. And one of the gauges has been swapped (one blue faced and one green) not sure what one if either is correct for a 73. I will be looking to get a factory seat for it, and I know it doesnt come with an exhaust.

 

[Biebs]

If it is free it is still not a good deal. You should be paid to haul that thing away.

better off to start with a complete bike.

I would keep looking.

 

[Pat_Beattie]

If it is free it is still not a good deal. You should be paid to haul that thing away.

better off to start with a complete bike.

I would keep looking.

As I said I'm not too scared of the mechanical side of things. Is there a particular reason you would stay away from this particular bike. I figured that worst case scenario I would have to source an engine if the current one isnt salvageable.

 

[Biebs]

Used engine complete with carburetors would be good , but as noted in your post speedo and tach faces are different - what other mismatched parts will you find??

yeah you can get an engine running but what about the rear axle from some Chinese scooter??

are you on the east coast USA / Canada ?? Much better choices out there.

I would understand if you lived on some island and there where not a lot of choices for projects.

 

[dirtdigger]

I would do complete tear down of the engine especially if its been sitting with just some rags covering. No telling what critters are living in the pan now. I am wondering if they cut the frame to get the head off or if engine was pulled then bottom end set back in. Kind of guessing frame was cut which means you have more work ahead then just engine. Have a starting point if its cheap enough and depending on how you are going to build it.

 

[Pat_Beattie]

The frame is unaltered. It was one of my buying requirements. The engine was set back on the frame after disassembly. I am planning to do a full inspection of the motor before reassembly. I am located in east coast canada and the picking are pretty slim out here right now.

 

[dirtdigger]

Good luck on the build, any questions on the engine dont be afraid to ask. Are you going back stock or anything special on the engine build?

 

[Pat_Beattie]

I am planning on stock but if parts need replaced than I will likely upgrade them. First priority is to get it running reliably. Mods that sound interesting would be electronic ignition, maybe a mild cam. Also debating exhaust options. 4 into 2 into 1 is the way I'm leaning because stock 4 into 4 opts dont seem to be readily available, and the 4-2-1 are apparently better for performance.

 

[dirtdigger]

Well I say you need to visit the cyclex website if you havent already. I 100% use there engine products.

 

[Pat_Beattie]

I have been there. Thats where i got the idea for the cams. Their big bore kit is interesting as well. I'm hoping to not have to replace the pistons but if the cylinders need bored than why not go bigger, right?

 

[Pat_Beattie]

So I ened up getting the bike.
The good things is that it doesn't seem to have any hack work done to it. The gauges and the tank are from differnt bikes but still 750k parts, and they are in good shape. The see any issues with the bottom end. I plan on inspecting the oil pump and the primary chains next time I'm at the shop. The rotating assembly feels really good. (I don't want to have to pull the rods where the bolts are supposed to be single use items but I will likely split the block to inspect the mains)

The bad news is that it needs to be bored. It came with two sets of cylinders, but both have issues. One set has some light rust in one cylinder and the other set has a significant score mark going down the cylinder. I haven't inspected how well the valves are sealing yet, but the camshaft and cam carriers look to be in good shape.

I am now weighing out my options to get the motor running.

1: The cheapest option would be to resleave the one bad cylinder and get a fresh set of rings.

2: bore to oversized pistons and rings. Yamiya seems to have some quality piston options

3: 836 big bore kit from cycle x. This option is interesting but what other suporting modifications would be sugested for long term reliability?

What are your guys thoughts? I plan on keeping this bike for a long time so quality parts are a must for me. Is there any problem areas that I should pay attention to with these engines?

Also if anybody has has a good running engine or exhaust that they are willing to part with in the northeast let me know.

 

[dirtdigger]

Cycle x has oversize std bore pistons as well. I would just go for the 836, only boring is need for these. For a street motor you can use stock rods but should upgrade the bolts no matter what size of engine you build. No matter what size you build I never assemble any 750 motor without heavy duty studs, heavy duty cam chain, at least new primary chains, new timing tensioner wheels, new primary chain tensioner, new rod bolts or upgrade rods depending on engine build. The 836 will run in an otherwise completely stock motor but they really run good with head work and camshaft.

 

[Pat_Beattie]

Cycle x has oversize std bore pistons as well. I would just go for the 836, only boring is need for these. For a street motor you can use stock rods but should upgrade the bolts no matter what size of engine you build. No matter what size you build I never assemble any 750 motor without heavy duty studs, heavy duty cam chain, at least new primary chains, new timing tensioner wheels, new primary chain tensioner, new rod bolts or upgrade rods depending on engine build. The 836 will run in an otherwise completely stock motor but they really run good with head work and camshaft.

I am along the same line of thinking. But I wasn't thinking about heavy duty studs. Are you talking about head studs or main studs? Are they a common failure point on uprated engines?

 

[dirtdigger]

The head studs. They are a weak point in stock engines. They are half the problem of oil leaking head gaskets. I never run stock head studs.

 

[Pat_Beattie]

So I ened up getting a cheap parts bike, which I believe has a 750f engine in it. I am now debating which cyclinder head would be the better choice to use for my bike. Both heads are going to need a valve job. Is there any performance reasons to go with the later f head over the k?

 

[Pat_Beattie]

http://www.hondachopper.com/engine/engine_mods/modifications.htmlSo based on the link above if the F motor is from an F2 or F3 it will have bigger valves. Sounds like a winner to me. Can the older style carbs be used on the F head as long as I have F carb boots?

 

[dirtdigger]

The F2 heads did indeed have larger valves but also had larger combustion chamber. If you put a f head on a K motor you drop compression quite a bit. You will have to get f pistons to get the compression up to a usable amount or use high compression k pistons and adjust as needed to get a good fit and correct usable compression. You also need to had sleeves installed in a couple of the cylinder head stud holes in the head or you will have oil leaks. There is a picture of them in my gallery somewhere. The F heads also had valve retainer issues and can pull through and drop the valve into the cylinder, upgraded titanium or cycle x retainers needed. Accelerated exhaust guide wear is and issue and I suggest the cyclex steel guides over the bronze.

You will have to figure out what boots would work as the intake size on the head is larger than the K heads.

The F2 series heads will have a 410 cast into the top of the head in between the two inner intake ports.

 

[Pat_Beattie]

The F2 heads did indeed have larger valves but also had larger combustion chamber. If you put a f head on a K motor you drop compression quite a bit. You will have to get f pistons to get the compression up to a usable amount or use high compression k pistons and adjust as needed to get a good fit and correct usable compression. You also need to had sleeves installed in a couple of the cylinder head stud holes in the head or you will have oil leaks. There is a picture of them in my gallery somewhere. The F heads also had valve retainer issues and can pull through and drop the valve into the cylinder, upgraded titanium or cycle x retainers needed. Accelerated exhaust guide wear is and issue and I suggest the cyclex steel guides over the bronze.

You will have to figure out what boots would work as the intake size on the head is larger than the K heads.

The F2 series heads will have a 410 cast into the top of the head in between the two inner intake ports.

Good information. I will check the casting number tonight. If it is an F2 I am planning on using the cylinders, pistons and head from the F2 motor and the bottom end from the k3. I believe that all the oil passages and compression ratio should be correct then. I will also be getting all my parts from Ken at cyclex to rebuild the head. Which should take care of the weak valve retainers and guides.

 

[Pat_Beattie]

So there is no casting mark on the head of the parts bike. The engine number correlates to a k4 I think. The whole bike seems to be Frankensteined together. Its got a K frame and F air box and oil tank. I am wondering if they mixed and matched part in the engine as well. Is there any other external differences to the F heads and cylinders?