Just picked up a Honda chopper project

[Liffa]

Hi everyone,

I am new on this forum and was hoping for some help from you cb750 experts

I just picked up a project with a cb750 engine and I have never worked on any cb engines before.

The engine ID number is RC04E-2305643. I did some research and I believe this engine came from a 1982 cb 750f.

I am not sure when was the last time the engine was running, but I assume it was many years ago.

The exhaust system is missing. What exhaust systems would fit my engine, any exhast from a cb750 dohc??

I do have the original carbs. I though I would give them a clean and somehow try to start the engine before I do anything else. I have all the electrics in a plastic bag in the garage

What are your recommendations regarding carbs on these engines? I will obviously not run with any original air intakes or exhaust system. Would a dynojet kit be a good option? I read that the original carbs dont run well with pods, even with a dyno kit?

Thanks!

 

[Motogrady]

I like it.
Kinda has the easy rider captain America look.

Don’t see why you could not put the wiring harness on it, throw a battery in it, run a fuel line, put the 10/40 oil in the motor, strap the carbs in and see what happens.

A few thoughts if I may.
The pods thing, you will hear yes you can and absolutely not. That bike, I’d figure out the pods. Keep it as minimal as possible.
The brat bike guys are using some kind of real small battery to keep their bikes open under the seat. I’d go that way also.
I believe a 750 dohc motor will swap exhaust with all other 79 to 83 motors. How they will mount to a custom frame like that is gonna be the question. As far as that, imho, the only thing to do is going with 4 pipes, 2 on each side, under the engine, ending up by your shoulders, with
fishtails. That, I mean, I’ve never seen that.
Since chrome is almost impossible anymore, the question begs itself.
What the color? Red, blue, green, na.
You could just clear paint it.
That would be an eye catcher.
I dunno, gloss white frame and tank, with flat black exhaust, black leather seat, yeah baby.

 

[Liffa]

Thanks! Yeah, it looks quite cool. I havent started thinking about colours yet. I have a long way ro go in order to make it a street legal bike in Sweden where I live

I will do like you proposed, hook it up and try to start it. That will be an exciting moment!

I have found some "header pipes" on ebay that I might get, see photo, but then I would also need a merge/collector pipe to make it 4 into 2 or 4 into 1. I havent found any of the for sale. I guess I could just run with open headers???

Can someone please help me identify the two parts I am holding in photo 1 and 2?

 

[Old_Crow]

Part #1 looks like a brake pedal pivot shaft. Part #2, I'm not 100% sure, but it sure looks like the rear engine mount off of an old Ironhead Sportster.

When you get that far, I've got a brand new in the box set of forward controls and the linkage rod kit to go with them that I'll never use. Just been waiting for someone to build a DOHC chopper. See at: https://www.vintagecb750.com/products/133/frame-body

 

[Old_Crow]

Yup, I was right about the Harley part:

https://www.lowbrowcustoms.com/coll...rley-davidson-ironhead-sportster-oem-16203-67

 

[Liffa]

Part #1 looks like a brake pedal pivot shaft. Part #2, I'm not 100% sure, but it sure looks like the rear engine mount off of an old Ironhead Sportster.

When you get that far, I've got a brand new in the box set of forward controls and the linkage rod kit to go with them that I'll never use. Just been waiting for someone to build a DOHC chopper. See at: https://www.vintagecb750.com/products/133/frame-body

Nicely identified Perhaps the previous owner intended to use this engine mount for the cb750 engine.

Cool, let me know what you want for the forward control things and I will think about it. I dont have an original cb frame though, or any other foot control parts. I dont know if these kits will help me?

 

[Old_Crow]

The forward controls themselves mount to the two lower engine mount bolts on the lower part of the front of the frame. Those bolts appear to be there (or at least the holes) on your frame. The only things you need to complete the kit are the stock brake lever and stock shift lever. These will both have to be modified per the instructions in the kit, so the reproductions available on the same web site I linked to will be good to use.
Before we go any further, let me check with my son back in Arkansas. The boxes got left in his shed and I need to make sure he didn't do anything with them.

 

[PWBII]

Howdy! One thing to check with this model is the copper wound ring on the crankshaft under the right side engine cover, these are known to go bad with this engine. You will need the regulator rectifier for the electric system you put on it, as well as deciding on lighting. Might I recommend the dual rectangular headlights for this build? You should be able to find the mount to fit your springer on ebay. Maybe the axle mounted Taillight / license plate holder as well. A tall sissy bar would fit this set up very well if you can get the fish tails mentioned above for the exhaust. I know chrome flat fenders are still available out there. Lots of choices for seats, but I don't like the solo seats myself. You will find hiding the wiring to be the hardest part for looks.

 

[Liffa]

Howdy! One thing to check with this model is the copper wound ring on the crankshaft under the right side engine cover, these are known to go bad with this engine. You will need the regulator rectifier for the electric system you put on it, as well as deciding on lighting. Might I recommend the dual rectangular headlights for this build? You should be able to find the mount to fit your springer on ebay. Maybe the axle mounted Taillight / license plate holder as well. A tall sissy bar would fit this set up very well if you can get the fish tails mentioned above for the exhaust. I know chrome flat fenders are still available out there. Lots of choices for seats, but I don't like the solo seats myself. You will find hiding the wiring to be the hardest part for looks.

Hi there! Ok, thanks for the advice. Would you be able to help me with a part number for this ring? I tried to find it out myself, but didnt find the part. I just ordered the valve clearance tool for taking out shims. My research tells me I should aim for a clearance between 0.10mm and 0.15mm.

I was given all the original electrics that came with the cb750. Do I have to replace the regulator rectifier if I want to run a small lipo battery? This battery I have installed on my vn800 bobber and was thinking of getting the same for the chopper:
https://www.caferacerwebshop.com/en...-iAx3bZwTPEAQYASABEgL2tvD_BwE#product-content

Yeah, the rectangular lights would probably look cool, I just saw someone selling tripple rectangular lights. I might get it.

My wife and kids are telling me I should/must get a passenger seat for this second bike of mine…I was thinking about a K&Q seat, with a sissy bar of course

I have just been in contact with SFRO*. Normally when you build a bike here in Sweden, you must be able to prove what frame material, welding methods etc. you have used for building the frame. Since I have no clue where this frame actually came from I will have to find someone that can run som tests (eg brinell hardness test) in order to verify that the frame metal is good enough.

*SFRO (Sveriges fordonsbyggares riksorganisation) is a Swedish national organisation founded in 1982 with the goal of making it possible to register amateur built vehicles. The goal of SFRO is to help builders to build their own vehicles and register them for road use.

 

[PWBII]

I'll have to double check my service manual on the valve clearance specs. But I found a bunch of part numbers for the alternator rotor, (31100425871, 31110425005, 31110425852, 31110426382) last number is the most current. Prices vary wildly for this part. FYI, I'm working on a 1978 SOHC on a Jammer brand, Amen style chopper frame. I had the bike all mocked up & running good, so now, time to make it pretty. Ask any other questions you have.

 

[PWBII]

I think you will find the frame is regular cold rolled steel. I'm not sure from the pic if you have drop forged, drop outs on it.

 

[Liffa]

I'll have to double check my service manual on the valve clearance specs. But I found a bunch of part numbers for the alternator rotor, (31100425871, 31110425005, 31110425852, 31110426382) last number is the most current. Prices vary wildly for this part. FYI, I'm working on a 1978 SOHC on a Jammer brand, Amen style chopper frame. I had the bike all mocked up & running good, so now, time to make it pretty. Ask any other questions you have.

Thanks PWBII! Any pics of yours?
I will attach a photo of the drop outs, welded in place. What does this tell me?

 

[Liffa]

The forward controls themselves mount to the two lower engine mount bolts on the lower part of the front of the frame. Those bolts appear to be there (or at least the holes) on your frame. The only things you need to complete the kit are the stock brake lever and stock shift lever. These will both have to be modified per the instructions in the kit, so the reproductions available on the same web site I linked to will be good to use.
Before we go any further, let me check with my son back in Arkansas. The boxes got left in his shed and I need to make sure he didn't do anything with them.

Hey! On the right side of the frame, where the engine mount bolts are supposed to go I guess, someone has welded both bolts going through the frame and the nuts. Not sure how to fit the controls (or an engine mount), but maybe I will figure something out. I might ave to remove the welded nuts.

 

[PWBII]

What I'm seeing is someone took the OEM engine mounts off the original frame (as well as the kick stand lug) & welded them to this frame. The job looks good from what I can see. The rear drop out seems to be plain steel as well, not cast iron. Forward controls for this model should bolt to the front & lower engine mount bolts (if you use those, but you will need longer bolts). Another idea is, from that era bike the Super Sport models uses an aluminum beam on each side that holds the foot pegs & originally the muffler. these will put the pegs in the normal position on the bike for driver & passenger. If you look @ me pics the used ones are from a 1978 CB750F which will allow you to put rear disk brakes on your chopper (a very good idea!). the master cylinder, reservoir & brake lever all go on this bracket you see here. Abe makes a 16" rear wheel which will take all the OEM wheel hardware to mount it. Or you can find the rear hub spoked up to a 16" rim. You will just have to add the brake stay mount to the frame, to use the system. These parts are all over ebay right now. As an aside, if you want to keep the drum brake, the same parts from the 1978 "K" model will give you the option for both OEM pegs, without the disk brake spot. CMSNL.com has lookups for all these old vintage bikes to get the old parts numbers if you need any.

 

[PWBII]

OBTW, the hole above the axle is for your sissy bar to bolt to (short or long bar). If you stay with the drum brakes, you will need the piece you're holding in the second set of pics. the brake lever goes on it for the back brakes. There should be a spring for it as well. Just a note on these DIY build, I prefer to keep the fasteners I use the same standard as the original machine when possible. So, for you keep it all metric nuts & bolts. This is for if you have a break down on the road, you just need one set of tools to work on it. I can tell you the frustration of not having the right tool on the side of the road in the pouring rain, trying to get a job done. Also, it's kinda tough to put a fender on a springer front end. I do NOT recommend drilling holes in those tubes! I have seen folks use hose clamps on plastic & fiber glass fenders, but I don't think that looks good.

 

[Liffa]

OBTW, the hole above the axle is for your sissy bar to bolt to (short or long bar). If you stay with the drum brakes, you will need the piece you're holding in the second set of pics. the brake lever goes on it for the back brakes. There should be a spring for it as well. Just a note on these DIY build, I prefer to keep the fasteners I use the same standard as the original machine when possible. So, for you keep it all metric nuts & bolts. This is for if you have a break down on the road, you just need one set of tools to work on it. I can tell you the frustration of not having the right tool on the side of the road in the pouring rain, trying to get a job done. Also, it's kinda tough to put a fender on a springer front end. I do NOT recommend drilling holes in those tubes! I have seen folks use hose clamps on plastic & fiber glass fenders, but I don't think that looks good.

Ok, cool. I was wondering if that hole was for a sissybar but it seemed quite small.

I believe I will have to check some service manuals and drawings to fully understand where all parts go.

My wheels must be from an older cb750? It looks like the front wheel has a speedometer gearbox missing? I saw that someone sells a piece that replaces the speedo gear for a more clean look 100usd shipped to Sweden is alot though for this little thing

Are the original engine mounts removable and bolted to the frame, at least on one side? I dont understand why they welded the nuts and bolts on the right side front. I believe an engine mount is supposed to be bolted to this location?

 

[PWBII]

I didn't notice that before, but yes! you have SOHC "K" model wheels on yours. The stock speedo drive should work fine. Finding a long cable will be fun though. Also the front wheel is on backwards, the disk should be on the left side. You will have trouble mounting the stock front brake caliper to the springer front end, as well as front tire may lock up too easily. Most choppers of this kind don't use a front brake. This is the main reason I recommended going to the stock rear disk brake for this build. If you lock the rear tire, you can still control the bike, not so much with the front. The sissy bar should mount with 8mm bolts in those holes in the back, You can thread them if you like (8-1.25). More to come.

 

[Liffa]

I didn't notice that before, but yes! you have SOHC "K" model wheels on yours. The stock speedo drive should work fine. Finding a long cable will be fun though. Also the front wheel is on backwards, the disk should be on the left side. You will have trouble mounting the stock front brake caliper to the springer front end, as well as front tire may lock up too easily. Most choppers of this kind don't use a front brake. This is the main reason I recommended going to the stock rear disk brake for this build. If you lock the rear tire, you can still control the bike, not so much with the front. The sissy bar should mount with 8mm bolts in those holes in the back, You can thread them if you like (8-1.25). More to come.

Haha, i didnt notice the front wheel being mounted the wrong way. Thanks for making me aware.
Yeah, I might remove the front brake later on, but I need one for the final vehicle inspection to make it street legal. I will probably get a springer break kit and hopefully there is one that fits my front wheel.
Also, I am trying to keep the cost down for this project I was hoping to not have to replace any wheels at the moment.

The sissy bar holes are threaded

 

[PWBII]

Cool! I am all too aware of keeping the cost down, choppers can empty the wallet real quick. The small disk brakes you normally find on springers won't go over the Honda disk. I don't think you can make the stock caliper mount work on the front end due to the springer action; the front wheel goes up & down with bumps. The disk will want to go in & out of the caliper. The caliper has to be mounted to a bracket around the axle to stay in-line with the disc. Can be made. But won't be cheap. I have a springer with a disc brake, I can send a pic, next time I go to storage. As to the engine mounts the right-side mounts are removable plates with holes on them originally, front & rear. If you go with forward controls you will need the holes in the frame to mount them. Originally the Honda frame had a small metal pipe in the frame tube for the bolts, so the frame tube doesn't get crushed when mounting the engine. Lastly, figure out what pitch those threaded holes are, I hope not 1/4-20".

 

[PWBII]

OK, I got some pics for you to check out. First one is what I run. Second shows the disc brake on a springer front end. Third another view. Fourth, a view of the disc on the left side. Fifth is a caliper I think will work for what you got going on right now. Just have to make the 3/4" axle hole in this fit your metric front axle (17mm?) & make sure the piston will open wide enough for your disc. Speedo drive will go on the other side of the wheel & you will need a way to hold it from turning as the wheel spins. Last pic is of a HD model original equipment setup. I suggest starting with a complete used CB750 wiring harness & slowly remove each circuit not needed or used. Adjusting lengths as needed as you go. This makes sure you have the right connectors where you need them to be. Do some checking on this engine, I'm not sure you can minimize the OEM amp rating of the battery, the rotor I mentioned earlier Is a required part for this system if I remember right. My brother bought the 1981 custom new. Lastly You need 2 small ball joints on the brake stay for the caliper as it moves as the springer moves but stays the same distance from the center of the axle.