CB550 Bobber-Junkyard Dog


Over 1,000 Posts
I've always wanted a hard tail bobber chopper. Last week I saw this hard tail frame on the local CL. Seller said he was selling it because the motor had locked up and he'd gone on to different things, it had a CB500 motor in it. I happen to have CB550 that I was working on making a cafe racer out of but the project is stalled and I've sort of lost interest. So what's gonna happen is I'm going to stick my motor in this thing and see try to get it going. There are some interesting things going on with this thing to sort out.

The hardtail looks to be well done, the welds all look very solid. All the factory welds seem to have been smoothed down and blended in. It had a cool purple paint job with some pin striping but it's been in the weather and has quite a few nicks in it. The fuel tank is actually below the seat and used to be a fire extinguisher. There's a tap the bottom of the tank and a small vacuum operated pump that apparently gets fuel up to the carburetors. The seller said it worked pretty well before the engine seized. The front end doesn't look like it came from a 500 or 550, due to the Comstar wheels and trailing bake caliper. Maybe from a 650?

So the plan to begin with is to get my engine in the frame and see if I can get it to run. From there we will see.


  • 20180120_162833.jpg
    2.3 MB · Views: 637
  • IMG_2933.jpg
    162.2 KB · Views: 1,258
  • IMG_2934.jpg
    169.8 KB · Views: 1,237
  • IMG_2935.jpg
    161 KB · Views: 1,234
  • IMG_2936.jpg
    173.1 KB · Views: 1,226
  • IMG_2937.jpg
    170.3 KB · Views: 1,261
Re: CB550 Bobber

This is stupid. Ill watch :)

Sent from my SM-G920V using Tapatalk
Re: CB550 Bobber

I think the chipped up frame paint is cool myself. Throw a lump in it and ride it as is. That's my meaningless vote anyways ;)

Sent from my SM-G920V using Tapatalk
Re: CB550 Bobber

Hardtails, they seem to be popping up all over the place.

I just picked up a SL350 frame, that I intend to hardtail it, if I can.............

I think what Hurco550 said is exactly what I would do, and sounds like sound advice.

I'm watching this, hope I can get something started with my frame.
Re: CB550 Bobber

I still don't know what to think of that fuel tank. And put shocks on it. That would make it unique.
Re: CB550 Bobber

Of course since I got it home I've had no time to get out to the garage. I did get the engine out of the cafe frame and moved over toward the bobber. Once I get the lump in there I'm gonna let the avionics tech at work perform some magic on the wiring. I'm definitely gonna give it a go with it's current fuel situation. I may eventually want to make some changes but for the mean time the goal is just to ride it.
Re: CB550 Bobber

I was thinking about the conversation about fuel pumps and carburetors and needle seats etc etc tonight while I was pulling this carburetor apart to rebuild it. It is a Tillotson brand carburetor, this particular one was used on a 1972 boa ski snowmobile. They're kind of a neat little deal. There is no Bowl, but instead where the bowl would be is actually an integrated fuel pump that runs on vacuum Pulses from the crank. It still uses a needle in a seat just like a normal carburetor would. Just something to think about

Sent from my SM-G920V using Tapatalk
Re: CB550 Bobber

I was doing a little reading about vacuum operated fuel pumps. Seems they're pretty common on lawn mowers when tank is located lower than the carb. That would make sense since mine has a Briggs and Stratton logo on it, also says Mikuni though. Seems they are usually operated by vacuum pulses from the crank acting on bellows like Levi mentioned. Not sure exactly how mine works yet since it doesn't have a vacuum line running to it. It does have a return line back into the tank, that might address Kileys concern about excess fuel overcoming the floats and flooding the carbs. I guess we will see.
Re: CB550 Bobber

they dont work on vacuum they work on positive windage pressure

just to help confuse you guys here is a write up from one of my mikuni sales catalogs

A pulse fuel pump has four tubes connected to it. One tube connects the fuel tank. Two tubes connect each carburetor and one tube connects the crankcase of the engine. When the engine revolves, the tube connecting the engine delivers fuel with a pulse of pressure at each revolution. The diaphragm pulsates with the pressure. At the top of the pump are two chambers. They are separated by a pair of one way valves. These valves ensure that the fuel that passes through does not come back. Fuel flows from one chamber to the other at one pulse and at the next pulse, fuel moves to the engine. A well maintained pump works at 5,000 pulses per minute.


The bottom end of the engine crankcase is continuously subjected to a high or low pressure with every pulse. This pulse is transferred to the fuel pump by a pulse tube. The pulse line connects the pulse chamber. The pressure of the crankcase is pushed by the pulse through a pair on one way valves and fuel passes continuously, and in a correct measure, to power the engine.

A pulse fuel pump has its own limitations. One cannot estimate the fuel pressure capacity while going uphill. The fuel is likely to rise when the vehicle climbs. The amount of fuel is always limited to the pressure or pumping value of the crankcase. It is impossible to create too much pressure with a pulse pump for the carburetors float level. Pressure of the pump slowly decreases as the crank seals and gaskets begin to leak over time.

The pump should be mounted, keeping a distance away from the crankcase. If the pump is close to the crankcase, the pulse tube could become damp and the efficiency of pumping fuel will be greatly reduced. The pump should never be mounted on the crankcase of the engine. This is because the engine vibrates. The vibration of the engine will affect the pulsating flow of fuel and the regulated flow of fuel enabled by the pump will be affected. The pulse line must be rigid enough to avoid becoming damp. The best way to mount the pump is to place it higher than the engine. This will ensure that fuel from the engine will not be able to flow back into the chamber of the pump.
Re: CB550 Bobber

I forgot to take pictures but I got the motor into the frame today. I took a look in that fuel tank and all was not well. It's rusted all to hell with bits of "liner" flaking off. Instead of dealing with a sketchy thing that needs cleaning and lining, and may or may not send bits of rust and junk into my carbs I just pulled it off. I'll be in the market for a tank now.
Re: CB550 Bobber

Engine is in the frame and the exhaust is even mounted. It's about time to let my sparky friend help me with some wiring. I now need to make a decision about what kind of tank I want and get some money together to make it happen.

The more I compare this frame to other hardtails I see online the more I like it. Some people cut out the rear frame down which eliminates your rear engine mounts. Others keep the swing arm as part of the hard tail. I don't like either of these ideas. This one seems to have been done properly.

The paint wasn't done very well apparently. It chips anytime I think about it very hard. I knocked a bunch off getting the engine mounted. It looks pretty thin, probably just one coat. Oh well, I guess that just means I'll get it powder coated next year.


  • 20180130_205131.jpg
    1.3 MB · Views: 444
  • 20180130_205159.jpg
    1.4 MB · Views: 441
Re: CB550 Bobber

Your bike is even making comstars look good. I just beg of you not to put a sportster peanut tank on it. My .02 is a Wassel tank.
Top Bottom