CB 450 twin. I said I needed a project


Been Around the Block
OK, I checked compression and I saw 140psi on the left cylinder and 90psi on the right. That seems like it could be a real problem. The left side is the one that has been running rich. I put a #30 pilot in and changed the main jets to 165. I started it and took it for a spin and there is still very little power when I twist the throttle. I could barely make it up a slight hill from standstill as it wanted to stall. I haven't checked the timing yet, I don't have a light and haven't used one before. When I set this up I used a test light and made the adjustment from that.


Over 10,000 Posts
140 is OK, 90, not so much. I think you mentioned that the motor was rebuilt. How long ago was thet and what was done to it? Lots of possible cuases from rusty or sticking rings to rust on a valve seat and other possibilities. I had a similar issue not too long ago. Seems that one valve must have dropped off the bench after it was cut and had a tiny little ding which was enough to cause it to leak.

A leakdown test would tell us more. Does your compression gauge have a hose that clicks on to teh gauge? If so, you may be able to connect that to a compressor so that with the bike in gear and brake on, let air into the cylinder and listen to each valve (ie each inlet or exhaust port and hear where it's leaking but be careful and have a helper and start with a low pressure - say 50 PSI so that the motor doesn't spin over and cause a problem.

The other way to test valves and seats is to remove the head and turn it upside down and pout kerosene (or something similar) into the combustion chamber on teh side with valves closed and wait and see if it starts to leak out of one of teh ports. Repeat on the other side.


Over 1,000 Posts
If it has been running rich on that side for a long time, they might both have leaks and the 140 just because of carbon build-up.


Been Around the Block
The rebuild was done a while back, completed in the Spring of 2019. When I originally picked up this bike, the cylinder head had a hole in it. A timing chain mount broke and the original owner rode it until the chain ate through the head from the inside. I replaced all the bent valves and the rebuilder did the guides. I did not like dealing with the guy that did the rebuild, he was non-responsive and it took him almost a year to rebuild the motor. At the end I was convinced that I wasn't going to get it back ever.

Once I did have it back it sat without oil for a few months. I was thinking that what ever build grease was used would protect the internals, but maybe it did get rusty. Doing a leakdown sounds like the next step, unfortunately I don't have the tools to do it. I'll have to figure that out. The idea of removing the head makes me want to cry. I guess there is no avoiding this though. I can't expect this thing to run/ last with the compression that far off. I'm never going to buy another non-running project again.


Been Around the Block
Update, my friend came over and we used his compression gauge to check my engine again. We got 150 in both right and left cylinders! The loaner gauge that I was using before had a universal end, and I thought I had it on tight, but I think I was just struggling to get it engaged well enough. So that let me breathe a big sigh of relief. I am going to work on it today @teazer and see if I can get it running with the #30 pilot and 165 mains. I did put the needle clips in the top most position, we'll see if it helps with the doggy midrange.

If anyone reading this has an old 450, have you ever re-packed your mufflers? Mine are completely empty and sound obnoxious. Not in a good way. They seem pretty well sealed up, not sure if I need to cut and re-weld to do the job?


Been Around the Block
And just a few short months later... :p
I spent some time tuning the carbs and it was just not running very great. I bought new mufflers (cheap emco things just to make it easier to work on without annoying the neighbors) and after watching a bunch of videos on tuning Mikunis (mikuni oz was the best) I realized that the cable the shop made when they put the new carbs on was about 40mm too short. SO I made a new cable this weekend and went for a test ride. It has never worked this well! I'm really excited to get to this point. I think I can finally get the inspection and get my plates to make it legal!
I made a video of the process of making the cable. I added sleeves to the inside of the metal elbows that go into the tops of the carb. There was so much friction in there and these totally helped.
Here's a link:


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