72 CB450 Carb tuning

[Nick4]

Hi Guys, I've got a 72 CB450 that I recently purchased non-running. I've replaced the points, set the timing per the manual (which i did without a timing light since I can't get it to stay running long enough to use one), taken apart the carbs, cleaned and put back together and I can now get the bike to start. Actually sounds fairly good, but I can't keep it running. It'll run for a few minutes if I hold the throttle slightly open, then after a bit it dies and won't restart. I have tried to dial in the idle to keep it running, but have had no luck.

So I guess what I was looking for is a carb primer of sorts that would show me where to start when trying to tune a carb. I have read the Carb Theory 101 posted here and understand the gist, just not where I should start to try to find and eliminate the problem.

Any tips would be greatly appreciated. Thanks guys

 

[cxman]

hang on first get it running again and when it dies

open a float drain on a carb and see if fuel comes out maybe your petcock is plugged and the bike is not getting fuel

Next

check your plugs for spark and if they are wet

that way we/you know you are chasing a carb problem not a spark one

 

[Nick4]

hang on first get it running again and when it dies

open a float drain on a carb and see if fuel comes out maybe your petcock is plugged and the bike is not getting fuel

Next

check your plugs for spark and if they are wet

that way we/you know you are chasing a carb problem not a spark one

Got it, I've gotta work this afternoon but I'll check it out in the am and post back. Thanks. I reran the fuel tubing, the old stuff was cracked and leaking so I suppose I may have crimped it or gave it too much slack.

 

[Parkwood60]

I believe those carbs just have a clip holding the bowl on. Turn off the petcock and pop the clip. Chances are there is a bunch of crud floating in the float bowl, especially if the fuel lines were going bad. If it will run with a little gas, but won't idle chance are the idle jet, and maybe even the slow jet are plugged with crud, and/or dried fuel.

 

[Nick4]

OK, it was a fuel issue. I ran the tubing a bit too tight. So, I reran it using clear tubing and now I can see the fuel going into the bowls.

She turns over now on the first kick (starter doesn't work). However, it'll idle around 1k for a few seconds then race up to 4.5k and not back off. but if I dial down the idle screws, it dies. That says to me I have a throttle problem somewhere, but I'll be the first to admit that I'm decently handy around a car, but this is my first motorcycle project and I am not that familiar with carbs, passed the basics.

Also a bit of useful info, previous owner ditched the airboxes and gave me 155 jets, although he never got the bike to run with them installed (electrical issues). He also gave me new 150 jets. The 155 is what is currently installed.

Thanks guys

 

[mydlyfkryzis]

Runaway idle is usually a vacuum leak, then possibly a timing issue...

Check for vacuum leaks especially around the rubber carb mounts/manifolds.

 

[Nick4]

OK, I've replaced the carb gaskets and now it'll fire up on first kick and idle right at 1.1k. However, if I rev above 3k it'll stay there. Also getting a popping out of right carb at higher RPMs. So, I am thinking timing issue.

Any thoughts?

Thanks

 

[Nick4]

Well, I let it be for a bit while I did a few other things. Went back out, fired her up and straight to 4k RPMs. So obvisouly I didn't fix much of anything. How to I find the vacuum leak?

 

[ubergeek]

So I just fixed a very similar issue I went through the normal suspects float adjustment, idle screw, timing, vacuum leaks. But it ended up being the auto timing advanced springs. As for how to check for vacuum leaks spray some carb cleaner on the gaskets on the engine intake side of the carbs if the motor revs higher there is a bad gasket letting air in you will need to replace it. This might be troublesome to test as the bike is already running at 4k but you should still see a bump up in rpm. If you find the the springs are not holding tight tension you should replace them. It took me a while to find some but I finally did on Mikes xs the xs400 spring are a direct match for a cb 450. FYI I have a 72' CL 450 the motor is exactly the same as the Cb.

 

[Nick4]

Hey thanks for the reply. I was starting to get a little worried. I'm haven't had time to mess with it much this week anyhow. But I certainly will this weekend. I did some googling about the advance springs and read a lot of similar issues being caused by those. Thanks for pointing me in new direction.