Honda Twins banner
1 - 8 of 8 Posts

·
Registered
Joined
·
43 Posts
Discussion Starter · #1 ·
Hello again all,
A few months ago I posted a thread asking for some help with a 1983 cb450sc nighthawk that I wanted to get running. I got as far as my time and patience would allow, so I took it to a local mechanic who I heard about through some friends. They said he can fix anything. He's had the bike for a few months, torn the carbs apart twice and still cannot get the bike to run on its own without spraying ether into the air box. Obviously this is a carb issue. He has checked the electronics and compression-both in good shape. He says that he believes the carb bodies are bad, thats why he can't get it to run. I know he's a pretty good mechanic just by word of mouth, but is it possible that something is being overlooked with these carbs? Is there a port that is still clogged up from the previous washing that might not allow the gas to flow through properly? Maybe there's a spring or a seal that's messed up? I have a hard time believing that the carb bodies are "bad" when the bike only has a little over 7,000 miles on it. How could I start from square one on my own now that I know for sure that this is a carb issue? What would you all suggest? Here is the link to my original thread before I sent the bike off. http://www.hondatwins.net/forums/63...9532-83-cb450sc-right-side-carb-flooding.html. I certainly don't want to part out or scrap this motorcycle. I know how rare these twins are becoming, but trying to find a "good" carb body would be like trying to find a needle in a haystack...make that a dozen haystacks :roll: Thanks for your help!
 

·
Premium Member
Joined
·
24,208 Posts
OK, I would start at the beginning then. Compression test, valve adjustment and check that the correct plugs are actually installed and correctly gapped. That'll cover the basics and insure you do in fact have a good starting point. From there I would pull the carbs and go thru them carefully making sure each and everything is correct. There is a sticky topic on rebuilding these that discuss's all the bit and pieces. It takes me about 2 days to go thru them so plan on taking your time to get everything right.
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #4 ·
Thanks, I've seen that article and will follow it religiously. The pictures will be a great help. The mechanic that has it started from the beginning. He's checked compression, valves, plugs, and electronics and he's confident that all that is good. Based on the fact that it will only run at mid rpms with ether is it possible that there is a vacuum leak somewhere? Are these carbs prone to air leaks? It's behaving similarly to when I was tinkering with it, except it ran with gasoline and on one cylinder when I had it. I sent it to him because it randomly quit running, which he found to be a bad kill switch. When he tinkered with it thats when it sputtered and ran only on ether. Is it also possible that the carbs got re-gunked up between the time I quit working on it and when I sent it to the mechanic (about 2 weeks)? I don't know the extent of his work when he tells me that he's "gone through the carbs twice."
 

·
Premium Member
Joined
·
24,208 Posts
Anything is possible and just because everyone says the guy is good doesn't make it so. Depends on what their standard of good is.
I had a situation 3 years ago of loosing the ignition. Local guy in NC, yeah 3000 miles away from home, that was "great" worked on the bike, swapped all sorts of parts and couldn't get it to fire. After hauling it to WV I found the plugs had been gapped to .001", fresh plugs and she fired up. So that being said I would start from the very beginning if for no other reason than to verify and know it's right. I can say that I have not seen any kill switches on this model go bad since it's a simple grounding circuit, no power to burn the contacts. Bad connections, broken wires on this circuit mean the engine can't be shut off except by the ignition switch.
The only possible air leaks are the carb insulator boots, O-rings that seal them to the head or the air cut valve diaphragms. There's been lots of bad gas recently and causing starting issues. I've had it 4 times on this trip. If the mechanic did a normal clean of the carbs then not much got accomplished. As I mentioned it takes me @2 days to do them right the first time.
I would get the carb rebuild kit from Honda as well as the float needles. Kit doesn't include those or the air cut's and accel. pump. Those pieces are fine being aftermarket. There should be no need for jets unless they are damaged. You may want to increase the primary main jet 1 size to help correct the inherent lean run condition.
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #6 ·
Awesome. Good information. Thanks for your help. So based on what I'm reading (not just here but in other threads), it's pretty rare for a carb body to go bad, but most likely there's a bad seal, diaphragm, or gunk that's causing a disruption of air/gas flow. Would the next jet up from a #120 be a #125? Would overriding the airbox with pods help after I get the carb super clean and rebuilt? I've seen some like that, but I know it would need to be re-jetted.
 

·
Premium Member
Joined
·
24,208 Posts
I believe that's where people have been going with the single main jet carbs. A one size increase would typically be a 120 to a 122.5/123.
One the air cut valves you will have the pintle type, not the ball end. You cannot cut a ball end type to make it a pintle, doesn't work.
 

·
Registered
Joined
·
43 Posts
Discussion Starter · #8 ·
Alright thanks. I'm not sure that I know the difference between the ball and pintle type, but I wasn't planning on cutting/machining anything anyway. Let me get the bike back from him and I'll take what I've learned so far and try rebuilding them again, this time taking a few days to clean and pamper them. I'll let you know what I find and what my results are. I think I'll leave the 120 mains in. That way I'm not changing anything too drastically since that's what the carbs originally had in them. If I can get it running right I may re-jet them with pods later on. Any last suggestions from anyone? Thanks so much for your help!
 
1 - 8 of 8 Posts
Top