Hi all, I'm having fuel issues and I'm stumped on what it could be, I've ran all different main jet sizes to play around with fuel after 1/2 throttle. Was running 117.5 mains in it and when I would get on it hard it would spit and sputter and my dad riding next to me says it would smoke alot, so I backed it down to a 112.5 and now I'm running lean, if I get on it hard it spits and sputter but if I pull the choke half way on it clears up and pulls great, I don't have major mods on it, has a sidewinder style 4 into 1 header with straight pipe and foam pod filters, I'm not sure about internal mods as far as cam or anything, when I first got the bike about 6 months ago I did all the time up stuff such as valves and plugs and timing, I don't really want to pull the carbs and adjust needles but looks like I might have to. Any ideas right off hand?
1976 cb750k fuel issues
- Thread starter shaunbe
- Start date
-
- Tags
- carburetors sohc
You are probably going to be in the 120 to 125 main jet range. I only adjust the jets in 5's 120, 125, 130 etc, its all I can get for jet sizes. To me the fraction sizes are not worth it because you get it perfect at 75 degrees but when its 50 or 95 your jetting is off anyway. The choke thing you did is exactly how I get close on my jetting. If you are 1/2 or 3/4 throttle and use the choke you are mostly checking the needle. To check the main jet you need to be wide open throttle. All of this providing the carb passages are clean. Sputter and what you are describing can also be from weak spark. I would throw some 120 mains in it and try. but you really should see what position your needles are in so you know, you need a baseline so you know what to adjust from. Also make sure your float level is high enough, carbs vacuum synced?
Yes carbs were synched when I did a complete tune up. And the bike will run all the way past redline if I let it. I can be running at 4k rpm in 3rd and just twist it wide open and it falls flat on its face, almost as if someone cut off the ignition, then if I do the same thing with a little choke it pulls great and runs hard. Float levels were checked with the clear tube from the carb drain method. Maybe I'll try going up again on jets and play from there. I don't think I have a 120 but I'm almost sure I have 122.5. I'll look and see what I can come up with.
I would put those in if thats what you got and try again. Still should verify where the needles are.
I am going to do that, I'm just dreading pulling the carbs apart and syncing them all up again, but I gotta do what I gotta do to get it running properly, I've been fixing oil leaks and such lately, so that is the last part of the puzzle is proper running engine, I've been thinking of putting velocity stacks as it helps with the way it runs as it pulls air in the factory style and should help it run better in theorie, but I want filtered stacks and am not able to find any.
These carbs work fine with pod filters, I prefer the cloth type over the foam type. You will still have to do jetting even with velocity stacks so you are not gaining anything. If you want simplicity go back to the stock air box.
I don't have a good useable stock air box, the foam filters were given to me when I got the bike. I know id have to rejet, I just figured with a more direct path of air flow the carbs would work better, maybe on a newer cv carbs it would help being that they have more air passages.
