Motor backfiring and dies at low idle

[illumastorm]

Hi all, new here and this looks like the place to find answers to get my bike up an running. I just picked up a 1980 CB750F and the guy said it ran good last year but he tried to get it running this spring and had no luck, so it has sat until now, I have been able to get it to fire up and run, anything above 1,100 or so Rpms it runs good and smooth, but lower then that rpm it'll backfire some and try to die, I can restart it but it won't stay running at idle. From what I know about dirtbikes that means the idle jets are plugged and needs cleaned. But this bike has 4 carbs but I bet it's the same. Also it has the stock air box removed and it has the little pod style filters. Any ideas from anyone? If I can get this problem fixed it'll be a cheap toy as I got it for $100.
Thanks

[illumastorm]

Got the carbs all apart and there was some gunk like I expected. Got em all cleaned but was wondering about the jet sizes, the idle is #68, the main #102, and needle is #53E and looks like needle is not adjustable. Anyone know if these are right sizes or should I change em since it has the pod style air filters. I have heard they flow more air and maybe I need to increase the jets or find a stock air box and put back on it. it has the original exhaust also.

[LukeM]

The Factory Service manual is available on this forum.  Download it, and look in the carb/fuel section.  The stock jet size should be in there.  If not, reply via this forum, and I'll look for it in my Haynes manual.

Luke M

[LukeM]

Also, the consensus on this forum is that these carbs tend to work best with the stock airbox.  It's not that they won't work with pod filters, but it's a LOT of work getting them running right with that setup.

If your current jet sizes are different from stock, and they're all the same size, chances are someone has already done the "hard" work for you.

Luke M

[TOOLS1]

Welcome to the forum. It sounds like you might have a vacuum leak to me.
TOOLS

[illumastorm]

Ok thanks, I am using my iPhone to post on here and it won't download the manual, I'll have to get my laptop over to my wides salon and use the wireless connection there and then I can download it onto that.
Hey tools1 about the only place for a vacum leak is around the carb boots and the head gasket on one of these bikes right? There is a small rubber hose that's just hanging from the carbs ( between 2/3 cyl) and I think it's just a vent line but next time I try firing it up I'll plug it and see if maybe thats a vacuum line.

[TOOLS1]

I am thinking it should have a vacuum line going to the gas petcock.
TOOLS

[ferroussphinx]

not to steal any thunder but  

I run velo stacks on mine and it runs tits  so to say what works best is preference

i do know that it wouldn be a bad idea to put atleast one tens on the main jet and see how that helps and also  


how is the electrical system and what kind of igntion is and please post pics   so that we can see what kind of  beast you have   thanks for your time

[illumastorm]

Well I can't post pics yet because I am using my phone for the Internet and it won't post pics on here for some reason,  but I did some more looking and I found all 4 plug wires were original and cracking, I had some leftover car wire so I thought I would try that for now until I get the right wires, got the plugs wired up and man what a difference that made, idles almost perfect, revs up like a raped ape, I do know I need to get the carbs synced and I bet this bike will be near perfect running condition ( to me anyway). I'm going to get new plugs for it also. And to think I almost gave up on it and gave it to my neighbor in trade for a mower lol. Not now!!!

[theninjaguy]

Hey were you able to fix it?

My bike does the exact same thing too.  I have myself a 1972 cb750 and i have to keep it at low revs to keep going.  If not, then the engine idles for like maybe 20 seconds and it just slowly cuts off.  I'm not too sure what it is, I was thinking maybe it's something with the throttle cable (it sticky, and I don't think this is the cause of the problem).  

Question, what are the vacuum valves?  From reading the posts, are they the tubes that are connected to the carbs which lead to pretty much nowhere? I have 4 tubes that are connected to the bottom of the carbs and they just hang and lead to the ground pretty much.  

[LukeM]

The 4 tubes connected to the bottom of the carbs are overflow tubes.  If the float needle valve hangs open, the bowls will overfill.  The tubes take the overflow and let it drain onto the pavement.

I'm aware of a vacuum line on the DOHC engines which connects to a fuel manifold valve.  Also known as the Spawn of Satan, it shuts off fuel flow from the tank if the engine isn't running.

Luke M