Topic: carbs wont idle and runnign way too rich

[bryon736]

Hi everyone.
I have been working on my 77 CB750 for a few years now. I am running a Carpy 4 into 1 exhaust, stock air box, the dyna clips are set at second from the top. I got it tuned from a local shop and they put in 108 mains in the 2,3 carbs and 105 mains in the 1,4 carbs. All the carbs are using stock 40 slow jets. The air adjustment screws are all set at one full turn out. The carbs have also been synced.

My problem I have been having is the bike is running way too rich and the plugs have been coming out black and sooty. From the research I have done this should mean it is running rich. I dropped down to a 102 main on all 4 carbs and rechecked the float high.

My biggest problem :
I am also have the worst time keeping the bike idling at a steady RPM. The bike starts up perfect no problems at all and will run great at higher RPM. It will also give a little bit of a back fire around 2 to 4 thousand RPMs. I know this is my slow jet system. And from what I can gather I am running way too rich. is it normal to have to reduce the slow jet size ? and will dropping down to a 35 slow jet help that much ?

I am really trying to get it to idle at a steady RPM I can adjust everything once it will idle at the proper 900rpm

thank you everyone in advance for the help
                 

[flybox1]

something doesn't sound right.  are you sure you're not LEAN?
look at my jetting setup in my sig, and I have a MORE restrictive exhaust than you are running.
what carbs do you have?

[harisuluv]

Your idle problem would definitely point to your slow jets, perhaps they are clogged slightly?

However, the range of 2-4k rpm is really slide needle dependent.  This is the range from 1/4-3/4 throttle.

Maybe at one point the needle clips were moved, have any confirmation on this setting?

Can you get the bike to idle correctly by adjusting the IDLE adjust screw (NOT air or mixture screws) and what RPM is that?

The jetting is a bit odd, from what I have seen, if people jet the outer carbs different than the inner carbs, it is due to pod filters and the change in air flow.  With a stock airbox, I don't think this is necessary.

[bryon736]

I attached a pic of the carbs. I was running pods and on recommendation of my mech I put the air box back on.  The jet clips have been moved to the second from the top inorder to lean out the carbs.

[flybox1]

you WILL run richer going from pods to a stock airbox 

[harisuluv]

Well, you have carbs that are round top, 71-76 style.

77-78 use the "PD" style carbs.  They tend to run leaner and are set up so.  They also have different needles, which means they aren't directly interchangeable, which you are experiencing first hand now.  The RPM range you have problems is regulated by the needle.  Even if you put the needle in the same position as a PD set, it's still a round top needle, and the taper will be different.

My advice:  Trade someone for the correct set of carbs (you will need PD style boots also) Looks like you might have an "F" bike or a K with an engine painted black.

Or you will have to fiddle with them for however long it takes.  What do you have the air screws set at?

[bryon736]

I do believe that the bike will idle at a really high rpm I will test this when I get home. And would it be possable that where the needles insert could be worn out and I am leaking gas into the engine at slow rpm?

[harisuluv]

Yes, it's possible.  It's also possible that all your jets are mislabeled from the factory because someone made a mistake and put them in the wrong bin, therefore they are all richer than what they actually say on the jet.  Anything is possible.

However, you have the *incorrect* carbs for your bike as explained.  To go chasing and speculating about some of the LEAST likely possibilities would be downright illogical when you have things staring you right in the face.

Again:  My advice, trade someone for the proper carbs, get some boots.

[bryon736]

I will do that this week. I have the old set of carbs (same model) I will bring both of them into my local shop and work out a trade. You are absolutly correct I should work with the proper carbs for that engine and then trouble shoot from there.
Thank you everyone for the advise!     

[martin99]

If I'm not mistaken the petcock is on the left side for PD carbs, so it looks like the tank has been changed too? Any chance of a full pic to help ID the bike a bit better? Not sure if you have a 77/78 K/F with early carbs and tank or earlier (70-76ish) bike with later engine? Or even an all-correct bike with a black-painted engine but with the '77' throwing us off?

[david 750f]

I see from your signature that you are from Edmonton. Isn't Edmonton at 2000+ feet elevation?

Once you get the proper carbs/jets cleaned and synced check if you are still running rich. You may have to go down one step on the mains.

I had to re-jet when I lived in Calgary (3500 feet elevation)..

[bryon736]

The bike is made up of a 74 frame and a 77 engine. Both K model and confirmed by serial numbers.  The tank is an older model and has double spouts on the petcock.


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[bryon736]

And I will be heading to Edmonton cycle salvage tomorrow to try and get the PD model carbs.  Has any one had better luck using the PD over the older round top models?


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[bryon736]

hey everyone just wanted to give an update. I just got the PD carbs yesterday. I have tore them down and am going to clean everything up. I have gasket rebuild kits and new rubber carb holders. I picked up a choke cable and a fule pump kit. I am woundering if i should return everthing to stock on the carbs or go a size bigger because i am running a 4 into 1 exhaust.   

[LesterPiglet]

Keep it stock until you know how it runs. No point intoducing another variable. A change of exhaust has negligable effect on jetting.

[harisuluv]

you should be ok with a 4-1

[bryon736]

Awesome! Thanks alot for the help guys. I am read as many of the treads I can find about rebuilding the PD carbs just to gain any more knowledge and trick and tips on rebuilding them. I really do appreciate all the help I will update after i habve cleaned and reassembled the carbs.