• Enter the March CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

Issues with my '79 CB750K. Seems like getting too much fuel?? or not?

1979GreenMachine

CB750 Member
Messages
13
Reaction score
0
Points
1
Location
SouthEast
Hi all,
Just joined the group. I hope to find some helpful steps to getting my bike on the road.

I bought a '79 CB750K last year for $250 with 5K original miles. I have a total of $500 in it now. I have bought alot of new parts (carb kits, air cut-off valves, front brake cylinder, tank petcock, etc.) and still can't seem to get it running right.
I have took apart the carbs and cleaned each one meticulously at least 5 times. I'm almost out of ideas what the problem might be.

When I bought the bike it would only run a few min with the choke on (seemed like a carb problem), I then cleaned/rebuilt all of them with new stock parts. The original exhaust was all rusted out and needed replacing.
I replaced the original pipes with a used set from a '81 Honda CB750C which seem to be a good replacement.
I have not changed the jets in the carbs. The bike will start and run strong a few min (sometimes), but then it seems like it's getting too much fuel. The exhaust looks gray and smokey as if running with the choke on. The choke is not on. After running like this for about a min it eventually bogs down (even with the throttle wide open) and dies. Note: the mixture screws are all set at about 1.5 turns out.


Have been trying to figure this problem out for several months now. Someone help!
 
Last edited:
Here are some pics of my bike
Note the change of exhaust. 553370_10151830025030319_1912442760_n.jpg 553547_10152220534515319_176416228_n.jpg 291868_10152299553145319_615747421_n.jpg 486357_10152299567290319_1105684155_n.jpg
 
Are you sure you are not loosing spark? There are problems with the ignition system and the spark plug boots on these. What are the spark plugs looking like? Was float level checked? Do all the cylinders seem to be rich?
 
Yes. All the cylinders seems to be rich. Strong gas smell from exhaust and lots of grayish smoke when accelerating or idling. I just adjusted the mix screws to 1/4 turns out, and still the same. I have spark on all cylinders, new plugs, and have trimmed back the coil wires 1/4in (for good connection)
 
I agree with Digger, in my opinion those ignition systems suck. I put a Dyna S and coils on a twin cam and the bike picked up 1/2 sec in the quarter mile with nothing else done to it!
 
I am just wondering if you are losing spark or the spark is getting very weak at the point when it dies. If that is good then you need to did back into the carbs. The only thing I can see in the carb is either too high float level or if the little black rubber plugs in the bottom of the carb are not sealing or left out???
 
dirtdigger: just checked the online carb diagram of the '79 model, it doesn't come with the black rubber plugs. As for the floats, the carbs aren't spilling over, so not thinking its a float issue. What about the vacuum needles? how often do they need replacing? New ones didn't come with the rebuild kit, but the old ones still look fine. How can I test the spark strength to rule out that possibility?
 
Almost all of the DOHC carbs need these plugs. There are a couple of oddball carbs on a couple of models that didn't need them, the 900 and 1100 are the only ones I know of. Looking at the bottom of the carb main body how many jets does it have? If you have 2 passages no plug if you have 3 you need the plug.
 
There are (3) passages in the center of the carb. Two of them are replaceable jets (102, 68). My carbs didn't come with those plugs, and I can't find them on the online carb diagram for a part #.
 
Thanks dirtdigger for that link. I just got them ordered. Hopefully this will be the right fix. Will update status again when I get them installed!
 
i bet that nobody has the correct answer yet, and i am gonna shoot for the Kewpie doll here. Your Vacuum safety shutoff on top of your carbs is leaking. the fuel goes 2 places. Into number 3 cylinder through the vacuum hose. And into the entire system through the vent line. eliminate it, and remember to shut the gas off. Problem solved.
 
on the very top of the carbs, your fuel lines run into it. and out of it, as well as a vacuum line and an air vent.
 
they were removed. very common practice as they are very expensive, and you cannot buy a kit, you buy the whole valve.
 
ok, as I look at the carb set... I have my fuel lines, vent (fuel spill-over) lines, and the smaller lines which feed the accelerator pump sprayers. Other than that, all I see is the vacuum piston cover/housing, and carb bowl
 
ok, yours has been removed. if you turn the gas on and walk away for a while is it leaking? did the new needles match the old ones? did you vacum or pressure test them?
 
No, the gas does not leak. The carb kits I bought did not come with new vacuum-piston needles. The old ones still looks clean and smooth. As for vacuum/pressure testing, I don't have that tool.
 
Back
Top