Topic: Number two cylinder not firing on my '78 CB750K. Absolutely at my wits end....

[BobbyR]

I think the point of all the suggestions you have been offer illustrate that the carbs are a system in which everything works together. All of the variuos orifices and mechanical components must work together. You have to go through them point by point. I tried quick and dirty and the result was not good. Since you and I have a single cable opening all of the carbs, synchronizing seems to be less of an issue. While that is true when we crack them open fully, I did find when i finally broke down and bought a $45 set of guages from Z1, my bike runs much smoother. Just go over your carbs point by point and you can get them working well.

[eldar]

Flat, a wire is FINE. Honda even sells wire cleaners. Berrymans cannot get the crud completely cleaned out of these things. If you need proof, go back and search and look it up. Many are the people here who have had to use a wire to clean their jets. You just do not use a LARGE wire.

As for your toilet analogy, remember, our shut off valve is ABOVE the float, not under it like a toilet. If you raise the float to say, 20mm or so, it will hang down FURTHER in the bowl and so would require LESS fuel to close the valve.

SO what you want to do is get some cloth and wrap it around the idle jet, grab it with a pair of pliers and twist while pulling. They will pop out then. Or should anyways. I had to wiggle mine which is ok IF you do not torque the hell out of them. Just a gentle wiggle while twisting and pulling BUT ONLY if the twist/pull method does not work. Last resort if you will.  Then use a fine wire, brass or copper as they are soft and will not have a chance of damaging your jet, unless you are severely ham-handed, and clean out EVERY hole. Once you have the holes cleaned, use carb spray to blow things out, followed by compressed air if you have it.

You will be surprised by how much berrymans can miss.

[Jonesy]

The big thing with using a wire to clean jets is many times if you pick out a wire that is larger than the size of the jet, you can end up enlarging the jet, which will screw up your tuning. Jet wires are sized according to a chart on the back of the case. You note the size of the jet you want to clean, then select the corresponding wire on the chart. This way you don't have to worry about accidentally enlarging the jet.

[medic09]

The big thing with using a wire to clean jets is many times if you pick out a wire that is larger than the size of the jet, you can end up enlarging the jet, which will screw up your tuning. Jet wires are sized according to a chart on the back of the case. You note the size of the jet you want to clean, then select the corresponding wire on the chart. This way you don't have to worry about accidentally enlarging the jet.

Yep, I actually went and bought a set of carb jet wires just for piece of mind.  You might try that.  GOOD LUCK!

[eldar]

Thats what I did and I just use the smallest wire usually then. It takes quite a bit though to open the holes up more unless you really do something dumb.

[flatblack]

Flat, a wire is FINE. Honda even sells wire cleaners. Berrymans cannot get the crud completely cleaned out of these things. If you need proof, go back and search and look it up. Many are the people here who have had to use a wire to clean their jets. You just do not use a LARGE wire.

Eldar, I won't argue that using a thin-diameter wire worked for you in cleaning jets.  That's great.  I just wouldn't do it.  Berryman's/MEK/shop air has always worked well for me, including some carbs others told me to junk.  That bike will start on the first kick, now.  Maybe we can compromise on some 6-lb. test monofilament?

 

And you're right about the toilet valve analogy -- too much time spent looking at carbs upside down on my bench.

fb

[BobbyR]

Flat, sometimes you just get a piece of junk that needs a bit of push. If you are careful you should have no problem with a thin wire. Copper is soft enough to prevent enlarging the holes.  may be a bit limp at that test.

[eldar]

Well flat, all I can say is that many here have used wire, not just me. For the earlier bikes, it may not be as common but for the 77/78, many of us have had to do it. But it is just like working on anything else. You have to be careful. But I think we would need a 10 pound test!

[realcrouton]

Number two cylinder is still not firing. I have done nearly everything you all have suggested short of removing the jets. Tomorrow I am going to check the compression. Maybe when I adjusted the valves I forgot to tighten up the tappet? I'm getting discouraged....

[eldar]

You have to pull the jet to clean it fully.  Aside from that, try switching the leads to plugs 2&3 and see if the problem stays on the same carb or if it moves. If it stays, then you know it is not spark. If it moves, then it is spark.

[Jonesy]

Don't give up. My CB750K3 used to do the same thing, dripping gas from the header pipe joint at the cylinder head at idle. You'd rev it up and it would act like it was running a flood out, and the pipe would warm up slightly. I figured it was a stuck float. Put in a new needle/seat. Still dripping. Hmm. Tore apart the carbs. Measured everything, set to spec. Still drips. Tore apart carbs again. Everything seems OK. Still drips. Some folks suggested bad coils. Swapped wires around, verified spark on all 4. Still drips. Reset valve lash, etc. New plugs. Still drips. WTF??

I tore apart the carbs for the umpteenth time. I was running my jet cleaning wires through the carbs and holding them up to the light (didn't do this before, as I was just checking the numbers on the sides.. did not look through them). When I held the idle jet for #2 up to the light, I found the problem. Someone, for whatever reason, drilled the jet out. The size is supposed to be a #40, but mine was drilled out to 130+. I say 130+ as my largest jet wire is a 130 and there was still plenty of slop in the jet bore!! I put a new #40 idle jet in and it idled nicely on all 4. Boy, what a head scratcher!

BTW.. did you try any of the ignition stuff, like check the plug cap resistance, swap coils, etc? Just wondering.

[realcrouton]

You have to pull the jet to clean it fully. Aside from that, try switching the leads to plugs 2&3 and see if the problem stays on the same carb or if it moves. If it stays, then you know it is not spark. If it moves, then it is spark.

Have not pulled the jet yet, but I will if the compression comes back OK. I did switch the #2 and #3 plug wires to no effect. #3 ran exactly the same with the #2 wire. So I gather from that it's not an ignition problem.

Don't give up. My CB750K3 used to do the same thing, dripping gas from the header pipe joint at the cylinder head at idle. You'd rev it up and it would act like it was running a flood out, and the pipe would warm up slightly. I figured it was a stuck float. Put in a new needle/seat. Still dripping. Hmm. Tore apart the carbs. Measured everything, set to spec. Still drips. Tore apart carbs again. Everything seems OK. Still drips. Some folks suggested bad coils. Swapped wires around, verified spark on all 4. Still drips. Reset valve lash, etc. New plugs. Still drips. WTF??

I tore apart the carbs for the umpteenth time. I was running my jet cleaning wires through the carbs and holding them up to the light (didn't do this before, as I was just checking the numbers on the sides.. did not look through them). When I held the idle jet for #2 up to the light, I found the problem. Someone, for whatever reason, drilled the jet out. The size is supposed to be a #40, but mine was drilled out to 130+. I say 130+ as my largest jet wire is a 130 and there was still plenty of slop in the jet bore!! I put a new #40 idle jet in and it idled nicely on all 4. Boy, what a head scratcher!

BTW.. did you try any of the ignition stuff, like check the plug cap resistance, swap coils, etc? Just wondering.

Wow, this is just what I am experiencing, that give me some encouragement since you got you problems sorted. I am a little nervous about pulling jets as I am still unclear as to what jet does what, can you give me a pic or a link to an illustration describing which jets I can pull out for a cleaning? I need to buy a Clymer and burn my Haynes!

Thanks for all your help and advice guys!

[Jonesy]

Click here to get the real Honda Manual: http://putstuff.putfile.com/42020/1753594

Couple other off-the-wall thoughts.
The air passage in the idle (or main) circuits could be blocked. This mixes air with the fuel at the jet. If these passages are blocked, the circuit will draw more fuel than intended, causing an overly-rich condition.

I remember someone posting on here about check balls in the carb with the accelerator pump. If these are to regulate when the fuel is pumped, if one was missing I wonder if extra fuel could be getting sucked through? (I'm not familliar with the K8 carbs, so sorry if this doesn't apply, just trying to help cover more bases...)

[realcrouton]

Thanks for the shop manual link, I just downloaded it. It is going to be soooo nice having this over the Haynes! 
 

[tinyrobot]

just sent you and email with another manual

[Jonesy]

This one may help too: http://www.cryscom.nb.ca/PDF_Docs/Honda_Technical.pdf

This is a Honda technical manual on motorcycle carburetion.

[upperlake04]

Some pics of your carbs

[realcrouton]

Thanks sooo much you guys. Those pics are perfect! I think I'm going to take tomorrow off and tackle this thing.

[tango911]

very nice thread, i have picked up a 74 cb550 and seems the #2 like yours is not firing.  ill be reading this thread more than once.

can someone plz list the Cylinder numbers for me.    Is is 1234  left to right as your sitting on the bike, or 4321  left to right?

thanks!

[BobbyR]

Left to right it is 1 2 3 4

[bollingball]

There is a check valve on #2 carb for the accelerator pump it is not made to come apart but can be done  and also cause your problem. To properly clean these 78 (pd42b) you must pull the slow jets an I agree you should run propper size wire thru it. There is a post here somewhere showing how to take apart the check valve.It has a ball&spring & plastic retainer.

[Spanner 1]

Tango guy.... start a new thread, folks think you have a '78 750, not a '74 550... look at the revived title you have resurrected