OKay, getting cranky here

[Seacat]

I have a 1981 CB750c.

I have the feeling that number three cylinder isn't firing right. The exhaust pipe isn't heating up at the rate that the others are. (The last owner put a MAC Four into One pipe on it.)

I have rebuilt the carbs more than once.

Pressure tests are great. All cylinders are within ten PSI of each other.

The coil tests out good. Good spark and no difference if I swap between plugs. Good spark. (I moved the leads from plugs 2 and 3. Three still doesn't heat up right.)

Now here is what I mean. After running for maybe ten minutes I can't touch the pipes from cylines 1,2 and 4. I can grab hold of number 3. The bike is idling at 1500.

I am getting the occasional backfire through number four.

Any ideas?

Cat

[Tech78]

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[cbRider]

With the compression and coil in order It most likely is a fuel problem, like not getting fuel into the cylinder.
Did you swap spark plugs? What does the spark plug look like? Is it wet or dry? Are you running an air box? If not, spray some starting fluid in to number three carb and see if it starts firing. If you are running with an air box you can spray starting fluid into the vacuum nipple on the cylinder head to see if it will fire.

Let us know what you find.

Karl

[Myumi-2]

I had a similar problem recently.  One cylinder was not working/not getting as hot as the others.  After working my way back from electrical through cylinder heads etc...

I was thrown by the fact that the spark plug was wet and I therefore concluded that it was not a fuel problem.

Turned out that there was a fuel blockage at the point where the fuel line joins up with the Carb.  It allowed some fuel to get through, but not enough to allow continuous operation.  So when I took the float bowl off there there was fuel and there was at least some fuel getting to the spark plug.  But not enough to keep it running.  

Im guessing it is a fuel issue.  

[Seacat]

This is what John at Daddy O's told me yesterday.

He told me to make sure the throttle slide was moving freely as well as to check that the small air intakes in the intake mouth aren't blocked by the boots. I'll be checking them tomorrow morning.

Cat

[karl liming]

the end of plug wire will unscrew from the wire, cut 1/4 inch off of wire and screw cap back on wire.