Almost!

[Braintree1]

Hi I'm new to this forum. I have an 82 cb750k. It had been sitting for most of its life,it only had 3200km on it this past May when I started cleaning it up. New 4-2 exhaust,cleaned carbs,synched carbs,new coils,wires,plugs. The bike looks mint!Starts good, idles great. As soon as I get it up around 3000rpm i dry foul my plug on my third cylinder. So close!Could it be magneto/rotor/points?the battery holds a charge.... help!

 

[Travis]

Did you just foul one plug or how many has this happened to on the 3rd cylinder? Dry foul meaning it's not black, wet, or oily?

Welcome to the site by the way.

 

[Braintree1]

Fouled a few plugs. They look sooty and black, not oily black

 

[Travis]

If you swap the plug wires from 2 to 3 does the problem go to the second cylinder?

 

[Braintree1]

If you swap the plug wires from 2 to 3 does the problem go to the second cylinder?

nope...stays on 3.

 

[amc49]

Run a compression check to make sure nothing wrong with 3, normal is around 170 psi. Could be valve clearance to drop compression, what makes many go after the carbs when nothing was wrong with them. Most even the shops never chased the valve clearances since they require shim changes to do.

FYI no magneto or points on those.......................

 

[Braintree1]

Compression test is 1 120 And 234 are all 147-150

 

[amc49]

That one being off that much says a problem coming even if firing there now. Run the valves on that one. DON'T go closer than .005" if possible and I don't care what the service manual says.

Normal compression and good ignition with carbs with correct jetting should be nowhere near black on plugs (D8EA), they will stay very light tan almost white if burning right. They take a while to color up too, if going black quick on all you got something wrong with ALL cylinders not just one. Just that one is the one that fails first.

Suspect carbs too, you have no idea how many (most, actually) clean these carbs 'perfect' to have instant problems. The idle is hooked to the primary circuit, idle then being lean from clogged causes the primary to go super rich (don't ask unless you got a half hour to listen) and the issue at that lower rpm. It gives people utter fits.

 

[Braintree1]

Problem ended up being carb. Needle came out and was in the bottom of float bowl... Runs great now