CB750f #4 cylinder slobbering

Posted by john stringer on Mar 06, 2013; 2:19am
URL: http://vintage-and-classic-hondas.81.s1.nabble.com/CB750f-4-cylinder-slobbering-tp4037429.html

I recently purchased a '75 CB750F that has been in storage since 1989. 15000miles on the clock.
I have  overhauled the carbs.set all float levels according to my honda shop manual.new carb kits ( jets,  etc.) new plugs, checked valve clearances etc. but cannot stop it slobbering raw fuel on #4 cylinder. swapped plugs, swapped #1 and 4 plug leads all to no avail. double checked float levels. checked downstream of #4 carb before the inlet valve; no flooding. so the cylinder is getting fuel in the correct manner but not burning it.
As soon as I remove the muffler, Bingo! I get fire in all four holes and my temperature gun shows reasonably balanced values. Noisy, throaty but all 4 cylinders firing.
suspect the muffler.  removed and on the workbench i can easily blow through it, no resistance at all. no bulging cheeks. 30 lb of compressed air and again  good air movement through the baffles.
Install it back on the headers and its all back to square 1.
i am tempted to take it for a good hard blast, plenty of revs. in a lower gear and see if it clears itself but at the same time don't want to wash down the cylinder walls and have an overhaul on my hands.
Any suggestions, recommendations would be most welcome.

thanks
john