CB750f #4 cylinder slobbering

[john stringer]

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

[shinyribs]

That is just bizarre! Good call on the cylinder wash issue,though.

Only thing I can think of is low battery voltage. These bikes really depend on a well charged battery for the ignition to work correctly. Still doesn't really make sense about the muffler,but that's all I got. Wish I could help more.

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Low compression on that cylinder?
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[shinyribs]

I thought about that,too. But wouldn't increased back pressure actually help bump the compression a tad?

I dunno. Strange one here for sure.

[john stringer]

Thanks for the feedback guys (and gal)
Forgot to mention: Compression is around 128 / 130 psi and on par with the other 3 cylinders.
-5*C in Calgary yesterday, bike has not been started for a week, choke on , one kick and all 4 pots light up with no muffler installed.  if I allow the engine to warm up until the oil gets up to temp. then install the muffler she'll run good, no slobbering. leave the muffler in place. start from cold next day and the symptoms are back  and won't clear no matter how long the engine is run.
I've worked on dozens of these 750's, went on the first CB750 factory training course in UK back in '71. Instuct engines for a living but this one's got me foxed.

[Ditchraneman]

Bummer, if your stumped it kind of explains why I am...

I bought my bike about a year ago. It isn't my primary ride so it tends to sit to much. I am having a very similar problem on the second cylinder of a 75 750K.  I noticed it wasn't running quite right. The exhaust was cold to the touch.  Good spark arc (the spark plug out). Reinstalled the plug. restarted it, no problem. Then a couple of days of sitting and the same thing happens. Also, once it warms up the problem goes away. That was November. I haven't worked on it since (school, life, work, lazy, cold, etc).  

 

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john stringer wrote

Thanks for the feedback guys (and gal)

Hey Shiney He thinks your a girl.

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[RodneyCB750K1979]

Was the throttle and choke wide open during the compression test?  I think your compression is on the low side. Under 150 is poor.  Try some Marvel Mystery oil.  Low compression may also be due to poor valve adjustment/seats.  The cylinder firing with muffler off might be a symptom you're right on the edge of having enough compression.

Muffler resistance can leave compressed gas in the cylinder and at the end of the exhaust stroke when the intake valve opens, you could be blowing back through the carburetor.  But you checked the muffler flow and it appears ok.

On the intake side, how is the air cleaner and is the choke plate wide open?

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john stringer wrote

-5*C in Calgary yesterday, bike has not been started for a week, choke on , one kick and all 4 pots light up with no muffler installed.  if I allow the engine to warm up until the oil gets up to temp. then install the muffler she'll run good,

This information makes me think the pilot circuit on that carb might be restricted, or a problem with the rubber boot. Both of these can be affected by temperature.
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[shinyribs]

  That's Mrs Shinyribs to you John....lol.

And he called me a "gal"?! Not even a lady!!

[Piute]

     Hey Mr.John
   L.O.L. I road my bike all winter and these / My bike hates the cold ,I know that it ran great before the cold started ,So just made sure she had plenty of warm up time ,I'm not say I road it while it warmed up no she idled for 15min most the time.We hit a few upper 50's and she perked up like a spring chicken Fo-Sho.I have std. plugs and everthing is stk.,My 2 cents would be the coil try switching them,I know there on the top of my list..
     Slobbering funny thats how I dicribed what it felt like when my batt. was low,cruzzing at low r.p.m.'s (parade,local in town runs)has caused it a few times since I changed my gearing.