Topic: #3 Cylinder Fouled Plug / Falls Flat at 1/2 + throttle - Advice

[andyvclifford]

Problem:
Recently been working on getting my bike running again.  Cleaned carbs and reset them to what they were intended to be with new internals - see thread below. 

http://forums.sohc4.net/index.php/topic,168064.msg1945801.html#msg1945801

Currently when operating the bike Cylinder #3 fouls a spark plug with black soot within a few minutes of riding.  This is not happening on the other plug on the same coil.  All of the other three are of a tannish color possibly verging on a bit too lean but still in a much more normal range.  When I open the throttle past 1/3 or so bike falls flat and wont accelerate without a lot of work.  Could one fouled plug / poorly tuned / problematic cylinder cause this and what could be the cause of the fouled sooty plug.  Below is information I have gathered to help diagnose.

Bike:  75 CB750  (verified vin number)
Engine # -  CB750E - 2380363

Carbs
Carbs  # -  064A -  ANB
Main: 105 mains (brand new kehein)
Pilot:  40 (Keihein original jets)
Clip: 3rd position
Floats: new Reproduction
Float Valves: New 2.0 mm honda genuine
Air Screw: 1 turn
Carb boots: New OEM
Air Boots: New Oem

Coils
Not Stock replacements
Primary test - 5 ohms
Secondary on both - 26k ohms with caps
16k without caps
Caps NGK SD05F and XD05F - Brand new each 5k ohms

[TwoTired]

step one:  find out why #3 carb is pushing more fuel to the cylinder than the others.

Your coils aren't the problem, according to your measurements.

Done a check on cylinder health?  Compression check or leakdown?  How do you know #3 is as healthy as the others?

Filter pods?  Rag in the airbox?  Choke fully opening on all carbs?

Cheers,

[andyvclifford]

step one:  find out why #3 carb is pushing more fuel to the cylinder than the others.

Your coils aren't the problem, according to your measurements.

Done a check on cylinder health?  Compression check or leakdown?  How do you know #3 is as healthy as the others?

Filter pods?  Rag in the airbox?  Choke fully opening on all carbs?

Cheers,

Hey two tired. Thanks !

StoCk air box with new oem boots for it. New air filter. No pods of course not sure what rag in the air box means ?  But no there is nothing in the air box but the air filter !  Choke works properly on all carbs. Have not done a compression test. Don't have that tool unfortunately.


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[robvangulik]

Main jet fallen out?

[andyvclifford]

Main jet fallen out?

Knocked on a neighbors door who I now works on his cars and borrowed s compression tester. Compresseion seems the same across all cylinders


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[andyvclifford]

Main jet fallen out?

You sir are a goddamn genius. Pulled float bowl from carb and what was sitting happily in a pile of fuel but my new main jet


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[robvangulik]

Main jet fallen out?

You sir are a goddamn genius. Pulled float bowl from carb and what was sitting happily in a pile of fuel but my new main jet


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Thank you, but not really, i've been there and done that.
I bet it'll run nicely now

[andyvclifford]

Just took it on a test ride tried to keep it at about 3-4k rpm the whole time but here in richmond can be hard to find just a big open stretch of road.  It ran quite well!  Now on to tuning the carbs in and vacuum syncing.  Ive never really understood what order to do plug chops in.  I believe my idle circuit is just fine so I'd prefer to not have to do a bunch of idle chops.  However trying to do chops while riding has always kind of baffled me. Do you only do it for WOT?  what if you live in a city and cant really just open it up for 2-3 minutes at a time like that unless i go on the highway which id prefer to not do especially if i have to basically shut the engine off while riding and slow to a stop.   Is there any other way to mke sure jetting is proper besides this way or is it the best way?

Thanks!
andy

[ekpent]

 Good call Rob !!  If it is now running good with no issues ride it and enjoy. After you get some miles on it check your plugs and see how they look. Your within stock specs on your carbs. Constantly taking out and putting in spark plugs is hard on the threads and not as fun as riding 

[flybox1]

Is there any other way to mke sure jetting is proper besides this way or is it the best way?

Thanks!
andy

Accelerate from a rolling start all the way to redline in any gear.  Roll on the throttle.
If your jetting is correct, it should smoothly pull all the way, without hesitation, and feel like it wants to go more.

[andyvclifford]

Thank you Fly I will try this today.  I also still have to do a vacuum sync.  I have the proper tool for it and have done it before however I never feel like I am doing it correctly.  Is there a "best practice" for doing this step by step.  For instance, should I start with Carb 2 where the idle screw is first and then move on to the ones around it?  I know that when you adjust one carb is affects the others and this always throws me off on balancing them properly.

THanks!
Andy

[flybox1]

Bring the lowest one up and then the highest one down....throttle blip, and adjust the idle screw to keep you at 1000-1300rpm. Repeat.