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Discussion Starter · #1 ·
I have 1970 CB350 that is running rich as indicated by black fluffy fouled spark plugs. Its starts easy, idles well, and runs strong until the plugs foul. Here is what I've tried so far:
- set the valves
- set the timing
- adjusted cam chain (although I just found out it should be set at 90 degrees AFTD (after top dead center) on the left cylinder instead of at TDC.) (was set on the compression stroke)
- set the float to 26mm
- checked that correct stock primary (105), secondary (70), and pilot jets (35) were installed with good O-rings on my "3D" carb. Pilot jet rubber cap is there but a little loose.
- installed new float needles and seats and o-rings
- installed new secondary, and primary jet nozzles, and slide needles.
- inspected the diaphragms
- I confirmed the choke was opening all the way
- confirmed the slides were moving freely.
- I synchronized the carbs
- I set the air screw to highest idle RPM one cylinder at a time.

The original air filters are still on it which look a little funky but I swapped them out for UNI clamp-ons and it did not help.

It does not smoke; black, white or blue. It just fouls the plugs.

I'm stumped!
 

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I have the same problem - 3D carbs, new jets, new O-rings, etc. A friend suggested there may be worn carb parts, such as the emulsion tube and/or slide needle. I have brand new air filters and I switched the needles and that didn't help. I'm about to resort to lowering the float levels 2 or 3 millimeters and see what happens. In the meantime, I'm running NGK B7ES plugs, which has stopped the plug fouling.
 

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Discussion Starter · #3 ·
I've been thinking about going to the B7ES plugs but I'm hung up on trying to find the problem.

A couple other bits of information; I replaced the engine not long ago and the problem started with the replacement engine. The engine had probably been sitting around for awhile. I don't think its oil fouling because the plugs are fuzzy and not oily. Also, there's no smoke indicating an oil problem.

I put Sea Foam in the gas to help clean the fuel path through the carbs before I replaced the carb parts. Could Sea Foam be causing the plugs to get black and fuzzy?
 

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Update on my rich problem: I lowered the float levels from 26mm to around 27.5mm and it runs great. Went back to the colder B8ES plugs and they have good light tan color, not black any more.
 
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