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Black spark plugs

Gediminas

CB750 New member
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Lithuania, Vilnius
Hello,

I have a problem with my Honda CB750 bol D'or 1981', It seems that it is running to rich, when hardly accelerating it looks that not all fuel is being burned and that confirms spark plugs, 3 are completely black and one is OK. Carbs were just rebuilt, cleaned in ultrasonic bath, all parts that came in rebuilt kit where changed, jets are stock (102 and 68), air filter is stock and new, air/fuel mixture screw is set same in all 4 carbs and set as in manual, stock 4 to 1 exhaust and no black smoke. Bike is starting just after pressing start button, no matter cold or warm, choke is working and is not stuck.
I am thinking whether is a fuel or spark problems, so I tried to check spark in all cylinders and compare with the one were spark plug is OK, but from visual side can't see any difference, battery is new and is charging OK (checked with multimeter). Maybe some of you guys had same issue or have some tips what to check next, because I am running out of ideas.
PS. Bike is stock and original and has VB52 carbs on it, it is the first time when bike is running in my hands, because I bought it with carbs issues already, carbs where leaking.
Thanks for any ideas.
 
Be aware that MANY rebuild the carbs more than once and don't get them clean, there are many ways they can clog where you can't even see it.

I personally feel 'ultrasonic' cleaning is a waste of time but then I have no trouble getting them clean to begin with.

If enough mileage on motor the valve seals will leak slight oil to color the plugs and act rich too.
 
Thanks for an answer, shouldn't i be blue smoke from exhaust if oil is burning? If I won't find any other solution I will try to remove carbs and clean them again.
 
Not if it burns completely enough. Catalytic converter cars rarely show oil smoke if cat still good but it won't be for long like that. The smoke only shows when the problem is excessive. You can darken plugs with oil and if ignition strong enough it shows at plug way before oil smoke. A light oil leak simply becomes part of the mixture.
 
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