Topic: Spark plug help... 1 & 2 look good but 3 &4 look black

[Nstillwell]

So I have a 74 CB550 that I bought this year and I'm going through. I just did a compression test and all the cylinders tested around 120-130 PSI.
But the brand new plugs I put in 10 miles ago but now they look very different. 1 & 2 look clean the way I expect plugs to look but 3 & 4 look pretty black. I cleaned and bench synced the carbs before I replaced the plugs. I was pretty meticulous and my bench sync had all the carbs opening at the same point. I sonic cleaned everything in carb cleaner and reused the OEM carb components. I visually inspected everything and it all looked OEM and clean.

I have not done an adjustment on the valves or timing yet but both things are on the agenda. The bike runs and ran when I got it. I'm hoping someone can give me a idea of what the issue might be.

I would assume an electrical issue but I would assume that it would be cylinders 2/3 or 1/4. So I am guessing that I screwed something up on the carbs... Thanks all.

The first pic is 1 & 2 and the second pic is how the plugs on 3 & 4 look...

[Don R]

 High float level, leaking float valves or drilled jets. Some of the things I've found in them. It looks like fuel to me, if they weren't firing at all they would be clean. Oddly, I've read that a vacuum leak may also cause a rich condition.

[PeWe]

A friend had pitch black plugs as carbs should overflow fuel.

After a few tries he mentioned what he had done to the carbs, cheap aftermarket needles, thinner so the known settings were no longer valid.

[scottly]

It looks like fuel to me, if they weren't firing at all they would be clean.

1+ There is something different with those 2 carbs. Plugged side holes in the emulsion tubes or plugged air-bleeds can also cause a rich condition.
By the way, once plugs get carbon fouled like that, they will continue to fail to fire, even after you correct any carb problems. You can sometimes burn off the black with a propane torch, instead of buying new plugs every 10 miles while you troubleshoot the carbs.

[newday777]

Did you buy the plugs on eBay?
What plugs are they?

[newday777]

Have you done a vacuum leak test with short spray of carb cleaner, starting fluid or an unlit propane torch (real slow feed) around all the carb boots to see if the idle changes?

[Nstillwell]

A friend had pitch black plugs as carbs should overflow fuel.

After a few tries he mentioned what he had done to the carbs, cheap aftermarket needles, thinner so the known settings were no longer valid.

I bought a carb kit but left the old OEM brass parts in. The needles worked before the carbs were cleaned.

Did you buy the plugs on eBay?
What plugs are they?

Nope. Bought NGK D7EA plugs brand new at my local auto parts store.

Have you done a vacuum leak test with short spray of carb cleaner, starting fluid or an unlit propane torch (real slow feed) around all the carb boots to see if the idle changes?

Not yet. But the plugs were good when I pulled out the old plugs. So something changed and I didn't touch any of the rubber piece between the carbs and the engine.

[Nstillwell]

It looks like fuel to me, if they weren't firing at all they would be clean.

1+ There is something different with those 2 carbs. Plugged side holes in the emulsion tubes or plugged air-bleeds can also cause a rich condition.
By the way, once plugs get carbon fouled like that, they will continue to fail to fire, even after you correct any carb problems. You can sometimes burn off the black with a propane torch, instead of buying new plugs every 10 miles while you troubleshoot the carbs.

Yeah I think you are right. It's got to be fuel. Maybe a plugged fuel coupling between one side or the other. I need to check all the carbs again. I'll check the emulsion tubes again. Thanks!!

[HondaMan]

You removed the carbs for rebuild?

Vacuum leaks at #1 and #2 can cause this problem when #3 and #4 are running right. Check closely the rubber hoses (and their clamps) at the carbs, and if you haven't already, change the rubber O-rings in the intake manifolds where they bolt onto the head. These get hard as plastic and don't seal for beans, especially if they were 'disturbed' after 50 years. These O-rings are the same size as those in your valve caps if you need some handy for a test.