Topic: 1974 CB550 Foul Plugs

[SisterRay]

Looking for some advise on sooty plugs.

I've had on going "issues" with my CB550 where the spark plugs constantly foul. From a riding perspective it hasn't caused too much issues besides having to ride the clutch a little when taking off as it feels like might stall but once the bike is going it feels responsive and fine. The colour between the plugs are pretty consistent besides Carb #4 which is half gray half white.

My question is, if the bike is running okay is this an issue or should I ride the bike as is. I'm unable to do a plug chop as I live in the inner city so am unsure at what range the plugs are fouling. If this is an issue I've mentioned the specs below which from what I've researched should be pretty much stock. The exhaust however is 4 into 2 instead of 4 into 4 is this an indication that I should go off spec as these aren't the original?

At one stage the carbs were rebuilt by a mechanic who used a rebuild kit but I've since swapped the main, slow and needle back to the OEM Keihin versions as recommended by many people in this forum.

The photo is of an off spec spark plug Autolite 4132 (recently was looked at by a mechanic who swapped them out) but prior to that I had D7EA plugs which looked identical.

I've checked the ignition timing, points, valves and sync'd the carbs. At this point should I be looking at adjusting the carb settings itself?


Specs:

Main #100 Keihin
Slow Jet #38 Keihin
Float height 22mm
Needle 4th notch (2nd from the bottom)
Air Screw 1.5 turns
New stock air filter from 4 into 1
New rubber boots from 4 into 1


 

[newday777]

Are you using ethanol gas?

[ekpent]

Stock air filter and how old is it ? A real old dirty and broken down stock style filter could be like riding with the choke on a bit.

[SisterRay]

Are you using ethanol gas?

No I'm using 98 unleaded

[SisterRay]

Stock air filter and how old is it ? A real old dirty and broken down stock style filter could be like riding with the choke on a bit.

The air filter is about 2 years old bought from the 4 into 1 website and hasn't seen too many miles, maybe under 500. Checking the air box the foam element looks worse for wear and air box doesn't have a drain pipe. Not sure if these have any effect on the air intake?   

[newday777]

Have you tested for vacuum leaks? The aftermarket rubbers are known for leaking.

[PeWe]

Those black plugs reminds me about floading carbs on my CB750.
Checked the reality with clear tube and saw that fuel levels were too high, 1mm over float bowl gasket instead of 2-4mm under.

One plug is much cleaner.
The only carb that is not floading?

[TwoTired]

Two years with a non functional breather drain and water blocked sponge could have collapsed the filter mesh paper making if it choke off the air supply.  Paper distorts with water vapor and clogs with oil vapor from the engine.  Honda recommends renewing the filter every 6 months and assumes the engine vapor recovery system is clean and functional.  Wouldn’t you be a little embarrassed if a new filter fixed your sooting woes?

However, there is no good reason for any cylinder to show different plug deposits than the others.  If the soot is coating the center electrode insulator, that provides a path for spark energy to avoid jumping the gap.

Check for carb coupler leaks, manifold oring leaks, Fuel level equal in each float bowl (clear tube check).  Emulsion tube hole blockage in the carbs, jets and needles same size in each with no wear marks.  Pilot screw position?

[SisterRay]

Those black plugs reminds me about floading carbs on my CB750.
Checked the reality with clear tube and saw that fuel levels were too high, 1mm over float bowl gasket instead of 2-4mm under.

One plug is much cleaner.
The only carb that is not floading?

Yeah that could be the case. A few people have recommend checking the fuel height with the clear tube I’ll swing by the store this weekend and try fix together some contraption to check it

 

Two years with a non functional breather drain and water blocked sponge could have collapsed the filter mesh paper making if it choke off the air supply.  Paper distorts with water vapor and clogs with oil vapor from the engine.  Honda recommends renewing the filter every 6 months and assumes the engine vapor recovery system is clean and functional.  Wouldn’t you be a little embarrassed if a new filter fixed your sooting woes?

However, there is no good reason for any cylinder to show different plug deposits than the others.  If the soot is coating the center electrode insulator, that provides a path for spark energy to avoid jumping the gap.

Check for carb coupler leaks, manifold oring leaks, Fuel level equal in each float bowl (clear tube check).  Emulsion tube hole blockage in the carbs, jets and needles same size in each with no wear marks.  Pilot screw position?

That’s a lot of food for thought. I’ll happily take the embarrassment if it fixes these sooty plugs! Thanks a lot for the check list I’ll make my way down the list and report back!

[PeWe]

A long check list is good. Everything else is just waiting to happen

[Deltarider]

That blow-by gas breather system on CB550s is notorious for wetting/fouling the paper filter element. Here dealers used to disconnect the tube. Others found a solution in using (inferior) foam elements which you can clean when it got dirty or wet. On my CB500 which lacks such system the paper elements last forever, so you can ignore the 6 months. That interval in the scedule was just a routine meant for dealers who ofcourse don't know what your filter has seen. It's just to guarantee you can continue for another year or 10.000km and the dealer can be sure you will not show up after a service with complaints. Now many of us do maintenance themselves. My OEM filter element is about 9 years old and, according to filter specialists in the automotive, now filters even better than a new one. Trapped particles actually enlarge the filtering area. Many don't know this. Too many good filters are replaced long before the end of their useful life.

[dave500]

check the resistance in the spark plug caps.