Topic: Carb tuning on my cb550 with spark plug pics and details (help?)

[RFogelsong]

Setup info:

I have a stock (with the exception of motogp werks exhaust) 1975 cb550k running 022a carbs with the stock idle jet (I've seen conflicting reports of 38's and 40's being stock, all I know is it's stock) and a 105 main.  Idle air screws are 1 turn out and the needle clips are stock.  I'm running the stock airbox with the standard filter (a little dirty but not bad).  The floats are all within a mm of each other and I've done the tappets and cam chain adjustments.  I also checked the timing, points gap, advance springs and all that, it's good.  Stock coils, wires, and plug caps too. 

What it's running like now:

I've got all the above settings (stock with the exception of a motogp werks exhaust, a 105 main jet and the IAS 1 turn out) and the attached pictures are after a 1/4 throttle plug chop and a full throttle plug chop respectively.  It was about 50 deg. f when I did these runs and these are D7EA NGK's.

It's not stumbling anywhere and just by feel, other than a slight burble on hard off throttle deceleration (lean condition I'm pretty sure) you wouldn't say anything out of the ordinary about how it's running.  This slight burble on hard off throttle decel can be cured by adding in like 30% choke if that gives you all some indication.

Do these plugs look acceptable or too lean? Any suggestions for what (if anything) needs to be changed?

[RFogelsong]

1/4 throttle

[RFogelsong]

full throttle

[harisuluv]

Thanks for providing info about your bike and pictures.  It's a little hard to tell with the lighting in the pictures but it doesn't APPEAR to be bad.  Is the "burble" on decel a backfiring or exhaust popping?  If so yes that would indicate a lean condition.  Not familiar with that exhaust but with the 105 I think it is probably fine, a lot of 4-1 work fine with stock main jets. 

I think you should try tweaking your mixture screw a bit.  I also show from the SOHC specs page that the stock setting is ~1.5 turns.  Maybe you should try some tweaking and see if that helps.

[matt mattison]

1/4 throttle looks lean. Dropping the clip down one notch might help a bit. The full throttle seems pretty close.

[TwoTired]

Fine tuning an engine with a dirty air filter?  I wouldn't do that.

Spark plugs must be new/clean prior the entering test operation.  Prior operation deposits can contaminate a reading at a different throttle setting.

Exhaust popping can be leaks in the exhaust system or a too-lean mixture, particularly with a 4 into 1.

Engine must be at full operating temp for spark plug reading.

Who put in the 105s?  If it wasn't you, how can you expect/assume the slide needles to be in stock position or the pilot jets to be stock?

How are the air box and filter box drains routed.  What about the engine breather tube routing? Is the breather element clean clear and operating?

Do all four of the spark plugs exhibit the same deposit patterns?

How did you measure 1/4 throttle?

Did you kill ignition and pull in clutch immediately after WOT operation?   About how long was WOT operation and under what loading?

[RFogelsong]

On the backfiring vs popping:

Not really sure how to tell between the two.  It's coming from the exhaust exit.  I guarantee it's not a leak. 

On the idle air screw:

They were at 1.5 turns out when I started testing with the 105's and I set them to 1 turn out and the spark plugs came better on the rich/lean scale.  I'm debating going to 0.5 turns out.

The spark plugs weren't new before "tuning".  I may get a chance to get some new plugs tomorrow, and if so I'll so the same thing and post the results. 

Engine was definitely at operating temperature, I'd ridden it for about 10 miles at 45 or so mph with some idling in there too. 

I put in the 105's yesterday after discovering the 100's were too lean.  So lean I didn't even waste my time showing pics of the 100's.  Both jets were the stock honda jets but I didn't pull the idles to see the number. 

the air box and filter drains are routed per the manual instructions and the breather element is operating (I checked all this over good about 3 months ago)

All four spark plugs are similar in coloring, I decided not to post all four because of this. 

For both runs I ran the engine at the prescribed throttle and pulled the clutch and hit the kill switch to kill it.  For both runs I was at the prescribed position for about 1/10-1/5 mi.  For full throttle I started from 4000 and pulled until I ran out of road (I think it was around 7 or so).  For 1/4 throttle I started from 3000 and pulled to like 5000 or so.  This was on flat ground (Ohio farmland-couldn't find a hill)

[dave500]

if its a random odd popping itll be ok,,if its severe and repeatable every time with the same rythm you might be lean on the slow circuit,i think those plugs look good,if you can rev the engine up and down(not severely!) in neutral and it dosent pop its ok.

[RFogelsong]

Yep, it's a bit random, doesn't happen when I just easily roll off from part throttle, just under hard engine braking. Neutral revving shows none of the sound and its smooth.

[Deltarider]

Just a tiny bit of water in the fuel can cause it. Is your fuel fresh and free of water?

[dave500]

itll be ok fogel.

[TwoTired]

On the backfiring vs popping:
Not really sure how to tell between the two.  It's coming from the exhaust exit.  I guarantee it's not a leak. 

Even at the head pipe to head join?

On the idle air screw:
They were at 1.5 turns out when I started testing with the 105's and I set them to 1 turn out and the spark plugs came better on the rich/lean scale.  I'm debating going to 0.5 turns out.

  Make smaller changes.  Go to 3/4 turn first.  As the screws get turned inward, the adjustment gets more sensitive.
However, don't expect the air screws to have much change effect above 1/4 throttle position.  I adjust these for best throttle response under load at low RPM.

Engine was definitely at operating temperature, I'd ridden it for about 10 miles at 45 or so mph with some idling in there too.   

IMO, you can't spark-plug-color fine tune any of the three carb fuel metering circuits if the engine is operated at multiple throttle positions.