Topic: 74 cb550k carb tuning help

[acer550]

Hello,
I'm new with the cb550 and I have a Lossa engineering yoshi style 4-1 exhaust with no baffle and uni filter pods. I was wondering what main jet, slow jet, and needle position should I go with to get a good starting point. Also I am located in southern California and my elevation ranges between sea level and 1000 ft.

Thanks guys 

[TwoTired]

100 main
38 slow
Needle clip in fourth position from top.

That's a starting position.

Fine tune with a dyno or a test track and plug chops.

Cheers,

[ChrisFMA]

@TwoTired what are plug chops?

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[BomberMann650]

I'm trying to remember the rule of thumb for choosing jets by elevation 

[acer550]

@TwoTired. Isn't that the stock setting on these carbs?

[BomberMann650]

I'm trying to remember the rule of thumb for choosing jets by elevation 

1,000ft of elevation won’t change anything.

@TwoTired. Isn't that the stock setting on these carbs?

It is. Start there and tune based upon plug chops. Else you guessing...

I really did want to know the elevation:jetting rules...

[TwoTired]

On the lycoming, leaning with the mixture control begins at 5000 ft. in the book instructions.  Fyi.

Cheers,

[BomberMann650]

Thanks TT!

[Scott S]

@TwoTired what are plug chops?

Do the same thing at Wide Open Throttle (WOT). Hold a sustained speed of WOT (at least 3rd gear) for 3 minutes. Pull in the clutch, hit KILL, and safely navigate to the side of the road. Pull the plugs, swap in a different set and ride home.

 I'm still trying to wrap my head around WOT in 3rd for three minutes. That's screaming...

[HondaYo]

What kind of roads with zero traffic or cops do You guys have where one can whip your poor little Honda for 3 min, +100mph....Pull the clutch and don´t rev the motor to death. Buy several sets of sparkplugs and ride like a maniac in different throttlepossitions for several days to get a reading of how to tune your carbs to perfection. Seems dangerous and stupid in my opinion.
 Is this forum not for that kind of info sharing. It´s a halflitre Honda not a spaceship...

I´ve tried a couple of different main-jets for my 550 with Norris-cam, open foam-filters and a 4-1 with an open can.
115/40 with the needle on the 4th position works well for my bike. At low speeds it stumbles a bit but the higher in the rev-range I get, the better it runs. #110/38 would probably be perfect, but I´m happy with status quo.

[TwoTired]

One thing a dyno gives you is operation under full load.  Fuel requirements do change under loaded conditions.
Be aware that using lower gears places less load on the engine and affects engine fuel needs.
Doing plug chops in or near a city isn't recommended, as imo, you need to be topped out with max wind resistance to have the engine operating under full load at WOT.
Of course the police would consider it wreckless, drag racing, street racing, exhibition of speed, or some other moving violation.  But hey, aren't all mc riders outlaws? 

Cheers,

[ilikepinkcrayons]

As my understanding serves me, the 3 minute "requirement" is to allow the plug to reach a steady state condition at that throttle position and air/fuel mixture. It takes a little time for any junk to clear off (or collect) on the plug, and for the ceramic to change colors.

To get around the 3 minute issue, a new-tech way of doing this would be to weld an O2 bung into each exhaust pipe, and use a wideband sensor and guage. A buddy of mine uses a PLX wideband sensor and gauge, which gives you instant feedback on the AFR, so you don't need to load it for long enough to "imprint" on the plugs, kill+clutch, and pull the plugs on the side of the road, rinse/repeat. I've never done either of these before, so I can't give you instructions, but there are alternatives if you can't find someplace to blaze at 70-90 or whatever for 3 minutes.

[GardenGnome]

On the lycoming, leaning with the mixture control begins at 5000 ft. in the book instructions.  Fyi.

Cheers,

What if I'm at 4,800 above sea level? I am a ways off (probably next spring/ summer) before I need to worry about dialing the carbs in. As I was rebuilding the carbs I just followed what was already there, no changes. I'm betting I'll be regretting that choice when I fire it up for the first time.

[TwoTired]

On the lycoming, leaning with the mixture control begins at 5000 ft. in the book instructions.  Fyi.

Cheers,

What if I'm at 4,800 above sea level? I am a ways off (probably next spring/ summer) before I need to worry about dialing the carbs in. As I was rebuilding the carbs I just followed what was already there, no changes. I'm betting I'll be regretting that choice when I fire it up for the first time.

The 5000 ft guideline is intended to keep the engine from fouling spark plugs.  It also specifies to go full rich when below 5000 and certainly while landing the craft.  When you forget, a go around can be butt clenching.

In practice, I'll lean the mixture while taxiing, so when I ask for max power on takeoff it is immediate and doesn't need to clear the plugs after a prolonged taxi at low cylinder temps where the plugs can build up carbon. 

If you will always stay at 4800 ft, and don't get the engine to peak temps routinely, you might want to readjust mixtures.  But, you should be mindful of where you wish to roam off the centric altitude.  Leaning for altitude can result in too lean for sea level, which could result in overheating issues.  Yes some of the fuel intake serves to cool the engine.  Vaporizing fuel absorbs heat for the liquid to vapor conversion.

If your bike is stock, stock jetting should work fine for you at 4800 ft. below and above that a ways.

Carburetors hardly ever provide optimum fuel ratios.  Barometric changes in weather, same as altitude, effect ideal mixtures.  Carbs provide an all around compromise, which allows the engine to make adequate if not ideal power.

Cheers,