Topic: lean CB550 - What size main jets to move up to?

[neudl]

I have a 1977 CB550k with a '72 CB500 engine.  All else is stock, including air box, exhaust, etc.  After cleaning the carbs, the bike was running lean, popping, and bogging.  I moved the needle clips to the bottom position, and the performance is much, much better, but still some slight popping at times.  It still appears to be running lean, and I was thinking of moving to bigger main jets.  The stock at 110, with 92 slow jets.  I'm wondering if this should solve the leanness problem, and how hard it is to find jets for the '77 carbs.

[mlinder]

92 slow jets? Do you mean 42? and 110 should NOT be lean for a 500.... What do your plugs look like?
And, this should be moved to the main forum.

[alltherightpills]

Wait, are your carbs from the 72 500 or from the 77 550?  The stock main jet for the 77 and 78 550 is 90.  On my 78 I'm running 105's with pods and it's about perfect.  110s on a 77 with the stock airbox and exhaust would be waaaay too rich. 

[neudl]

Sorry - that's what happens when I start typing before coffee - the mains are 90, slows are 42.  The plugs are definitely lean.

[neudl]

Carbs are from the '77.  Except for the engine, all else is stock '77 550k.  I'm wondering if the smaller displacement has something to do with the leanness.

[mlinder]

"and how hard it is to find jets for the '77 carbs."

550 Carbs.

How lean are they showing? Describe them, please.

[neudl]

Pulled this pic off the internet, but it's a pretty good approximation.

[mlinder]

I believe the 500 was jetted a tiny bit larger, but, they were different carbs, too. Try 100 mains, and return the clips to middle position. I don't think getting the press-in idle jets is very easy for these carbs. You may want to buy 95's and 97.5's at the same time you buy the 100's, just in case 100's are too rich.

You don't happen to have the 500 carbs, do you?

/edit: hard to tell fromt he picture, but those don't look terribly lean... course, it isn't a picture of you plugs, anyway

[neudl]

Thanks - Don't have the carbs - previous owner put in the engine.

[Sam Green Racing]

Are you at a much diferent altitude?

Sam.

[DanEarl]

my '72 500 has 100 mains.  pretty sure they're original. 

[TwoTired]

Honda made A CB500K3 for the European market.  I put specs for their carb tuning in the carb FAQ.  But, FAQs just don't get that personal attention, I guess.

They used the same fuel jets as the Cb550K3;  #90 and #42.
They specify a different taper slide needle, clipped in the 2nd groove. They also have a smaller air jet for the emulsion tubes and the main mixture, which would make metering on slide needle and main richer.

The 500's smaller displacement doesn't suck as hard on the carbs, making less vacuum in the carb throats, and therefore less draw on fuel metering devices.

[neudl]

Thanks TwoTired - would the small displacement and weaker draw account for the leanness?  Pardon the confusion, but I'm certainly no expert in these things.  Moving the needle clip to the lowest position has certainly improved the bogging and popping.

[TwoTired]

Thanks TwoTired - would the small displacement and weaker draw account for the leanness? 

Didn't I just say that? 

The 500's smaller displacement doesn't suck as hard on the carbs, making less vacuum in the carb throats, and therefore less draw on fuel metering devices.

Pardon the confusion, but I'm certainly no expert in these things.  Moving the needle clip to the lowest position has certainly improved the bogging and popping.

My assumption is that to make the 500 run as well as Honda tuned for that displacement, the recipe for tuning to that displacement is the same as the CB500K3.

You should make sure the float height is 14.5 mm.  You have to use the same air jet, #120 instead of your current #130, and you would also use the slide needles for the 500 configuration which are E2350F clipped in the 2nd groove, instead of the CB550 slide needles which are E2349F clipped in the 3rd groove.  Do that, and it should run perfect, right out of the box, like it did for Honda off the showroom floor for the CB500K3.

You'll probably have to get the needles, maybe the air jets, from a European source.  Maybe BryanJ can help you with part numbers.  Or, get creative about modifying what you have.  I don't know how to decode the Honda needle numbers to determine their taper angle, overall length, and starting diameter.

Otherwise, keep futzing with your current carb adjustments and trying different parts, until you are happy with how it runs.  But, essentially you have the same problem that others have when they change to pods or a different exhaust system, where a dyno is the fastest way to the dial in the proper carb settings.

Maybe putting another groove in your needles, or shimming them to raise them even more will help your run issues.  I wouldn't change the main jet size unless a WOT plug chop said it was necessary.  And, I note that Honda thought a bigger main jet WAS unnecessary.  They solved the midrange issues with a different slide needle taper and position, along with the smaller air jet.  The smaller air jet also richened up the WOT throttle position, too, I expect.

You did resynchronize the carbs after changing the needle position right?

[neudl]

Thans TwoTired - a great help as usual. I did re-synch after changing the needle clip position.

[neudl]

TwoTired - are the air jets even removable on these carbs?  It has the push-in slow jets and screw-in mains, but I've never tried removing the air jets.

[TwoTired]

On my PD46c carbs they are pressed in brass things at the mouth of the carb.  They weren't expected to be changed, just like the slow jets.  I've never changed the air jets.  I've heard of people filling them with solder and drilling a new smaller hole.  Haven't done it myself, though.

#130s are 1.3mm diameter.  1.3 mm = 0.051181 "
#120s are 1.2mm diameter.  1.2 mm = 0.047244 "

That's probably a #56 vs. a #55 drill bit.  Not very precision.  But, close.  Probably better precision using jet reamers and gauges. 

I suspect the needles will be harder to get/modify, and will have a greater impact.  Just a guess, though.

[neudl]

Thanks - I did find the OEM needles for the CB500F in England, and hope I can have them shipped.  I think the air jets might be damn near an impossibility.  In any event, thanks for the advice.  I have a spare set of identical carbs.  I might try seeing if I can pull the air jet and try the solder trick. Worth a try.