Topic: 1978 CB550K running lean

[urbanspan]

Hello Ladies and Gentlemen,

I recently purchased a 1978 CB550K......runs terrific, but after reading the posts on the forum, I've come to the conclusion that my bike is probably running a bit lean (high rpm during idle.....bluish spots on the top of pipes, 3 plugs are tanish, 1 has a spot of tan, but the rest of plug is white).

I've not worked on a bike before, and am usually more comfortable taking it to somebody with experience....so forgive me if my question is elementary.

As I believe the bike is running a bit lean, would it be completely unwise for me to slightly adjust the air/fuel screw to help richen the mix?  Or would this cause a myriad of other possible synchronization issues etc.

Also, per the Clymer manual, to richen the mix on a 78 CB550 is says to turn the screw clockwise.......but I've also heard this is incorrect, and that the screw should be turned counter clockwise.

Any advice would be greatly appreciated!
J

[luvhonda750]

Yeah, its very hard to get to those mixture screws, a very small screwdriver is needed, I agree, I'd start by turning the screws in 1/8 of a turn and then seeing how it runs and checking the plugs. So the further the screws go in, the less air is allowed, out = more air. You can also get a visual of where the screws are at by the # of threads sticking out. Turn them all the way in, then back out exactly one turn. Thats the default position.

It could be float levels also. Or even needle height, but all of this gets more complicated.

[urbanspan]

Thank you very much for your advice.....I'll give it a try and let you know!

Much appreciated!

[zoo mob]

Good to see you on the board Joe. Let's get that girl running right so we can ride!

[TwoTired]

First of all, understand that the Clymer manual was/is based on Honda's original Cb500 shop manual.  But, it doesn't reliably address specifics of the later 550 models. In particular, the Clymer manual is completely ignorant of the 77-78 model PD carbs.  So, I would discount any carb advice/direction given by Clymer for your carbs.  The Honda shop manual has supplement that specifically cover the PD carbs.

Your carbs have idle mixture screws that control fuel flow.  Turning them out (CCW) richens the mixture for that carburetor.  Turning them in (CW) begins to close off the port that feeds the idle fuel mixture into the carb throat, making the idle mix leaner.  These screws have little effect on mixture with throttle positions greater than 1/4 of the total twist grip travel.  Also, beware that you only LIGHTLY seat the mixture screws. They have a VERY shallow ramp angle and can be wedged tightly into the seat with very little pressure.  This can distort the needle and seat making future fine adjustments very difficult, as well as break the tip off the needle when they are stuck in tightly.

If you have changed the air filter type/design or the exhaust type/design.  You may well have many carb tuning adjustments ahead, depending on how different the new parts behave compared to the originals.

The first area of concern is why all four plugs don't have the same type of deposits.  Could be:
- Valve clearances unequal across the cylinder bank.
- Carb slides are not adjusted for equal vacuum at the intake runners.
- Some carbs have blockages/restrictions in the fuel passages internal to the carbs.
- Air leakage in the carb couplers to the intake runners.

You should probably state when the last complete and thorough tuneup has been performed on the bike (s/b every 3000 miles).
This should most certainly be done before mucking with carb mixture settings/adjustments.

Check for coupler leakage.
Vacuum balance carbs.

See if new correct heat range spark plugs (D7EA) exhibit the same deposit behavior.

If so, then check float height settings and internal carb cleanliness, or go in and compensate for air filter and exhaust changes from stock.

Cheers,

[Jonesy]

Two common issues I've seen with teh same type of CB550: 1), the upper two exhausts tend to be rotted out from the inside (less backpressure=leaner running) and 2) the airbox boots tend to harden and chunks break off (again, this mainly occurs in the outer two boots). My CB550 had the same issue, with heavily bronzed header pipes, which got worse after I ported the head. The lean running was mainly at high engine speeds, so I had to run a larger main jet. Don't start fiddling with the jets until all tune-up items have been addressed first.

[urbanspan]

Thanks everbody for you response!  The wealth of knowlege on this site is extraordinary....very useful for a newbe like me.  All of your feedback and advice is greatly appreciated!

[bryencass]

So, if bike is choking out while throttle then it's too lean.  Too lean means to "open" the air mixture screws up?  This is what I'm gathering. 

[TwoTired]

Choke is what you use to enrich the mixture for starting a cold engine.

If you are snapping the throttle open from idle with more than half available twist, it is going to wheeze and die due to mechanical slides and sudden loss of vacuum. Normal.

This thread is about the 78 cb550, known to run hot and lean from factory.  Loss of high pressure stock exhaust makes it even leaner.  The IMS screws have limited effect and mostly only at idle closed throttle position, though it may help with low speed throttle twist response.  It will not cure a lean condition in mid and full throttle operation.  For that, you need slide needle and main jet orifice increases.

[bryanj]

I suspect that if your exhausts are colouring that they are aftermarket and single skinned as against the double skinned originals which rarely, if ever, discolour.
Also as TT said the pd carbs as fitted are finicky and designed to run very leanto apease the USA(especialy California) emission rules of the time plus the 550 does not have the accelerator pump the 750 got

[bryencass]

@TwoTired I appreciate what you are saying.  I rebuilt my carbs and needle is in 2nd from top position.  100 main jet, stock air box.  I'm running a aftermarket exhaust 4 to 1.  IMS screws were set to 1 1/4 turns off the bench.  Bike would run beautifully at idle and full speed, just with full choke only.  Yesterday I closed up and then reopened the IMS screws to 1 turn, now runs worse.  Worse meaning it wont throttle even with full choke sputters and dies.  My thought is to open the IMS screw up 1 more full turn.  Thoughts?

[grcamna2]

What brand/type 4 into 1 exhaust are you running ?
A restrictive/quiet baffle will work best to give back-pressure for less lean-running on those carbs.

[TwoTired]

Clarify if you have PD carbs and a choke knob rather than a lever at the carbs.

Slide needle 2nd from top drops the needle into the metering orifice making it leaner throughout midrange.

PD IMS turns out to enrich idle.  Enriching idle helps off idle pick up because you have no accelerator pump on cb550 carbs.

What do deposits on the plugs tell you?

Have you checked for air leaks on carb couplers and manifold o rings?

Have you completed the entire 3000 mile tuneup checklist?

[HondaMan]

My 'antennae' went up:

I rebuilt my carbs and needle is in 2nd from top position.  100 main jet, stock air box.

If you rebuilt the carb using non-Kehin parts, then it IS running lean, very lean. The non-Kehin parts out there are Keyster parts. Their jets run about 7% leaner than the number that is stamped on them (i.e., your "100" jet is actually a "93" equivalent jet in practice) and the needles are FAR too lean, and large, to use at all. I have had to fix a LOT of these. The needles cannot be succesffully used unless the mainjet is resized into the 110-115 size range, which then causes other problems like poor transition from idle to mid-range (i.e., falls on its face at 2500 RPM).

Start right here: reinstall your OEM needles and jets. Use the OEM needle slot (probably #4 for the top). Then try running again and see how it does.