Topic: 1975 CB550K idling very rich - can't figure it out. HELP!!

[reddyvv]

I'm just about done with my project and trying to get this bike running right but I am running out of ideas. The bike is running very, very rich at idle - sooty plugs, black smoke, hanging at 2,500 to 3000 rpm when revved. I haven't tried riding it very far or fast, just up and down the lane because of the hanging idle but can sense it starting to clear up the rich condition when the revs start to rise. The transition is terrible and not rideable.

Everything on the bike is OEM stock - 022A carbs, jets, air box, new air filter, etc. The only non-OEM item is the MAC 4-1 exhaust.

CARBS
I completely tore down the carbs and cleaned everything out - jets, emulsion tubes, verified carb cleaner squirting through all the passages, idle circuit, etc. New OEM gaskets and O-rings. Standard settings applied on the rebuild.

CAM TIMING & VALVES
I double checked the cam timing and it is spot on. Valves have been lapped and I double checked valve clearances.

IGNITION
New 5K caps. Coils measure at ~5ohms primary and ~25ohms secondary including caps. Nice fat blue sparks.
Static timing has been checked and rechecked and the advance has been cleaned and lightly oiled. Tried two sets of new plugs.

Nothing that I have tried is getting rid of the rich idle. In desperation I changed float heights from 22 to 24mm and tried even 2-2 1/2 turns out on the air screws but it made negligible difference. If it was only one or two carbs showing the symptoms one could presume it is blocked passageways on the idle circuits but what chance of all four doing that? And I did clean them again when I changed the float heights.

The only thing I have not tried yet is to check the dynamic timing with a strobe to see if the advancer is sticking. Could the springs be so weak that it jumps to advance even before I rev the engine past idle? Even then I don't see why it would run so rich.

It's definitely a systemic problem since it's affecting all four cylinders and I have run out of other ideas. I'll be damned if I'm wrong but I'm pretty sure that none of the obvious culprits like upside down floats, jets, etc. apply. I'm starting to conjure up conspiracy theories of misaligned alignment notch on the cam and other crazy ideas!! 

[LesterPiglet]

Air screws?

[reddyvv]

I wrote idle screws earlier which I've updated to air screws. Started at standard 1 1/2 and even tried going out to 2 1/2.

[Dave-and-his-550]

I'm by far not an expert about these bikes, but as far as your wandering idle goes, I had the same issue with my '76 550F for awhile and after I vacuumed sync'd the carbs my bike is running great now, and no idle issues. If everything else is in spec and you checked the timing/point gap/valves/cam chain tension etc, then I would say you need to vacuum sync the carbs next.

[reddyvv]

Sorry, I forgot to mention that I synced the carbs as well. The idle doesn't wander, it starts and idles very consistently although very burbly because of the rich mixture and when it hangs up at revs it is steady then too.

[Untold]

Is your air filter clean and clear?

[LesterPiglet]

Turning the airscrews in or out should adjust the idle mixture. If they dont work you havent cleaned your carbs properly.

[reddyvv]

The air filter is brand new OEM Honda - tried without the filter too to check.

The air screws are making a difference in that if I turn them out to 2 1/2 it gets rid of the high residual idle problem but it is still rich. And why should I have to go so far out of spec despite the 24mm float heights? I can't figure it out especially since it's all four carbs.

[TwoTired]

What size pilot jets do you have?  Does the orifice size match the stamping?
It should have #38, which is .38mm or .0149 inches.  Do you have a .015" guitar string?  It should not pass through the pilot jet.

How have you determined it is running rich?  Show us a spark plug end so we can verify you are looking at the proper part of the plug.

The idle is purposely set on the rich side to aid in throttle transition.  A lift of the slides temporarily raises the carb throat pressure, which is what pulls fuel from the jets.  Poor jet draw makes a lean mixture and the engine can't accelerate.

Did you happen to note the numbers on the slide needles?

[reddyvv]

TwoTired,

Jets are standard 40 and 100, I don't recall the needle numbers. Is it possible the jets have been drilled? Sure, but since this bike was bog standard last parked in 1980, no sign of the carbs or anything else having been wrenched on I think it is highly unlikely. I guess I will have to go down the path of measuring the jets as you suggest as a last resort to totally eliminate the possibility. Before I rip the whole works apart again is there anything else that you think may be the likely culprit? The dynamic advance? Cam timing?

I'll try and get pics tomorrow but the entire plug surfaces are sooty on all cylinders, accompanied by black, stinky, eye-stinging exhaust. With the air screws dialed out to 2 1/2 the plugs get a little better, more black than sooty, but still very rich.

[TwoTired]

Honda says a 1975 CB550K should have #38 pilot jets.  Yes, I have found #40 in some of them.  But if you believe the idle is too rich, it would seem like getting factory Keihin #38 pilots would be a logical step.

[reddyvv]

Thanks for looking up those numbers but they seem to be for '76 onwards? The Honda manual I downloaded and the link here http://www.sohc4.net/cb550-faq/ show the pilot jet for the 022A carbs as a 40 standard. I guess I'll have to open up the carbs again to check the jet orifices as you suggested but before that I will get hold of a timing light today, as mine is broken, and check the dynamic timing. That is the only thing that has not been checked yet.    In the meanwhile if you have any other ideas, however remote, feel free to throw them out.

[Black 750K8]

I don't know if this is the case with your bike but it was with mine 750K 1978. Look to see if there is a supplement in the back of the manual they may have said 40 at first then changed to 38.

TT usually knows what he is talking about.

[yozuri54]

I had 2 bikes with similiar issues (both with MAC 4 into one exhausts)

1972 CB500: The PO drilled out the stock idle jets - found a set of stockers and all was fine.

1975 CB750: although the jets and carbs checked out - the plugs came out sooty. I went down one size of main and pilot jets and had to move the needle clip one step leaner - ran perfect after that.

My theory was that the MAC exhaust and modern gasoline combination can make the old SOHC fours  run richer?? Does seem to be a common issue......

[TwoTired]

Thanks for looking up those numbers but they seem to be for '76 onwards?

No they are for CB550, K1, and 76.
 The commas are separators, so it applies to each of these models.  The first two models used 022a carbs and the 76 had 087a carbs.  The spec booklet applied to all these models.

The CB550 was introduced in 74,  It was retroactively designated as the K0 when the K1 model cam out in 1975.
The 1976 model was technically a K2.   But, Honda was juggling designations with various documentation during this period and it was a bit muddled.

Here is the front cover for the booklet is posted earlier.

The Honda manual I downloaded and the link here http://www.sohc4.net/cb550-faq/ show the pilot jet for the 022A carbs as a 40 standard.

I'm the one who submitted that chart many many years ago, based on what I found in my known original 74 Cb550's 022a carbs.  None of my other Cb550s have 40 pilot jets, they all have #38.

My 74 does idle rich and coats the muffler insides with soot if I let it idle a lot.  But, it gets better with the throttle opened up.  Next time I happen to be inside those carbs, I'll put in the #38s, like all the rest of my 550's have.

[Sable]

This does appear to be a common issue for many of us lately. Hope you get it figured out and can post up your solution afterwards.

[reddyvv]

TwoTired,

Thanks for clarifying the information about the settings. I'll probably end up pulling these carbs again tomorrow so will confirm exactly what I have. In case I don't recall exactly

I picked up a timing light today and I must be the first person to be disappointed to have the dynamic timing absolutely spot on!! I could not have got it better if I tried. I was hoping it would be something related to the timing

So I took the bike out for a ride of a few miles and it is just terrible. Transition from low to med revs is very bad, runs well at 4-5K. If I slow down or stop the engine keeps trying to push the bike along at ~3k revs. Just not usable.

My plan tomorrow morning is to take the plenum chamber off and blast the Idle air intake holes with my compressor. If it was running lean I could imagine blocked jets and passages as they are more intricate, but the air intake through the screws and onwards is fairly clear so I am baffled as to how all 4 carb air bleed circuits are blocked?!!

If that doesn't show any results it's carbs off and another complete going over I guess.

[reddyvv]

Also, thanks for all the other comments and suggestions.

Yozuri54,

I'll probably have to try your suggestion if I can't get the standard jets to work.

[reddyvv]

So I took the carbs off again today. The idle jets are #38 after all  and I measured the orifices to be the same, so they are not drilled. Mains are #100 and the needles are set at 4 grooves from the top. I reset the floats back to 22mm and verified again that the idle circuits are absolutely clean, no doubt. Started up the bike again and similar results.

I then took a look at the advancer again and found that there was a tiny bit of free play in the springs, not quite holding the arms down tight. So I took half a coil off the springs to tighten them back down. After starting the bike up I could feel a clear difference, it idled much smoother and did not hang when revved. The timing light showed it advancing fully at 2500 rpm so all looks good there. However, as soon as it warmed up I was back to a more uneven idle and hanging revs. In summary, I think that the little bit of creep on the advance at idle that I eliminated helped to reduce the rich idle symptoms but obviously not solving the problem.

So now I have to decide on next steps. I am inclined to try dropping the needles by 2 grooves tomorrow as that should help the transition off idle. Then decide if I also need to order new idle jets - #36?

Any suggestions?

[Dave Voss]

Which spark plugs are you using?  Are they the correct heat range?  An issue with the ignition system can sometimes cause an incomplete burn, resulting in what looks like an overly rich mixture.

[reddyvv]

I have finally solved my problem. The best I could get it after trying a lot of settings was everything standard except air screws at 3 1/2 turns and no air filter element. Even with this it was idling very rich, stumbled badly upto 3000rpm but ran great above that once it was off the idle circuit.

So I ordered a set of #35 jets from harisuluv. I put them in this evening and at 1 1/2 turns out it is running just beautifully. No hesitation or stumbling, can crawl away from idle, no hanging idle, pulls cleanly in every gear, even in 5th gear from 25mph/2000rpm. It even seems to have got rid of a brief stumble the bike would take at 3/4 throttle when transitioning off the needle to the main jet.

I know that #35 is not the prescribed/recommended idle jet but at the end of the day after trying everything I could think of here I am. I can't help but think the rich idle circuit was caused by the exhaust although no one else seems to have any problems. Or else some bikes just have their own idiosyncrasies!! I may fiddle with the air screws a little after checking the plugs after a longer ride or two but it is great as it is.

Thanks for all the comments and suggestions and I hope this helps others if all else fails.

[Dave Voss]

Glad to hear that you were able to sort it out, thanks for posting the outcome.  In hindsight, I would suspect that your size 40 idle jets might have been previously drilled out to a larger size, as a change from size 40 to size 35 doesn't typically make such a dramatic difference.

[reddyvv]

Hi Dave,

The jets were actually 38 and I measured them to be so, I mentioned it further down in the thread. So it was actually a smaller change but it made all the difference. I guess it doesn't take too much to change the idle mixtures.

[HondaMan]

Some thoughts:
1. The MAC pipe (or any pipe, for that matter) won't cause this problem.
2. The most common culprit: the O-rings in the castings for the carbs, that bolt to the head, get old, hard, and leaky. This makes the idle circuit lose much of its vacuum until about 1/2 the intake stroke has increased the velocities in the throats. They are the same size O-rings as the valve caps, easy to get from Honda or someone else (like me: I have a whole bag of these...). This loss of mixture control makes the idle circuit run very rich, regardless of air screw setting.
3. If new, undersized pilot jets have improved things a lot, then take a very close look at the emulsifier holes in the old pilot jets, they are likely partially clogged with calcium deposits. Drill them out with a small drill bit. Also, run a bigger drill bit down the inside of this tube, to remove the buildup along the insides of the perf'd tube. Also repeat up inside the hole in the carb body where it screws in: use of gas "preservers" always causes partial blockages in these little holes.

Also, if smaller jets have helped the problem, see #2.

[reddyvv]

So I have one final post to cap this thread.

The bike was running fine with the #35 jets during the warm weather although a little lean. A little sluggish below 3000rpm but I rode a few hundred miles. However after the weather turned colder it started to hesitate and stumble - too lean now. So I switched back to the old #38 jets. Back to square one, smoking, hanging idle, etc.!!! Tore half my hair out. 

Finally, I ordered a new set of OEM Keihin #38 jets. Put them in today and at 1 3/4 turns out she is idling smoothly, plugs look great and she runs like a scalded cat.

Frustrating lesson to be learned - it's not that easy to measure jet orifices to accurately determine their size. I thought I had  . If in doubt BUY A NEW SET OF JETS.