Topic: 1976 CD550Fss high idle when hot

[550Fss]

As the subject indicates, my 1976 CD550Fss increases idle after its well warmed up. She is cold blooded at start-up, and needs a bit of throttle even with the choke on until a little heat builds. She runs well and very responsive to throttle inputs. The plugs indicate she's running on the rich side.

Here are some of the technical details/setup...
Electronic ignition conversion
I am at sea level
Valves adjusted
Carbs fully cleaned
Vacuum synced
I think the main jet is 98 (not positive)
Slow jets are #32
Floats at 22mm
All carb o-rings replaced 2016
Carb boots in good shape, no cracks or evidence of leaking

One odd experience is that I have never been able to get any idle change when adjusting the air screws. I have pulled them and the passages are clear.  I do know the slow jet should be 38. Last summer I purchased #40s (this is what my factory manual says is standard, I see from doing some reading on these forums the 550F specs 38) and symptoms were much worse. When hot, I was getting way too much fuel flow after closing the throttle and had high rpm run out until the fuel burned off and then it just idled high. I went back to the #32's which were a bit clogged when I pulled them. Put them back in after cleaning them. I have the same high idle when hot as I did before.

Trying to determine if I should try either #35 or #38 slow jets or perhaps I am not even going in the right direction chasing this down? Maybe my floats are not at spec and too full?  Any thoughts are welcome.

[newday777]

"One odd experience is that I have never been able to get any idle change when adjusting the air screws. I have pulled them and the passages are clear. "

Did you get the small washer and oring out from below the springs of air screws and replaced those orings?

[jaytee-nz]

Fairly sure the 76 and earlier carbs did not have the o ring and washer, as these were only fitted on the later PD carbs.
The fact that you don't get any change to the idle speed when adjust the air screws indicates that those circuits are blocked or partially blocked. Did you check them by blowing carb cleaner through the circuits ?
The standard slow jet is a 38, not a 32 and around 1.5 turns out is the normal setting.

[newday777]

I was thinking the 76 might have PD carbs.

[HondaMan]

"One odd experience is that I have never been able to get any idle change when adjusting the air screws. I have pulled them and the passages are clear. "

Did you get the small washer and oring out from below the springs of air screws and replaced those orings?

That, and...have you checked the O-rings in the intake manifolds where they bolt to the head? They may be hard now, and not sealing well.

[denward17]

Could the slides be set too high during the vacuum sync?

[jonda500]

My 550F runs fairly well with #35 slow jets (genuine keihin) - but still a little rich at idle (slight stumble just off idle if I wind on the throttle too slow/too little). Where did you find the #32 jets?

No 550F models came with PD carbs!
John

[Deltarider]

As the subject indicates, my 1976 CD550Fss increases idle after its well warmed up. She is cold blooded at start-up, and needs a bit of throttle even with the choke on until a little heat builds. She runs well and very responsive to throttle inputs. [...]

This is normal behaviour for all CB500/550s. The Owner Manuals give the right instructions.

The plugs indicate she's running on the rich side.

What part of the plugs? Only the ceramic noses can give you a clue. Realise that CB500/550 plugs with oldstyle Keihin carbs can foul quickly at idle.
All CB550Fs have the oldstyle Keihin carbs. Consult the overview for standard settings.
Is the airfilter element clean? When the crankcase breather tube is attached to the airfilter case, the element may get humid by condens or even fouled by oil resulting in decreased air intake and performance. Keeping oil level maximum at half way between the upper and lower mark can be a remedy.
When all slow circuit components are clean, turning the airscrews will have an effect on the idle rpm. When you change only one airscrew's position, it may be minor and hard to detect, but with all four changed, change in rpm should be noticeable. It will take the engine at least 20 seconds to react. Realise that these pre 1977 models need a relatively rich idle to give a satisfactory acceleration (driveability). Leaning out the idle may result in an engine purring nicely like a kitten, but acceleration will be poor.
#32 slow jets are too small IMHO and they may get partially clogged when bike is not operated daily.

[Stev-o]

One odd experience is that I have never been able to get any idle change when adjusting the air screws.

Just to be clear, you are not using the air screws to try to adjust idle, right?

There is a thumb screw on the right side for that.

[BenelliSEI]

Not sure there is a problem…. I constantly adjust the idle on my SOHC-4’s all the time. Up a bit when cold and back down once it’s good and hot. Keep the threads on the adjuster clean and lubed and reach down anytime. It’s a fine adjustment and I play it a few times on most every ride…….

[550Fss]

"One odd experience is that I have never been able to get any idle change when adjusting the air screws. I have pulled them and the passages are clear. "

Did you get the small washer and oring out from below the springs of air screws and replaced those orings?

Yes I did when I tore down the carbs and cleaned them. I also removed them all again last summer and sprayed carb cleaner through the holes to make sure they were all 100% clear.

[bryanj]

If you got a washer and O ring out either the carbs were built wrong OR its got the wrong carbs, as stated before those parts were only on PD carbs which were only fitted th K3 models(and 750 F2/3)

[550Fss]

Fairly sure the 76 and earlier carbs did not have the o ring and washer, as these were only fitted on the later PD carbs.
The fact that you don't get any change to the idle speed when adjust the air screws indicates that those circuits are blocked or partially blocked. Did you check them by blowing carb cleaner through the circuits ?
The standard slow jet is a 38, not a 32 and around 1.5 turns out is the normal setting.

Hmm, need to double check, you have me second guessing.  I have a '78 Hawk 400 as well.  Either way, I did pull the screws and blasted the holes with carb cleaner and they were not blocked.
The bike came with the #32 slow jet, when I saw that, it is what made me think this might be at least part of the problem. 

[550Fss]

"One odd experience is that I have never been able to get any idle change when adjusting the air screws. I have pulled them and the passages are clear. "

Did you get the small washer and oring out from below the springs of air screws and replaced those orings?

That, and...have you checked the O-rings in the intake manifolds where they bolt to the head? They may be hard now, and not sealing well.

They seem to be in good shape.  I was worried they might be leaking, so I have sprayed carb or brake cleaner around them while running and did not observe an rpm change. They are not rock hard and flex reasonably well, but are stiff.

[550Fss]

Could the slides be set too high during the vacuum sync?

I was worried about this, so I found the description of using a 1/8 drill bit to bench set the slides, then went back and re-performed the vacuum adjustment.

[550Fss]

My 550F runs fairly well with #35 slow jets (genuine keihin) - but still a little rich at idle (slight stumble just off idle if I wind on the throttle too slow/too little). Where did you find the #32 jets?

No 550F models came with PD carbs!
John

Definitely have the keihin carbs, the bike came from previous owner with the #32's.

[550Fss]

As the subject indicates, my 1976 CD550Fss increases idle after its well warmed up. She is cold blooded at start-up, and needs a bit of throttle even with the choke on until a little heat builds. She runs well and very responsive to throttle inputs. [...]

This is normal behaviour for all CB500/550s. The Owner Manuals give the right instructions.

The plugs indicate she's running on the rich side.

What part of the plugs? Only the ceramic noses can give you a clue. Realise that CB500/550 plugs with oldstyle Keihin carbs can foul quickly at idle.
All CB550Fs have the oldstyle Keihin carbs. Consult the overview for standard settings.
Is the airfilter element clean? When the crankcase breather tube is attached to the airfilter case, the element may get humid by condens or even fouled by oil resulting in decreased air intake and performance. Keeping oil level maximum at half way between the upper and lower mark can be a remedy.
When all slow circuit components are clean, turning the airscrews will have an effect on the idle rpm. When you change only one airscrew's position, it may be minor and hard to detect, but with all four changed, change in rpm should be noticeable. It will take the engine at least 20 seconds to react. Realise that these pre 1977 models need a relatively rich idle to give a satisfactory acceleration (driveability). Leaning out the idle may result in an engine purring nicely like a kitten, but acceleration will be poor.
#32 slow jets are too small IMHO and they may get partially clogged when bike is not operated daily.

Need to take another look at the plugs, cannot remember exactly what they look like.  I'll get back to you.
I have used both a foam and a paper style air filters, running the paper style at the moment.  Same symptoms either way.
Certainly have tried air screws one at a time as well as changing the turns on all four similarly (all 4 a turn in, or out, etc.). Can't seem to detect a change in idle.
#32 seems too small to me as well, and #40 was way too big.

[550Fss]

One odd experience is that I have never been able to get any idle change when adjusting the air screws.

Just to be clear, you are not using the air screws to try to adjust idle, right?

There is a thumb screw on the right side for that.

Ha ha, indeed, talking about air screws, not the idle adjuster.

[550Fss]

Not sure there is a problem…. I constantly adjust the idle on my SOHC-4’s all the time. Up a bit when cold and back down once it’s good and hot. Keep the threads on the adjuster clean and lubed and reach down anytime. It’s a fine adjustment and I play it a few times on most every ride…….

Yup, this is exactly what I have been doing.  It seems to work, but also seems like it should be able to reach a better setup.  My CD400T Hawk is very stable.

[550Fss]

If you got a washer and O ring out either the carbs were built wrong OR its got the wrong carbs, as stated before those parts were only on PD carbs which were only fitted th K3 models(and 750 F2/3)

Hmm, yeah, I need to double-check, maybe I am thinking of the keihin carbs on my Hawk.  Going to go pull one of the screws this afternoon and see.

[550Fss]

Attaching a few pics.  The air screw is not the washer/o-ring type with the long needle on the end,, just a hole and spring.  My bad, confused with my other bike.

[550Fss]

Plug, black soot, but not super awful. Definitely rich though.

[550Fss]

other side

[550Fss]

carbs

[550Fss]

The bike