Topic: Stumbling when returning to idle

[CarbonK]

Is that the correct word? I got a 77 cb550K that, after blipping to a quarter throttle, will bring the idle way down to the point of stalling or almost stalling if I don't give it a bit of gas. What could cause this?

Dyna Electronic ignition, PD46 Carbs with velocity stacks and open headers. Carb settings are as follows.
Mains-110
Slow-42
Needle-2nd from top
IMS-3 turns out

[flybox1]

inadequate vacuum sync
A/F mixture at idle slightly off.

What color are your plugs at idle, and when was your last vacuum sync?

[CarbonK]

What color are your plugs at idle, and when was your last vacuum sync?

Last sync was about a month ago after I replaced spark plugs and switched out 45 slow jets for the 42s. Was still having the same problem back then also. I'll hook up the carb tuner tonight and see if all my vacuum readings are the same. Need to get back to you on the plug color.

[flybox1]

#42 pilot jets are stock for your PD carbs.  Best guess is you are too lean at idle for your stacks and open exh.... even with 3 turns out on your mixture screws.
Warm up the bike (as jetted) and reproduce the symptom.
Add 1/3 choke and blip again....still there or better?
Add more choke and try again.
If you are lean at idle, the symptom should be less apparent as the choke is added.

[DaveBarbier]

PD46C pilot is 42 ('78 K4)
PD46A pilot is 38  ('77 K3)

TT posted this in my build thread a while back I think.

[flybox1]

Nice one, Dave.
OP will need to list which PD's he has

[CarbonK]

It's PD46A. I got time right now, gonna check the plugs and fiddle with the choke.

[CarbonK]

Damn, here's a dumb question. On the side of my carbs is stamped PD46CAPI with the A inside a square. Am I correct in assuming this is PD46A or have I been wrong this whole time? My float levels were set to PD46A specs. Checked my plugs at idle and they are black with soot.

[CarbonK]

Kill me, these are PD46C carbs. Pulling them and correcting the float level now.

[DaveBarbier]

After you set the floats, do the clear tube method to verify floats are about 3mm below the bowl seam on all carbs. With the PD46 carbs it's easy because you just connect a tube to the overflow nipple and crack the bowl screw.

This ensures that the fuel level is right on the money. It can be affected by aftermarket float needles and floats. Setting the floats to factory is like bench syncing the carbs, close, but not done.

[alacrity]

Is that the correct word? I got a 77 cb550K that, after blipping to a quarter throttle, will bring the idle way down to the point of stalling or almost stalling if I don't give it a bit of gas. What could cause this?

Dyna Electronic ignition, PD46 Carbs with velocity stacks and open headers. Carb settings are as follows.
Mains-110
Slow-42
Needle-2nd from top
IMS-3 turns out

Hey - I've had a few of these bikes and my most recent project bike (running the '77 type Pd46a carbs - stock airbag) has had that hanging idle problem... intermittently... and I THOUGHT it was temperature related.  But - it was fuel quantity related.  Weight vs. gravity = fuel pressure (or in this case, the lack thereof). 

Does yours work correctly when the tank is full (or at least,  does it work better?) IF so, look at your petcock, fuel feed line (did you add a fuel filter inline) and possibly at the shared feed tube for the carb rack itself. You might simply need to remove and clean the petcock and screen.

My anecdote here:   At the end of a couple long rides it ran poorly and wouldn't maintain idle properly. I checked for air/vacuum leaks and had none. Tight as a drum (idle/running unaffected by carb cleaner spray around the boots and manifolds). The next day, it was better but not right, and I figured I'd have to pull the carb rack and go suss it all out, doing the things mentioned correctly above. But I got busy. Wednesday evening I went for a ride - had about 1/4 tank of fuel to start and the bike ran ok when hard on the throttle but wouldn't drop properly down to idle.  Later that evening, I filled up.  It ran well and dropped back to idle correctly.  I am running a handmade aluminum tank that had a dodgy liner in it, and so I added a fuel filter.  Turns out it was gummed and not flowing well *(unless there was the weight of 4 gallons of gas pressing to escape behind it.)  I pulled the filter, ran a new line from the petcock to the carb rack, all better. I "fixed" my carburetor problem without touching the carburetors.

[Deltarider]

Alacrity, all CB500/550s are sensitive for even the slightest change in the fuel lines. Over the years I have been pretty consistent in my recommendation to restore length/diameter of the fuel lines to their original state and to abstain from inline fuel filters. Our carbs are gravity fed and there is not much column to speak of. A little change (also in routing) can already hamper fuel delivery. If the petcock filter is still there and in good order, an inline 'extra' fuel filter is useless and I myself had nothing but trouble with them, be it intermittent which made it even harder to detect. I reckon soon there will be a reaction here saying: O, but mine does just fine. Well... then you're lucky (as long as it lasts). I do not recommend them. Of some models we can learn in the parts lists what length and diameter the fuel lines are supposed to be. Also all vent tubes must air free. Particular the K3-4 models are sensitive in this respect. They should be checked at their ends and if needed shortened an inch or so.

[alacrity]

Yes. Thanks Delta.  I never used one before for just those reasons. This tank was a mess, and I didn't want my emulsion tubes gobbed up... It has a few tankful a through it now so i am comfortable with just the petcock filter (it's an aftermarket petcock and the filter screen is a bit too coarse... )

[CarbonK]

After you set the floats, do the clear tube method to verify floats are about 3mm below the bowl seam on all carbs.

Will do.

Does yours work correctly when the tank is full (or at least,  does it work better?) IF so, look at your petcock, fuel feed line (did you add a fuel filter inline) and possibly at the shared feed tube for the carb rack itself. You might simply need to remove and clean the petcock and screen.

Nope, no inline fuel filter. I'll fill up the tank and see if it makes any difference.

[CarbonK]

Fixed the float level, double checked with the clear tube method. I fitted fresh spark plugs in and let it idle for 7 minutes. I pulled out the plugs and they all were a bit rich except for cylinder 1 which had no carbon deposits at all, looked basically new. It wasn't wet with gas and was firing. I'm gonna pull of the carbs and clean up the jets, hopefully it's just a clogged idle jet. What could it be if it is not? Also, can I tweak the idle mixture on just one carb or do they all have to be turned to the same amount equally?

[flybox1]

Fixed the float level, double checked with the clear tube method. I fitted fresh spark plugs in and let it idle for 7 minutes. I pulled out the plugs and they all were a bit rich except for cylinder 1 which had no carbon deposits at all, looked basically new. It wasn't wet with gas and was firing. I'm gonna pull of the carbs and clean up the jets, hopefully it's just a clogged idle jet. What could it be if it is not? Also, can I tweak the idle mixture on just one carb or do they all have to be turned to the same amount equally?

Once you are certain the carb passages tubes, jets, etc., are all clean, you wouldtune each carb idle mixture  individually, using the A/F mixture screw to achieve a light tan plug color.

[CarbonK]

Made sure all passages were clear on #1 Carb and I'm still getting unequal readings at the plugs. 2,3,4 are black while 1 is very lean. After setting #1 IMS to four turns out while keeping 2,3,4 IMS at three turns out the plug on #1 got a hint of tan. It's not normal for the IMS to be off by this much is it? If I'm going to correct the rich mixture on 2,3,4 the IMS would be off by even more.
Also I think I've reached the end of the adjustment range on #1 IMS at four turns out. Should I swap in bigger slow jets?

[flybox1]

After 3 turns out on the IMS, they lose their effectiveness.
you bank of carbs should all have the same jets and needle position.  the only variable should be the IMS
Continue looking for the reason you have such a huge difference between 1 vs 2/3/4

How is that o-ring on your main jets. Are they new?  or old and hard allowing fuel to pass?
Remind us of the brand of pilot and main jets you are using.

[CarbonK]

Keihin slow jets and the "oem equivalent" main jets from jetsrus.com. I wasn't aware that the main jets had an o ring. Just popped open the carbs and I'm not seeing any inside either carb. They would be under the emulsion tube, correct? I appreciate all the help flybox.

[harisuluv]

Flybox is thinking of the earlier style carbs.  If you have PD carbs, there is no main jet o-ring.

[DaveBarbier]

PD46 carbs don't use the main jet o-rings that previous models do. In the carbs that use it it goes between the main jet and the emulsion tube. Not needed in our carbs.

Lower fuel level in the bowl I think would cause this issue, but you said that they're all good. A while back I had a weird carb issue and nothing was making sense. An older friend of the family that knows carbs took a look at it and said that the pilots were drilled out much larger. Instead of a 42 it was like a 90. Because I'm not experienced I would have had no idea this was an issue. If these are new jets then I'm sure it's not the issue, but if these are the ones that came with the your bike, who knows what the PO tried to do with them.

[flybox1]

please post a picture of all your plug tips.  all lined up in one picture 1-2-3-4

[CarbonK]

please post a picture of all your plug tips.  all lined up in one picture 1-2-3-4

1 2 3 and 4. Number 4, the last one on the right, was wiped clean prior to taking the picture because I was planning on running the engine again. It had equal amounts of carbon as 2 and 3.

I noticed that only one carb, number 3, has both the washer and o-ring that fit under the IMS spring. Number 1 is missing the o-ring, 2 and 4 the washer.

The order goes o-ring, washer, spring, and last IMS screw, right?

[flybox1]

I noticed that only one carb, number 3, has both the washer and o-ring that fit under the IMS spring. Number 1 is missing the o-ring, 2 and 4 the washer.

The order goes o-ring, washer, spring, and last IMS screw, right?

Fix this first before anything else is done!

[DaveBarbier]

+1 to Flybox.

You can't expect the carbs to perform the same when some are missing parts. Especially o-rings.

Washer goes in between the spring and o-ring so the spring doesn't cut the o-ring.