Topic: CB550 won't take throttle without choke- any ideas?

[NewGuy]

Hi everyone, I wonder if I could avail myself of your collective wisdom for a probable diagnosis of a problem I'm having...

First some info about the bike:
It's a 1977 CB550, but the motor is from a 1976 CB550K2. It has the PD46A carbs that were original to the bike (not the motor).  The jetting is all stock.  It has a MAC 4-into-1 exhaust, full intake manifold and stock aribox with stock filter.  The ignition system is 100% new, plugs, coils, wires, wire caps, timing plates etc., and has been set with a timing light.  Carbs were thoroughly rebuilt (by me) using an ultrasonic cleaner.  The idle jets, main jets, and needles are OE.  The rest of the carb parts are from a 4into1 rebuild kit.  Subsequent checks with compressed air and carb cleaner show that all the passages are clear.

It starts easily with the choke on, but revs quite high (no tach at the moment, but I would guess it's around 2.5K).  The throttle is snappy and responsive with the choke on.  Very light backfires if I snap it shut.

With the choke off it will idle, but not very stable, and if you look at it funny, or even thing about touching the throttle (or idle speed adjustment) it dies right away.  It's a gentle death- like hitting the kill switch.

I did a bench sync of the carbs, then tweaked it when idling with the choke on. 

There is no black smoke, or "8-cycling".  It does not load-up/clear-out, and the plugs are coming out clean- perhaps a tiny bit on the grey side.

My magic 8-ball sez "Too Lean", but the condition is totally insensitive to the air bleed adjustment.  From full open to full closed it does not appear to have much effect.

Do I needed to move up from the size-42 idle jets?

Thanks in advance.

[NewGuy]

...forgot to mention: it's a pretty cold day and I'm working in an unheated garage.  I can see my breath.

[Don R]

 Re check the float levels and the emulsion tubes/jets. I gently use a torch tip cleaner or guitar strings to open the tiny holes in the emulsion tubes. Also check for a vacuum leak at the carb manifolds.

[NewGuy]

Thanks for the suggestions.  I followed your advice (update at the end), and am now battling one cylinder with a dead miss.

Here's what I've done:

Re-set the float heights to 13mm, instead of the clymer's recommended 22mm (all cylinders)

Checked the elmusion holes and jets on the offending carb: all good; removed them from the carb and used carb-cleaner and compressed air to confirm that they are clear.

Confirmed spark on the bad cylinder using quick-start- a little spray at it revs right out.

Checked the intake for leaks with quick-start: none found.

I attempted to check the "live" fuel float levels with come clear tubing in the drain screw holes, but that didn't give me a very clear picture and it made a big mess......

Any more ideas?  Your input is deeply appreciated.

[harisuluv]

When you spray into the cylinder with starting fluid, you have fire.  You have a fuel delivery problem (carb problem!)

First you need to accept the fact that your jets that you think are clear might not actually be clear.  Spraying carb cleaner at an idle jet and compressed air will do just about nothing if it is clogged up with lacquer.  You do not verify an idle jet is clean this way.  You LOOK through it and see a perfect circle of light on the other end.  If you don't see that, it's not verified clear.  You will even find that you can clear it with a guitar string, go all the way through, then look through it and see nothing.  You verify the slow jet VISUALLY, period. 

Change your float height back to spec (22mm).  Where did you even get the 13mm number, it is advisable not to just throw things and try to get something to stick to the wall.  The float height is only valid if your floats aren't tweaked to the point where the entire geometry is thrown off.

Sounds like the clear tube method gave you a VERY clear picture of the internal level:  a mess!

You most likely have a clogged idle jet, but you won't be able to fix it unless you accept the possibility it's clogged.

[Gene]

^^^^^ Trust him, NewGuy.

[NewGuy]

Thanks for your input guys.

Ok, ok:  *hand to heart*: "I accept the possibility that my idle jet may be clogged".

That said, the slow jet on the offending carb on the offending cylinder did pass the visual test.  I held it up to my shop light, as it does shine through with a nice circular signature.  I also pressed the jet into a peice of clear tubing, filled the tubing with carb cleaner, then blew it though.  The stream of carb cleaner out the other end was a nice even narrow cone.  I then reversed the jet's direction and I got a similar effect when blowing through the other way, this time accompanied by a vegas-waterfall effect from the emulstion holes. No apparent asymetries.  "I accept the possibility that my idle jet may be clogged", but I've been as rigourous as I know how to be!  Short of ordering a new-in-box jet, I can't think of what else to do!

UPDATE:  I had bought a "cheap" rebuild kit (-I can hear your collective eyes rolling-) but re-used the OEM slow jets, main jets, needles, etc.  I really just used the gaskets from the cheap rebuild kit..... and the air bleed screws.  I swapped back to the OEM air bleed screws and my dead cylinder came back to life!

Could this have been the issue all along, or is it likely to be incidental?

[harisuluv]

Who knows for sure.  The cheapie air screws aren't great but not to the point where it wouldn't even fire because of it. 

All I will say is that I am satisfied with your testing methodology of the slow jet and from what you said i agree it would indicate it is clear.

Regardless, good job, you fixed it!

[HondaMan]

I'd like to toss in my "I find this often" tip with the Mid-Four: the O-rings in those castings that bolt the carb hoses to the head. If you haven't changed them, they are hard as plastic now, and leak vacuum a bunch. This often shows up as balky throttle response. These re the same size O-rings as are found in the valve caps.

[Deltarider]

Change your float height back to spec (22mm).  Where did you even get the 13mm number

Didn't he say he has the PD46A carbs? Then 13 mm is much closer than that 22mm that is prescribed for the oldstyle carbs. I fear by now we have an other example of unnecessary messing with floats. It's epidemic in this forum. I have yet to se the first 500 or 550 that needs to have it's floats adjusted. The floatheights are ALWAYS right unless someone has messed with it. Chances are high that someone is a reader of this here forum. Ipse dixit.

[harisuluv]

Yeah the PD46A carbs would be ok at 13mm.  It seems weird that after a rebuild he would change just the one float, does that mean that the other ones are at 22mm? 

In the first post it does say he has a earlier engine with PD carbs.