Topic: Odd trouble with PD Carbs on 1978 CB550

[aguerra]

Hello all,

I recently rebuilt my engine on my 1978 Cb550 K4, using the PD carbs. The carburetors worked well before the rebuild but, now it seems that an issue has sprung up. When I'm starting up cold and at full choke, it seems like the bike is making no attempt to fire (i.e. no power from engine when cranking). When I remove the choke, the same thing happens. I was smelling gas during these cold starts so I figured I was flooding the engine. When I drain the float bowls and subsequently remove the choke, the bike fires up (I keep my hand on the throttle and adjust the idle screw accordingly to let it warm up). The peculiar thing is that it will hold an idle (@ ~1000 rpm) after this point and I've put over 50 miles on it running it with this cold start-up situation. The other odd thing is that during my rides, when I stop (let it cool down for an hour or so) it will start back up with no issues. It only needs me to drain the carbs when I leave it over night. I have tried starting it (cranking for first time without the choke at all) to see if this was a factor. However, it behaves the same way. Only when I drain the bowls will it fire up.

When its running, I have made 1/4 turn adjustments to the pilot screws on the bottom of the float bowls to see if they have an affect on the cold start but, they seem to change nothing other than low end throttle response.

Things I did post rebuild and specs on the equipment, just to give some ideas as to what I made sure to cover:
- De-glazing of cylinders using ball hone
- New piston rings
- Valves cleaned and honed against their original seat (made sure not to swap like valve types around)
- Valve clearances set per clymer manual
- Converted over to Dyna ignition system (issue still existed with stock points though)
- Cam chain adjusted with engine stopped and cold @ 15 degrees ATDC
- Cam shaft/valve timing set per clymer manual via notch on right hand side of cam
- Carb sync done at ~1000 rpm after rebuild (somewhat of a struggle to start up but, thought this was do to lack of sync | bike ran better after sync but, start-up didn't improve)
- Stock NGK D7EA spark plugs (verified spark on all plugs before first run)
- 2 x VD05F - Spark plug cap for cylinders no. 1 or 4
- 2 x XD05F - Spark plug cap for cylinders no. 2 or 3
- Stock 5 ohm coils
- Stock paper-style air filter
- Stock air intake box with rubber boots (no pods)
- no in-line fuel filter, only stock tank mesh filter

It should be noted that the only thing that isn't stock on the bike is the exhaust system. I got it from the PO with these pipes and I have no clue who made them. They are a 2-into-1 configuration and it appears that they are reasonably baffled (i.e. not crazy loud like the free-flowing megaphones). I know that detail regarding the pipes isn't very helpful but, I'm just not sure about where they came from. It is worth mentioning that the bike did run with these pipes and carbs when I bought it but, I do understand that with the changes in compression/vacuum the interaction between the pipes and carb system will change (just not sure how the cold start is the only thing that is obviously affected). Also, you should note that the carbs were in storage while the rebuild was taking place but, I  made sure that they were drained and cleaned out with carb cleaner before wrapping them up and putting them away.

Apologies for the novel but, I figured more details would be more helpful.

Thanks in advance for your help!

[flybox1]

Thanks for all the information.  It is appreciated.
We also need your pilot and main jet size, needle clip position (if not in stock slot) and idle mixture screw setting.

Pull all your spark plugs.  Take a closeup picture of the insulators/tips (all plugs in one pic, lined up 1-2-3-4), and post it here.
I believe you are just struggling with proper air/fuel mixture, making starts more difficult than need be.

[aguerra]

Flybox1,

Thanks for the quick response. Regarding the extra information, my ignorance will show regarding these carburetors. I actually don't know how they are setup inside due to the fact that they were a rebuilt set I purchased later on during my ownership of the bike (original purchase had the wrong carbs and some hack-job on the manifolds).The replacements were clean and practically new so I never cracked them open and they ran good after a good sync (before rebuild). I assume that settings were stock given that the guy who rebuilt him knows what he is doing (trust the guy who rebuilt them). However, I will still definitely need to confirm the settings in order to give you a true base line.

That being said, can the pilot and main jet be accessed through the underside of the carburetor bodies (i.e. remove float bowls and floats, remove jets and inspect for a stamp with number) or do i need to pull out the entire assembly from the bike in order to access the jets and needle clip (assuming needle clip is accessed from top)?

I know this is a question that stems from laziness but, just want to know what would be the best/simplest way to get this information. In short, would pulling the carbs out make getting this info easier?

Also, idle screw mixture settings are 1 and 1/2 from fully screwed  in. Pictures of plugs will come soon.

Thanks again for your help.

P.S. I don't mean to sound like whiny baby regarding pulling the carb assembly out. From my limited experience I've noticed that I tend to do things the hard way for no reason sometimes (i.e. inexperience)

[flybox1]

OK.
For now, dont worry about the needle clip position.  Drop one of the carb bowls and access the pilot and main jet.
Airbox removal is not really the fastest process, but it makes carb access much easier.
If you just want to drop a carb bowl, thats fine too.  No need to remove the float.  Just remove the main and pilot jet, and look for a stamped size, and possibly a stamped logo.  A "K" is a good thing.  No logo is not.

[RevDoc]

Flybox has you started in the right direction but, one simple check I would recommend before pulling float bowls and jets is to do
a clear-tube test to confirm that your float levels are properly set.  The mm settings from the factory are only a general guide.
The clear-tube test will get you right on the mark.

[Bankerdanny]

OK.
For now, dont worry about the needle clip position.  Drop one of the carb bowls and access the pilot and main jet.
Airbox removal is not really the fastest process, but it makes carb access much easier.
If you just want to drop a carb bowl, thats fine too.  No need to remove the float.  Just remove the main and pilot jet, and look for a stamped size, and possibly a stamped logo.  A "K" is a good thing.  No logo is not.

You may recall that dropping the bowls on a 550 is not like a 750, there are 4 very small screws holding it on, not a clip. You can do it in place, but it is a major PITA.

[flybox1]

OK.
For now, dont worry about the needle clip position.  Drop one of the carb bowls and access the pilot and main jet.
Airbox removal is not really the fastest process, but it makes carb access much easier.
If you just want to drop a carb bowl, thats fine too.  No need to remove the float.  Just remove the main and pilot jet, and look for a stamped size, and possibly a stamped logo.  A "K" is a good thing.  No logo is not.

You may recall that dropping the bowls on a 550 is not like a 750, there are 4 very small screws holding it on, not a clip. You can do it in place, but it is a major PITA.

Yes.  I know its a PITA...my K8's carbs have 4 screws, too 

[Deltarider]

You can do it in place, but it is a major PITA.

Not if you've replaced them by allen screws, which you all should do. And leave your needles alone unless you have serious indication a PO has changed the clip position. Not many undertook and undertake this, although you may get a different impression in this here forum.
Clear tube test is best done with engine running to check the level remains constant.

[aguerra]

All,

Thanks for the help. I want to pull the main and pilot first since I have a clear path for doing so. However, the clear tube test seems nice since there is no disassembly. However, do I need a special tool to rig this up or is there a set manner that this can be done on the PDs in particular? At first glance its not clear how to do it without a fitting to go in the hole drain hole on the bowl. When I rig up the tubes I can take some pictures or video when doing it to make things clear.

Any details on rigging it up?

Thanks again

[aguerra]

All,

Attached are the pics I took of the pilot and main I believe. Both have the Keihin K stamped. The main read 90 and the pilot read 42 (upside down in pic). I didn't pull the pilot out because it looks like its pressed in. I got lucky based on the way it was oriented when installed.

Any thoughts from this?

Thanks again.

[scottly]

It only needs me to drain the carbs when I leave it over night.

Are you turning the petcock off before leaving it overnight? If not, start doing so.

[Deltarider]

However, the clear tube test seems nice since there is no disassembly. However, do I need a special tool to rig this up or is there a set manner that this can be done on the PDs in particular? At first glance its not clear how to do it without a fitting to go in the hole drain hole on the bowl.

If my memory serves me well it can be done quite easy on your model. Replace the black rubber overflow tube temporarily by a transparent one and open the screw right where it's attached and keep the end of the tube high enough ofcourse.

[aguerra]

Delta,

This sounds like a reasonable solution. I'll give it a try tonight to see if I get any good info from the test. Any thoughts on the main and pilot jet numbers? Look good?

Scottly,

I have built the habit of closing the petcock just about every time I stop the bike (won't close it if I'm just stopped at a gas station for a few minutes). I definitely shut it off at the end of a days ride. I know that there are problems that can stem from leaving it open for extended periods.

Thanks again for the help. Hopefully the clear tube test will bring some clarity.