Topic: Pilot jets plugged on 1978 CB 550K

[5youngs]

Does anyone out there have any ideas on how to clean the plugged up pilot jets on the carbs for the 78 CB 550K? All 4 of them are plug full.

Thanks

[Bob Wessner]

Try soaking them in carb cleaner for a while, then, without letting them dry, poke some fine soft wire through.

[5youngs]

I got them unplugged!! I used a .020 drill bit on the end of a dremel and it got all four of them cleaned out. We should have the bike running by next week.

Thanks for the help so far.

Here is a picture of the latest work done to it.

[TwoTired]

It's a good looking bike you have there.

 I hate to be a wet blanket on your fire.  But, you just changed your the pilot jets from  #42 (0.0165354 inch)  to #51 (.020 inch or 0.508 millimeter).

The jets are pressed in and there is not simply just a hole in the jet (now very jagged from the twist drill).  Attached to the jet inside a chamber in the body behind the slow jet is a small tube (called an emulsion tube) with very small holes cross drilled into it.  These cross drilled holes emulsify the fuel from the jet orifice with air, premixing the fuel before it is delivered to the idle mixture screw and then the carb main bore.  If the pilot jet was plugged, it seems likely that the emulsion tube holes are also plugged and perhaps the small chamber in the body as well as the passageways to and from the chamber to the air jet, the idle mixture screw and the carb bore.

You can expect the engine to idle very rich, perhaps to the point of fouling plugs.  You might be able to compensate with the idle mixture screws, but don't expect the book adjustment values to be proper for your bike anymore. The idle mixture screws may also be very sensitive for correct setting.

Lastly, as the new fuel flows through the carbs once again, the solvent action may clear some of those pilot jet emulsion tube holes, adding more air to the idle mix on random carburetors.  This may cause the engine to suddenly change it's idle behavior. And, the idle mixture screws will need to be adjusted again.

Or, maybe it will all work out for you anyway. As this year bike ran leaner than previous years at idle anyway.

Though you should know...

[5youngs]

It's a good looking bike you have there.

 I hate to be a wet blanket on your fire.  But, you just changed your the pilot jets from  #42 (0.0165354 inch)  to #51 (.020 inch or 0.508 millimeter).

The jets are pressed in and there is not simply just a hole in the jet (now very jagged from the twist drill).  Attached to the jet inside a chamber in the body behind the slow jet is a small tube (called an emulsion tube) with very small holes cross drilled into it.  These cross drilled holes emulsify the fuel from the jet orifice with air, premixing the fuel before it is delivered to the idle mixture screw and then the carb main bore.  If the pilot jet was plugged, it seems likely that the emulsion tube holes are also plugged and perhaps the small chamber in the body as well as the passageways to and from the chamber to the air jet, the idle mixture screw and the carb bore.

You can expect the engine to idle very rich, perhaps to the point of fouling plugs.  You might be able to compensate with the idle mixture screws, but don't expect the book adjustment values to be proper for your bike anymore. The idle mixture screws may also be very sensitive for correct setting.

Lastly, as the new fuel flows through the carbs once again, the solvent action may clear some of those pilot jet emulsion tube holes, adding more air to the idle mix on random carburetors.  This may cause the engine to suddenly change it's idle behavior. And, the idle mixture screws will need to be adjusted again.

Or, maybe it will all work out for you anyway. As this year bike ran leaner than previous years at idle anyway.

Though you should know...

I appreciate your input. Do you know what the air settings should be for this bike. I set it at 1 1/2 turns out.

While I drilled out the low speed jets, i noticed it was only plugged just a little bit into each jet. I am assuming it was because the fuel level was only allowed up so far becuase of the floats and float valves doing their jobs correctly.

I an assuming the passageway from the low speed jet to the air screw adjustment is clear or I hope so at least.

Any more feedback on this matter will help me very much.

Thanks again, Mark

[kghost]

Twotired is right.

Also if you have any doubt as to the passages being clear (and the only way to know is to blow something through them) you better check now.

Else you'll be doing the job again.

[5youngs]

I spoke to a person that said you can be sure it is unplugged if you remove the air adjustment screw and blow into the low speed jet and air should come out the air screw hole.

Does that sound right to you??

Thanks

[TwoTired]

There are no air settings to adjust on these carbs.   Air jets are fixed orifices.  Theer are two of them at the mouth of each carb.  The idle mixture screws meter fuel.
The book values are 1 1/2 turns with the stock jetting.  But these were supposed to be fine tuned for max idle RPM, which is interactive with the main idle stop screw.  An RPM meter with 50 RPM descrimination was also very helpfull for those who don't have very descriminating ears.

You'll notice the pilot jets are kind of long.  The hole you see in the end is not the jet orifice.  Down deep in the hole is a restriction which is the metering orifice.

As far as clear passageways are concerned, I guess you'll find out.  If the all the head pipes won't stay hot below 2K RPM, the cold ones are attached to plugged carbs.



I usually test each passageway with the nozzle of the aerosol carb cleaner can for fluid flow.

Good Luck,

[TwoTired]

I spoke to a person that said you can be sure it is unplugged if you remove the air adjustment screw and blow into the low speed jet and air should come out the air screw hole.

Does that sound right to you??

Thanks

No, not for your carbs.  The pilot system consists of the slow jet, the slow air jet, the idle mixture screw and the exit hole in the carb bore.  With the mixture screw removed, there are 4 holes in the system that all have to be clear.
But, be aware that behind the idle mixture screw is a spring, a small flat washer, and small o ring.  These small parts can be lost to gravity and blasts of carb cleaner or air.  Also, some carb cleaners can damage that small o-ring.
Cheers,

[5youngs]

If the end I see is not the jet orifice that encourages me!!  As stated before I just found a small ammount of restriction in the begining of the holes and did not push hard at all. After I got through the restriction I lightly ran the drill down and then remove the drill and pushed it in manually to double check for any more blockage.

I will try the carb cleaner trick for fluid flow.

Also I will check out the hot pipe idea when idling.

[5youngs]

I spoke to a person that said you can be sure it is unplugged if you remove the air adjustment screw and blow into the low speed jet and air should come out the air screw hole.

Does that sound right to you??

Thanks

No, not for your carbs. The pilot system consists of the slow jet, the slow air jet, the idle mixture screw and the exit hole in the carb bore. With the mixture screw removed, there are 4 holes in the system that all have to be clear.
But, be aware that behind the idle mixture screw is a spring, a small flat washer, and small o ring. These small parts can be lost to gravity and blasts of carb cleaner or air. Also, some carb cleaners can damage that small o-ring.
Cheers,

Yes, I did take apart the adjstment screw and found the spring and washer, but the o-rings stayed inside.

This has been informative to say the least......I have been enjoying doing the carbs.

Mark

[Bob Wessner]

I have been enjoying doing the carbs.

Wow, will you do mine next time they need it? 

[5youngs]

I did end up taking all 4 of the low speed jets out and cleaning them. Thanks your help TwoTired, Bob Wessner, and kghost. We might get it running tonight.

[Bob Wessner]

You will develop something of a relationship with your carbs. It may not be the best one you ever had, but it will be enduring. 

[5youngs]

That has been the most difficult job thus far on the restore of the bike!