Topic: Syncing carbs - Starting point when no #2 master

[sscardefield]

I am having a hard time syncing the carbs on my stock '76 CB550F. These are the carbs that do not have the #2 master. I did not know this at first and was trying to sync the other three to #2. I could get them completely in sync, but only at a high idle (like at 2000rpm), and that would be with the idle screw all the way out (not even touching the plate). My throttle cable is nice and tight, but I wouldn't say too tight. There is still at least a 1/4" of play in the throttle. The air screws are at 1 1/4 turns which from what I have read is stock. At this point all 4 idle adjustment screws have been moved around and I think everything is just out of wack now. I just want to start fresh and follow the proper steps to a T from the begining.

With these style carbs, how do I get back to a good "starting point". I would prefer to not have to take the carbs back off the bike if possible, but I have a feeling the answer is going to be to take them off and bench sync them. If I do need to take them off and bench sync them, with these carbs is the proper way:

1. Back the large idle screw all the way out so it is no longer touching the plate
2. Pick one carb and and set it using the drill bit or light method
3. Tighten the lock nut on that adjustment screw and use that carb as the master
4. Put the carbs back on the bike and sync the other 3 carbs to the master.

If that is correct, once I throw the carbs back on the bike what do I do with the large idle screw? Screw it back in until it just makes contact with the plate, then sync the carbs? That doesn't seem right to me though since that wouldn't then give you any adjustment out. It seems like you need to start with the screw at least a couple turns in so that you have some adjustment in both directions.

Sorry for the lame questions, but all of the reading I have done all points to syncing to the #2 master, which doesn't apply to me. I am just frustrated at this point.

[TwoTired]

As there are 4 adjustments for slide positions, it is possible to skew all the adjusters in a way that prevents them from reaching the bottom of possible travel and have the ability to close off all air feeding the engine.  This also renders the main idle knob ineffective.

With the idle knob backed out, you need to pick a carb to assign as master, and ensure that this carb's slide can reach the floor of travel.  With the stock air box this is not possible to see while installed.   But, it can be inferred with a manometer reading.  With the engine running, adjust your "master" carb (pick one) to the highest vacuum reading you can achieve. This signifies that the slide is more closed that the others, as the highest vacuum possible is when the slide is fully closed.   It's a bit tricky, since it will be running on three and you'll still want the lowest idle speed you can get.  Once the "master" has the highest reading, lock off that adjuster, and don't mess with it again while adjusting the slides on the others to the same vacuum level as the master, even though the "masters" vacuum reading will change.  Once you get all the slides to an even reading, the Idle knob should be effective again to set idle speed.  But, if all the slides have been skewed to the extreme, you may have to go through the whole process again with a lower idle idle speed while setting the "master" position.

I don't recommend using drill bits for syncing these carbs.  Far too easy to have the exact problem you are encountering.

Cheers,

[sscardefield]

Perfect! Thanks for the info. I'm going out to mess with it now. I'll update tonight (hopefully after a nice ride).

[Untold]

TwoTired, when you say you get the master carb to the highest vacuum reading, is this an arbitrary number? Like, as high as you can get it without the engine dying? Or just so that it's higher than the other 3? Or the highest reading on the actual manometer? 

[TwoTired]

TwoTired, when you say you get the master carb to the highest vacuum reading, is this an arbitrary number? Like, as high as you can get it without the engine dying? Or just so that it's higher than the other 3? Or the highest reading on the actual manometer?

The vacuum source is the cylinder (piston falling).
The air intake is where the air comes from to fill the vacuum and rise the pressure.
The carb slide is what impedes this action. So, the more closed it is, the less equalization occurs, or the deepest vacuum is measured at the port where the manometer is sampling the intake runner vacuum.

The engine will run on three, if the throttle is held open enough (rapid idle speed).  If the speed is held constant, the lowest slide will effectively not allow the cylinder to contribute to RPM.  Then the slide is lowered to peak vacuum, which should be when the slide is closed or very nearly so, as it stop letting the intake air supply to raise the pressure (highest reading achieved the manometer).
This procedure is only to recover or determine a "master" carb without looking into the carb bore.  Once that is determined, the vacuum sync process is resumed, making the other carbs the same as the assigned master.  Then, the idle knob should be effective at setting idle speed.

I haven't done it this way in years.   But, this is the theory and what I can remember, when I blew it in 1975 chasing the adjustment on all four.  Since then, I've always used the light method on the bench, which results in always having the idle knob determine idle speed.

Cheers,

[sscardefield]

Man, this is getting frustrating. I gave it a go. I tried for like an hour, but it just seemed like the bike either wanted to die or idle at 4500 RPM's. So I said enough with that and I pulled the carbs back off. Now I'm even more confused. This doesn't seem right. Here are some pictures of the slides positions, large idle screw, and idle adjustment screws.

http://imgur.com/a/3rqtL
 
All 4 idle adjustment screws are all the way in. With them all the way in, I would think the slides would be down further than that (no space showing at all). Is that correct? Also, as you can see in the picture the large idle screw is all the way in, and it's not even touching the plate. I'm guessing that's just because all 4 idle adjustments are all the way in/down, but not sure.

I'm not really sure what to do at this point. I would think that I should have some more play in the idle adjustment screws to bring the slides down further. Any pointers would be greatly appreciated.

[luap]

someone please help him so he get this bike out of my garage

[Untold]

3 of those adjusters are missing the nut.  I'm assuming this is just temporary while you're getting it set?  Are you sure they're screwed in as far as they would go, and not just stuck?

[sscardefield]

Yeah, they are just off while I was adjusting. Number two has the nut on it because I am going to use that one as the "master".

From what I can tell, they aren't stuck. I can't tighten them any more. They all spin very freely up until that stopping point, so I don't think they are gummed up or anything. And they all seem to stop at the same spot (the same amount of threads exposed).

[TwoTired]

Are the carbs off the bike now?

1. Back the idle knob way out.
2. Pick a carb to be a master.
3. Adjust the individual adjuster on the assigned master so that the slide bottoms in the bore.
4. verify that the idle knob will raise the master slide about 1/16 inch.
5. lock down the master so it won't lose it's setting.
6. Back out the idle knob again 
7. adjust the other carbs so that their slide reach the floor of travel.
8. give the big idle knob a tweak so that it will raise all the slides about 1/16 inch
9. Install the carbs back on the bike and adjust each of the other carbs in turn to match whatever vacuum the master has, while idling at 1000 RPM (adjust the big idle knob to achieve this).
10.  Ensure all carbs have same vaccum after lock each adjustment.

[sscardefield]

Are the carbs off the bike now?

1. Back the idle knob way out.
2. Pick a carb to be a master.
3. Adjust the individual adjuster on the assigned master so that the slide bottoms in the bore.
4. verify that the idle knob will raise the master slide about 1/16 inch.
5. lock down the master so it won't lose it's setting.
6. Back out the idle knob again 
7. adjust the other carbs so that their slide reach the floor of travel.
8. give the big idle knob a tweak so that it will raise all the slides about 1/16 inch
9. Install the carbs back on the bike and adjust each of the other carbs in turn to match whatever vacuum the master has, while idling at 1000 RPM (adjust the big idle knob to achieve this).
10.  Ensure all carbs have same vaccum after lock each adjustment.

Yup, they are off the bike now. Thanks a lot for the steps. I've got a good feel for the process now. My problem is at step 3. I have chosen carb 2 to be the master. I have the individual adjuster for 2 as tight as it will go. Even with it as tight as it can go, you can see in this picture that the number 2 slide still has a good amount space between the slide itself and the carb body. With the slide bottomed out in the bore you should see no space, right?

[TwoTired]

The slide have a cutaways on the inlet side.  They should bottom out on the engine side, which effectively cuts off the air to the engine.

Engine can't run without air. A little air means idle speed.