Topic: Carb Tuning - seeking opinions

[pmpski_1]

I think that my carbs are running rich. I'd like to run the symptoms and my conclusion by y'all before I dive in.

1. Plugs are sooty and black. This is the main reason that I think its rich.
2. A little black smoke. Not much, but enough to notice.
3. Flat above 5k RPM. I haven't ridden it enough to see if it comes back sometime after that, but it seems kind of weak.
4. Will only run for a few seconds with the choke on. After that it won't run or runs very badly.

I'm thinking rich. Sound right? 1974 CB550, just rebuilt the carbs, bench sync, air screws are within spec, full airbox and brand new filter. Compression is on the low side, but all the cylinders are within ~5 of each other. Valves have been adjusted and are within spec. I am going to follow the steps in the carb FAQ and adjust to lean it out.

Another question, if I can squeeze it in: When I start the bike it idles by itself at 1k. However, after I ride it for a bit, the idle goes up to 2-3k. Is this related to my theory about running rich, or is just that my main idle is set too high? Should I adjust the idle after the bike is warmed up?

Thanks in advance. It looks like I only have a few more kinks to work out before I'm riding every day!

Brian

[TwoTired]

Your description does fit a bike running too rich.  Follow the FAQ.
Read your plugs. The chart at:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
will help you read them for mixture indications.

This bike doesn't have a fast idle cam coupled with the choke.  So, when cold, you have to hold the throttle open to keep it running with the choke on.  The main idle setting is done after the bike is fully warmed up.  The 74 has a friction adjust for the throttle grip.  This can be set to hold the throttle open while it is cold.

One thing I didn't find in the FAQ (maybe it is there, I just didn't see it) is that carb tuning is the last thing you want to do.  Perform all other tune up items first, cam chain adjust, tappets, new correct heat range plugs, spark timing, new filter, etc.  Then carb sync and dial in the mixture.

[pmpski_1]

Thanks. I'll adjust the idle when the bike is warm.

All the other work items have been completed, so I'm well on my way.

One more question - if the float valve is set too high, where there is too much gas in the bowl, could that cause a rich mixture as well? I'll take the carbs back off to adjust if necessary ... of course, I'd rather not ...

[oldbiker]

The fuel level in the float bowls is quite important. It is well worth while taking some time to get it spot on. When you have got it right, drain one bowl into a measuring jar and note how much fuel you have.  In the future you can check fuel levels without taking off the bowls just by measuring the fuel in each bowl.

[Raul CB750K1]

There is a simpler method to do that. Attach a piece of clear tube to the drain plug. Hold the tube upwards, touching the bowl on the outside. Open the drain screw; the level in the tube will match the level inside the bowl. Mark the bowl with a permanent marker or a sharpie, but as gas will clean it a tiny screwdriver scratch should be enough. Then you will have a reference.

Anyway, in my opinion it is very unlikely that the float height can be modified by itself. As there is no adjusting screw or similar, there is no way that they can loosen up.

Raul

[Harry]

One thing I didn't find in the FAQ (maybe it is there, I just didn't see it) is that carb tuning is the last thing you want to do.  Perform all other tune up items first, cam chain adjust, tappets, new correct heat range plugs, spark timing, new filter, etc.  Then carb sync and dial in the mixture.

Hey, its Lloyd, fellas! 

Good point, have added to the Carb FAQ.

So, are your CB550 sites up and running again?

[TwoTired]

Sorry Harry.  Mistaken Identity.  My first name is Lloyd.  I am not Rob Lloyd of web site fame.

If it's any consolation, you're not the first to confuse us.

Cheers,

[Harry]

Sorry Harry.  Mistaken Identity.  My first name is Lloyd.  I am not Rob Lloyd of web site fame.

If it's any consolation, you're not the first to confuse us.

Cheers,

Good Lord! its hard to tell you two apart 

[oldbiker]

Harry, surely you mean 'Good Lloyd, it's hard to tell you two apart'