Topic: Erattic idle on barn-fresh CB550

[ChopperCharles]

I just got all the parts to get my 1978 CB550K running. I replaced all of the needle valves and cleaned all the jets, including the slow jets and pilot passages, with carb cleaner and compressed air (the carbs weren't that bad to begin with). I've verified that all the clamps on the carb boots are secure, and all the nuts on the intake manifolds are tight. The throttle is not sticking. All four slides move together without problems.

Here are my symptoms:

The bike starts right up easily. It runs on choke a bit, and then once it warms up I turn the choke off. Idle is at 2000rpm. If I rev it rpms come down slowly, and they may stay at 4000rpm or so. I use the idle screw to lower the idle, rev it again, and now idle is barely 1000rpm, and it's really rough and wants to stall. Adjust the idle back up and all is good, until the next time I rev the motor. At idle I get a few quiet pops out of the exhaust.

These are all the signs of an air leak or a way over-lean condition.... but the bike RUNS great! I took it out for a spin and it was smooth as silk, had power everywhere, and was fun to ride (more fun considering I had no front brake and my clutch cable hadn't come in yet, so no clutch...)

When I bought this bike it came with four really grungy pod filters and a 4-into-1 exhaust. I replaced the grungy filters with new filters... but I have no idea if the PO rejetted.
 I wonder if this would be solved with larger slow jets. Any ideas on how to track down the real source of this problem?

Thanks!

Charles.

[750goes]

What are your plugs like - are they showing up rich or lean - you could have one sticky slide in there as well.

have you synched the carbs ??

[MrZxp]

x 2 on the carb synch. After I did mine, I was impressed with how smooth the little 550 idled, clutch rattle excepted! (this was after doing the timing etc, where it was idling ok, but not as good as after synching with my Morgan CarbTune.)

Cheers, Phil

[TwoTired]

Agreed, vacuum sync the carbs.

[Ichiban 4]

Another vote for synching your carbs.  Doubt it has much to do with jetting..unless your living above 3500 feet.  I'm @ 5000 feet..had to go up to next larger main jet size.  Smaller jets shouldn't effect overall smoothness though.  Biggist thing I saw was running lean, hot, blueing pipes some.

Your description of how runs, idles differently @ different times though..makes me wonder if your throttle cables are clean, have been lubed recently.  Stripping out cables (or @ least running solvent through, then light oil) cleaning, lubing..made big difference in getting consistent idle for me.  Can't hurt anyway.

Let us know what works.

[ChopperCharles]

remember, I'm running a 4-into-1 and pod filters, so it could be jetting.

Throttle cables are fine, and no slide is sticking.

Does synching really make that big of a difference? I've never bothered to sync any carbs on any other bike I've owned... but those have all been CV carbs.

Bike hasn't been run long enough to really show up anything on the plugs.

Charles.

[joeb]

Sync your carbs and check for vacuam leaks. Depending on what kind of 4 into one exhaust you are using will determine wether you need to re-jet. mac's claim to fame is no rejetting is nessecary. 

[bryanj]

If this is a 77/78 it will have the push in pilots and no ammount of carb cleaner/compressed air will clean them properly.
There are lots of hreads about getting them out and its not difficult , also i believe they are now available pattern but cant remember where from (also in another thread). Get them out, clean them properly, especialy the cross drilled holes, and then sync the carbs and i bet you will be OK

[TwoTired]

Here are my symptoms:

The bike starts right up easily. It runs on choke a bit, and then once it warms up I turn the choke off. Idle is at 2000rpm. If I rev it rpms come down slowly, and they may stay at 4000rpm or so. I use the idle screw to lower the idle, rev it again, and now idle is barely 1000rpm, and it's really rough and wants to stall. Adjust the idle back up and all is good, until the next time I rev the motor. At idle I get a few quiet pops out of the exhaust.

Does synching really make that big of a difference? I've never bothered to sync any carbs on any other bike I've owned... but those have all been CV carbs.

CV carbs position the slides based on the vacuum they experience.
Mechanical slides are positioned based on the the throttle position.  At idle, mis-positioned slides cause the cylinders to fight for domination of the crankshaft rotation.

Yes, vacuum synching makes a difference. Particularly at idle.

Cheers,

[ChopperCharles]

Those push in pilots were quite easy to remove, and they are nice and clean, as are the passages behind them.

I'll do a sync... I bought a dial synchronizer years ago from JCW and haven't had a reason to use it yet... now is the time!

Also, is it normal to have to use a ratchet strap to get the carbs to seat into the manifold boots, or do I need to buy four new ones?

Charles.

[bryanj]

You need some new ones, but again the 77/78 are different and not as easy to get or as cheap as the earlier ones

[ChopperCharles]

Well, 1,2, and 3 were spot on, but #4 was off by quite a bit. I adjusted the #4 carb, and now all four are pretty close to each other. The bike idles noticeably better... but still not perfect. I'm going to run a full tank of gas through and see if that makes any difference when its done.

Charles.

[750goes]

By some fuel injection cleaner (car stuff)...and run a little bit through for every tank of fuel you use...
it's amazing how a little consistent running will smooth things out... and clean those little passages...