Topic: 1977 cb550k Rejet

[mke_design]

I recently picked up a 1977 Honda cb550k with around 9000 original miles. The bike is stock other than the 4-1 MAC exhaust that was installed 2 owners ago. The previous owner pulled the carbs and had them cleaned. I've been riding the bike on weekends and have noticed a few issues that might require adjustment.

The bike idles a light fast, at about 2000rpms. Keeping it at 2000 requires keeping the choke out about 1/4. Anything lower than 2000rpms causes the bike to start running a little rough. When riding, the bike is a little rough under 2500rpms, but after that point, pulls real strong all the way up. There is also some bluing on the pipes, indicating a hot and lean condition. I pulled the plugs and they look alright. The tips are blacker than they are tan, but definitely not oily or greasy, and no white spots indicating an overly hot engine. Im not quite sure on the age of the plugs, so they might just be old too.

Ive done a little searching on the forums and found that, although the MAC website says there is no need to rejet, its probably more realistic to assume that the bike is running a little lean once they are installed. The bike is running with the stock airbox installed, and as far as I know, still has the stock #90 main and #38pilot jets in the carbs.

Has anyone had a similar situation. Im thinking I either have the fatten up the jets a level, or pull the needle out a little. Its not off by much right now, but I want to tune it so its running smooth all the way through!

[Swede]

Have you had the carbs off the bike? I know that you said the po had them cleaned but I have experienced first hand how a po can screw a bike up even when he seemed competent. 

[TwoTired]

The 77 and 78 K bikes had PD carbs with pressed in idle jets to discourage people from changing to larger jets and cause more pollution.
Still, they can easily become plugged with gunk as the jet is only about 0.016 in in diameter, a bit bigger than a High E guitar string.

For a proper cleaning, the idle jets still have to be yanked from the carb body and reinserted afterwards. It's the only way to know they are clean and flowing properly.

Check the head pipe temps at idle (1100RPM).  The cylinders with the cooler pipes likely have plugged slow jets, or the carbs need to be vacuum synchronized.

Of course, all the standard 3000 mile tune up items should be addressed before ANY carb tuning is performed, including a clean as new air filter.  Fine tuning carbs to an out of tune SOHC4 engine, is pretty much a futile endeavor, IMO.

[mke_design]

Have you had the carbs off the bike? I know that you said the po had them cleaned but I have experienced first hand how a po can screw a bike up even when he seemed competent.

No, I haven.t pulled them yet. Things are running fairly smooth, so Im hoping the amount of screwed-up-ness has been kept to a minimum!

The 77 and 78 K bikes had PD carbs with pressed in idle jets to discourage people from changing to larger jets and cause more pollution.
Still, they can easily become plugged with gunk as the jet is only about 0.016 in in diameter, a bit bigger than a High E guitar string.

For a proper cleaning, the idle jets still have to be yanked from the carb body and reinserted afterwards. It's the only way to know they are clean and flowing properly.

Does yanking these jets destroy them? Im assuming the mains screw out? Looks like I have a little more reading to do...Also, TwoTired, you keep mentioning that the slow jets may be plugged, but nothing about the mains. Given the way the bike is running, do you think that there is a chance that the mains are running clean, and that there may be no need to rejet, but just make sure those idles are cleared out?

Check the head pipe temps at idle (1100RPM).  The cylinders with the cooler pipes likely have plugged slow jets, or the carbs need to be vacuum synchronized.

What's a standard method for figuring out the temp? Is there a certain kind of thermometer made for this? Im assuming this process is fairly straight forward and can be done in my garage. Vacuum syncing is another story...

Of course, all the standard 3000 mile tune up items should be addressed before ANY carb tuning is performed, including a clean as new air filter.

The bike could probably use new plugs, oil, oil filter, and a cleaned up air filter anyways, so that makes good common sense to swap those out before any tuning. The next step is to do a little searching to figure out how to clean the air filter that came in the stock air box. Manual says to "tap it lightly and blow out with compressed air". Is that it? Any experience with that?

[TwoTired]

Does yanking these jets destroy them? Im assuming the mains screw out? Looks like I have a little more reading to do...Also, TwoTired, you keep mentioning that the slow jets may be plugged, but nothing about the mains. Given the way the bike is running, do you think that there is a chance that the mains are running clean, and that there may be no need to rejet, but just make sure those idles are cleared out?

The pilot/idle/slow jets are a parallel fuel delivery circuit that work in concert with the mains (and slide needles that they feed).
The pilot jets are #38 which is 0.38mm in diameter.  The mains #90 which is 0.90mm in diameter dominate the mixture above 1/4 throttle position.  Which jet would you think is more likely to get plugged?
Yes, a plugged pilot jet can effect mixtures in the more open throttle positions.

Check the head pipe temps at idle (1100RPM).  The cylinders with the cooler pipes likely have plugged slow jets, or the carbs need to be vacuum synchronized.

What's a standard method for figuring out the temp? Is there a certain kind of thermometer made for this? Im assuming this process is fairly straight forward and can be done in my garage. Vacuum syncing is another story...

I use the flash touch method.  Assume the the header will burn you.  But, so will an open flame ad you can pass your hand/fingers through a flame if you do it it quick enough.  Do the same with the header, you feel how hot it pipe is after you have released it with a very brief touch.  However, just because the the first one is cool doesn't mean the next one will be.  Several rapid short touches will give you a sense of pipe temp.  If the headers are turning blue, they are certainly hot enough to give blisters or worse.  So, you will have to be fast.  The bodys heat sensors are slow responders, so you have to wait for it to "register" with the brain.  Also the heat you interpret is not the heat specific but rather a change in heat for what it was.  When you touch a hotter surface, you register how rapid a change has  occurred a few seconds ago.  Let the info register in your brain well after you have let go of the pipe.

The slide openings are very small at idle setting.  If they aren't the same, it can make some cylinders work far harder than others.  In fact, if one slide is closed that cylinder won't fire or contribute heat to the exhaust pipe.
This should explain the importance of a vacuum carb sync which evens the opening position of each carb's slide.

The next step is to do a little searching to figure out how to clean the air filter that came in the stock air box. Manual says to "tap it lightly and blow out with compressed air". Is that it? Any experience with that?

That procedure is for the 3000 mile tune up, and assumes the filter was new 3000 miles ago.  The paper filter is scheduled for replacement every 6000 miles/ 6 mos.
I find I can stretch the use of the paper filter to once a year.  After that, and it becomes too restrictive even with monthly cleanings.
They are too expensive for me to replace at that frequently.  So, I switched to a UNI NU4055 Foam filter that is cleanable/re-oil-able/reusable, just for the long term economy aspect.  The first UNI I bought lasted 15 years.