Topic: 78 CB550 Revs High at start.

[Habbyshaw]

So I have narrowed my assumptions down about the previous owner of my 78 CB550. He was either a Rocket Scientist or an #$%*. I have a feeling the second one is closer. He Removed the air box and slapped on some pods that don't fit and chopped the exhaust at the baffle. That's not even the funny part! when I bought it he looked me dead in the face and said. I'm not sure why... but it doesn't run as well as it used to.  Hhmmmmmm... Wonder why.

Well... Good news is I reinstalled the stock air box and swapped the exhaust off a 71 CB500. Bolted right up with no problem. Not sure if the restrictions were more or less back in 71 but I figured anything is better then nothing... Now I have back pressure and proper air flow. And guess what... It runs better! but still not well enough.

Symptoms: wont idle, Revs High into 5000RPM range with the choke on,

Good news: fires right up when the choke is on.

Discoveries: While installing the air box I noticed that the rubber mounts that go from the carbs to the air box are no longer sealed by the factory glue. Is this an issue? Does everything have to be SUPER tight? or is there some wiggle room? The bike has PD carbs so there is no air screw just a few mixture screw. Any troubleshooting tips would be great.

Thanks again,
Ryan In Boston

I should also mention... I recently cleaned the carbs and tightened the headlight screw.

[Hondawggie]

There is a round thumbwheel  near the 'butterfly' pulley where the throttle cable attaches at the carbs --- this thumbwheel sets the idle RPMs.  Try backing that off and see if the idle drops.
The thing you're testing here is whether he inadvertently screwed in this thumbwheel too far and hence made it rev too high.


To *really* figure out if this idle thumbwheel is the 'high revs' culprit or whether instead it's an air leak at the carbs -- unscrew this thumbwheel all the way out.  Under normal conditions, this would drop the RPMs at idle to such a low level that the engine would stall under normal conditions.

If you do this (back out the thumbwheel quite a ways) and you still have an excessively high idle you might have an air leak, usually between the carbs and the cylinder head in the region of the rubber intake manifolds.

[dave500]

the po might have changed the jets or anything,the mixture screw is underneath towards the front of the carb,it points vertically downwards,isnt the airscrew on the side at an angle?

[Habbyshaw]

I will double check the thumb screw position today.

The jets are stock and the PD carb's don't have air screws, just mixture.

thanks for the advice,

Ryan

[vance]

It should rev high with the choke on because there is a fast idle cam that bumps up the thottle linkage when you pull the choke. It should be between 3 and 4k rpm though. You can adjust that, but really you need to set the idle with a warm engine with the choke off first.

[TwoTired]

The PD carbs have a choke knob, and the cable moves the choke plates AND a fast idle cam.  While the fast idle cam is engaged, the large idle knob doesn't do anything.  As mentioned, there is a separate fast idle speed adjust screw.

You should get the K3 Honda Shop manual addendum, which has information about the PD carbs. (Chapter 19)

The 71 exhaust has less back pressure than the Stock 77-78 K3 exhaust, but it is way closer than an open pipe.  These bikes were finely tuned to please the EPA.  The higher back pressure of exhaust allowed the carbs to be set leaner and put less hydrocarbons out the exhaust.  Lowering back pressure makes it run leaner, and run hotter.  It may be possible to fix the idle with the idle mixture screw, turn them out some.  This should also help the the pick up response off idle when opening the throttle, too.

In theory, there should be some tweaks to the slide needle and main jet, too.  An A/F map made with a dyno would be the best indicator.  But, you could also read spark plug deposits after power testing ("plug chops") to infer mixtures at different power settings.  Got a test track?

Are you using the paper air filter?
To state the obvious.  The stock (book value) carb settings and adjustment positions only apply to bikes in stock configuration.

I believe the factory glue on the air box was an installation aid for factory line workers, rather than a requirement.  Clearly it should not leak air, though.  If you are comfortable with the air seal, it should be fine.  I never worried about it in my bikes.

Cheers,

[Beastman]

I have the same bike/carbs.  Installed a 4 into 1 exhaust, larger main jets, new airbox rubbers (no glue on the new ones).  Doesn't seem to be an issue.

After I bench synced my PD carbs, I had that idle knob way high.  Started the bike and it revved up to 5k, scared the #$%* out of me at first.  Backed it off and things went back to normal.

Where in Boston are you?  I'm in Cambridge, let me know if you wanna cruise some time.