Topic: CB550 air fuel mixture

[iancb550]

Ok I guess that's just how the old gal is then. Must have just been an oddity with the vacuum leak that allowed it to do that before. I've had an older Honda before that warmed up just stationary but it was about ten years younger, an nc30 so just thought that would be normal.

[phil71]

i was speaking in general,do you think its normal to do it twice?

if you've never done it before.. yes.

[TwoTired]

Ok I guess that's just how the old gal is then. Must have just been an oddity with the vacuum leak that allowed it to do that before. I've had an older Honda before that warmed up just stationary but it was about ten years younger, an nc30 so just thought that would be normal.

Info for the NC30 is hard to get here in the states, as the VFR400 sold for nearly the same price as the VFR750.  And, the only importation of the VFR400 was the grey market.
Searching some part lists reveals they had CV carbs which are quite removed in operation and features from the mechanical slide types used on the CB550F.
My guess is that the NC30 had a choke knob at the bars, that when pulled, deployed both choke and fast idle in proportion.

If you insist on stationary warm up for your bike to maximize neighbor annoyance, then add a throttle lock such as the Vista cruise.  It will hold the throttle at any position you set during warm up.

However, it is unnecessary in my view, unless you are going jump straight on to the Autobahn from the parked position.)

Cheers,

[phil71]

I have to respectfully disagree about the warm-up period. I've gotten many 550s to start easily on choke, idle at half choke for a few minutes, then come off completely within a few minutes.
I'm not talking about arctic climes, but Derbyshire is not the north pole.
I think it is an attainable goal to not have to diddle your throttle .

[TwoTired]

I have to respectfully disagree about the warm-up period. I've gotten many 550s to start easily on choke, idle at half choke for a few minutes, then come off completely within a few minutes.
I'm not talking about arctic climes, but Derbyshire is not the north pole.
I think it is an attainable goal to not have to diddle your throttle .

These were stock 550s, with non-PD carbs?

I have 7 Cb550s that disagree with you, including a 74 I got in 1975.  While it doesn't get THAT cold here I've ridden regularly to work with temps in the 30s.  They will run fine on partial choke, but you have to hold the throttle a bit open.  No "diddling " required, just keep it off the idle stop.

 Then there is the physics thing... about poor fuel atomization when cold and poor combustion efficiency.  And induction air leak would equate to opening the throttle a tad.

It IS interesting that many carbs have a fast idle cam coupled with choke.  Wonder what that was for?

You know, I recall reading lots of threads where people routinely twiddle the Idle stop screw for stationary warm up.  Latter when the engine is racing during idle at a stop, they reach down and tweak it back.

Do share what you had to do to make them idle cold in stock form and with no fast idle cam.  Did the MPG (overall efficiency) suffer from such tuning?  Such motorcycle-fu would be of benefit to us all!

It will be interesting to see if Ian can achieve your goal.  None of my F models can do what he wants.

[phil71]

Sorry, I misunderstood .. i thought you were saying they don't idle at all. I did just finish a 550 (non-PD), new boots, o-rings full cleaning/tuning etc.. Even in new york cold, it starts on full choke without throttle, can move to half choke within a minute. I do run the idle in the 1500 range, which is a little high, but I tried to make it as easy to use as possible. This guy was starting to get frustrated by the bike, and honestly, for him an old bike may not have been the best move (more of a fashion statement than a desire to know how to maintain it). However, it's been 2 weeks and I saw him last night in the neighborhood and he said it's been very easy to start, and seemed very happy.

[TwoTired]

The book says to set the idle at 1000 when at full operating temperature.  (I use 1100 for these old engines with the modern fuels available)
When the engine is cold, the fuel doesn't atomize well, and the engine is inefficient.  Thus, the idle position for the slides set for warm operation at 1000 RPM, will be in the 500 RPM -700 RPM range when the engine is cold.  And, it is unusual to get a 550 to idle reliably in this range at any temperature.

To achieve 1000 RPM idle at any choke setting when cold, you have to bump the slides up, either with the idle stop screw or the twist grip.

If you use the twist grip, you don't have to futz with the idle stop screw later in the warm up cycle to get the idle speed back down.  Once I learned how reliable the CB550 is to drive on partial choke when cold, I eliminated any stationary warm up in the commute routine, and very seldom need to readjust the idle stop setting.  Saves on gas, too. Zero MPG is certainly worse than some MPG.   I also found that the warm up was faster with the engine making more power (moving itself and me on side streets) than simply turning itself over at the crank.  By the time I reached a freeway, the engine was warm enough to not need any choke and able to produce cruise power and full power acceleration.

It's a pretty sweet, well mannered street bike, really.  But, it does take more operator prowess than the modern autos with computer controlled everything.

Cheers,

[pangloss]

And induction air leak would equate to opening the throttle a tad...??;

[TwoTired]

And induction air leak would equate to opening the throttle a tad...??;

This engine/carb combo idles on the rich side.  An induction air leak adds air, increases RPM.  Equivalent to raising the slide a tiny amount, which would also add air to the idle mix.  Neither situation changes the carb throat vacuum pulling fuel up from the bowl.

Was that the explanation you were looking for?

Cheers,