Topic: irrational high revs while warming up

[crobar]

I have a 78 cb550. When I fire it up, i have the choke on, and turn the idle up a bit as it usually won't start without it. it's slow to warm up.

So it starts up, idles around 2000 rpm and then for no reason it jumps up to 4,5,6000 rpm. Then when turn the choke off it dies or drop the idle. It's a balancing act between keeping it alive and stopping it from over revving while it warms up.

Any ideas what might get it warming up normally? Everything in that area is stock.

[flybox1]

do you have a fast idle cam on those carbs?
If so, you're getting it out of balance by increasing the slide height with the idle set screw AND using your choke at the same time.
get your bike up to good operating temps.  set your idle and LEAVE IT ALONE.  no need to adjust it any more.
Now, only use your choke when starting, and slowly decrease the amount you use as your bike warms up....

I can pull the choke, gear on, and leave, and by the time i get to the first light, i can press my choke in about half-way.  second light, it goes all the way in as my bike is warm, and my idle set screw holds my idle.

[Shane72]

I have a 78 cb550. When I fire it up, i have the choke on, and turn the idle up a bit as it usually won't start without it. it's slow to warm up.

So it starts up, idles around 2000 rpm and then for no reason it jumps up to 4,5,6000 rpm. Then when turn the choke off it dies or drop the idle. It's a balancing act between keeping it alive and stopping it from over revving while it warms up.

Any ideas what might get it warming up normally? Everything in that area is stock.

Hey there,

It really sounds to me like your symptoms indicate that a thorough carburetor cleaning is in order.

Clogged jets are the first place I'd look, in particular, the intermediate jets--they have the smallest orifices, clog easily, and control nearly all of the fueling in choke-off slow and mid-speed running.

What is probably happening when "for no reason it jumps up to 4,5,6000 rpm", is that between the increase in idle speed (by turning the idle adjustment in during startup), and having the choke on, the engine pulls enough to start feeding off of the main jets.  If your intermediates are clear, it should idle normally soon after the choke is not needed for startup.  Turning up the idle speed at every cold start should not need to happen.

Intermediate jets, slow jets, etc...all refer to the ones I'm talking about.

All the best,

Shane

[TwoTired]

When was the last routine 3000 mi tune up completed?

When was the last vacuum carb sync performed?

When was the fast idle cam screw tweaked?

Does the bike have the stock induction set up?

Does the bike have stock exhaust?

Have you  checked for induction leaks?

Have you verified smooth operation of the timing advance?

There IS a reason for the RPMs to advance.  You just don't know it yet.  Partly it's due to the fact that as the engine becomes more efficient as it warms up.  The wide variation is a set up/ adjustment problem?

The is no good reason why you should be tweaking the main idle knob after it has been set properly.

[FillionJ]

My 77 550k does exactly what you are explaining crobar. Although I'm getting some gas leaking from carb ones overflow tube, when it does run it sounds like it's not on all cylinders. So I thought maybe my float 1 was sticking or is stuck? I just rebuilt these carbs last winter and have not ridden much this year so I find it weird. I have a stock airbox with a 4-1 Mac exhaust.


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[FillionJ]

Could it be that just one of my spark plugs are shot ? Cause it runs sometimes but quite rough


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[crobar]

Yes, also at first mine sounds like it is not on all cylinders, but when warmed up everything is fine and my plugs are good.

As I said, everything is stock. If we back up all the way to the beginning. Each time I ride the bike, the idle has been set for normal operating temp once it warms up. The next time I go for a ride, if I try and start the bike then without adjusting the idle, choke is all the way on, it won't start. Turns over but never kicks in. Does that indicate anything?

[Don R]

All the above, plus check for a vacuum  leak. Mine idles up after a highway ride, I suspect the timing advancer. One of these days, Ill check it.