Topic: 77 cb550f fickle idle

[000jeremy]

Hi guys,

Ive had my 77 cb550f for a week or so now.  When I start it up it seems to want to die right away unless i give it a bit of throttle for a while.  The previous owner had told me that he would adjust the idle screw (the thumb screw under the right side of the tank) in and then gradually back it out as the bike warmed up and this is how Ive been dealing with it also.  After a couple minutes it seems to warm up and I can back the idle screw most if not all of the way off and it idles nice right around 1200rpm. 

Im new to these bikes but I cant imagine this is normal.  It has k&n pod filters, stock jetting, and stock exhaust with some higher temp plugs.

Im guessing that it needs to be re-jetted, or at least the air screw needs to be adjusted to smooth things out?  Could the previous owner have adjusted the air screw in so that there was less air in the mix to compensate for the increased flow from the pod filters?  Could this be starving the motor of air at start-up?  Could I just back off the air screw back to normal and put in slightly larger jets?

Thanks in advance!

[razor02097]

First off welcome to the forums 


the 550 MAY have a fast idle adjustment but not entirely sure.  It adjusts the fast idle when the choke is pulled out.

With stock exhaust you should be OK with stock jets.  If it runs and idles good now I wouldn't mess with it.  However installing the correct NGK plug should be done... hotter isn't necessarily better.

[TwoTired]

Cold engines don't atomize fuel well.  That's why the carburetors have a choke.
Cold/choke engines aren't as efficient as ones that are warmed up.  Many carbs have have a fast idle mechanism to increase the idle speed setting when the engine is cold.

The 77 CB550F has a choke, but no fast idle mechanism.  The rider is expected to hold the throttle open during the brief warm up periods that the bike requires to come off the choke.

I have F model 550's with foam filters in the stock air box.  If you are using the stock D7EA spark plugs (hotter than D8).  You just suit up, get on the bike, apply choke, start, and then back off the choke enough for the engine to take some throttle.  You hand is on the throttle while you drive off with the engine still cold.  During the first few blocks, you reach down and incrementally back off the choke until the engine is warm.
The idle knob is set to idle at correct speed when fully warmed up.  I don't bother resetting it cold.  It's on the same side as the throttle and I'd rather have my hand on the throttle than on the idle knob.

Your pod filter may have leaned the bike more than the stock or foam element filters do, making the cold/choke thing more sensitive.  If you won't change the pods, then you may be able to compensate by turning in the air screws a small amount.  I prefer to adjust these for proper operation with an engine at normal operating temperature.  But, if you are using your bike mostly in the warm up regime then adjust accordingly.

Since these carbs have no accelerator pump, the engine needs an over-rich mixture at idle to overcome the wheeze when the slide first open on acceleration.  It should be able to take up to one half total throttle travel without engine stumble.  The air bleed screws adjust the idle mixture to achieve this acceleration response.  If you go it too far, then you will start fouling spark plugs will prolonged idle.

I can't imagine the stock #98 mains or the stock main needle setting will give good top end power with K&N pods.  But, maybe the stock muffler is dominating the mixture requirements.  Still, I'd check your plug deposits for too lean mixtures and signs of pre-ignition.