Topic: 1975 CB550K dies when I'm idling at a light or stop sign

[tkarahalios]

Hey guys,

My 1975 CB550K is pretty much stock. The only thing is I've added a MAC 4 to 1 canister exhaust.
I've never opened the carbs myself (bought it a year and a half ago and it had a 4:2 straight pipe exhaust on it).

It starts up easy and seems to drive really good. The only thing is that after its been running for a bit, 30 minutes or more, and its well warmed up, it tends to die when I stop at a light and let it idle.
It starts up again tight away, I just need to give it some gas to give if from dieing.

Any ideas of what I may want to do? Is it simply the idle screw that I need to tweak? any other thoughts about what may need to be checked out?

[flybox1]

its probably a good idea to clean your carbs(and do a complete 3000mi tuneup for that matter) but yes, turn up your idle set screw to keep it from dying for now.  dont skip the tuneup.

[Sprocket75]

Hmmm... Have you checked it on a set of vacuum gauges after it is warmed up? Fuel flow? Fuel filter if installed? (clogged) Adjusted the mixture? Checked the plugs? (wet? dry? etc.) It should comfortably idle around 950 or 1000 RPM when the idle screw is backed out. (not touching the plate) Or at least mine does. A little more info may help.

[bjbuchanan]

Set the idle screw so you don't stall when hot. Your idle will be lower on a fully warmed up engine, aim for 1200 rpm.

The fact that it starts right up and still runs great is the biggest clue it is an idle screw adjustment

[Duanob]

You might check the condition of your plugs to see if how it's burning fuel. Too Rich or too lean can lead to idle problems but mostly too rich.

Also what is the choke position? Off (lever down) or on (lever up). I've left the choke partially on a couple of times. It runs fine when cold but after it warms up it ran like a dog. Until I looked down and turned the choke all the way off then it ran perfect.

[DWS]

1978CB750F3
4 into 1
#35 slow jet
#110 main
4900 feet elevation
Timing is at --1&4 F mark now
advance,  not sure what i was seeing timing light.
Number one cylinder running around 190 to 210 Deg
Number 2, 3, 4 are running close to each other, around 530 Deg
Not sure why?  maybe valve adjustment?
i have not synced the carbs yet either need tool.
timing jumps a little  on 1&4 and 2&3 when looking at it with light.
changed plugs from one and it does not follow the plug and or plug wire.
when i rump on the throttle it die's.
At idle just above 1000rpm it rev up with very small turns with the idle screw and sometimes it will settle back down on its own.
and have to use the choke to control the rpm in till it gets to operating temp.

[grcamna2]

I think it's recommended to service your ignition system/points/cond. also along with your advance mechanism.

[flybox1]

1978CB750F3
4 into 1
#35 slow jet
#110 main
4900 feet elevation

= lean.

...check your plugs!

[calj737]

You might check the condition of your plugs to see if how it's burning fuel. Too Rich or too lean can lead to idle problems but mostly too rich.

Also what is the choke position? Off (lever down) or on (lever up). I've left the choke partially on a couple of times. It runs fine when cold but after it warms up it ran like a dog. Until I looked down and turned the choke all the way off then it ran perfect.

+1. Check the choke!

[DWS]

switched plugs and did not follow, plugs all new, switch wires and did not follow.
carbs have been rebuilt completely, choke is fine.(checked)

[bjbuchanan]

I don't understand what you are looking to fix...You said the bike runs totally fine but stalls at a stop after running. Only then does it die, then starts right back up like normal. Simple idle adjustment knob fix

Now if it isn't running perfect then time to dig. My first inclination is to tweak your advance springs to get rid of timing flutter from old weakened springs. Trim a coil from one of the springs, just one coil from ONE spring and see how that does. Advance shouldn't begin until 2500 rpm up. That will cure that hanging idle and dancing timing light at idle.

[mono]

My 2 cents is to adjust your air screws.  My 550 did the same thing, would fire right up every time from cold start, then after warming up got all sputtery and would puke at standstill.

[evanphi]

When I put my new exhaust on this year I had to fiddle with my screws.

I would start up fine from cold (with choke), but then when warm I would die at lights or when decelerating sometimes (that was fun the first time!). I was running too lean.

[DWS]

took air box off and found all four carbs over flowing over / thought the main jet and out the intake of the carbs all four. I have carbs off bike, took apart and really did not find anything really wrong, floats are at 14.5 and level on all and put back together and no leaks, dont get it.
But now my excelerator pump not working, all holes are clean. it looks like the rod it not getting depressed, weak spring maybe?