Topic: 74 cb550 air screws out 3 turns

[zx12r]

74 cb550 air screws out 3 turns. thats were it idles, runs best, but still not good. after you rev it up it stays around 2000rpm then slowly goes down to 1200rpm, and idles rough. above 1/4 throttle it runs great. going by this info do i need to go richer with pilot jet or leaner?

[TwoTired]

Stock air filter and exhaust?

Carb synched?  Air filter clean, less than a year old?

Are the carb couplers leaking air?

Is you timing advance mechanism operating smoothly?

Less than 3000 miles since all the tune up items have been checked?

Any info on spark plug deposits?

Is the fuel clean?  Are the carbs clean?

3 turns out an the air screws of my bike will mean that the throttle response will be dismal, as this makes the idle mix far too lean for effective pickup from low RPM under load.

Cheers,

[zx12r]

Stock air filter and exhaust?

Carb synched?  Air filter clean, less than a year old?

Are the carb couplers leaking air?

Is you timing advance mechanism operating smoothly?

Less than 3000 miles since all the tune up items have been checked?


Is the fuel clean?  Are the carbs clean?

3 turns out an the air screws of my bike will mean that the throttle response will be dismal, as this makes the idle mix far too lean for effective pickup from low RPM under load.

Cheers,

Stock air filter and exhaust?
mac exhaust, foam filter, not new, better breathing, so you would think it would be running lean instead of rich

Carb synched?  Air filter clean, less than a year old?
carb synched= needs it. air filter clean

Are the carb couplers leaking air?
sprayed them with carb cleaner= don't seem to be leaking

Is you timing advance mechanism operating smoothly?
it seems to be. could the springs be weak, the idle seems to have a mind of it's own.

[rbmgf7]

3 full turns or 3 turns where it appears so because you're referencing by the slot on the screw head? i thought it was just 1 to 1.5 full turns out? yeah, 3 full turns will make it leaner.

[TwoTired]

carb synched= needs it.

Synch the carbs and put the air screws back to 1.5 turns out from seated.  If your idle issues persist, then report back.

Sync has the most effect on smooth idle.  If carb 1 wants an idle of 800, and carb two wants an idle of 1000, carb three wants an idle of 1300, and carb four wants an idle of 900, hows the engine going to decide where to idle?

The unsyncronized effects are far less prominent when the slides are farther open for higher RPMs.

Cheers,

[a.raia]

just got my 77 cb550 runnnig good, had some trouble finding right spot for air srcews, put in new plugs,  again new plugs , my bike took almost 2 turns, I turned 2 full and backed offt the space of slot in srcew head, drive bike in rpm range that is not running right in, mine was 4200/ 5200rpm pull over and adjust a little at a time  and drive bike again keep playing with it , after every adjustment drive bike , small adjustments at a time, and start with new plugs , if carbs are clean and synched and everything else is in order this could help

[zx12r]

carb synched= needs it.

Synch the carbs and put the air screws back to 1.5 turns out from seated.  If your idle issues persist, then report back.

Sync has the most effect on smooth idle.  If carb 1 wants an idle of 800, and carb two wants an idle of 1000, carb three wants an idle of 1300, and carb four wants an idle of 900, hows the engine going to decide where to idle?

The unsyncronized effects are far less prominent when the slides are farther open for higher RPMs.

Cheers,

synced carbs yesterday and it helped the idle alot, smoothed it right out, but now throttle response at closed to 1/4 open is worse, it really stumbles till you give more gas. adjusted the air screw in to 1 1/2 turns (tried all settings 1/2 to 4 turns out) and it was worse. i took the carb bowl off the #1 carb (while still on the bike) and everything looked good, pilot jet, main jet clean, float bowl was shining. i did notice the spring for the float needle seems weak. would this cause my having to turn out the air screws to 3 turns out to make it a little better (fuel level to high)?

[TwoTired]

Temporarily remove your air filter.  Does the pick up from idle improve or worsen?

Alternately, if you apply partial choke, does the throttle respond better from down low?

Can you describe your Spark plug deposits?  Or post a picture?  Are they all the same?  Do all the head pipes get equally warm/hot at idle?

Describe the throttle response symptom in more detail.  Does it wheeze like you turned off the ignition and catch later?  Or, does it spit and blubber and slowly pick up in an irregular way?

You are checking the throttle response and full operating temperature, right?

Cheers,

[zx12r]

Temporarily remove your air filter.  Does the pick up from idle improve or worsen?

Alternately, if you apply partial choke, does the throttle respond better from down low?

Can you describe your Spark plug deposits?  Or post a picture?  Are they all the same?  Do all the head pipes get equally warm/hot at idle?

Describe the throttle response symptom in more detail.  Does it wheeze like you turned off the ignition and catch later?  Or, does it spit and blubber and slowly pick up in an irregular way?

You are checking the throttle response and full operating temperature, right?

Cheers,

thanks for helping, yes shes running rich, plugs black with carbon. tried the no filter and it was better, still not right tho. i got her up to temp and turned the fuel off and after a couple of minutes of reving in the 1/4 throttle range the bogging was gone, nice throttle response (till she was running out of gas). so i'm guessing fuel, float level to high?  thanks

[a.raia]

put in new plugs ,go 2 turns out on air screws and start from there again,I did same thing with my petcock ,and I thought the floats were off too, took off carbs checked floats they were good ,the plugs in my bike only had a couple of miles on them but when they  get fouled there done.

[TwoTired]

thanks for helping, yes shes running rich, plugs black with carbon. tried the no filter and it was better, still not right tho. i got her up to temp and turned the fuel off and after a couple of minutes of reving in the 1/4 throttle range the bogging was gone, nice throttle response (till she was running out of gas). so i'm guessing fuel, float level to high?  thanks

Too much fuel possibilities:

Emulsion tube air bleeds clogged.
pilot jet drilled too large.  The 550 ought to have #38, which is right around 0.016 Inch.
Throttle valve needle too narrow and/ or Throttle jet needle too large, worn or drilled.
Slow air jet blocked or slow Air jet passage blocked.
Main Air jet  blocked or passageway restricted.
Float level too high.
Is your main jet #100 and undrilled?

The risk here is that the plugs can get sooty from a main jet too large or throttle valve too big, etc., and still the pilot system can be too lean.  That is why I asked you to describe the the off idle stumble in better detail.
I find it best to let test data lead the way to a cause and correction rather than assumptions of what the problem is.

Good luck with your investigation.

[zx12r]

The 550 ought to have #38
That is why I asked you to describe the the off idle stumble in better detail.
[/quote]


the pilot jets are #40 (this would explain my problem, right. would 4 single air filters match the #40 pilot jets). the off idle stumble= it bogs and no power till you move the throttle above a 1/4 turn, above a 1/4, or over 3000rpm it runs fine, it goes from no power and stumbling, missing, to over 1/4 throttle (almost like a switch) running fine. last time i had the cb550 out, i had  her going a little over 100mph. thanks once again

[TwoTired]

I think my 74 550 has #40 pilot jets. Certainly later ones had #38.  I assumed that since my bike was early manufacture, they used #40 like the the 500 had.  It has the Uni foam filter and stock 4 into 4, and runs quite well.

So, while it may be a contributor to your problem, my gut say's that it is not THE problem.  ...Unless, of course they were at some point drilled to a larger orifice size without changing the markings.

Your recent stumble description does indicate the pilot system as providing too much fuel.  Were it my bike, I would certainly scrutinize the pilot air jet , pilot jet for orifice alterations, air screw (should be hollow tipped), and the pilot passageways between these devices for obstructions.

Interesting puzzle you have there.

Cheers,

[L.A. Nomad........]

Sounds like he's using single 4 air filter pods?

[mgautreau]

I had very similar problems with my 75 550, poor off idle performance and constantly fouling plugs, until I dropped the needle jet two notches.  The clip was in the bottom slot and I raised it to one from the top.  Now it runs like a champ and plugs are burning a nice tan color.  It's kind of a pain to get to the needles but ya gotta do what ya gotta do to get them running right. 

I got the idea from reading some past posts on 550's running rich.  Seems to be a common problem if all else fails.

Good luck.

[zx12r]

I think my 74 550 has #40 pilot jets. Certainly later ones had #38.  I assumed that since my bike was early manufacture, they used #40 like the the 500 had.  It has the Uni foam filter and stock 4 into 4, and runs quite well.

So, while it may be a contributor to your problem, my gut say's that it is not THE problem.  ...Unless, of course they were at some point drilled to a larger orifice size without changing the markings.

Your recent stumble description does indicate the pilot system as providing too much fuel.  Were it my bike, I would certainly scrutinize the pilot air jet , pilot jet for orifice alterations, air screw (should be hollow tipped), and the pilot passageways between these devices for obstructions.

Interesting puzzle you have there.

Cheers,

update, took the air box completely off, to the open carbs in  back (was that a pain), guess what, it now is running alot better in that 1/4 throttle range (still not right), air screws are still around 3 turns out (started at 1 turn, still ended up at 3), but at least now i can tell there adjusting the air. cleaned the carbs (they were clean), checked the floats (all 4 at 22mm). ordered 4 emgo 39mm pod filters on ebay, said they fit cb550, then that nite i read on here the 39mm emgo filters will hit the frame, go measure the clearence and sure enough, not enough room, he did cancel my order, so thanks to the person that posted that (saved me $40). definitely not putting the air box back on, it's running better without it. i'm thinking of putting #38 pilot's in  instead of the #40, thats in there now and looking for pod filters.
thanks once again

[zx12r]

update-  put #38 slow jets in and it's now running good in that 1/4 throttle range. (i think my #40 slow jets were drilled out) thanks