Topic: 1975 CB550 Carb Tuning Advice Sought

[sniper1]

I have just put my '75 CB550 back together. I am looking for advice on carb tuning. My setup is as follows:
 - MAC 4 - 1 Exhaust
 - Stock Airbox
 - Stock Cam
 - Stock Carbs
I have valve adjustments set, timing is correct, cam chain set, new plugs, etc. etc. I just synched the carbs and noticed that it runs pretty rough. It is slow to respond and chugs a bit when getting on the throttle. At higher speeds (45-50 mph) it feels like it is struggling. It ran better before I synched the carbs, which were way out of whack.

Any advice on what jet sizes I might need if I have to go that route? I am at sea level

Do the carbs need to be taken off in order to rejet? or can I do it with carbs on the bike?

Any advice is appreciated.

[flybox1]

did you bench or vacuum sync them? big difference there....
do you have the appropriate level of fuel in your carb bowls?

with less back pressure from your exhaust, you'll probably make changes to your AF mix screw, your jets, and probably change needle clip position. 
the only way you'll be able to determine, will be to do a plug chop in several throttle positions.

[adamlwvdc36]

Idle air screw will do nothing to correct this problem... it is used during idling.  that said, what i would recommend is to do a plug chop at the speed/rpm you are having problems... start with new, clean plugs... I know you said you just replaced them, but get new ones anyway.  Run the bike at the rpms and speed you are having the problem for about 5 minutes, keeping throttle steady.  right at the end of this time without changing the throttle at all, simultaneously pull and HOLD in the clutch and turn the bike off using the red off-run-off switch on the right, coast to a stop on the side of the road, and pull 2 plugs, one from either 1 or 4 and the other from either 2 or 3.  this will tell you what the problem is.   If the plugs are nasty and black sooty then you are running rich in that throttle position, if they are white (HOT) you are running too lean.  Let us know what you get (pictures would help) and let us know what throttle position you kept it at (0%-100%).  from there I or someone else can better tell you exactly what jet needs to be adjusted and how

[TwoTired]

For a stock bike with only a MAC 4to1 (with baffle) change there should be no need for ANY jetting modification from stock settings.
Stock settings can be found in the FAQ.

You will NEVER be able to snap the throttle full open from idle and get reliable power.  One to one half of total throttle travel will get reliable power advance.  This is because there is no accelerator pump for these carbs.

A new air filter is recommended before ANY tuning refinements are attempted.

If you are having carb issues, I expect something inside the carbs are not stock (like dirt, varnish, corrosion, prior meddling, non-stock parts, etc.)
Perhaps you have carb coupler leaks?

Do take a look at your plug deposits to get a read of what's happening inside the cylinders.

The pilot screw does far more than adjust for idle, it is also adjusted for throttle advance response.  However, with the setup described, the stock setting ought to be VERY close to correct.

Cheers,

[DJ_AX]

I've got the same setup on my 550. Carbs are set all to stock jets and settings. Runs great!

[lucky]

1975 CB550

Your set up:
MAC 4 - 1 Exhaust
Stock Airbox.
Main 100 mm.
Idle jet 38 mm.
Needle clip position 4th down from the top.
Mixture screw 1.5 turns out.

My solution.
I would say you can have good throttle advancement from idle all the way up.


MAC 4 - 1 Exhaust
Stock Airbox.
Main 105 mm.
Idle jet 40 mm.
Needle clip position 4th down from the top.(stock)
Mixture screw 3/4 turns out.


 

[dave500]

im with two tired,i think it should be running ok on the stock jetting,,a few terms you use like "chug"that means rich to me?sure your not cracking the throttle too suddenly at low rpms?it feels like its "struggling"at 50 mph?,,these engines really start to hustle at 50 and up,they rev hard at speeds other bikes do nearly half the rpm at,as flybox asked,was it a vacuum sync or the "ball park"gap/light or other type?

[sniper1]

To answer some of your questions:
- I vaccum synched the carbs.
- Carb floats are good.
- New air filter.
- I backed out the air mixture screws 1 1/2 turns out from full in.

Here is my process:
 I rode it around for a bit to warm it up. Removed the tank, removed the screws in the manifold, removed tank, hooked up the carb synch dials. Started the engine.

A problem I have with the vaccum gauges is that they bounce like crazy until I roll the throttle until it gets up to about 4,000 RPM. I hold it there and then adjust until all 4 read evenly. Then retighten lock nuts, and put everything back together.

It ran a little bit better this time. The new problem is that it doesn't seem to want to settle back to a good idle. If I adjust the idle adjusting screw to 1100, then blip the throttle, it stays revving at about 2000 - 2300 rpm for a while, until I redadjust the idle adjusting screw.

I need to read up on the plug chop method to see what is going on.

[lucky]

Held the rpm at 4,000 rpm!!!!! wow.

If you do not have little valve dampers on your vacuum gauges you need to get them. If you already have them and the gauge needles still bounce around it is possible you need new valve guides.
When you sync the carbs it should be at idle.

Bouncing vacuum needles can indicate worn valve guides.
How many miles are on the engine?

[TwoTired]

Here is my process:

I'm glad you shared that, because...

A problem I have with the vacuum gauges is that they bounce like crazy until I roll the throttle until it gets up to about 4,000 RPM. I hold it there and then adjust until all 4 read evenly. Then retighten lock nuts, and put everything back together.

You need to sync the carbs at idle, where the carb's slide position is most sensitive.

Your gauges should have come with inline flow restrictor valves which nearly eliminate the needle bounce.  As you have seen, the inlets pulse the demand for air.  The restrictor knobs average these pulses so you see where they pointing on the dial.

Go back a vacuum sync again at idle position.

The new problem is that it doesn't seem to want to settle back to a good idle. If I adjust the idle adjusting screw to 1100, then blip the throttle, it stays revving at about 2000 - 2300 rpm for a while, until I readjust the idle adjusting screw.

I expect you twiddled with each adjuster of the four, and have now skewed all the adjusters so that the slides can't reach their opening floor, making the idle knob loose complete control over this function.

...Or, you still have uneven vacuum sync at idle and the cylinders are fighting for dominance.

Check to see if slide can reach the floor of travel.
Tune your gauges to give you useable information at idle.

Oh, and have a fan blowing air over the hot motor, or the cylinder's will have uneven runaway cooling while stationary (which can also effect carb sync).

Cheers,

[bollingball]

http://www.sohc4.net/index.php/cb550-carb-sync-by-mrmaxstorey/

This reminded me of you The bike sounds like it is running to fast and I don't see restrictors on the gauges.
Ken

[bollingball]

http://www.petmountain.com/product/aquarium-pond-airline-valves/11442-509821/lees-airline-valve-control-kit.html

If you don't have these you can get them at WallMart cheap better than nothing.
Ken

[klinger44]

almost identical 45 50 and im bogging down horribly.