Topic: 1973 CB550 MPG help

[Jimsun]

Hey everyone.

I hope that someone can help shed some light as to how I can improve my MPG from 29-30 MPG to hopefully over 40+!

Carbs was bench synced to 1/8 bit size. I'm running full stock everything except the pipes (26" EMGO cone 4-4) and the air filter (using the foam one TwoTired uses)

Now these pipes and the stock ones have been giving me the same MPG. I don't fuel when I hit reserve, I fuel after 30 or 60 miles, and do the math with the corresponding fuel it has taken me to top it off at the same volume.

I've recently done the following:
Oil/filter change last 1k Miles
Tensioner, valve clearance, point gap, timing, spark plug cleaned and regappped done in less than 500 miles ago



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[BomberMann650]

bloo got 30mpg if I was really hard on the throttle 

[Bankerdanny]

Are you sure that you have stock jet sizes? Have you done a plug chop to see if maybe you are running rich at wider throttle settings?

29-30 is definitely low unless you are just crazy with the throttle or do a lot of stop and go. My previous 550 routinely did low 40's in mixed riding and upper 40's at steady highway. I even broke 50 a couple times with a windshield.

I haven't measured my current F much, but was getting 37-38 the few times I did check and that was before I finally gave the carbs a basic cleaning. I expect that to improve, but haven't run through a full tank yet since I cleaned them a couple weeks ago.

[flybox1]

40+ should be the norm 
No carb vacuum sync as part of that service?
After a ride, what do your plugs look like?   Black and sooty maybe?
With maintenance done and correct...IIWM, id work on tuning my carbs better.  Clear tube test bowls and verify jetting w plug chops.

Is your speedo/mileage counter working properly?  Might want to test its accuracy against a gps.

[TwoTired]

I hope that someone can help shed some light as to how I can improve my MPG from 29-30 MPG to hopefully over 40+!

Carbs was bench synced to 1/8 bit size. I'm running full stock everything except the pipes (26" EMGO cone 4-4) and the air filter (using the foam one TwoTired uses)

Now these pipes and the stock ones have been giving me the same MPG. I don't fuel when I hit reserve, I fuel after 30 or 60 miles, and do the math with the corresponding fuel it has taken me to top it off at the same volume.

I've recently done the following:
Oil/filter change last 1k Miles
Tensioner, valve clearance, point gap, timing, spark plug cleaned and regappped done in less than 500 miles ago

First the foam air filter needs to be clean and NOT over oiled.

An oversize front tire (taller) will make the odometer lie, which would effect the calculations.  Changing drive sprocket ratios can make the engine turn more revs per mile.  A stiff chain eats surprisingly more HP.  Well lubed is better.  Dragging brakes front or rear? Easy to check.
Do you warm up with choke on and standing still?

Er, I much prefer a vacuum sync.

Would be helpful to see what the spark plug deposits look like.

Also would be nice to know what the predominant throttle setting is during your rides and what gear you are in mostly.  Not going to get good MPG in stop and go city traffic, remaining in first gear.

These factors in mind,  I can tell you that the CB550 in stock trim used to get 50 MPG in highway cruise before the Gubmint took energy out of a gallon of gas by force adding ethanol.  Must use up more fuel to make the same HP.

Still, I think 45-ish is a reasonable ask.

So, since you've addressed most everything outside of the carbs.  Lets look in there to see what is or could be.

A leaking main jet oring ?  Emulsion tube holes blocked/ restricted?  Float bowl fuel level too high?  Worn slide needle jet and or slide needle.  Wrong taper slide needle?  Slide needle clipped in too high position?
Some of the above can be inferred by reading the spark plug deposits.

Of course, non Keihin Brass bits may have the wrong dimensional measurements and contribute to poor MPG.

When all the above are addressed and confidence of correctness achieved, you might just make a modification to the mains emulsion tube by adding more holes at the needle positions related to cruise setting, to lean the mixture at that point.   This will be a try and test procedure.  So, best to have spare originals to fall back on, though you could just solder up some of the holes you added.
Then, I think the spark plug deposits patterns will tell you if it should be better at MPG.

Cheers,

[Jimsun]

40+ should be the norm 
No carb vacuum sync as part of that service?
After a ride, what do your plugs look like?   Black and sooty maybe?
With maintenance done and correct...IIWM, id work on tuning my carbs better.  Clear tube test bowls and verify jetting w plug chops.

Is your speedo/mileage counter working properly?  Might want to test its accuracy against a gps.

I haven't done the vacuum sync yet.
Here's a picture at low rom less than 2.5K RPM



Here's an image at 5K RPM steady for 20 seconds with a warmed up engine



The speedo is working. I have tested it with my phones' GPS +/- 5KMH

I've taken apart my carbs in pieces - literally. I know which pieces goes where short of thing. I made use all passages/jets/tubes are clear. I've used thin gauge copper wire - similar to what you'd see in a wounded transformer. New gaskets as well

Are you sure that you have stock jet sizes? Have you done a plug chop to see if maybe you are running rich at wider throttle settings?

29-30 is definitely low unless you are just crazy with the throttle or do a lot of stop and go. My previous 550 routinely did low 40's in mixed riding and upper 40's at steady highway. I even broke 50 a couple times with a windshield.

I haven't measured my current F much, but was getting 37-38 the few times I did check and that was before I finally gave the carbs a basic cleaning. I expect that to improve, but haven't run through a full tank yet since I cleaned them a couple weeks ago.

Base on the model and year I have stock sizes.

Please see above images of the spark plugs.

My carbs are clean no junk at the bottom of the bowls.

I hope that someone can help shed some light as to how I can improve my MPG from 29-30 MPG to hopefully over 40+!

Carbs was bench synced to 1/8 bit size. I'm running full stock everything except the pipes (26" EMGO cone 4-4) and the air filter (using the foam one TwoTired uses)

Now these pipes and the stock ones have been giving me the same MPG. I don't fuel when I hit reserve, I fuel after 30 or 60 miles, and do the math with the corresponding fuel it has taken me to top it off at the same volume.

I've recently done the following:
Oil/filter change last 1k Miles
Tensioner, valve clearance, point gap, timing, spark plug cleaned and regappped done in less than 500 miles ago

First the foam air filter needs to be clean and NOT over oiled.

An oversize front tire (taller) will make the odometer lie, which would effect the calculations.  Changing drive sprocket ratios can make the engine turn more revs per mile.  A stiff chain eats surprisingly more HP.  Well lubed is better.  Dragging brakes front or rear? Easy to check.
Do you warm up with choke on and standing still?

Er, I much prefer a vacuum sync.

Would be helpful to see what the spark plug deposits look like.

Also would be nice to know what the predominant throttle setting is during your rides and what gear you are in mostly.  Not going to get good MPG in stop and go city traffic, remaining in first gear.

These factors in mind,  I can tell you that the CB550 in stock trim used to get 50 MPG in highway cruise before the Gubmint took energy out of a gallon of gas by force adding ethanol.  Must use up more fuel to make the same HP.

Still, I think 45-ish is a reasonable ask.

So, since you've addressed most everything outside of the carbs.  Lets look in there to see what is or could be.

A leaking main jet oring ?  Emulsion tube holes blocked/ restricted?  Float bowl fuel level too high?  Worn slide needle jet and or slide needle.  Wrong taper slide needle?  Slide needle clipped in too high position?
Some of the above can be inferred by reading the spark plug deposits.

Of course, non Keihin Brass bits may have the wrong dimensional measurements and contribute to poor MPG.

When all the above are addressed and confidence of correctness achieved, you might just make a modification to the mains emulsion tube by adding more holes at the needle positions related to cruise setting, to lean the mixture at that point.   This will be a try and test procedure.  So, best to have spare originals to fall back on, though you could just solder up some of the holes you added.
Then, I think the spark plug deposits patterns will tell you if it should be better at MPG.

Cheers,

I've followed the manufacturers way of how to apply oil. I've bought their branded oil as well.

Please see picture above for the spark plug result.

Spirited. A lot of stop and go downtown. I always stay around 5-7k rpm

I've used the K&L carb kit before but I went back to Kehin's stock jets and parts. No restriction. I used a caliper to measure the height. It's pretty bang on. I used new gaskets in the carbs. Clip position is second from the top or 4th from the needle side.


Instead of adding holes, can't I just unscrew the air fuel mixture more? Right now I'm sitting at 2 full turns from seated position.



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[TwoTired]

All that extra soot on the spark plugs is where your MPG goals are getting wasted.  I see a slightly cleaner plug after higher rpm operation.  This is to be expected and an artifact of the design.  But, it still looks like more fuel is being delivered to the cylinders than necessary.

The fuel requirements at higher RPM change a great deal when the engine is under load.  Also the plugs get hotter under load and tend to burn off the soot deposits.

What heat range spark plug are you using?

Instead of adding holes, can't I just unscrew the air fuel mixture more? Right now I'm sitting at 2 full turns from seated position.

The air screw is a part of the pilot circuit.  And it operates in parallel with the throttle valve circuit.
So opening the air screw has no effect on fuel delivery from the throttle valve pathway.  If you want to lean the throttle valve pathway, make the emulsion tubes work harder, or use smaller fuel metering orifices.

Did I miss where you stated the actual fuel level in the carb bowls?  That's another way to lean all the mixtures from the carbs, lower the fuel level, making it harder for the fuel to rise into the carb throat.  It's an overall leaning effect.

Cheers,

[Jimsun]

All that extra soot on the spark plugs is where your MPG goals are getting wasted.  I see a slightly cleaner plug after higher rpm operation.  This is to be expected and an artifact of the design.  But, it still looks like more fuel is being delivered to the cylinders than necessary.

The fuel requirements at higher RPM change a great deal when the engine is under load.  Also the plugs get hotter under load and tend to burn off the soot deposits.

What heat range spark plug are you using?

Instead of adding holes, can't I just unscrew the air fuel mixture more? Right now I'm sitting at 2 full turns from seated position.

The air screw is a part of the pilot circuit.  And it operates in parallel with the throttle valve circuit.
So opening the air screw has no effect on fuel delivery from the throttle valve pathway.  If you want to lean the throttle valve pathway, make the emulsion tubes work harder, or use smaller fuel metering orifices.

Did I miss where you stated the actual fuel level in the carb bowls?  That's another way to lean all the mixtures from the carbs, lower the fuel level, making it harder for the fuel to rise into the carb throat.  It's an overall leaning effect.

Cheers,

I figured that even if I'm getting the bronze effect, the darker areas around the spark is wasted gas/rich.

I'm aware that the bike uses more under load

I use D7EA.

I don't quite understand the pilot circuit area but what you said about it not altering the fuel delivery makes sense. I thought that opening the air screw filter even more will adjust the fuel intake to the cylinders.

I measured the float level before (read one of your posts on how to measure it). Maybe I should alter the float so it's range will shut the fuel supply to the bowl sooner?


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