Topic: How much effect do the air screws have on mileage?

[Scott S]

 Swapped a '76 CB550 engine and matching 087A carbs into my CB500. Engine has been tuned, timed, etc. Carbs were cleaned TWICE, once by me, once by my friend who is a carb specialist. As far as I know, jets are stock. Floats set, bench sync'ed, stock needle clips, etc.

 Right now, I'm just past 1 3/4 turns out on the IMS, by about the width of the slot on the screw. I just found out that the manual says 1.5 turns, +/- 3/8, so I still have a little more to use.

 The bike starts fine, idles well and runs/pulls hard. There are two issues:

 1) When gearing down or coming to a stop or, say, letting off the throttle for slower traffic and rolling back on, there's a slight hesitation. There's a tiny burble or cough and I have to clear the carbs. Not bad....but it's there. It's just not crisp.
 It's actually stalled a couple of times at a stop b/c I didn't clear it out.

 2) Checked mileage today and it was abysmal; 28 mpg on the first tank and that was with some 5th gear, 55-60 mph riding. Second tank was slightly better after adjusting the screws, but we had been in the twisties and I was keeping the bike on the boil. Mostly 3rd gear, high RPM stuff.

 Made another adjustment on the way home and every adjustment makes it better.
 If I get to two turns out, should I start looking at the slow jets? If I'm really rich on the air screws, will that cause the poor fuel mileage?

[grcamna2]

Hi Scott S,
     Are you running CB550 air filter box & exhaust ?

[dave500]

ive always found 38 slow and 100 main works,the 40 was a little rich,i use any amount of turns listed as a starting point only,i mostly start with about two anyway and go from there,i also have a 550 engine in a 500 frame with airbox,i have the 069a carbs though.

[Scott S]

Hi Scott S,
     Are you running CB550 air filter box & exhaust ?

 I'm running the 500 air box with a Uni filter. I do have the 550 tool tray (my '71 500 didn't come with an air boc cover/tool tray. Only the later years, but I ran a tool tray for many miles with the 500/069a carbs.) Only other change from stock is a MAC 4-1.

[grcamna2]

Scott,
    Just for grins..try installing a paper filter just to see what that does to your pilot circuit;maybe you have a decent used paper filter or O.E. ...,otherwise you might just keep readjusting you're pilot screws.

[Scott S]

 I don't have a paper filter. After reading the replies, I thought about removing the tool tray to see what happens, but I really like having it.

 The CB550's came with the tray. Is the actual airbox that it sits on any different on the 500? I know some 550's came with some EPA type vents, but is actual volume any different?

 I'm going to keep going out on the IMS's. If I get to 2+ turns, is that OK if it runs well?
 Will that help my mileage or do I have something else going on?

[grcamna2]

I think it's fine to keep "fine tuning" all you're pilot screws .Do you have an inductive electronic test tachometer to hook up to +1 or +4 spark plug wire ? I would use that and keep an eye on the digital readout...every time you adjust the screw a little(w/ a small throttle blip to "clear it")keep an eye on the readout and just keep tuning each one until you have each screw as far in as they can to stay in the Optimal range where the rpm's will not bounce up or down too much.You may get them all out somewhere between 2-2.5 turns out from closed.I thinks it's good to remember that each pilot screw is a separate & different adjustment....when you watch that digital readout...just do each one separately.I would set up a good house fan(or 2) in front of the motorcycle and just spend 15-20 minute intervals fine tuning all 4 pilot screws..one by one...,shut her down to cool off for 10 minutes if it runs too hot and go at it again until you have the rpm's on you're digital readout as STEADY as they can be with the least amount of "stumble" after every time you lightly blip the throttle...and the quickest overall recovery from each throttle "blip".
Make sure you have sufficient oil in the motorcycle and do all this on the centerstand or the motorcycle "centered" in your front wheel vise on your lift(it could starve the #4 lobes on the camshaft for oil when parked running on the sidestand for too long...conceivably )to keep the levels all "even" in your carb bowls.
I hope that you have your "air filter of choice" installed before you begin this long fine tuning procedure along with you're exhaust baffle of choice installed.
Scott S, this is the last step in true "fine tuning" that you do after you have done a FULL tune up(and usually after installing a jetting kit w/ different size jets,needles,slides drilled,carbs all completely redone w/ all floats in good operation and all set to the SAME height;make sure you have the correct pilot jets in there and carbs are 100% clean before you take this long "journey" )w/ valves all adjusted,compression tests done and marked results,new plugs and caps,points fully tuned and timing spot-on w/ advancer checked for good operation and  spring tension ,all exhaust flanges tight and all joints checked for a "fully sealed system" front to back,carb slides all synchronized to the same spec.,etc. I mean a FULL TUNE UP . I then do this last part of an excellent fine tuning by doing the pilot screws to the most smooth idle I can get after spending LOT'S of time.Scott,I could only afford to do this to my own bike ...nobody could afford that much labor for me to do that much sweet fine tuning on their "put it in the shed for the winter" "old" motorcycle.I really enjoyed my own bike ...but fine tuning is very time consuming and you can truly be "chasing your tail" if you don't do all the rest of the FULL tune up BEFORE you fine tune the pilot screws.......it's a process.    ENJOY !

[Scott S]

 I'm with you. All the other tune-up stuff has been done. I guess I was just concerned that I was getting close to, or going past, the "max" turns according to the manual. But after hearing you and Dave say to do whatever the bike likes, I'll keep tuning. I mean, it's not like something is WAY wrong and I'll be 5 turns out or anything.

 Will that help the mileage, or is there something else I should check?

[grcamna2]

Scott,
    If it's fully tuned up and the carbs are set up w/ all the right stuff(float height,correct jetting as per plug color,etc.)and fine tuned,and...the advancer 100% along with the ignition,you should be good to go.Have you ever tried driving to get the max mileage? I've found when I shift...if I go at it gradually..initially and just open the throttle progressively to match my speed(these aren't CV carbs which compensate for too much throttle opening..;you can flood the ports slightly)..I'll get the best mileage that can be achieved for that particular terrain.I learned to only open the throttle as much as the engine can take the extra revs...I think I learned this best from riding 2 strokes for a number of years ;they'll bog out if you give 'em more than they like for a given terrain. How is you're plug color ??

[Scott S]

 I'm at 2 turns on the air screws and every little bit helps, but I still have to "clear the throats" of the carbs when decelerating unless I'm very conscious of matching revs when downshifting, etc. If I sit too long at a red light, I have to blip the throttle a bit to clear them out to make for a smooth take-off.

 Acceleration is fine, cruising is fine, highway speeds are fine, any speed at any given gear is fine. Only when coming DOWN through the gears do I need to clear out the carbs a little bit,

 Will lowering the needle one clip help me in this mid-throttle range?

[Scott S]

 Also, I'm a little confused.....the engine is a '76 and I "thought" these were the matching carbs. However, a '76 is a K2, right? Well, this website and a couple of other sources list the 022A as the carbs for a K2.
 
 I have a factory manual and some supplements, but they don't refer to the #'s on the carbs, just "K1", K2", etc., or simply "550".

 I know the slow jets changed over the years, as did the needle taper and the IMS settings.
 Some carbs list the air screw setting as 1 1/2 +/- 3/8. If that's the case for the 087A carbs, then I still have room to go.

 Are the 087A carbs correct for a '76 550 engine? What are the stock slow jet, needle taper and air screw settings?

[w1sa]

There has been some discussion on the 087A carbs and 550k engine before.......search 'carb ID' (about March 07)...........and also check the carb id sheet info in FAQs .

From a quick scan it appears the 087A may have been used in some late '76 550K bikes just prior to the release of (EPA) PD carb versions in '77(?)

Apparently the 087A configuration is very similar to the 022A, but someone had identified a possible difference (at least) in the airscrew spec.............which could be contributing to your problem................might be worth checking out.

[lucky]

Swapped a '76 CB550 engine and matching 087A carbs into my CB500. Engine has been tuned, timed, etc. Carbs were cleaned TWICE, once by me, once by my friend who is a carb specialist. As far as I know, jets are stock. Floats set, bench sync'ed, stock needle clips, etc.

 Right now, I'm just past 1 3/4 turns out on the IMS, by about the width of the slot on the screw. I just found out that the manual says 1.5 turns, +/- 3/8, so I still have a little more to use.

 The bike starts fine, idles well and runs/pulls hard. There are two issues:

 1) When gearing down or coming to a stop or, say, letting off the throttle for slower traffic and rolling back on, there's a slight hesitation. There's a tiny burble or cough and I have to clear the carbs. Not bad....but it's there. It's just not crisp.
 It's actually stalled a couple of times at a stop b/c I didn't clear it out.

 2) Checked mileage today and it was abysmal; 28 mpg on the first tank and that was with some 5th gear, 55-60 mph riding. Second tank was slightly better after adjusting the screws, but we had been in the twisties and I was keeping the bike on the boil. Mostly 3rd gear, high RPM stuff.

 Made another adjustment on the way home and every adjustment makes it better.
 If I get to two turns out, should I start looking at the slow jets? If I'm really rich on the air screws, will that cause the poor fuel mileage?

We cannot help you if you do not tell us what kind of exhaust and intake you have.
Idle jets only control the first 1/4 throttle with the pilot air screw /mixture screw.

Everything can effect mileage. Even your weight.

[Scott S]

 Stock 550 engine with fresh tune-up, Pamco ignition, MAC 4-1 with baffle and stock airbox with Uni-filter.

 This same set-up (Uni, Pamco, MAC) on the 500 engine/627B carbs ran crisp and returned easy mid-30's MPG, going as high as 41 on the hwy. at times.
 I did swap the rear sprocket to match the engine. Stock 550 sprocket.

 I'm thinking that I need to lower the needle and/or go down one size on the slow jet.