Topic: Another CB550F mpg question?

[BuffaloBill]

This is a great forum by the way!  A few weeks ago I bought a 1977 CB550F with (now) 13K miles on it.  The previous owner took great care of the bike.  The carbs are the 069 calibration.  The jetting is stock I believe (38 pilots, 98 mains, needle clip in the 2nd position from the top).  The airbox, exhaust and sprockets are stock also.  I ride at an altitude around 700ft and I weigh about 170lbs.  Temps have ranged here in Wisconsin from the upper 50s to 80's F.

Here's what I've done so far:

Disassembled the cabs, soaking them in the Gunk cleaner and verifying all passages are open, etc. I replaced the o-rings around the mains and the fuel distribution tubes.  The float heights were all set to the specified 22mm.  The float bowl vent tubes are not obstructed.

I set the valve lash, adjusted the cam chain, installed new points and condensers, set the dwell and timing to factory specs.  The advancer is stock and the advancer springs seem to be functioning, in that they return the points cam to the idle position.  I can see with the timing light that full advance is reached around 2800rpm on the tach.  I have new D7EA plugs.

The brakes are not dragging and the wheel bearings seem fine.  The chain is adjusted and well lubed.

I vacuum sync'd the carbs and set the air screws at 2 turns out from seated.

At this point the bike started and idled great but ran poorly when trying to maintain a hwy cruising speed.  At WOT it pulled OK.  It took me a while to figure out if the bike was running rich or lean.  I was disappointed in myself that it took me so long to figure out that the stock airfilter, although looking OK, was partially plugged, causing the bike to run really rich.  Before I realized it was running rich, I experimented lifting the needles one notch; that made it worse. I experimented running the bike with the filter removed; a plug reading showed me it ran quite lean in the midrange that way.  I stopped by 3 Honda dealers but none of them stocked the factory air filter.  I ordered one on-line but in the mean time, cut the paper element out and made an oiled foam filter.  The first one I made was about 1" thick the bike still ran rich.  I didn't have any specific filter foam; this foam came from a donor gun case!  The 2nd one I made was about 1/2" thick and the bike (for the first time since I owned it) ran pretty cleanly at cruising speeds.

I received the new factory air filter the other day and have since run a couple tanks of fuel (91 octane premium) through it.  Commuting to and from work, a 22 mile trip, 1/3 city, 2/3 hwy) I am getting 36mpg.  I'm thinking it should be capable of better mileage than that, and wonder if it's still running richer than it should be.  I'm riding fairly conservatively, keeping up with traffic but not goofing off.  I get 45mpg with my Hypermotard for comparison.

I have not done a compression test but the bike does pull pretty well.  I've run it up to an indicated 95 but didn't push it for any more than that.

I've read on the forum that the 77 and 78 actually would benefit from raising the needles, unlike the earlier models that ran rich.  I'm not sure if that comment applied to the F model also or just the K?  Do you think I should drop the needles (there's one higher clip position to go)?

Have any of you experimented with running the carb vent tubes to the airbox (to make it run leaner)?

Fire away with any questions or suggestions.  I may have forgotten to mention other things I've checked, etc.  I've had a lot of fun already with this $700 bike and my wife, who hasn't ridden with me for at least a decade, is having fun on the back.

Thanks, Bill

[TwoTired]

I can't see why a stock CB550F with all stock components air filter/exhaust wouldn't run best with the factory carb settings.

All mine do.  Except, I use the Uni NU-4055 oiled foam filter rather than the paper ones, simply due to cost issues.  I haven't calculated MPG recently, but 45-50 was routine with a mix of city/highway driving.

There is a chart in the FAQ with carb settings.  I don't see why you would set the air screws to 2 turns out, unless it was to compensate for a restrictive air filter.

The F model uses the earlier carb style.  The lean issues as with the PD carbs on the 77-78 CB550K do not apply.  Were I you, I'd put the needles back to the 2nd groove.

Do you have the stock sprockets/tooth count? 

Tires fully inflated?  Near stock sizes?

Assuming you have the stock muffler (quiet) is there any chance of a mouse nest inside?

Have any of you experimented with running the carb vent tubes to the airbox (to make it run leaner)?

No. And, I think this is a bad idea.

When you cleaned the carbs, did you extract the emulsion tubes (behind the Main jet) from the carb bodies?  Those air bleed holes must be clear.

Are you using gasahol?

You don't need premium, btw.  The compression, even when new, only required regular.  In theory, it burns a little hotter, and may help your MPG.

[05c50]

I had some of the very same problems as you when I first got my 550F. After a lot of messing around, I finally got it running good with decent gas mileage (40mpg), but the greatest improvement came when my wife started riding it. She consistantly gets 49 mpg. I've decided that fuel economy is directly related to the operator's desire to get there fast!

.........Paul

[BuffaloBill]

Thanks for the replys guys.

TwoTired,
Yes, I saw the FAQ on carb settings where the air screws are nominally set for 1 1/2T out.  I can't say I ran it with that setting long enough to measure fuel mileage but if I understand correctly these screws control air flow, not fuel.  Wouldn't 2 turns then be leaner than 1 1/2?  Do I have this backwards?

The sprokets are stock as I'd mentioned in my post.  The tires are close to the original size (110/90-18 and 100/90-19) and are aired up.

The tires are aired up, no ethanol in my fuel.  I'll try the 87 octane then on the next tank.

I perhaps didn't word my post very clearly: I do have the clips in the needles in the 2nd groove from the top (making the needles in the 2nd from leanest position).  I understand this to be the factory setting.  And yes, when I clean a carb I think I do a thorough job.  The emulsion tubes were removed and cleaned.

I was having trouble trying to use the quoting technique (never tried it before...lol).  I'm not sure if I answered all your questions but will send this and check again.

Thanks for the questions and I welcome any other suggestions.

Bill

[BuffaloBill]

I missed a couple of things:

I have not removed the muffler to check for an obstruction but the bike does run well and sounds like I'd expect it to.  I"ve put about 700miles on it and don't smell burning mice.

I mentioned the venting of the carbs to the airbox because that's how SkiDoo's temp and altitude compensation systems worked on the sleds having their "DPM" system.  They have a solenoid valve that's modulated to control the strength of the vacuum in that line that's driven off their engine computer.  It's a fairly simple system (as compared to fuel injection) but it seems to work quite well.  I'm sure you're right though that to do so might need different baseline jetting.

Thanks for clarifying that the F model wasn't included in the "lean 77/78" discussion.  Knowing that makes me think about dropping the needles that last position but like you, I question why it wouldn't get better mileage with all the factory settings.  It makes me wonder if something else isn't right or that I did something incorrectly.

Bill

[TwoTired]

TwoTired,
Yes, I saw the FAQ on carb settings where the air screws are nominally set for 1 1/2T out.  I can't say I ran it with that setting long enough to measure fuel mileage but if I understand correctly these screws control air flow, not fuel.  Wouldn't 2 turns then be leaner than 1 1/2?  Do I have this backwards?

No, turning them out make the idle mix leaner.  But, their effect on midrange throttle positions is very small to non-existant.  Have the idle air bleed screws out too far WILL effect engine response under load at low RPM for a throttle twist.  If it's like my 550s, in gear, at 1500 RPM, you should be able to give it up to one half throttle twist and the bike will accelerate smoothly.  More throttle than that will cause a "wheeze" reaction from the motor.  The Idle air bleed screws effect this behavior.

I perhaps didn't word my post very clearly: I do have the clips in the needles in the 2nd groove from the top (making the needles in the 2nd from leanest position).  I understand this to be the factory setting.  And yes, when I clean a carb I think I do a thorough job.  The emulsion tubes were removed and cleaned.

If you are convinced the stock settings and configuration is the cause of its mileage issues, you could try adding another row of holes to the main emulsion tubes, which would add more air to the throttle valve needle (and Mains) delivery.  Basically all throttle positions from about 1/4 and up to WOT.

I mentioned the venting of the carbs to the airbox because that's how SkiDoo's temp and altitude compensation systems worked on the sleds having their "DPM" system.  They have a solenoid valve that's modulated to control the strength of the vacuum in that line that's driven off their engine computer.  It's a fairly simple system (as compared to fuel injection) but it seems to work quite well.  I'm sure you're right though that to do so might need different baseline jetting.

I think this would be tricky to do and not risk the engine, mechanically.
The fuel jets flow according to the pressure differential between the carb throat and atmospheric pressure supplied by the carb vents (there are 6 vents).  The filter box and the air plenum chamber are all under negative pressure from the very same carb throat source.  This negative pressure has a lot of root causes, slide position, engine RPM, and various elements in the intake duct tract, including the air filter whose contribution changes with the amount of dirt collected by filter element.  The latter would deepen the vacuum (making it closer to it's source) in all places between engine and filter element.  The closer the differential pressure gets between carb throat and "vent" pressure, the less fuel will be pushed through all the jets.  The mechanical risk from a resulting "too lean" condition is high head/valve train temps, pre-ignition, and detonation.  Are you willing to sacrifice the engine in the spirit of experimentation?

Thanks for clarifying that the F model wasn't included in the "lean 77/78" discussion.  Knowing that makes me think about dropping the needles that last position but like you, I question why it wouldn't get better mileage with all the factory settings.  It makes me wonder if something else isn't right or that I did something incorrectly.

I think it should get better with factory settings.  I've got two examples that do, for sure.  So, I lean toward your last statement. 

Fishing....
Maybe your riding style/technique has something to do with it.  What are your RPM/MPH choices?

Do you warm the bike before riding?  How long do you keep choke on?  What's your ratio of standing idle to "on the move"?  Is the mileage related to two up riding?  (No, I'm NOT saying your passenger is a fat ass.    )

You know, I will say that all my 550 Fs have a windshield/fairing on them.  Perhaps that actually improves MPG?  I never did a "before and after" experiment.

Another thought... Is your engine breather element clean/clear?  Told you I'm fishing...
Might as well ask about the element drain hose and the air plenum drain hose.  Those stock, too?
Might you have rag under the seat...precariously close to the air intake?

Cheers,

[noobie dave]

Hi,

Just to chime in with info from another thread I posted a few days ago (see "CB550 Horrible Mileage"), some people were suggesting that my front brake was dragging too much and lowerin my MPG.

I checked my brake and found that it was dragging pretty severely; with the front wheel lifted it wouldn't rotate 3/4 a rotation even with a strong push.  I haven't ridden the bike since working on the brake so I don't know if I've solved the problem, but it might be worth a look.

[BuffaloBill]

I had a reply typed in and pictures attached.  Then I seem to have lost it all when clicking on post.  Something went wrong.

I've been up-shifting on average about 500rpm and ride about 64 on the speedo (about 60mph).  I haven't had a rider during the last tankfull.  I warm the bike for maybe 1/2 min with the choke part on, then ride off with the choke fully open. The  breather element looks pretty clean although my airbox drain hose was missing!  The plenum drain hose was cracked and in a rotted condition.  I capped them both off for now until I get new hoses.

The wheels turn pretty freely.  If there is brake drag it's very light.

I'll try pics on the next reply.

Thanks.

[Tretnine]

I drive mainly in town and get 35-40 mpg. I do drive with pretty high RPMs, because 1. my battery is never charged and 2. I like to be in the power band when some a-hole wants to try to sideswipe me. My mileage is better on the highway, but I've run with 2 or 3 different stock carb racks on this bike and never had much difference in performance, either in torque/HP or MPG. I would say that I'm definitely NOT tuned to anything close to peak performance, but the bike starts on one kick every day.

[BuffaloBill]

 Is this the correct airbox lid for an F model?

[BuffaloBill]

I missed a "0" on an earlier post!  I don't up-shift at 500rpm...lol...meant 5K

Here's the bike:

[TwoTired]

The  breather element looks pretty clean although my airbox drain hose was missing!  The plenum drain hose was cracked and in a rotted condition.  I capped them both off for now until I get new hoses.

Ahh, this might be the smoking gun...

The plenum drain hose is a reduced diameter hose, whose end is NOT capped (the filter box drain is capped).
Anyway, the plenum hose does let in some unfiltered air, and may well effect the carb throat vacuum in a way to increase fuel draw through existing jets.  Certainly the stock jetting was set for this hose/air inlet to be in place.
Although, I haven't got direct experience to say capping that causes worse MPG.  It is certainly a difference to note compared to my F models.

Comparing my operation with your description.  I flog my bikes worse than you do, RPM wise.  However, I generally suit up, jump on, apply choke, start up, back off the choke enough for the bike to accept throttle and drive off.  The choke comes off as I ride incrementally, just enough to keep throttle response for a few blocks until its all off and will idle and runs well with the choke off.

That's a nice looking bike.  My 77's are the blue versions.  I've only seen the maroon ones in pictures.  My 76 is currently the Orange crush color, but I can make it metallic blue, as I also have the tank and side covers to change it over.  The 75 was metal flake orange when I got it, but the bottom rusted out of the tank, so it has a blue tank and orange side covers.  It also has rotted carbs, and I've lacked the time to put the replacements on it.  It will get it's turn eventually. 

Yes, all the F models had that inlet horn/lid on the filter box.  The 77 and 78 K models did, too.  Earlier K models, no.

Cheers,

[BuffaloBill]

TwoTired,

Thanks for the compliment on the bike!  I ordered this evening the missing hoses from Service Honda.   I flogged the bike harder today with a little over 100 miles on the trip meter and haven't yet had to switch to reserve.  I'll check mpg on this tank when I fill up in the morning.

I like the orange colored 550F bikes.  I've seen one go by on the street in Winona, MN.  I think it looks a bit more sporty than the maroon.

[Tretnine]

TwoTired,


I like the orange colored 550F bikes.  I've seen one go by on the street in Winona, MN.  I think it looks a bit more sporty than the maroon.

Are you a minnesota boy? I originally hail from there.

[BuffaloBill]

TwoTired,


I like the orange colored 550F bikes.  I've seen one go by on the street in Winona, MN.  I think it looks a bit more sporty than the maroon.

Are you a minnesota boy? I originally hail from there.

I'm from across the river in Wisconsin.

Darn it, the parts I ordered from Service Honda are discontinued.  Anyone know where I could find the plenum drain hose and the drain hose for the bottom of the airbox (for a CB550F)?

[TwoTired]

I just happened to have this window open in my browser.
http://www.cmsnl.com/honda-cb550k3-four-usa_model1015/partslist/F++16.html

I don't know if they actually have the parts available.

You know, though, you might simply remove the plenum hose to see what effect it has on MPG.  Or, just put a restrictor in the end of a hose that fits on the existing nipple.

Just a thought...

[noobie dave]

following the developments of this thread, I JUST placed an order with my local dealer for the plenum drain hose (#17 on the air box fiche).  He said it was available and should be here in a few days.  Who knows, maybe he's mistaken and will call me back soon with the "oh, sorry...".  

But it might be worth a try.

[BuffaloBill]

following the developments of this thread, I JUST placed an order with my local dealer for the plenum drain hose (#17 on the air box fiche).  He said it was available and should be here in a few days.  Who knows, maybe he's mistaken and will call me back soon with the "oh, sorry...".  

But it might be worth a try.

Dave,
I hope you get your hose.  I'll ask the Honda dealer next time I'm nearby if they can order me the parts.

TwoTired,
Well, this evening I dropped the needles the last remaining notch (clip in the highest position), re-sync'd the carbs, set the air screws back to 1 1/2 turns and took it for a 5mile test run.  The bugs were bad or I'd have gone farther.  It sounds a little cleaner running while cruising at hwy speed now and accepts throttle inputs well off idle and when increasing throttle opening while cruising.  I'll have to check mpg with this calibration.

The air temp was about 65F and the bike didn't exhibit any signs of running lean that I could pick up on.  Perhaps I should take a plug reading after a mile or two at a 60mph cruise.  On snowmobiles (2-strokes) I install thermocouples in the exhaust, 100mm from the pistons to measure EGT for tweaking in the jetting.  I'm not sure if EGT really works well on a 4-stroke as a means of determining proper air/fuel ratio, and I'd hate to drill holes in the header anyway.

I still wonder why my bike would run richer than your F bikes and other people's with the stock #2 clip position.  I'm going on the assumption that this is why my mpg is down but I'll continue to think on that and please throw me any ideas you might think of. If the Uni is a little less restrictive than the factory element I suppose that would make some difference.

My needles (If I am seeing the numbers correctly) are 28004.  The jetting chart I found a while back didn't list the needle part number.  Is this the correct one?  The slide has "2.5" and "103" numbers. 

[Pinhead]

Man you "other" SOHC4ers (non-CB650 riders) really have it rough; if I ever got anything below 47 mpg I'd be really worried about the internals of my engine!

[Tretnine]

Man you "other" SOHC4ers (non-CB650 riders) really have it rough; if I ever got anything below 47 mpg I'd be really worried about the internals of my engine!

Too bad the bikes stopped looking nice in '78, though.

[BuffaloBill]

Man you "other" SOHC4ers (non-CB650 riders) really have it rough; if I ever got anything below 47 mpg I'd be really worried about the internals of my engine!

Do you mean the cylinder walls having the oil washed off them under an extremely rich condition?  I understand that can be an issue if running very rich.  I'm pretty sure my 550 isn't to that point though.  Speaking of mpg, I used to have a 1981 GPZ1100 that'd get 52mgp and a 78 Hawk 400 that got 40!

[JohnG]

For whatever its worth:   many years ago I had a girlfriend who had a box-stock 550F.  I think 1976. Had a metallic blue tank.  Gas mileage and performance were modest  (mid 40s?).

We never touched a thing save for one mod:  we put a stock replacement K&N air filter in the box.  Best money ever spent!  The mileage went up into the 50s and the change in performance was great!  Mid range became far more responsive and the whole bike become far more fun to drive.  Never changed any jetting and have no idea what jetting was in it but I assume it was factory.

Only problem we had was the shafts the rockers rotate on themselves rotated in the valve cover and wore it out.  Had to get a new valve cover and drill/tap for some set screws.


                 John

[crazypj]

26~43mpg on mine when it was stock, depending how I rode it, (usually closer to lower number  )
 With 591 kit, cam, exhaust and K&N filters, consistent 41mpg, 54mpg if I took it easy
being in too high a gear at lowish rpm when accelerating wastes a bunch of fuel

PJ

[TwoTired]

26~43mpg on mine when it was stock, depending how I rode it, (usually closer to lower number  )
 With 591 kit, cam, exhaust and K&N filters, consistent 41mpg, 54mpg if I took it easy
being in too high a gear at lowish rpm when accelerating wastes a bunch of fuel

Didn't you change the carb jetting, sprocket gearing, too?

[BuffaloBill]

The day started off on a sour note.... the bike got knocked over in the in-law's driveway.  Turn signal damage, scuffed mirror, broke the ball end off the brake lever, and scuffed the #4 headpipe.  This 32yr old bike prior to this showed no evidence of ever being down  :-(

On a positive note, after dropping the needles last night, on today's ride with my wife on the back, we got 42mpg bucking wind for a good portion of the trip with 1/2 the miles on back roads with lots of dugways (the roads from the valleys to the bluff tops as we call them here in the Mississippi River valley). 

Do any of you know how well the aftermarket turn signal lenses match the original Honda lenses (p/n 33402-377-671)?