Topic: 550 misses after a short ride...?

[hapsh]

On my '76 550 I am currently getting 51 mpg with 4-1 Mac, Uni-pods and Dyna S ignition.  My jets are stock other than my needle clip dropped to the 4th slot (one richer than stock).  When I got the bike with stock points and the jet needle on the 5th clip I was getting 38 mpg.  Installing the Dyna S made my mileage jump to 46mpg.  I then raised my jet needle clip to the 4th position (to fix a flat spot off idle).  That jumped my mpg to 51.  I do commuting that is 30% city (25 mph) and 60% highway (75-80 mph).  I am very happy with how it runs.  However, on my quest for higher gas mileage I am going to install a stock airbox.  I am just waiting for a uni filter element for the box.  I plan on putting the jet needle back to stock when I do so.  I live at 2600 ft, so my bikes usually run a bit rich in stock config, so I figure the increased airflow of the uni filter should bring me right in.  I also am going to try to leave earlier to work and keep my speed at or below 65 mph and see what that does on a tank of gas.  The bike really runs very smooth and calm at 65 so I figure that efficiency should be pretty dang good..

[edbikerii]

Do vehicles generally get better mileage at your elevation?  I can see how "thinner air" would cause a carbureted engine to run rich, so you might be able to get by with stock jets, even with pods and exhaust.

I noticed the other day, when I was driving a friend's Honda FIT in HighPoint, NJ (elevation 1570 feet) that when we got near the top the car had significantly less power.  Funny, I would have thought that a fuel injected, CPU-controlled engine with at least one oxygen sensor would be better able to handle elevation changes.  I know it was due to the location, because the same thing happened when we drove by on the way up, and again on the way down.

On my '76 550 I am currently getting 51 mpg with 4-1 Mac, Uni-pods and Dyna S ignition.  My jets are stock other than my needle clip dropped to the 4th slot (one richer than stock).  When I got the bike with stock points and the jet needle on the 5th clip I was getting 38 mpg.  Installing the Dyna S made my mileage jump to 46mpg.  I then raised my jet needle clip to the 4th position (to fix a flat spot off idle).  That jumped my mpg to 51.  I do commuting that is 30% city (25 mph) and 60% highway (75-80 mph).  I am very happy with how it runs.  However, on my quest for higher gas mileage I am going to install a stock airbox.  I am just waiting for a uni filter element for the box.  I plan on putting the jet needle back to stock when I do so.  I live at 2600 ft, so my bikes usually run a bit rich in stock config, so I figure the increased airflow of the uni filter should bring me right in.  I also am going to try to leave earlier to work and keep my speed at or below 65 mph and see what that does on a tank of gas.  The bike really runs very smooth and calm at 65 so I figure that efficiency should be pretty dang good..

[TwoTired]

I haven't calculated my CB550 mileage in a good long while.  And, since retirement, I don't drive a regular route anymore, and I don't stick to one bike either. 

When I bought my 74 550 in 1975, I recall the PO proudly and exuberantly stating the bike got 50 MPG on trips he used to take before he wrecked the bike.

Because of the wreck, I put 4 into 2 exhaust on it to make it road worthy for the least cash outlay.

It was my only bike for many years and my mileage, when I used it for commuting 17.5 miles away, was about 45 MPG with a 50/50 mix of 70-ish MPH cruise and 25-30MPH stop and go.  I didn't keep good records.  But, I recall refilling the tank about once a week.  I'd drive it to my usual gas station, near home, after I switched to reserve.  The 74 550 has a 3.7 gal tank.  35 miles times 5 days with say 3.5 gals of gas is about 45-50 MPG.

When I put the big Vetter fairing on it (6 AM rides in January were, too cold) I thought the mileage would get worse.  But, I didn't notice any change in fill up frequency.  This surprised me.  But, it makes sense because with the Vetter, I felt air pressure on my back instead of my chest, though at a reduced magnitude.  I speculate the big Vetter actually improved the bike aerodynamics, offsetting the added weight.

Anyway, when I started riding the F models with the bigger tanks, I found I could go longer between fill ups.  So, I believe with their stock pipes and uni filters in the stock airbox, they get at least as good gas mileage as that old 74.  But, that's without any real numbers recorded.

I went on a ride with other club members a few years back.  We rode for the day and I was using my 77 CB550F for this ride, and my wife was on the back.  The group, led by 750 riders, of course, all pulled into a gas station, so I followed.  I hadn't switched to reserve yet, but I figured while I was there...  Anyway, I filled it up, and the 750 rider next to me noticed final amount.  His incredulous comment was, "That's all it took!?".  We'd both ridden the exact same amount and filled just before the outing.  We were both riding two up.

My 78 Cb550 hasn't gotten the regular use my others have had.  (Which is why I'm selling it, btw)  But, when I WAS riding it, it sure seemed more frugal than the others with gas.  It also doesn't seem to have the power the other have, though, either.  I suspect the stock 4 into 4 exhaust is the cause for both the fuel economy as well as the power anemia.  But, it's just a feeling I have.  No data from controlled testing.

FWIW
Cheers,

[edbikerii]

Oh, yeah, just realized that 20W50 oil is probably holding my mileage down somewhat.  I sure as hell don't want to argue about what oil you are using, but I'm curious to know what weight you use?

[TwoTired]

I use 10-40W.  But, I've used 20-50W at times in the past.  I never thought it had much impact on fuel economy.  But, I suppose it could on close tolerance engines.

Related discussion:
I never let the 550s sit and warm up before setting off, unless, of course there is some sort of problem.

I suit up, start, and drive off immediately, letting it warm on the way, with gradual choke let off.  This has to save fuel over letting it sit and run to warm up, particularly with the choke on.
It is also the reason why I prefer the 10-40W oil, as that will circulate and lube better when the engine is cold, and I was riding year-round.  For those few months of really hot weather where a thicker oil would be of particular benefit, I ran 20-50W Castrol for a few summers.  But, I alway felt bad "wasting" oil that was changed before being fully used up (not enough time/miles) when the weather turned cold in morning start-ups.  This was particularly prominent when I started getting multiple bikes in the collection.  What I did was switch to semi synthetic 10-40W, with the reasoning that during very hot months, the synthetic retains lube qualities at higher temps to protect the inner workings, and still defeat the cold morning start up lube issues.
Yes, the oil is more expensive, but the trans shifts noticeably better, and expansive oil is still cheaper than replacing pistons, etc.

I know.  More than you wanted to know.  Sorry.

Cheers,

[edbikerii]

Not at all more than I wanted to know.  Thanks, TT.  I was just afraid that somebody else would take this as an opportunity to argue about oil.  I'm just gathering information.

I googled CB550 mpg and I've found a wide variance of reported mileage from 23mpg up to 50mpg.

Obviously the average is somewhere in between 23 and 50 mpg, which makes me more comfortable with my 39.32 out of my last tank of gas.  I know I had at least two cold starts with choke on that tank, and some putting around at a lot of stoplights on route 9W along the Hudson river for ~40 miles, plus more than a couple 90+mph runs on the highway.

[edbikerii]

Another mileage update.  131 miles since last fill, bike took 3.317 gals., so I got 39.49 MPG.  This time I was doing mostly city riding, with lots of stop and go in Manhattan traffic.

Lately I've been noticiing a little intake popping, which seems to improve when I put the choke on a little.  So tonight I richened up the idle mixture screws 1/2 turn tonight to 3 turns out.  That was an improvement, not perfection.  So, I'm soon going to raise my needles one notch, and see how that goes.

[TwoTired]

Any chance you could reduce the main jet size a smidge when you raise the needles?

[edbikerii]

Rode to Marcus Dairy today.  Nice ride.  Saw at least three CB750F2s up there.  Lot's of old GoldWings, and stuff too.

Point is, I have another mileage update:

124.7 miles on 2.679 gals. for 46.55 mpg.  Almost completely highway miles.  Two full-choke startups on that tank.

50mpg is sounding more and more possible to me now!

TT, I should have mentioned that the slight popping in the intake is only at about 1/4 throttle at constant speed.  I think that throttle range is most impacted by the main jet.  Also, all seems good at WOT.  You know, TT, I can't seem to find that chart that you keep posting.  Can you post it again?  This time I'll save it to my local drive.

[TwoTired]

When your bike was 1st delivered by Honda, I expect that it got 50mpg or better then. 

I this the chart you want?

[edbikerii]

Yes, thanks TT, that's it.

For various reasons (cost of a new stock exhaust, mainly), restoring to stock is not a cost-effective option.

Any idea what "Straight Dia." means?  That and throttle valve cutaway seem to perfectly match the throttle range in which I have the popping back through the intake.  Since I do not wish to change the throttle valve cutaway, and I'm not familiar with what is meant by the "Straight Dia.", my next closest adjustment would be to raise the needles, I believe.

Once I raise the needles a notch, then I'll lean the IMS.  I suspect that would help tremendously in stop-and-go city traffic where one idles the most.  We'll see.

Thank you,

E

When your bike was 1st delivered by Honda, I expect that it got 50mpg or better then. 

I this the chart you want?

[TwoTired]

My under standing is that the top three factors are all about the jet needle.
Clip position - obvious
The straight diameter of the needle is still smaller than the needle jet orifice, providing flow capability.  When the needle is inserted into the jet orifice far enough, the straight section is what restricts fuel flow.
Then there is the taper that increases the needle jet flow capability as the needle is withdrawn from the orifice.

If you wanted to increase the fuel mixture in the [straight diameter] area, you would actually reduce the needle diameter in the straight section area.

I understand you won't buy the stock exhaust.  But, if your quest is for fuel mileage, you might consider adding back pressure to the exhaust you've chosen.
Then you get to do the carb jetting over again. 
Or, set it to stock and keep restricting the outlet till it runs right again.  If you are interested in persuing this, I can measure the outlet diameter of the stock exhaust pipe.

Cheers,

[edbikerii]

Jeez, the thought of "engineering" the needle taper or straight, or the throttle valve cutaway, does not appeal to me, as those changes are irreversible short of cannibalizing a perfectly good set of carbs.  I've always operated under the assumption that there was enough adjustability in the carbs to cover every possible situation short of engine displacement changes, but I suppose that they were flexible enough for Honda to provide different hardware to adjust it as required on the drafting table.

If it isn't too much trouble, could you measure the diameter of one of the four exhaust outlets, and also verify that they are all the same?  I'll gladly do the math to figure out the cross sectional area.  I've measured the single outlet of my MAC 4-1 at 35mm or 1 3/8".  This gives a radius of 17.5, and an area of 962.115 sq mm  (17.5mm * 17.5mm * 3.1416 = 962.115 sq mm).

Luckily, the MAC exhaust has a very simple, easily removable baffle that can be wrapped with fiberglass or stainless steel wool.  Or I could temporarily attach a reducer to the baffle's inlet or outlet.

If you have a 77 or a 78 CB550K (with identical stock exhaust, I think) that would be best for my purposes.

My under standing is that the top three factors are all about the jet needle.
Clip position - obvious
The straight diameter of the needle is still smaller than the needle jet orifice, providing flow capability.  When the needle is inserted into the jet orifice far enough, the straight section is what restricts fuel flow.
Then there is the taper that increases the needle jet flow capability as the needle is withdrawn from the orifice.

If you wanted to increase the fuel mixture in the [straight diameter] area, you would actually reduce the needle diameter in the straight section area.

I understand you won't buy the stock exhaust.  But, if your quest is for fuel mileage, you might consider adding back pressure to the exhaust you've chosen.
Then you get to do the carb jetting over again. 
Or, set it to stock and keep restricting the outlet till it runs right again.  If you are interested in persuing this, I can measure the outlet diameter of the stock exhaust pipe.

Cheers,

[TwoTired]

The muffler outlets on my 78 CB550K are 16.6mm in diameter for each one of the four (or 0.654 in), according to my Mitutoyo Caliper.

An aside...
Since the 74 was right there, I measured the two right outlets of the 4 into 4 on that, 19.1mm and 18.7mm.  However, the outlets are slightly recessed in the flare end and hard to measure using a vernier caliper with consistency.

With either exhaust, there is some possibility that internal restriction exceeds the end diameter cross section.  But, don't ask me to cut up my exhaust systems to find out. 

Something to note.  Fluid dynamics are tricky.  It won't just be the cross section area to deal with.  You also have to calculate the outer pipe wall surface area for each system.  For here, is where boundary layer resistance is at maximum.  (the layer nearest the wall is always at less velocity than that of center of pipe flow.  Four smaller pipes will have more boundary layer effects that a single larger outlet.

I think what this means is that the single outlet diameter must actually be smaller cross section the the four individuals to achieve the same resistance to flow, with the added complication that relative fluid velocity makes a difference, meaning rpm will make a difference.

But, it is probably more complicated than that.

Cheers,

[edbikerii]

Yes, we both know that it is more complex, but I'm going to try to stick to simple stuff that won't make my brain hurt.

8.3 * 8.3 * 3.1416 = 216.424824 * 4 pipes = 865.699296 sq mm.

So the total cross sectional area of the outlets on the stock exhaust is 865.70 sq mm vs. 962.115 sq mm for the MAC.  96.42 sq mm more surface area.  That is 11% more surface area for the MAC.

This seems consistent with my jetting change from 90 to 105 which is 17% more cross sectional surface area in the fuel jets.  Considering that my bike was running too hot (lean) with the 90 stock mains already, I think the additional 6% may make it 1 or 2% too rich?  Of course, I'll live with 1 or 2% too rich if it means keeping my engine from burning up.  Of course I'm ignoring the specifics of a lot of other factors, but I think this all kind of makes sense for a "ballpark", no?

Also, I can't pull off the baffle from the MAC right now since I'm at the office, but I seem to remember that the baffle consisted of two pipes that were welded offset to each other and then squeezed together slightly to fit in the can.  This also reduces the cross sectional area by some small amount as well.  I have no knowledge of the baffles inside the stock exhaust.

So, in simplest terms, I'd need to have an outlet reducer with a diameter of SQRT(865.699296/3.1416) x 2 or 33.2 rather than the MAC's 35mm in order to approximate the stock exhaust, if fluid dynamics, and all else are ignored.  Or more simply, an obstruction with a cross-sectional surface area of 96.42 sq mm could be placed in the exhaust outlet to restrict flow.

This would be a square with 9.82mm sides (again ignoring other more complex that cannot be quantified without testing.

All that is fine and dandy, if the exhaust were the most restrictive part of the entire system.  However, I believe that the exhaust is not the bottleneck.  I think it is the airbox and filter.  My head is starting to hurt a little now, though, so I'll have to think about that a little more.

[TwoTired]

But, wait there's more!

I recommend a ramped restriction diameter in the exhaust, rather like a venturi.  This will get the orifice size you desire but not insert a reflective baffle to upset flow dynamics.

Re: intake vs exhaust restriction.  The carb throat venturi's are 22 mm.  Both the stock and pod filters have much larger cross sectional area than they do.  The factor you are missing (I think) is pressure differential.

The pipe and muffler are filled with hot expanding gases under pressure.  Whereas, the induction system does not.  Remember the vacuum sync gauges you put on the intake runners?  This pressure differential from atmospheric is what draws air into the engine, through the restriction of the filtration and carb venturi.  The combustion process puts far more pressure differential into the exhaust pipe, and this is why there can be more flow out a restricted exhaust orifice than an intake orifice.  It's not simply an air volume in vs air out equation.  There is the temperature, fuel, and chemical conversion products, as well.

Increased heating effects. 
Higher pressures in the exhaust mean less scavenging of the cylinder.  The remainder volume of the gas displaces fresh oxygen on the next intake stroke.  To match the available oxygen, less fuel is supplied for the burn cycle.

When you put the open exhaust on your bike, you made more oxygen available for the burn.  Stock jetting became "lean" and the engine temps went up accordingly.  Pod filters reduced the vacuum in the carbs, so less fuel was drawn from the metering jets, making the cylinder burn even more lean.

Ready to yell UNCLE?

Cheers,

[edbikerii]

Yes, agreed, there is a vacuum created inside the airbox that is lower (higher air pressure) with pods.  That is the whole point of pods, after all:  increase the air pressure in the intake charge into the cylinders to get more power.

Yes, restrictor in exhaust should be cone shaped or something that ramps up in order to avoid reverberation.

My MAC exhaust was not intended as a performance improving (free flowing exhaust) for me.  It was a cheap replacement for a rusted out stock exhaust.  MAC advertises that no re-jetting is required with the MAC system.  Obviously, this was not true in my case due to the lean/hot running condition.

Since I cannot revert back to a stock exhaust cost effectively, and I like my MAC exhaust because it is lighter, has better ground clearance, and looks good, I have to live with what I have and try to adjust the mixture.

Perhaps it would just be wiser to try raising the needles one slot and see what happens.  If I have a very slight pop back out of the carbs at around 1/4 throttle, and closing the choke helps, does it seem reasonable that raising the needle one notch might solve my problem?