Fuel economy tips for the CB550.Slightly over inflate tires. Less rolling resistance = better fuel economy.
Wide tires have more wind resistance. Soft rubber rolls with more resistance than hard rubber. Narrow tires with high mileage ratings should also have better fuel economy. Cheng Shins might just be the best choice for a fuel economy goal. Though, I would seriously consider some of the Continentals for the MPG goal, as I've had some very long wearing front Continentals on the CB550. I think Michelin touts a high mileage tire that could be a furtherance toward the MPG goal, as well. But, I don't have experience with those. A radial tire is also desireable from a mileage/MPG standpoint, too. Again, I haven't tried this but the theory is sound. However, I don't think the SOHC4 was suspension tuned for a radial tire. It is a pity that tire manufactureres don't routinely distribute meaningful data such as rolling resistance.
Keep a clean or new stock air filter. Or, better yet a slightly less restrictive Uni Air filter, kept clean of course.
Let your plug deposits be your guide. The lighter the tan, the better for fuel economy. But, avoid white insulator appearance. Altitude is also a consideration. As these carbs do not compensate for pressure differences with altitude, if you tune your mixture for optimal burn at say 5000 ft above sea level. Then driving down to sea level may put your bike at risk for detonation. <--- very bad for engine.
Open the spark gap to .030 -.032. More is better for fuel economy. If you have the stock points you may need to shorten the service interval to maintain the mileage improvement. If you start getting engine miss, it's tuneup time for sure.
Idle air bleed screws, IMS screw adjustment: These tune the idle circuits for a mixture balance between economy and throttle response. If you open the Air bleed screws, you will improve economy at idle settings. However, this will also effect how the bike responds to twist of throttle from low speed. Turn the Air bleeds out till you can't stand the throttle response.
The 77-78 models have IMS screws, these were factory tuned for emissions/economy during idle. But, turning them in reduces fuel flow.
The idle circuits flow fuel at all throttle settings. They don't contribute much, percentage wise, at high throttle settings, but it is still a contribution. Reducing it improves fuel economy.
Float levels: Lower fuel levels in the carbs, translate to leaner mixtures across the throttle range. Watch those plug deposit colors!
Don't use engine braking. It sucks more fuel through the carbs than an engine simply running at idle RPM.
Exhaust: Almost all performance exhaust is intended to scavenge as much as it can from the cylinder during the exhaust cycle. This includes any unburned hydrocarbons (fuel). A restrictive exhaust holds some of these in, to be burned during the power cycle. The best I've found for the 550 is the stock F model 4 into 1 muffler. All my F models get better fuel economy than the early 4 into 4s or any other aftermarket exhaust I've encountered for the 550. They also have the smallest orifice main jet of any of the early model carb equipped 550s. Of all the early model carbs, the F model's 069A carb have the best potential for fuel economy, provided the exhaust can match it.
The 77-78 model carbs have even smaller main jets to go with the stock 4 into 4s for those years. These 4 into 4s were even more restrictive than the earlier 4 into 4s and quieter, too. This setup has the best potential for fuel economy than any of the CB550 models.
I don't know of any aftermarket exhaust that claimed or realized economy as a performance benefit. Generally speaking, a loud exhaust is almost certainly worse for fuel economy due to its lack of restriction.
Gearing: Go up a tooth on the front, or use the 500 rear sprocket of 34T vs the 37 T of the stock 550. Although, spending more time in the lower gears can easily negate any benefit from this. Further, if your vehicle loads are high, sprocket changes can reduce you mileage rather than improve it, as reduced engine RPM may put it in a range where the engine can't make enough power to overcome rolling resistance, wind resistance, and vehicle load.
Keep Chain well lubed. A dry, stiff, chain eats more horspower than a smooth flexible one.
Thinner oil takes less HP to drive the oil pump, and interferes less with the meshing gears in the trans.
If you are using 20-50 now, go to 10-40.
Reduce electrical load: The alternator drag takes energy from the crankshaft. The harder the alternator works, the more drag and horsepower are need to spin it. If you've added electrical load to the stock configuration (Higher wattage headlights, more running lights or replaced with higher wattage bulbs, radio, heaters, etc. You can expect that to have some impact on fuel economy. If you have a lighting switch, simply turning it off should help. However, safety and some state laws prohibit that.
Dragging brakes. Not much to worry about for the rear unless it is seriously maladjusted to provide extra drag. But, that front disc can drag a lot. Jack up the bike and give that front wheel a spin. Any resistance you feel is robbing your fuel economy. Address as appropriate.
Use 1970s fuel. :-) Climb into your wayback machine and bring back the actual fuel these things were designed run well using. Be carefull with the dates on your currency.
My 74 routinely got 45-50MPG on trips back in the early days. 40-45 is more likely what I get today. Part of it I'm sure is the engine is more worn and tired. But, I think the bulk of it is caused by gasoline reformulation.
My mileage noticeably dropped when the industry changed the octane rating system in the US. I believe the oil companies used that to lower the energy contant of the fuel sold to the masses and increase their profits. What is certain, is that ALL gasoline that has added alcohol definitely DOES have a lower energy content. Your mileage WILL decrease using fuel with added alcohol as alcohol has about half the energy content that gasoline should have. You will have to open the throttle a bit more to make the same amount of power as with the old gas, and that makes the liquid flow faster for the same distance traveled.
Side effects:
A lean burn engine, ideal for fuel economy, can blue your chrome exhaust pipes.
The closer to white spark plug insulators you get, the more significantly pronounced the side effects become.
A hot engine engine burns fuel more efficiently. It also wears out faster, and fatigues the oil with larger heat cycles quicker, too. You might actually need that oil cooler to prolong oil life. Or, change oil more frequently. Synthetic oils may evaporate more rapidly, leading you to believe the engine is burning oil. Certainly any oil that is heat damaged, loses some of its lubricative properties which are so desireable for engine longevity.
The bane of all air cooled engines is the dependence on the surrounding air for a consistent cylinder to ring and piston fit. Running the engine hotter for fuel economy, means you are also closer to the upper limit of cooling fin capacity. Hot pistons expand at a greater rate than the cylinders do. Once you run an engine hot for a while, the break-in process stabilizes for fit at that temp. If you later run the engine at a lower temp., the clearances become larger, which means that combustion gasses "blow by" the rings, contaminate the oil supply, and the oil supply seeps past the rings to be burnt in the combustion process, behaving like a worn out engine.
Finally, a bike tuned for max fuel economy, probably won't be competitive on the track, offer the best handling or cornering performance, or make the best power for acceleration. In the engineering community, we call these "tradeoffs".
Cheers,
