Mikuni BS 32 Carbs Don't Get the MPG They Used To

My 1982 KZ650 used to get fantastic gas mileage some years ago. When gently riding around town without any significant stops, I got from about 48 to 57 MPG. Now my mileage ranges from about 40 to 45 for the same riding. This is still good mileage, but I believe something has changed within the carburetors.

All these factors remain unchanged: Main Jets; pilot jets; jet needles and settings; float level; idle mixture adjustment; stock air box & filter; ignition timing; gearing / sprockets; Valve adjustments; my compression is way better now than before, etc.

I believe my mixture is richer than before, due to the following:
1. I use the choke for cold starting, but immediately pull out onto the street, shift to second gear; open the choke, and the bike pulls away nicely without any hesitation whatsoever. (It used to take a mile or so to warm up).
2. The spark plugs are way too black in color. They are not damp, but not the light chocolate or tan color that they used to be.
3. I had experimented with jet needle settings in the past to get more performance, but now they are back to the stock position with no shims, washers, etc. THE THING THAT I NOTICE, IS THAT THE FIRST TIME I TOOK THE TOPS OFF THE CARBS, THE DIAPHRAGMS WERE DRY. ON LATER OCCASIONS, THEY WERE ALWAYS WET WITH FUEL. Perhaps they are leaking, but I would think this would simply delay the slides from raising, and not bother gas mileage for easy riding.

I just cannot figure what has changed. Does anybody have any ideas? Thanks.

How is the voltage going to the primary side of the ignition coils, as per battery voltage? Much of a drop? This is a common problem on the older bikes, and cleaning electrical connections, and the fuse clips can lift the voltage to the coils. Which equates to more spark energy.
Otherwise I agree with your diaphragms having fuel on the top would be bad. Must have a tear, or pin hole there.

WABBMW wrote: ...remain unchanged...float level...believe my mixture is richer than before, due to...spark plugs are way too black in color....

float level -- what about fuel level, as determined by the clear-tube test?

Good Fortune!

Motorhead:
I'll check the voltage to the coils. All connections were cleaned back 6 years ago, but that was over 20,000 miles back. The bike runs great and never misfires. I'll check it out anyway.

Patton:
Good idea about the fuel level. I'll see where it is, using the tube method. The present float level settings were done with mechanical measurement, rather than the fuel level method. I had replaced the needle and seats back 6 years ago. Never have had the carbs to flood, however the fuel level could be too high.

Well I checked the voltage at the coils and found it to be about .3 volts lower than the battery voltage. For example, with the motor off, but the ignition switch and headlight on, the Battery voltage = 11.9; Voltage at the coils = 11.6. I don't know whether that is good or bad.

I also checked the fuel level, using the clear tube method, and found the level to be between 2 - 3 mm below the bowel upper mating surface. This might be a touch high, but within spec.

I did think of one other item that has changed: On the original exhaust system, pipes 1 and 4 run straight back and into the muffler. Pipes 2 and 3 turn almost 90 degrees before entering pipe 1 or 4. I felt that this was terribly constrictive. Also the inside diameter is small - about 1 1/8". Then there is the crossover pipe between the two sides.

I modified the system by welding in a "Y" configuration into each side, so that pipes 1 & 2 entered the "Y" and on into the muffler. Ditto for 3 & 4. The "Y" was about 1 3/8" for each leg, and then emptied into a 1 3/4" collector on to the muffler. The crossover pipe remained unchanged, as well as the internals to the mufflers. The only advantage to the original system, might be that the velocity of exhaust pulses might remain higher than with my modified arrangement. I just don't know whether that would decrease gas mileage any. Anybody have an opinion about this? Also, how about the coil voltage, or fuel level? Thanks.

Does your state mandate ethanol mixed with the gas? That would account for a 10 percent drop in milage. Just a thought.

Yes indeed, in fact the Ethanol gasolines are all that I have been able to find for a few years now. I believe that is definitely a contributing factor to the poorer gas mileage.

I hate that stuff. I also believe that it slowly melts the rubber gaskets in my vacuum operated petcock. I have to replace these every year or so. All so we can live an extra 2 months longer due to cleaner air.

WABBMW wrote: Yes indeed, in fact the Ethanol gasolines are all that I have been able to find for a few years now. I believe that is definitely a contributing factor to the poorer gas mileage.

I hate that stuff. I also believe that it slowly melts the rubber gaskets in my vacuum operated petcock. I have to replace these every year or so. All so we can live an extra 2 months longer due to cleaner air.

I think you have your answer...it is the ethanol in the fuel. For example, in a car I have had for 12 years (2001 KIA), I went from a steady 34 mpg to a consistent 31 mpg after the introduction of ethanol laced gasoline. Ethanol belongs in a beer bottle, not as a fuel additive.

In this case it's probably not the ethanol. If it were, my mpgs would be down as well.
Last weekend, I got the usual 52 mpgs on a long ride (81 KZ650-CSR).

I'll suggest installing 750 cams, 750 timing advancer, new Dyna coils & spark plug wires, B8ES spark plugs, new airbox ducts, maybe new carb holders, repair any cylinder & cylinder head gasket leaks, do the carburetor service fuel level check using a proper measuring device (not eyeballed), jet/adjust the carbs as necessary - perhaps using the 750 BS34 base settings as a starting point?. Or sell the old clunker & only ride the FZ1. hee.