28 mpg

[jetmechmarty]

I'm getting 28 mpg most of the time. I rode the bike all weekend with my daughter on the pillion and my best was 30 mpg. I was running it easy all weekend, 4th-5th gear. Most (not all) of my fuel stops were ethanol free.

The bike sat for two weeks. I pulled the cover, turned the key and mashed the button. It started immediately, no choke. It settles to idle after only a few seconds. It runs great, no dead spots, and does not have popping on decel. Idle jets are stock size, mains are 115 across. All jets are new genuine Mikuni. The float needles and seats are new. The float height/fuel level has been set, checked, rechecked, and rechecked again. IIRC, the fuel level is about 3 mm below the gasket.

I went to a rally over the weekend, and I was getting feedback that I'm running rich, as in somebody smelled fuel from my exhaust. This bike runs great, but only my big block El Camino gets poorer fuel economy.

Could the mixture screw settings have anything to do with it? I replaced them too, but I don't remember the setting. 28-30 mpg can't be right.

[WMarshy]

I'm getting 28 mpg most of the time. I rode the bike all weekend with my daughter on the pillion and my best was 30 mpg. I was running it easy all weekend, 4th-5th gear. Most (not all) of my fuel stops were ethanol free.

The bike sat for two weeks. I pulled the cover, turned the key and mashed the button. It started immediately, no choke. It settles to idle after only a few seconds. It runs great, no dead spots, and does not have popping on decel. Idle jets are stock size, mains are 115 across. All jets are new genuine Mikuni. The float needles and seats are new. The float height/fuel level has been set, checked, rechecked, and rechecked again. IIRC, the fuel level is about 3 mm below the gasket.

I went to a rally over the weekend, and I was getting feedback that I'm running rich, as in somebody smelled fuel from my exhaust. This bike runs great, but only my big block El Camino gets poorer fuel economy.

Could the mixture screw settings have anything to do with it? I replaced them too, but I don't remember the setting. 28-30 mpg can't be right.

Do you have stock exhaust and air box/filter? I can run heavy handed around the back roads and average 30 all day. Get out on the freeway and it only improves even if I run up to 75... If your running rich then its going to be your mains not your idle as your into low to mid RPM range. If anything, I would adjust float height to lean it out, your jetting should be fine and if your rich I bet your rich all across the board. What do your plugs look like?

[crazy steve]

Make sure your vacuum advance is connected and working properly. The bike will run fine without it, but a hit on fuel economy is one of the symtoms...

[jetmechmarty]

Steve, I forgot about that.

Wade, The bike has a stock airbox with a K&N filter in it. It has a 4 into 1 exhaust. I expect to find black plugs, but I'll look. White plugs were what got my snowball started.

[DGXSER]

Marty, I have been playing around with a second set of carbs, so I am certain I pitched at least four sets of bowls of gas on the driveway, with that, at my first fuel stop over the weekend I was getting 28 MPG. From there I rode some very hilly, twisty backroads through Ky for about 70 miles. I bet I was either throttling up or gearing down for 80% of those miles. I found I was at 40 MPG. From there it was a mix of highway and backroads and straight cruising 2 lanes. Have not refuled to check it, but I tend to avg around 35-37 MPG.

I am running aftermarket 4-2 (Jardine I think, but who knows for sure) stock air filter, 120 mains, 42.5 pilots, floats right at 23 mm. No popping on decel, plugs look good in color.

I am guessing steve has it on the vacuum advance.

[fredintoon]

Make sure your vacuum advance is connected and working properly. The bike will run fine without it, but a hit on fuel economy is one of the symtoms...

Hi Steve,
like Marty, my XS11SG only gets 28mpg, and that's Imperial gallons!
I've always reckoned it's pulling the sidecar that was soaking up the gas but now it occurs to me that although the bike's vacuum advance has always been connected I've only assumed that it was actually working.
How do I check that?

[jetmechmarty]

OK. I pulled the vacuum line off of the #2 carburetor. I applied some suction and I hear clanking under the timing cover. I'm going to assume it works.

I pulled the plugs. #1 & #4 are very dark brown on the porcelin. #2 & #3 are brown and #3 looks the color I think I'd want it to be. It used to be jetted 110-120-120-110, and looking at the plugs makes me think that was right. Now 115-115-115-115. I don't think jetting is my problem.

Maybe my timing is too far retarded? I can't remember when I last checked the timing, if ever.

[crazy steve]

Fred, pulling that sidehack around is undoubtably hurting your fuel economy. But checking the advance is fairly easy....

Pull your ignition cover off, then turn the crank until the timing pointer is on the timing mark. Disconnect the vacuum line to the advance from the number two carb. If you have a XJ, the bike needs to be running and you'll need a timing light and have to check at certain rpm points. But if you have an XS, do this....

First, make sure the timing plate moves smoothly and fairly easily through the full range of the vacuum advance travel by physically moving it. If it doesn't, you'll need to clean/lube or replace the timing plate.

If it does, connect a vacuum pump/gauge to the hose. Start applying a vacuum; you shouldn't see any movement of the timing plate until you reach 50 mmHg (2 inHg), then you should see smooth movement as you apply more up to 150 mmHg (5.9 inHg), when you should see about 16 degrees of advance. This should be the maximum advance; more vacuum shouldn't increase it. This is for the '78-79 bikes; the 80-81 bikes should start at the same 50 mmHg, but should reach maximum advance at about 125 mmHg. There's a bit more 'slop' in the later spec, if you're close it should be OK.

No advance, your vacuum diaphram is bad or the hose is leaking. Not enough, probably the same issue. Either way, this will hurt fuel economy under 'cruise' conditions. The only fix is replacing the vacuum advance unit as they're sealed and not repairable as a rule. Keep in mind the early and late units look the same but aren't, so try to match any replacement or you may have pinging problems.

[crazy steve]

Maybe my timing is too far retarded? I can't remember when I last checked the timing, if ever.

Retarded timing can kill mileage too....

You should be at 5 degrees BTDC if you have the original ignition components in there. If you're running a higher-grade fuel (something better that regular), you can 'cheat' the timing a bit more advanced and that will help mileage. I'd try sneaking up on it, advancing the timing a degree or two at a time until you hear pinging under full throttle then bump it back. Note that this will make the motor run a bit hotter (burn the fuel better = more heat) but shouldn't hurt anything.

I'd still check the advance, just because you hear 'clanking' doesn't mean it's working right...

[bikerphil]

The float height/fuel level has been set, checked, rechecked, and rechecked again. IIRC, the fuel level is about 3 mm below the gasket.

I went to a rally over the weekend, and I was getting feedback that I'm running rich, as in somebody smelled fuel from my exhaust.

Your float height may be too high, only thing I can think of. Not sure about the spec. you mentioned above. What does that translate into using a float height measurement with the carbs upside down on the bench?

[Yard Dogg]

Maybe your timing chain is worn out. When was the last time it was changed?

[GARTHXS]

Too slow maybe these biks like to go fast I always get better mileage on the slab going 80 mph

[James England]

28mpg doesn't seem unreasonable to me... especially since you are talking a US gallon, presumably? They are smaller than the imperial ones we use here. That's for an old, heavy bike and a pillion. Add a few hills, a bit of speed, maybe a headwind....

[Yard Dogg]

I got almost 40 on my midnight. Our gas here isn't loaded with ethanol yet though. There are a lot of additives that vary from place to place that are not mentioned at the pump. I know people in Oklahoma, Texas really get bad mileage due to their fuel.

[CatatonicBug]

28mpg doesn't seem unreasonable to me... especially since you are talking a US gallon, presumably? They are smaller than the imperial ones we use here. That's for an old, heavy bike and a pillion. Add a few hills, a bit of speed, maybe a headwind....

28 MPG actually is a bit low. If I don't twist the throttle hard and stick to the speed limit, I generally get around 40mpg - 44 if I ride a whole tank at 55mph. If I get more agressive, it'll drop to 32 MPG sometimes, but not on a regular basis.