Tires and Gas Mileage

[Sidehopper]

As long as I can trust that the GPS apps on my phone are accurate, it appears that moving to Shinko 777 HD tires has corrected my speedometer inaccuracy. I have been getting about 35-38 MPG and my trip odometer shows about 180 miles when I have roughly a gallon of fuel left. Math is not my strong suit but I imagine I would probably only "lose" a couple of miles per gallon since the speedometer/odometer accuracy isn't changing by a huge margin. This leads me to believe that I'm getting medium-poor mileage in general.

Are there any common issues with the fuel system that lower mileage, such as a vacuum leak on a specific hose? I haven't found anything specific on the forum, but the service records from the previous owner mentioned a vacuum leak being repaired at some point. I know I need to get my carbs synced (at least) as I'm hearing some backfiring when I nearly close the throttle, but not on all cylinders. It shows up with something like 5-10% throttle open when cruising down a 25MPH road, like an unknown number of cylinders are closed while the rest are open properly. The backfiring is the same kind you hear when you roll down a hill at speed with the throttle closed - my dad always calls it "popcorn" since it's very poppy and crackly.

I am going to swap my spark plugs at some point, but in general all 4 cylinders are working well with the throttle opened enough and all 4 headers have the same temperature after a ride (verified by infrared thermometer) so there doesn't seem to be a huge amount of de-synchronisation or a weak cylinder. Most of my riding is at about 60-70, closer to 60.

[cushman eagle]

That milage is 10+MPG less than what I usually get.
Is the vacuum line from carbs to the ignition module hooked up?
If not, your spark timing will be retarded at cruise throttle setting.
It is also possible someone richened your carb mixture,both of which will hurt fuel economy.
Let us know what you find

[ekap1200]

Hello from NJ, Lee I have read your post and agree with Loren that your not getting the best mileage that these engines will deliver. At 180 miles you should have used around 3.9 gallons. Both of us have been known to travel great distances, I have a 2000 with 115 K on it and still get consistent 48 to 50 mpg. As Loren mentioned , check out your vacuum lines for leaks and that the advance vacuum line is connected to the igniter. Carb sync must be checked with a good set of gauges, that have been verified to be very accurate. If you think little fingers have been in the carbs, just get an inspection mirror and look under the carbs to see if the anti-tamper plugs have been removed. Thats a good sign that someone had been into them. How is the air filter ? And all the duct work going to the carbs. And also have you been draining the bowls and seeing any debris flush out. Let us know what the plugs look like, and condition of the wires and caps. That ( popcorn ) sound on deceleration should be hardly noticeable . Again if someone has opened the pilot screws more than the correct amount she's going rich on you when your decelerating .
I would start with looking at the plugs , draining the carbs and checking the sync. However make dog-gone sure you Blow out the spark plug wells , with compressed air. One grain of sand is as bad as 10...
Gene K.

[cushman eagle]

Hi,Gene,My VXII only has 98,543 miles on it sooo yours has a lot more experience than mine
I checked my gas milage for the 2915 miles on the rally trip + 1000 after I got back and it came out to 51.6
Not bad for a full dresser for mostly 2 up over 4,000 miles.

[cushman eagle]

Hi,Gene,My VXII only has 98,543 miles on it sooo yours has a lot more experience than mine
I checked my gas milage for the 2915 miles on the rally trip + 1000 after I got back and it came out to 51.6
Not bad for a full dresser for mostly 2 up over 4,000 miles.

Update on milage,now it is 98,614,making a years total of 7,013
Still have a loooong way to go to match yours

[buzzcut]

I also get 51.6 mpg (Canadian) travelling 2 up 70-75 mp/h

[SgtSlag]

We've averaged 43 MPG since 2010, when we first bought our Voyager XII (a 1993 model). We tour, two up, around the USA and Canada; we ride with empty trunk and empty saddlebags, in diverse weather conditions, and diverse terrain, on day trips. We still average 43 MPG, no matter the conditions, no matter the load on the bike.

I just had a kit put into the carbs: fuel rail O-ring was leaking between the carbs (second time it has happened, went for a complete overhaul kit, this time, replacing all of the O-rings between the carbs, not just the leaking O-ring...), and one of the Floats was stuck. Haven't ridden enough yet, to see if the MPG changes (had dropped to 34), but I doubt it will. As long as we get 43, we are happy with the bike. Cheers!

[Nails]

We've averaged 43 MPG

So about the same as buzzcut. And me. (Mine needs some tune-up attention, recently down to about 40.)

I've said before that the gas mileage can go way up (mid-50's) on the kinds of roads that are actually fun to ride, like the 45MPH Blue Ridge Parkway or similar mountain roads. And I fall into the 30's when I'm in a hurry on the superslab (85mph or so).

Sometimes I ride to get somewhere; and sometimes to be there already.

[GrandpaDenny]

the gas mileage can go way up (mid-50's) on the kinds of roads that are actually fun to ride, like the 45MPH Blue Ridge Parkway or similar mountain roads. And I fall into the 30's when I'm in a hurry on the superslab (85mph or so)

Same here, plus city traffic - stop and go - really kills the mileage. Best mileage I've seen was after a day thrashing her on the Dragon - 51.4 mpg. Worst I've seen was about 25mpg following a C14 on the PA turnpike at around 90 mph. Must've been all downhill (it was) otherwise she wouldn't have gotten that fast.

Gotta pull them carbs this winter. I've got the kits. Don't have the money to pay Gene to do it (gee, thanks, 2021 Trip To Florida). I've done a few car carbs, like 30+ years ago. How do these compare besides being smaller?

[Nails]

I've done a few car carbs, like 30+ years ago. How do these compare besides being smaller?

Don't know if your experience includes CV carbs, which are simple, but a difficult kind of simple. They get the right fuel mix in a rather indirect way that's hard for me to wrap my head around. That is, the throttle only affects the flow through the carb (a butterfly valve). The carb sorts the fuel mix by itself (the vacuum piston). So, it's a touchy bitch to set-up right.

(The basics: The flow through the carb goes past a little hole to the top side of the vacuum piston gasket. This creates a vacuum that pulls the piston up against gravity and a weak spring. The fuel needle is hooked to the piston, and it's tapered. So the higher the piston goes up, the smaller the needle that partially blocks the jet. Thus more air flow leads to more gas flow -- after a slight lag. And obvious why vacuum syncing is so important.
Edit: When down, the vacuum piston also mostly blocks the throat of the carburetor. Opening the throttle increases the air flow/velocity, which raises the piston, which unblocks the carburetor throat, which slows the air velocity. This negative feedback maintains a ... wait for it ... constant velocity.
These are also called "constant depression" designs. The Bernoulli's vacuum relies on the air velocity. Keeping that fairly constant also stabilizes the "suck" that draws fuel through the jets. And if the carburetor doesn't keep the velocity constant, it's still constant depression, just the psychological kind rather than the barometric kind.)

I recommend taking them apart, going after it with a toothbrush and degreaser, and then finding them some quality time in a quality ultrasonic cleaner. There's some Black Art to getting the throttle valve (vacuum piston) to slide right, which is totally essential. (I know people who swear by polishing the pistons with aluminum foil.) And on the bottom side of the carbs, they're notoriously sensitive to setting the float level exactly right, exactly the same for all four carbs.

As always, replace rubber bits (o-rings), BUT the vacuum piston gasket is hard to find and silly expensive. And if it leaks, the carb won't work. I had plastic t-connectors rot totally through, but they were only in drain lines. (MaKa wants a lot for these little things, too. Just find something that fits.)

Putting the individual carbs back into the set can be a puzzle, with some interference-fit fuel connections. Getting the carb set back in the bike is difficult. Go at it from the left side of the bike. The carb boots also are silly expensive, so try not to cowboy on them too hard. The throttle cables are hard to hook up -- you'll want some large roach clips. I find the fuel line to be difficult to hook up -- has to go on an interference-fit t-connector that spins to the side.

Tons of vids for "keihin carb rebuild", especially among your favorite people, Harley riders.

Two things I do:

1) Nuke that cruise control switch, which I think does no good thing, safety or otherwise. I've already disabled this switch, and the next time I'm in there I'll remove it entirely. I think that'll help with hooking up the throttle cables.

2) While the carbs are out, bottom-out (gently!) the slow jet (pilot jet) screws (count the turns!) and then mark how the screwdriver slot lines up on the carb body. I mean, use a dremel to grind a little mark on both sides of the screw. (I used a sharpie, but now it's hard to see.) This is to avoid having to bottom the screw while the carbs are on the bike, which is a total PITA. (You can buy special screwdrivers for this, but they don't work on our bikes -- way too little clearance. I fabricated a tiny screwdriver device, described in a thread somewhere on here. It works, barely.) You want all four carbs to be the same number of turns out. (Because I live at a high altitude, I set mine further in than factory specs, and I turn them out a bit when I go to the flatlands.) I think these pups are adjustable for a reason; and I carry a dentist's mirror, my tiny screwdriver device, and a little flashlight.

Others may have other hints.

[Wormys_Dad]

Mine typically gets around 43-44 local ,with a lot of stop and go back and forth to work, and 46 and upward fully loaded two up on longer constant speed rides. If it started getting lower than 40, I definitely would be concerned.


Stay safe out there.

[Sidehopper]

I had "replace spark plugs" on my to-do list since I got some replies on this post, but finally got around to it last week after suddenly losing what sounded like two cylinders. The old plugs were wet when I pulled them. She ran fine for a bit after the new plugs (apparently PO replaced just one spark plug - the others were rounded off and pretty old), but started having issues again yesterday. I was running Seafoam in the gas and wanted to make sure I wasn't just having issues with throwing the octane off, but two gallons of fresh gas added to the tank didn't seem to make any difference. I took a longer ride this morning to see if it just needed to clear something or reach proper operating temp on the highway, but when I got back cylinder 1 and 3 were much cooler on the headers than 2 and 4 were.

From what I'm seeing on the diagrams, one coil runs cylinders 1 and 4, and the other does 2 and 3. Unless a wire got swapped around or something it seems like the issue wouldn't be one of the coils since it's affecting cylinders (normally) run by both coils. It also seems like it has more issues when the temps are cooler, but it was only about 68 F when I had issues the first time so I wouldn't think a small change like that would have a huge impact since thermal expansion isn't very different with only 10 degrees difference.

I haven't checked the alternator yet, but it has never had issues charging the battery and the tender doesn't charge it for long before turning green. It's getting colder so ride time is more limited. Maybe it's time to (finally!) pull apart my fairing so I have more room to work and test things.

[cranky]

... too late now, but if ever again.... I put in iridium plugs and could
detected a noticeable improvement!!!! FYI...

[Sidehopper]

... too late now, but if ever again.... I put in iridium plugs and could
detected a noticeable improvement!!!! FYI...

I definitely considered Iridium plugs, but couldn't justify the increased cost currently. My standard plugs only cost $11 for the set though, so I won't consider that wasted money if I do go to Iridium plugs next year! I used an in-line spark tester to make sure I was getting spark, but those might not be terribly accurate because I could be getting enough spark to light up the tester, but not enough to ignite everything well.

[GrandpaDenny]

I resisted putting in Iridium plugs, too, but after going through the hassle once of changing plugs, I decided the cost was worth probably never having to mess with them again. So I jumped on the Iridium bandwagon.

[SgtSlag]

My 1979 Honda had weak coils. Iridium plugs were far cheaper than two new, dual coils. The Iridium plugs were amazing! Put them in my Voyager, even though the coils were strong. I likely will never need to change them again. Cheers!

[Sidehopper]

Sorry I haven't been posting, life gets loud sometimes. I was still having issues with my cylinders acting poorly, so I pulled the fairing off for some troubleshooting, but didn't find anything obvious. It was good practice, and I only had to buy a couple of screws to get everything buttoned back up - not sure if I lost the screws or if previous owners lost them and they were never on my bike in the first place. I did some testing and research and found that it ran a lot better after pulling the vacuum line from the igniter.

I've seen some talk about people adding a T to the hose so they can add a screw for a controlled vacuum leak, but I just pulled the small in-line air filter and shaved a flat spot in the nipple, and that seems to have fixed the issue. It lets the vacuum leak enough that all 4 cylinders are running smoothly again. Maybe my carbs have some gunk in a vacuum passage? Either way, I'm riding again and my Mom was pleasantly surprised that I was able to get everything back together again

Now I need to give her a bath...

[GrandpaDenny]

That's interesting about adding a vacuum leak helping the bike run better. I had a '76 (IIRC) Datsun 260Z (IIRC) that I think had a bad MAP (or was it MAF? FIIK, it was 35 years ago) sensor. Blew black smoke like crazy. Pulled a cap off a vacuum nipple on the top of the engine, put a K&N peanut filter on it, and it ran fantastic. Wouldn't pass the visual part of the Arizona emissions test, but then again the inspector might not know what they were looking at. Had that happen with my '85 J-10 pickup - the inspectors were all goo-goo ga-ga over it that they forgot to put the hose in the tailpipe.