Low Gas Mileage

[jellyfish5714]

Since I bought my Voyager a few months ago, it has only been getting 20 miles per gallon. I've tried a few things to make it better, like readjusting the fuel mixture screws and changing spark plugs, but that didn't help at all. As far as I can tell there aren't any leaks in the system, so i'm not sure where in the carburetors the problem could be coming from. I haven't torn the carbs apart yet, but was wondering what sort of problems to look for that could be causing such low mileage.
Something else that might be linked to the issue is that the idle jumps from 600 rpm to around 3,000 when any throttle is applied. It goes back down if I release the clutch to bog down the engine low enough, but otherwise stays at 3,000.
Is there any chance these could be fixed with various carb adjustments or will I need to do a full carb rebuild?

[ekap1200]

Wow, that is about 25 miles below average.. First question I have is , what did the plugs look like ? And I do hope you used compressed air to blow out the spark plug wells before removing them. Do the carbs appear to have been off recently ? Perhaps someone had their little fingers messing with them and have things way off specs. My experience is to leave the fuel screws where the factory has set them.. Are the cables too tight ? Look very closely at all the boots , I have seen more boots folded over upon a inexperienced person installing the carbs. Have you drained the carb bowls , one at a time in a baby food jar and marked the quantity of fuel of each one ? Again someone may have had their fingers messing with the float setting. Is there a problem with the air filter ? Prolonged storage can invite rodents to make a nice home in the air box.. Your plugs should have shown you some tale of what the carbs or one carb is doing wrong. Carb sync is also very important, and don't scrimp on cheep sync gauges. Small adjustments make big changes..... And just knowing that you can get to the fuel screw indicates , someone has had them off and removed the anti-tamper cups. Reach out to a club member that may be in your area and ask for some help.
Gene Kap

[GrandpaDenny]

When I got Gertrude (2000 XII) she was only getting about 25 mpg. Took a combination of things to get the mileage up.

• New plugs

• Found a carb boot folded over

• Lower air box had been melted and had a fairly large hole patched with electrical tape. It's now patched with double-sided tape

• Drained the fuel bowls

• and throughout this I kept using Sea Foam

After draining the fuel bowls, I was running up rt. 32 / rt. 611 towards the Poconos. I don't remember quite where it happened, but all of a sudden she roared and almost ate a Toyota in front of me. HUGE increase in power and revvability. She now gets about 45 mpg average. It varies with speed and terrain. 80 mpg with cruise on, about 38-40 mpg. 55-60 mpg on cruise, right about 45. Romping through the hills and twisties in 3rd and 4th gear, usually around 49, with a high of 51. Can't 'splain that one, but it's been consistent. Even on this last trip, when I filled up in Asheville after running on the Interstate, it was 38 mpg. Next fillup was in Knoxville after spending a day Dancing The Dragon and other twisties, 49 mpg. Same as I'd experienced through my weekly adventures closer to home. I reckon it's due to wind resistance.

[SgtSlag]

My 1993 has always, since I bought it in 2012, averaged 43 MPG... Almost always: it has dipped a few times, down to 38, but I think those were bad gas fill-up's, as they disappeared after a day or two of riding (on multi-day tours). We ride two-up, 99% of the time, which means our XII carries a heavy load, which might explain why we average less than most riders on this forum.

I would be concerned with checking the choke, in addition to going through the carbs. Make certain the choke is working properly, being full off, not being stuck partially on, dumping excess fuel into the engine.

I am a huge fan of SeaFoam, as it keeps varnish deposits out of my fuel system, due to the fact that I use it on a regular basis. I also use it to stabilize the fuel for the long winter storage periods. It is not a miracle product, but it is effective, for certain applications. Marvel Mystery Oil, is equally as reputable, performing virtually the same functions.

I recommend using SeaFoam/Marvel Mystery Oil on a regular basis, regardless of climate. Add the appropriate amount to your fuel tank, once per month, during the riding season. It will absorb small amounts of water in the fuel system, dissolve varnish deposits, as well as stabilizing the fuel, keeping it from deteriorating, up to 12 (MMO) to 24 (SeaFoam) months. Some folks say modern gasoline can go bad in just a month. Don't know if that is true, but it might be. SeaFoam/MMO will not harm your bike. They are relatively inexpensive. My experiences with using SF, on three bikes, since 2006, has been entirely positive. Friends and family have also had very good experiences with both products. Cheap insurance, IMO. Cheers!

[tunnelz]

I'm a Newbie too, and I see that you have been given some good advice already. I would suggest that before you tear in to the carbs, back up and start with the simple things. ekap 1200 asked what your plugs told you and mentioned that your cables could be too tight. Sgt Slag said he suspects the choke. That is where I would start. The high idle problem may be cables. Service manual page 2-8 shows the Idle adjusting screw, and Appendix page 16-8 has cable lubrication. The choke system on these CV carbs is actually an "enricher" plunger ( someone correct me if I'm wrong ) like 1990's Harleys use. This starter plunger may not be fully closing resulting in a rich condition. Cables stuck or too tight? Also- clean or replace the air filter. Do one thing at a time, and let us know if it helped. Then we all learn. Respects, tunnelz

[jellyfish5714]

Wow, that is about 25 miles below average.. First question I have is , what did the plugs look like ? And I do hope you used compressed air to blow out the spark plug wells before removing them. Do the carbs appear to have been off recently ? Perhaps someone had their little fingers messing with them and have things way off specs. My experience is to leave the fuel screws where the factory has set them.. Are the cables too tight ? Look very closely at all the boots , I have seen more boots folded over upon a inexperienced person installing the carbs. Have you drained the carb bowls , one at a time in a baby food jar and marked the quantity of fuel of each one ? Again someone may have had their fingers messing with the float setting. Is there a problem with the air filter ? Prolonged storage can invite rodents to make a nice home in the air box.. Your plugs should have shown you some tale of what the carbs or one carb is doing wrong. Carb sync is also very important, and don't scrimp on cheep sync gauges. Small adjustments make big changes..... And just knowing that you can get to the fuel screw indicates , someone has had them off and removed the anti-tamper cups. Reach out to a club member that may be in your area and ask for some help.
Gene Kap

I replaced the plugs (I made sure to blow any debris or dust out before taking them out) and didn't notice any wear or excess carbon buildup on any one spark plug. They all looked in similar shape to each other. I also did a compression test on each cylinder, and each was showing about 125 psi (not sure if it's relevant, but no cylinder had lower compression than another). I had taken the carbs out to be able to get to the fuel mix screws (I checked for folded boots and made sure they were installed properly) and most of the screws holding the float chamber on looked rounded off. I'm guessing a previous owner messed with the float and messed it up. The air filter and air box looked to be in good condition and clean. The throttle and choke cables had some slight slack in them, so they shouldn't be causing any issues. I haven't drained the carb bowls, I'll give that a try and see if that helps.

[jellyfish5714]

My 1993 has always, since I bought it in 2012, averaged 43 MPG... Almost always: it has dipped a few times, down to 38, but I think those were bad gas fill-up's, as they disappeared after a day or two of riding (on multi-day tours). We ride two-up, 99% of the time, which means our XII carries a heavy load, which might explain why we average less than most riders on this forum.

I would be concerned with checking the choke, in addition to going through the carbs. Make certain the choke is working properly, being full off, not being stuck partially on, dumping excess fuel into the engine.

I am a huge fan of SeaFoam, as it keeps varnish deposits out of my fuel system, due to the fact that I use it on a regular basis. I also use it to stabilize the fuel for the long winter storage periods. It is not a miracle product, but it is effective, for certain applications. Marvel Mystery Oil, is equally as reputable, performing virtually the same functions.

I recommend using SeaFoam/Marvel Mystery Oil on a regular basis, regardless of climate. Add the appropriate amount to your fuel tank, once per month, during the riding season. It will absorb small amounts of water in the fuel system, dissolve varnish deposits, as well as stabilizing the fuel, keeping it from deteriorating, up to 12 (MMO) to 24 (SeaFoam) months. Some folks say modern gasoline can go bad in just a month. Don't know if that is true, but it might be. SeaFoam/MMO will not harm your bike. They are relatively inexpensive. My experiences with using SF, on three bikes, since 2006, has been entirely positive. Friends and family have also had very good experiences with both products. Cheap insurance, IMO. Cheers!

I will double check the choke operation. The cable has play and moves freely, but I didn't look into the actual mechanism on the carbs. I had just started putting seafoam in the gas to see if cleaning it out would improve mileage, but it hasn't been long enough to get noticeable results.

[jellyfish5714]

I'm a Newbie too, and I see that you have been given some good advice already. I would suggest that before you tear in to the carbs, back up and start with the simple things. ekap 1200 asked what your plugs told you and mentioned that your cables could be too tight. Sgt Slag said he suspects the choke. That is where I would start. The high idle problem may be cables. Service manual page 2-8 shows the Idle adjusting screw, and Appendix page 16-8 has cable lubrication. The choke system on these CV carbs is actually an "enricher" plunger ( someone correct me if I'm wrong ) like 1990's Harleys use. This starter plunger may not be fully closing resulting in a rich condition. Cables stuck or too tight? Also- clean or replace the air filter. Do one thing at a time, and let us know if it helped. Then we all learn. Respects, tunnelz

I didn't know the choke acted as an enricher, that would make more sense for bad fuel economy if it is slightly stuck open. I'll look into that, thanks!

[Scott-(Altoona, PA)]

Seafoam is to me 1oz per gallon of fuel. I use it at a 1.5 - 2 oz/gal mixture. I was only getting 35-38 mpg till I drained my bowls and doubled the Seafoam treatment for a full tank of real fuel (ethanol free). Rode 150 miles and mileage jumped to 46 mpg.
Seafoam now at 3-4 oz/per tank and maintaining 45-46 mpg.
Good Luck!!!!!