Next thing to improve MPG?

Hey all,

I have my '80 650 maxim running well, except I'm only getting about 32 city maybe 35 highway MPG. I was hoping to get closer to 38-40. I generally run between 3500 and 4000 RPM around town. I have clean carbs, synced and colortuned. New air filter, no leaks in carb boots.The plugs are all a nice tan. Fresh oil and gear oil. Brake pads don't seem to be rubbing.
So the question is, what else should I look at to improve the mileage? Oh yeah I've run regular mid grade and premium gas with no noticable difference.

 

what about tire pressure?

 

Odd that it's so low - my bike has an add-on backrest/trunk & windshield, is jetted larger with 4-into-1 exhaust, has a K&N air filter, and I'm a 270lb beastie, and I still get ~45mpg on a boring diet of regular fuel.

I don't think tire pressure could account for 10mpg, but it's certainly worth checking.
Are the wheel bearings all spinning freely? Possibly some drag there?

[edit] - I just noticed you're in Denver - elevation 5280 feet. I'm in London, Ontario - elevation 823 feet. Could this have an effect? But if it were jetting, I'd suspect it'd show up on the plugs, but you're saying they're nice & tan.....

 

Tire pressure is good, if anything it's a bit high. I'll have to check the wheel bearings, as far as the alitude I don't know how that effects me for sure, but I know it does.

 

Check your compression, i found i had a bad cyl and my MPG went down to 30

 

Compression is a good call - but again - I'd expect to see that reflected in the colour of the plugs, as you'd get oil blow-by burning.

 

At the bottom of this page:

Jetting help

there is a correction table for the effects of alitude and changes that may need to be made, the higher you are the thinner the air, the richer the bike runs then temperature has a effect as well. I don't know if it will have much of a effect oh, on your MPGs that is, preformance wise the bike should run better.

 

My XJ700N was only getting about 38mpg....my bike ran real well and my plugs looked good too.......then I changed out my defective petcock.

Now, I get in the low to mid 40'S

 

I'd say ... given that the Plug Read sounded good and the Bike is running real good.

Don't do anything that will cause the Apples to start falling off the cart!

Make sure you have sufficient Intake Air happening.
I'm a XXL Guy. I'm pretty sure I was bending the seat down onto the top of the Airbox.

I stacked some Flat Washers (3) ... between the seat locking mechanism and the seat frame ... raising the seat about a quarter-inch. Now, I know the Intake is getting an "Unobstructed Air Path" and can breathe the way its supposed too!

(Put 3 on the front stud and 2 on the rear and the Seat Locks nice and tight; too!)

 

Thanks for the link Kiwi.

Hvnbnd, I had a bad petcock too. I rebuilt it but the vacuum still doesn't work right so I have to run it on prime and make sure to switch it off. Don't know if that would use more fuel or not?

Rick, That sounds like a pretty easy fix, I"m about 200, probably 230 with gear and backpack.

Anybody else wanna throw out ideas?

 

If you are in Denver then it's running too rich unless someone already rejetted it for the high altitude but if you are getting low mileage then most likely they didn't. If I ride up to Flagstaff I have the same problem, the mileage goes way down.
If the jets are just slightly too large or you have leaking float valves or anything like that it makes the mileage go down while the bike may still run fine and everything seems to be all right. I assume you don't have yearly emissions tests there like we do or you would have to fix the problem in order to pass because the emissions, specifically CO, would be too high.
I suggest checking the mixture at all speeds and adjusting the carb jets accordingly. If it's only an altitude problem and no modifications have been made to the stock air box or exhaust system you probably just need to decrease the jets one size. If there have been any modifications to the bike it won't be that easy and you may have to run it on a dyno with an exhaust gas analyzer to see what you need to do.

 

AZSteve (or anyone else) would rejetting make that much of a difference? I ask out of ignorance. Could I get the same thing with a higher flow K&N filter as opposed to the stock one? I read a post here that said you might need to increase the jetsize 1 with a K&N filter. I'm kind of leery of pulling the carbs apart as all of my help is in another town.

 

I never thought of that but a high flow filter or pod filters might solve the altitude problem. Normally if you increase the airflow you have to also increase the jets to keep the mixture correct but in your case where you need to decrease the jet size for altitude then it may work backwards for you by giving you the correct amount of air with the same jets. I can't verify if that will work here since I'm only 1200 feet above sea level but it's worth a shot.

 

You night even try taking the Rubber Goose Neck out of the Airbox inlet and seeing if that gives you a few more Cubic Feet of Intake Air.

I know some guys drill holes through the Top of the Airbox looking for more air.

I don't recommend doing that.