Gas Mileage Problem?

Bought my first motorcycle last week (1981 xj760 seca) to ride to work figuring would save some money on gas since I have to go 35 miles one way to work everyday. But when I ran through my first tank of gas and checked the mileage I got 24 mpg!!! I am thinking this cant be right?? It ideals fine, and goes down the road good (60-70mph). Carbs were cleaned, air filter looks good, ran some seafoam though this last tank (24.5 mpg). Any Ideas why it is getting such bad gas mileage? My 93 sonoma gets 25 mpg so I am going to drive that till i get this figured out.

 

A little info to be found here:

http://xjbikes.com/Forums/viewtopic/t=7 ... leage.html

 

Another Minnesota XJ'er!

So I'm not the only one!

 

You may also want to examine your tire inflation, wheel bearings and brake calipers. If your tire isn't properly inflated, your rolling resistance will increase rapidly. Wheel bearings will kill gas mileage if they are toast. And a sticking brake caliper is a really bad thing. As a second guess, you may want to change out your air filter even if it looks "ok". Clogged air filter will suck fuel too. Check your plugs, if they are rich, I'd swap out that filter.

 

Thanks for the info. I know tire inflation is good, but will check the bearings and brakes out. It must be running rich though cause the plugs have a black soot on them. I also noticed that the rubber boots that go to the carb are cracked. Would that cause poor mileage?

 

Give her some new plugs and an oil change too. Also make sure your battery is nice and healthy. Does the idle RPM's climb drastically when the bike is warmed up? If so, then yes, the boots may be cracked badly enough to cause an air leak. However, this would not hurt gas mileage, it would slightly increase it (the bike would be running too lean).

Chad
1981 XJ750R

 

The spark plugs and battery are new this spring and oil/filter was changed. I did lift it up last night and the front wheel didn't spin freely. Sounded like the brakes were rubbing. It is running rich because I cleaned the spark plugs and then drove to work and back and they were black again. So I don't know if putting a new air filter in and adjusting the brakes will get the mileage back up? Is there any adjustments that can be done for it running to rich?

What kind of mileage does anyone else get with the xj750??

 

Sounds like you and Hess need to get together and hold a carb clinic. The colortune plug will help reset the rich mixture but it only works at idle. I would change the air filter and check the valve clearances.

 

Bryan, my 82 750 seems to average right around 36mpg.

 

I'd change that air filter then ride for a short bit and check my plugs again, if still rich (dark soot) then your carbs need to be leaned out.
To find those adjustments look at the link below
http://www.xjbikes.com/Forums/viewtopic/t=1586.html
Hope that helps.
Be careful when drilling out the screw caps

Mike

 

I would replace the air filter, do a brake flush, and clean that caliper out. I would also suggest sealing up any cracks in your carb boots. and then i would get your carbs synced and tuned. If it's STILL getting terrible gas mileage.... well then it beats me.

my 81 xj 750 seca gets between 38-44mpg
check your math perhaps??




just for the record:
running lean, does not increase your gas mileage because there is less gas in the mixture. if you spent a lot of time idling this may be true. But under normal load it is the opposite. less than optimal fuel/air mixture means you have to use more throttle to get the same amount of power from the bike, so you infact get a lower MPG.

granted lean is better then rich as far as the MPG thing goes.

 

Ok I think I have some time to work on the bike. Just wondering where anyone usually gets their parts, online or dealer? Right now I am looking for an air filter and then fork seals. Any particular brand that I should go with?

 

I've used BikeBandit.com with good results. You could also check out OldBikeBarn as well. Their link is listed under our stealership/shop forum if I remember rightly (and I hope I did). Google them if you need too. Parts-n-More is another place for the old bikes. They just have lousy customer service. All the online places have good stuff, just a bit cheaper than the stealership and you usually get the parts faster anyway.

 

The adventurous one may of course make his own air filter

http://fj600.blogspot.com/2006/05/air-filter-deed.html
http://fj600.blogspot.com/2006/05/air-filter-update.html

I've been using my high throughput air filter since then and it seems fine so far.

Best wishes,
Relu

 

I've built three air filters using the methods described by the fellow in the link you provided Ohmega. A few points of difference: I purchased a sheet of UNI foam for my applications. I use RTV to bond the pieces of my foam together. Any color will work but I use clear. I cleaned just as he did but I use the UNI or K&N foam filter oil so I don't have the problems he described. I don't glue the entire sheet into place, I purchase long springs and use them to hold the element in place. I need to post some pictures for you all.

 

That guy is me Robert ... that's my journal of repairs. I'd buy the Uni oil, but I just couldn't find it here in Toronto. From what I understand the K&N oil (spray) is for cloth, not for foam; but if you say it works, I think I'll get a can and use it.

I too wanted to buy the Uni foam. But then I asked myself what properties such foam should have. And found this "dense" type of foam at work, so I thought I may as well give it a try.

 

I'm not aware of the different characteristics of the foam in question but I will say this. Some foam cannot endure exposure to hydrocarbons and oils so I made the $14 USD investment in materials I knew would stand up to the rigors of a motorcycle. I've built three filters with the one sheet and have enough left over for at least two more (got one to build right now too). You might have a perfectly useful chunk of foam there, I'd just watch it rather close for a few months for breakdown of the media. Best of luck on that one!

 

http://cgi.ebay.com/Yamaha-XT550-XJ700- ... dZViewItem
Here's the fork seals

http://cgi.ebay.com/ebaymotors/Yamaha-H ... enameZWD1V
Heres the filter

Prices seem in line

Good luck
Mike

 

I just got some carb rebuild kits to see if that will help me w/ my gas mileage problems. Got a couple questions though. The new kits that i got don't have the screen on the end of float seat and the end is smaller then the ones that I had so the screen will not fit on there. Do i need the screens? Also was wondering about the Pilot Screw Blind Plug. I can not see where that would go, do i need it?

While reading through some of the forms too guys were talking about the washer for the main jet. Mine has it and is running rich, if I take that out will it lean out or am I reading them wrong?

 

Bryan:

I recently cleaned the carbs on my baby Seca.

After cleaning, I originally set the pilot screws about 4 turns out. The bike made awesome power, but my fuel mileage went down about 25% (from 50mpg to 38 mpg)!

I then adjusted the pilot screws to about 2 1/2 turns out, and the gas mileage returned to 50 mpg. Not as much fun riding, but definitely cheaper on the pocketbook.

Given your rich condition (as indicated by the sooty plugs), you may want to check your pilot screws and see if they are backed out too much.

Just one more possibility...

Herb

 

My '81 750 Seca has been getting 38-41 for the past year (except for the past three weeks that it's been parked waiting on a fuse block replacement).

 

Checked the pilot screws and they were out 3-4 turns on all of them, so i put them back to 2 1/2 turns out. Drove and still no better on the gas mileage. I the only thing i havent done is do the floats because that screw is pretty much welded in there(broke two screwdrivers already). Can the floats being off cause the abundance of gas usage?

 

Yes. Although I would think it prone to flooding out at idle.

 

Thats what I am not getting is that it seams to be running ok. It will sit there and idle, goes down the road, but uses the gas. My drive to work is 40 miles one way and by the time i get home the tank is almost empty. Roughly 25mpg is all that I am getting.

 

Any clue as to what jet sizes you have in the carbs? stock I believe the pilot jet is a 40 and the main is a 120. If your main is larger than that I would say that would definately contribute to the poor fuel economy. Also what plugs are you running? Should be the NGK BP7es plugs gapped at .32 I believe. Please correct me if I am wrong guys. My 82 gets around 38 right now,but I have to rejet the pilots, as it doesn't Idle at all at 2.5 turns out. Previous owner put on a Mac 4 into 1 header and a K&N air filter on her. I have been trying to sort all that out since I got it. Living at 6000 feet really puts a new twist in it. Should get better after that. I will hopefully get that done in the next few days. I will post up and let you guys know.

 

Finally got the pilot jets, size 41 and installed them. sinked the cabs again and it ran worse. Plugs started fouling. nice black soot. Reinstalled the originals and all is back to normal. I believe I wil get some larger air jets for the top end of the carb and see what happens. I will post up and tell you what happens. Anyone need any 41 pilot jets? LOL

 

I guess I don't have a reason to complain. My XJ700 has sat for 4-5 years and got about 38mpg on the first tank I ran through (which included about 3/4 gallon of gas from when it ran last!) That was also with the old plugs and filter 8O , and haven't even touched the carbs (yet). I just put a new air filter and plugs in day before yesterday and noticed a difference in performance. I will need to get gas on my way home tonight, but think it will be at least somewhat improved. I'm thinking that maybe I shouldn't have read any of this string as I am feeling less motivated to dismantle, clean, and synch the carbs now 8)! I imagine I'll do it anyway sometime soon :cry: