Fuel mileage woes, sprocket changes?

XJers:

Allright guys, I need some advice...

First two tanks of fuel through my "new" Seca 400 yielded exactly the same mileage, 49-50 mpg. I have to admit I was somewhat disappointed.

My commuter car (94 Escort) was getting close to 40 mpg, and I was hoping for a little more of a difference. One reason I went with the smaller bike was to get better mileage, but even those of you with the 750s get mileage in the upper 40s, right?

Magazine reviews of the 400 when it was new (1982) reported mileage ranging from 52 (when it was "flogged") to 68 mpg, so I was kind of hoping to at least hit the midpoint at 60 mpg. It (recently) occurred to me that those reviews were written when the speed limit was 55 mph, so maybe their mileage results were higher than I can expect at current interstate speeds.

My 40 mile commute is about 35 miles of high-speed interstate (60 - 75 mph), and 5 miles of country roads/city streets. On the way home about 3 miles of the interstate driving is typically stop and go, bumper to bumper.

My bike seems to run very well, so I don't think I have any glaring carb or tuning issues that would hurt the fuel mileage significantly. So I've been considering a gearing change to lower the RPM at highway speeds.

The engine is currently turning 6300 rpm at an indicated 70 mph. Do you think if I made the sprocket change(s) to lower that to about 5500 rpm I would create more problems than I solve? (stop and go hassles, fried clutch discs, etc.)

That would be about a 13% change in gear ratio. Does anyone have any experience with sprocket gearing changes to know if that's too severe of a drop?

One final question -- "back in the day" when I rode dirt bikes, I remember the magazine gurus advising to change both sprockets and the chain together, because an old chain or old sprocket would quickly wear out the other that was new. My bike only has 5000 miles on it, and since I'm already tapped pretty dry from buying the bike, gear, repairs, etc., I'd rather just buy one sprocket (front?) and give it a try. Any thoughts?

Thanks for your patience with this long posting, and for any advice you can offer.

Regards,
Herb

 

Does the bike have a fairing ?

Fairing + higher gears seem to do wonders...

Check out craig vetters site and the results of his MPG challenge.

V twin harley's with fairings and higher gears were getting 90MPG..


Aero's of a UJM seem to be it's biggest MPG challenge.

 

Knick:

No, my bike doesn't have a fairing. I kind of figured my flat chest trying to push through the air at 70 mph was one of the problems. I know sometimes the buffeting gets pretty severe.

I've looked a little bit at the Rifle fairing site. They make a couple of models that would fit my rectangular headlight. Of course, there goes another $200 or so.

So anyway, thanks for the response, and I'll be sure and check out the Vetter site.

Happy streets,
Herb

 

Herb, your efforts are worthy, keep at your goal. I understand that the bike is running well but unless you "just" got a tune up, you should still clean up the carbs, tune and sync as well. This should help establish a baseline for you to work from in your quest for optimization. I'm pretty much a "by the book" kind of fellow and I'm going to advise against a single sprocket change unless the other components have only a few miles on them (like 500 miles or so). Accelerated wear is what they mean when they say it, and how. Fish around on FleaBay for a used fairing. Given the recent floods out in LA, you might even find a used one cheap at the salvage yard or the want adds.

 

Thanks Robert.

I see a lot of wisdom in your response. You stated what I probably knew deep down, but needed someone with experience to confirm and reinforce.

Thanks again for taking the time to help a newbie!

Herb

 

Pain is best shared, so is joy come to think about it. Save up and get the correct parts as a group if your not certain of the age of the parts. Only a very practiced eye or machinist could tell you if a used part is good enough to re-use, seek out that kind of advice. Sorry I can't help you beyond that. JT makes very inexpensive sprockets and RP chains are a good buy (just did my FZR). The complete set up was only $130 delivered. Check with BikeBandit.com , they may be able to hook you up.