Help me figure out my crappy fuel economy!

LOL rick.

gas leak
plugged air filter
valve adjustment
poor compression
oversized carb jets
choke not releasing
carb fuel level too high
weak spark
plugged mufflers
SPEED, SPEED, AND SPEED.

do a comparison between constant 75 and constant 55.
if you don't get at least 55 mpg at 55 mph. then you know somethings wrong. i can vouch for that mileage cuz that's what i got with i had the 82 maxim. and the speed limit was 55 at that time.
my 86 magna got pathetic mileage. at 75 mph. a windshield added 5 miles to the gallon.

 

Well, it's not the petcock. I'd say there was an 'essence' of gasoline but certainly no liquid...

 

Geez, haven't rode for a while, but rode a couple times lately and still poor mileage. Valves are all adjusted. Thoughts?

 

Dude...look at your signature....a poorly running bike that is fast??? Sounds like a recipe for poor mileage.

Until you say something like "I drove 100 miles on the interstate at 65 mph and got x mpg" all that we hear is "I don't like my mileage, but I would rather make complicated adjustments that may not be necessary than slow down and gather valid data about my motorcycle."

20-25 mpg might be GREAT gas mileage...we can't say until you standardize your test. Fill up, go to a highway, drive 55-65 mph for 50 miles, fill up again, post your result.

"Driving to work" doesn't give us the required info. Do you full throttle it uphill both ways and stay in 5th gear the entire time, or is it flat with few stoplights/signs and mostly on a deserted highway with a 45 mph speed limit that you strictly adhere to?

Then we can START.

I don't mean to be too harsh, but sometimes the truth is hard to hear. The truth is that we first have to make sure there is a problem before we try to find solutions.

 

It isn't running 'poorly' now, save for the bad fuel economy. It starts right up, has plenty of power, and doesn't stumble, stall, or race.

That is just in reference to it being a pretty quick bike for an old 650cc carb'd bike. Then again, I haven't ridden that many bikes. And I don't mean MY bike, I mean these Maxims.

The normal range for these bikes, from what I've read, should be 30-50 MPG depending on driving conditions, driving habits, condition of the bike, speed, idle time, etc. Getting 20-25 MPG on the highway doing 60 MPH on my way to work is NOT a sign that I am just heavy on the throttle or don't know how to drive. Getting the same mileage as my 4-door sedan just seems a little ridiculous to me, when it should be capable of almost double that.

 

Two questions:

-how did you check/adjust the float levels?

-how many miles since that initial valve adjustment?

OK, two and a half more:

-YICS motor or no?

-what sync method did you use/what type of manometer and was the YICS blocked for the sync? (if applicable)

Poor mileage for me on my 550 is when it drops down to around 43-45 from its normal 52-56 because of city traffic jams. I think you're justified in doing some investigating.

 

Fitz,

I have used both the dry set (measuring the float height with tiny ruler) and the wet set (clear tube to the drain outlet) methods to set the float heights. Both were a pain! But I feel like I was pretty meticulous about setting it.

Not a lot of miles since I just adjusted the valves. Are you thinking there's some 'break in' time that would ensure the pucks are fully seated or something?

It is a non-YICS motor, I used the two glass baby bottle sync and was able to get pretty consistent vacuum across all four carbs. I also colortuned before the valve adjustment with what I'd call moderate success (as I don't know what I should be looking for since I'm new at this!) but have since resynced.

 

OK, so the floats were dialed in "wet" the valves don't need any "seating time" you're fine there (5000 mile interval) from the way you describe its manners your sync is probably close (especially since you don't have YICS even in the picture) so all my theories so far go right in the circular file.

Crap. But I don't give up easily.

OK, so more questions: Stock airbox?

Condition and age of air filter?

Stock exhaust?

 

my 81 xj650 got 35 mpg. turns out poor compression with engine blowby. my compression was 140.

there are lots of things to cause poor mileage. but getting 20-25 is extremely excessively poor. you said you did a compression check but you didn't post the results.

the manual says standard compressions readings should be 156. minimum would be 128. the lower your compression the lower your gas mileage.

most people have been posting 120 compression. that's pretty low by specification standards. those bikes are also 30 years old.

they got 60 mpg back when they were new. you 40 mpg below standard. that's pretty extreme to be just a carb problem or valve adjustment.

you've either got a gas leak or a very poor condition motor.

 

The next time you go out for a spin put your bike up on the center stand get the front wheel up off the ground and give the front wheel spin compare that to the spin when its stone cold.

 

I was having the same problem. The solution had nothing to do with the engine. My petcock was leaking, but only when it was revved up. Couldn't see it stopped or at idle, but as soon as I revved it, gas poured out.

If you have no performance problems, it is unlikely that valves, carb, cylinder (except low compression), etc. is having a problem. Like rick said, if your valves were messed up, you would know it. And even a poorly tuned engine should do better than that. My mileage didn't change after I cleaned my carbs, but it did run much better!

You might try a 0-60 test to ensure you don't have a performance issue.

Also, try cruising around at a slower speed and see if the mileage gets better or worse. Leaks are time dependent, not mile dependent. Your mileage should get better at 50 mph due to less wind resistance, but if it gets worse its because an hours worth of gas spilled out in 50 miles, rather than 65 miles.

And of course check your oil for fuel dilution. And after a ride check all around for liquid, even if you have to clean all the old oil off first.

That's all I got for now. Good luck and good looking out using your mileage as a guide. Not everyone does that.

 

The test for fuel in the crankcase is simple:
On centerstand, fill until the window shows a little bubble.
3 OZ of gas in the crankcase will make the bubble go away.

When I first cleaned my 750's carbs, I accidentally folded over a diaphragm. It idled perfect, but went down the road on 3 CYL and got 32 MPG. Now gets 46.

You need to start looking at non-obvious stuff (cam phased correct? any carb tampering, correct main needles)

You might want to pull your mufflers and do an accurate mileage run.
As mentioned earlier, no mileage guesstimates. Tank up, ride 50 miles, tank up to the exact point in the tank. Calculate.

Change one variable at a time, and re-test.
Post Pictures of your plugs, and your compression #'s

 

My new thoughts are:

-we're assuming a carb/fuel consumption issue; how accurate is your ODOMETER?

-I've read through this and your other thread-- what are your compression numbers?

-what do the plugs look like?

Looks like TIME beat me to it...

 

OK, so just to be clear, my stated mileage wasn't a guess. I've filled up and rode around and calculated it several times. I'm good enough at math that I can just do it in my head when I fill up. The mileage my odometer reports is consistent with mile markers on the highway and distances that I know.

Having said that, I took her for a spin today (even though it's a little chilly) just to be sure I wasn't full of crap! I did 60 miles, 30 on the interstate at 65mph and 30 on a state highway between 45-55, with a few stoplights and a couple of towns with 35mph zones. I filled up before I got on the interstate, and filled up right off the freeway when I got back. 26MPG. I took it very easy and just drove in the slow lane at 65mph the entire time.

This number echos my 20-25mpg range that I get mixed around town and highway.

So, I redid my compression check. And, I found my answer. When I did it before, it was around 135. A little low, but tolerable. Now, it's 120, 120, 90, and 100. Pretty awful. This is after the bike was warm from my ride, then sat for about 15 minutes before I did the test.

So, it looks like I need to replace the motor or rebuild it - I'm guessing I just need to check the cylinder bores and replace the rings, possibly with a set of oversize pistons? I suppose if anyone knows of a either rebuilt or low miles motor, let me know! Also, are parts available (gaskets, oversize pistons, rings) to rebuild my motor?

Thanks for everyone's help in figuring this out. I am very glad that it wasn't something stupid I did (besides not checking the comp.) with the carbs or ignition system.

Cheers
Aaron

 

Thats to bad but before you start tearing her apart why not take it down to a qualified mechanic

 

Or at least do further diagnostics.

The first thing I would do would be to re-check the valve clearances, just in case.

Then re-run your compression tests; do a "wet" compression test too, to help discern between valve or ring problems.

Finding enough "hard parts" (oversized pistons, rings and the like) to actually rebuild one of these motors can be a challenge plus quite expensive; if it comes to that I'd look for an '82 motor to put in your '82 frame. You should be able to find a decent motor with good compression numbers for under $200 (or even a parts bike) whereas the parts alone for a complete rebuild could end up being three times that.

 

When you checked compression did you have the throttle pulled all the way open? Also did you do a wet test. (Squirt of oil in cylinders) to see if the compression comes up?

 

Throttle was open. I didn't do a wet test, will try to get to that. If it jumps up that means the rings are suspect or there is too much clearance between pistons and cylinders?

 

Yeh, higher 'wet' numbers indicate wear to the bores and or rings worn or stuck.

 

worn rings are also evident of engine of blow by from the crankcase breather which leaves an oil residue inside the air filter box if all the equipment is still there.

wet test always a good idea for low compression problems.