Help me figure out my crappy fuel economy!

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.

 

Have you checked the air pressure in your tires?

 

^^^^Another great piece of advice here in the tech forums. ^^^^

 

Fastnova,

Your plugs could tell a story here too. If your running rich for some reason it could be washing the oil from your rings and giving you those readings. If you do a wet test it might just give you part of the answer. I would post a picture of the plugs so we can see and let us know if you have access to a leak down tester. That will give you a percentage of leakage in each cylinder. The air will be coming out where its leaking too. Easy to see if its coming out of the exhaust, intake or crankcase..

 

MN-
You should have read the entire post. He has looked at the plugs already... Unless he is blind HE KNOWS what the plugs look like.

READ THE POST

 

We would like to see them for ourselves. A picture would be nice.

BillB I was addressing the person who started this post. Not you sir.

 

you have addressed me many times darling

 

his compression readings were 120, 120, 100, 90.

standard compression should be 156. low compression is 128.
basically he's running on 4 dead cylinders.

he's found the answer to his problem. now he just needs to figure out why and how to fix.

since he's getting under 25 mpg. it would be a safe bet all his plugs are black. pics would still be nice to see though. so that we may all learn his problem and maybe form an opinion to an answer.

 

Snowwy,
If the compression is that bad would the bike still run fast like his signature says?

 

Fast is relative, and subjective. Even a tired soggy XJ is still an XJ and if it's firing on all 4, it'll still get up and go.

However, it would suck gas like a pig... oh, wait...

Snowwy is right, this is basically a bank of 4 really tired cylinders, with or without some burnt valves. It could also be a nearly unworn motor with rusted/stuck rings from its (years long?) dormant period.

fastenova I don't mean to talk about you like you're not involved.

Here's my most sincere take on your situation at this point, with "what would Fitz do" advice.

You've already got a "Frankenbike" an '82 with a 1980 motor.

That 1980 motor is not a happy camper anymore. IF it needs a "serious" rebuild, then tracking down yet another replacement motor makes more sense.

You've put a lot of hard work into the bike (and probably learned more along the way then you expected.) What would I do?

First, a couple more compression tests; borrow (or buy, they're relatively cheap) a different compression gauge and run two sets of three or four tests with each gauge, "wet" and dry.

Then, I would yank the head and have a look-see. Run the pistons down and inspect the cylinder walls, check the degree of "lip" at the top, use a bore gauge and see what's going on. RINGS can be got; pistons start to get expensive unless you get lucky on eBay. You could have a relatively low-mileage motor with rings rusted stuck, or it may be thrashed. If it just needs a valve job, and that's quite possibly worth it. It was a transplant motor to begin with, you can't know without looking.

Then and only then I would decide whether to replace the mill or fix it. If it needs a rebore or pistons, then I'd be looking for another mill. For me personally, I'd go ahead and do a valve job, but then again I have a whole drawer full of valves that I paid like pennies on the dollar for off eBay.

The bottom line is this: You know you've got a soggy motor; figure out what's exactly wrong with it then decide whether it's worth repairing or replacing. There are good mills out there; my 650 blew 150+ across the board cold, and it's a POS.

 

Correct me if i'm wrong but if the rings are shot and its drinking gas like a pig won't the oil be full of gas.

 

Well you would think. I was trying to see if he was washing his rings out and making the compression numbers look weird. Thats why I suggested a leak down test.

 

Yes and no;

They're not that shot, he's still got 100-120 it's just an all-done motor (maybe.)

The gas it's sucking is getting burned just not very efficiently at all because it's not being compressed properly. It will build up a gasoline smell in the crankcase but it will take some time before it actually dilutes the oil. The oil has probably been getting really black in relatively short order though.

Test Test and Teardown. Then we'll know.

 

Have you checked to see if maybe your front brake is dragging?
Did you check the rear brake for delamination?
Choke definitely not stuck on?

Dragging brakes can significantly reduce MPG, however, the way you ride it probably makes the biggest impact on MPG.

If I ride like a bat out of hell, I get around 20-25mpg, but if I baby her and take it easy I can get around 40-45mpg. It could probably be better, but I colortuned a tad on the rich side, and am happy with her performance. Plug chops look good, and the power is there when I need it. Better to run a little rich than lean.


When you shift, do you rev her out to at least 7k before each shift, or are you shifting it like a Harley? So many people are scared of letting these girls wind out. Every time I ride with my buddy who has a Harley full dresser, he always comments on how I shift. He says "man your gonna blow that thing up!"... I always laugh, because these Maxims don't red-line till 10k+. Revving out to 7k is just getting into the powerband of these bikes. You don't really feel the power of these beauties until after at least 7k rpm. By simply changing your shifting pattern to shift at a higher RMP, these bikes go from being a kitten, to a cheetah.

By not shifting properly, and giving her more throttle than needed by being in too low of a gear, can suck down the fuel. As day7a1 said, if your staying in 5th or even 4th during an uphill climb, or even lazily not downshifting properly when someone turns in front of you, that could contribute to your MPG problem.

Good luck man, I hope you figure it out. Nothing worse than being "that guy" that the rest have to stop for so he can fuel up every 75 miles...

*Edit* Just noticed there are some new posts at the time I submitted. Looks like your compression test may have revealed the source of your problem.

 

just how fast is fast?

what may seem fast to some will seem like a pig to others. specially if they don't have a lot of years riding to know the difference between bikes.

my 81 felt pretty good. till i got upwards of 75+ then it was a dog to go any faster. and it most certainly lagged compared to the 82 i had brand spanking new 25 years ago.

you've got a 30 year old bike. that's a lot of years.
it doesn't have any miles on it. OR, it gets driven for a year or two. then sits for 5 years. or lets say that it is driven on a daily basis but sits in the winter time.

what happens when the bike sits for prolonged periods of times?????

oil drains off the cylinder walls. and dries out. you crank her over. OUCH OUCH OUCH. dried piston rings scratching dried cylinder walls.

put 100,000 miles on the engine OR 10,000 miles on the engine. either way it's 30 years old and worn out.

some of the people on this sight have very good condition engines while some of us others aren't so lucky.

don't ask me for the secret to keeping a motor in good shape after all these years. cuz i don't know. and most times one bike will switch owners at least 10 times in 30 years.

good strong 30 year old motors are out there. but a dime a dozen they are not.

 

Good points.

The secret for keeping a motor in good shape for years and years is to: Use it regularly (like for instance, my Norton) and keep the cylinders oiled while in layover storage.

Not all XJs are 10-owner bikes; I'm the third owner of my '81, the third owner of my 650, and the fourth (possibly) fifth owner of my '83.

But you are absolutely correct. There are LOTS of good strong motors still out there, but you do have to be careful; luck plays a role but you can attenuate the luck with careful inspection, compression testing, etc.

 

hi, my 81 650 special has just gone over 39500kms,my 60 year old mechanic told me it looks beautiful inside after he just did the valves and carbs,im the 3rd owner and the other 2 were mature riders who knew how to look after a bike,it goes like a bat out of hell but i dont feel the need to consistently rev it much over 5 or 6000 RPM,although once in awhile i will push it to 10000RPM,,he said if i dont thrash it it will last for a very long time,PS at this stage i am not inclined to learn how to be a mechanic as i have a good one at the right price who knows these bikes like the back of his hand and i can afford him ,for now anyway,i hope he lives to be 110 years old LOL

 

fastenova, did you ever get this sorted? I'm having vaguely similar issues, but with good compression and wondering what you may have learned.

 

I thought you were gonna get your valves in spec and set your float levels, re-sync the carbs and see what it does for your mileage.

Your compression test showed one cylinder "out of spec low" compared to the others but still pushing decent numbers, which generally indicates a valve needing to be adjusted.

You already know what you gotta do.

 

Oh yes, that is exactly my plan. I was just curious, and also bored at work...