Help me figure out my crappy fuel economy!

Hey everyone...

Thanks to you all and Len, I've been able to get the bike running pretty well. However, my fuel economy is still crappy - I've been getting between 25 and 28 MPG. I'm calculating this based on my trip odometer and topping the tank off at each fill up. Not sure what's going on, and I could use some help!

I've done the following related services:

Replaced fuse box
Rebuild and clean carbs
Replace airbox boots
Replace intake manifolds
New plugs - NGK BP7ES
New plug caps
Checked coils/pickup coils for proper resistance
Valve adjustment
Checked Tire pressure
Fresh engine oil
Fresh final drive oil
Tune each carb using colortune plug to just barely on the yellow side of bunsen blue
Synchronize all four carbs using a glass baby bottle sync tool
Set idle to ~1100 RPM


Right now, the bike doesn't seem to resist me moving it at all when the transmission is in neutral, so that makes me think the brakes aren't rubbing. I have fresh pads and shoes installed anyway, and the caliper/drum were in proper working order. I am not getting any noise from the wheel bearings, but I'm not sure of the best way to check them, and would not be completely opposed to changing them out just for good measure as I believe them to be original. How should I go about checking them?

The bike seems to run great. The idle is smooth, it usually starts with just a little choke and warms up quickly, and seems to have plenty of power. But I can't figure out why I'm getting <30 MPG. I'm about 200 lbs, no windscreen, and ride fairly agressively but I would expect mid to high 30's riding it hard, and low 40's for easy freeway riding.

What am I missing?

Thanks!
Aaron

 

Also - I seem to remember reading it somewhere, but I can't find it again - When tuning the bike with the colortune, I usually take it for a 10 minute ride to get it nice and hot before I come back, park it in front of a large boxfan, and start using the colortune. That sounds correct, right? Then I sync it immediately after and adjust the idle as necessary...

Aaron

 

Plug chops are your best friend at this point.
you gotta see what the mix is like at load around 4k

 

Synch first, then colortune, and re-synch after each cylinder is colortuned. The adjustments you make while Colortuning will affect the synch.

Repeat this process twice and you're in Fully Tuned Heaven.

 

Sorry, but I'd have to agree about the plug chops... unless you have access to a dyno. Colortune at no load is only going to tell you about your pilot jet settings; it won't help you if your metering needles are too small or high, or if you main jets are too big. Plug chops under hard accelleration should give you a better idea if you're running too rich under higher loads.

 

The bike is completely stock, with stock jets in the carbs. Is it common to have to modify the jets as the bike ages? It's got 30K on the ticker.

For some reason I was under the impression that doing plug chops was more appropriate for bikes with modified intake/exhaust systems.

And are you all saying that having it not be in perfect tune would cause it to get 10MPG less than what is common? I had figured it would only be a few MPG...

 

in a finely-tuned swiss watch, one tiny gear will affect the entire watch significantly.

 

what plugs are you running? stock?

 

Yes, as I specified in my initial post, stock plugs - NGK BP7ES.

 

I missed that part

 

Are you the original owner? Is this low MPG thing a new development? If you're not the original owner, and the bike came with this problem, then there are still things that could have been done to the carbs. For instance, were there any washers under the needles? Could the main jets have stock numbers on them, but actually have been drilled out to a larger size?

And, yeah, if you riding it pretty hard, I could easily see an over-rich high load condition sucking down 10MPG.

 

I'm not the original owner, but the previous owner told me he was getting around 40 MPG before he stored it for two years.

These carbs, however, are another set I picked up from craigslist, so who knows... There weren't any washers under the needles, and the main jets LOOKED to be the same size as the set that was on the bike, but LOOKED is a pretty feeble comparison when you're talking minutia.

I'll see what I can figure out as far as plug chops, and if they look very rich, first think I'll do is replace all the main fuel jets with new ones. Does that sound reasonable?

 

you should compare the numbers stamped on them to make sure.

 

That only holds if they haven't been drilled out. Some people do drill them to save money and time. Ideally, if you drill them, you also file off the numbers, but you can't count on PO's doing the right thing.

What happened to the original carbs? If the bike was getting 40MPG with those, it sounds like a good cleaning for them might be a strong option as well.

Also, besides the jets, try to compare the needles to known stock needles.

 

Sorry, I wasn't clear. The numbers matched, but as to whether they had been slightly drilled out, I can't say. The did not look like it, but I suppose for the $12 for a new set of jets it's probably not worth questioning it.

 

Are you sure that your speedometer and odometer are accurate? Check them against another car or bike on your next trip to see if they ae close. You are right you should be getting in the upper 30s for mileage. Reading through the posts everything looks to be in place.

 

ride it like a cop is following you for a tank, well a half tank, and see what it does
the difference on mine is drastic

 

I tend to ride mine like I stole it... well, for the price I paid, I may have stolen it
:lol:

 

Could you elaborate on "drastic"...?

I certainly don't ride like a grandpa, but I'm not full throttle to 9K all the time either...

 

i don't really watch the gallons i watch the $ and sometimes it's a buck or buck and a half more
are the choke plungers all the way down, not even touching the lifting fingers

 

How about putting a throttle lock on that bike (cruise control) . Go out for a 200 mile trip and lets see what it really gets.

I set my bike at 60 mph on our southern MN cruise and I've been able to get over 50 mpg with my 1100. My son got 53 mpg with a 750 Maxim

MN

 

It's not about going from 40 MPG to 45 MPG... it's about the bike getting well under the accepted travel distance on a tank for a seemingly healthy motor. I say seemingly healthy motor because I did do a compression test, but not a leakdown test, and this is the first time I've adjusted valves on a bike like this (although I have done shim-type valve adjustments on a 4V DOHC auto engine before). I am confident in my mechanical abilities but know that I'm still just a shadetree mechanic and I can make mistakes more easily than a seasoned pro =]

I looked at the main jet needle on carb #1 today, comparing the old set vs the installed set, and the installed set has no numbers stamped on them. I am not sure whether they're aftermarket (I assume somebody made aftermarket needles at one point?) or whether they're just extremely worn. I'll try swapping all four needles over and see where that leaves me. It's totally possible that somebody pulled this rack of carbs from a different bike, and only installed the air/fuel jets and neglected the main jet needle or installed an incorrect aftermarket one, which would cause a rich condition at throttle.

Then I'll do plug chops, and if it's still rich at 4-5K, I'll install a new set of main fuel jets.

The bike does NOT smell rich at idle, unless the choke's on and it's warming up. However, in group rides others have told me it smells rich going down the road. This does indicate something related to the main jet / main needle jet / diaphragm to me.

 

There is always the PO factor you have to deal with. If the guy that was in there before you was changing things, You'll have to start from scratch. I always like the guys who use thumb drills to reem out the jets. Maybe thats what you got going on there.

MN

 

Haha, true. I thought I'd be making my life simpler by getting a second set of carbs to rebuild then swap into the bike... Lesson learned! But, I suppose that once I get this all sorted out I'll be in good shape.

Rule 1 of working on motorcycles... Never trust the PO!

 

Wanted to share some insight from Len, may be old news to some of you but it's good info nonetheless:

The correct needles for your '82 XJ650 are stamped Y-10 on the head of the needle, just under the plastic "cap" on the fattest part of the shaft.

Other common versions of these needles are Y-12 (Canadian model 650's), Y-13 (750 engines), Y-17 (some other 650's, most overseas), and Y-20 (700 models).


The plastic cap should be pushed "down" as far a possible onto the shaft....it is supposed to sit tightly against the largest diameter part of the shaft below the cap.

Caps that have moved "up" the shaft indicate that at one time the needle was seized to the emulsion tube, and as someone or something tried to pry the vac piston up into it's bore (to remove it), the needle held tight onto the emulsion tube and the vac piston---as it was being pried upwards----took the cap with it and left the needle behind.......


If you have some good digital calipers, you can measure your fuel jets, the "sizes" are millimeter markings, so a #110 fuel jet would be 1.10mm diameter, a #40 fuel jet would be 0.40mm diameter.

 

Well, all of the needles in the installed set of carbs were unlabeled, and I was unable to verify whether they were correct or not, so... I swapped them all with labeled Y-10 needles from my spare set of carbs. I took the bike for a ride and once warm, it was down on power quite a bit, felt lean to me, although no backfiring, and the exhaust was much hotter than usual.

This is probably due to not being tuned after installling these needles, which makes me think they're different than what was in there. I'll try and get her tuned and synced today and see where that leaves me.

 

If the needles that were in there came from a jet kit, then they may have had special emulsion tubes that went with them...

 

Well, I just ordered some new main fuel jets and those should be here in a few days. I'll see where I'm at when I install those, but I may swap out the needle jets from my spare set of carbs as well.

The fuel economy went down slightly =\ And, it doesn't seem to be running all that well. I tuned it via colortune and synced before and afterwards. As always, it fires right up and idles a little rough for a minute or two until warm, then seems fine. But power is down. Not sputtering, just down.

I'm close to the point that I want to take it to a shop and let them look at it, because I feel like I'm missing something big. But, then again, I know that I won't learn much if I do that, and I'd love to be able to figure this out myself (with everyone here's help, of course!)

Thanks SQLguy for pointing out that 'obvious' possibility that didn't even cross my mind! When I rebuilt these carbs, one of the emulsion tubes was slightly damaged and I ordered a good used one from Chacal. Didn't really think that they might be different! But I'll inspect them. Are they normally stamped or labeled in a way that would let me find out what they are?

Cheers!
Aaron

 

Oh, and I also read somewhere that sometimes people drill out the holes in the bottom of the piston as part of a jet kit to make it respond faster - anyone know what diameter those small holes are supposed to be so I can verify that as well?

 

The air bleed holes should are approximately 1.70mm and 1.75mm in diameter (two different sizes) in stock HSC32 vacuum pistons.

 

So, I received the new main jets and it did indeed look like the size of the new vs. the old were a hair different. I installed them, as well as put in some of the copper washers I had sitting around that were a little more flat, did a quick colortune, and took it for a ride, and it pulls great at full throttle now.

I still need to sync, colortune, sync, colortune, sync but right now it's hesitating a bit from a stop if I don't rev up and slip the clutch a bunch. Does that indicate a lean condition on my idle mixtures? Or rich? I haven't ridden enough to get an idea of fuel economy yet, but man, it feels great at WOT. It's like a different bike! And this week is supposed to be in the mid-80s so I should be able to get her dialed in.

Thanks Len for the fast and ACCURATE service!

 

Oh, also, the bike seems to do a little better when it's not completely up to temp. So doesn't hesitate when it's cool vs lots of hesitation when it's hot.

 

I would also recommend thinking about how you drive it. When I burn through an entire tank driving around the city in mostly stop and go kinds of traffic and accelerating pretty hard between stopping I get less than or around 30mpg. When my motorcycle is tuned well and I drive through an entire tank on the highway (but not at 65-70, at more like 55) I get as high as 62mpg.

 

if it runs better while it's cold then gets worse as the temp comes up, it's a little rich
if you still have the snorkel it the air box lid, try removing it for a bit more air

 

I rode to work and back yesterday... about 20MPG.

I must be missing something here...

 

I'm bringing this thread back from the dead... A winter has passed, and I've started riding again, and my mileage is still poor - 20-25 MPG.

Would incorrectly adjusted valves cause this bad of mileage? Those are the only thing that I haven't gone back to. I've been through these carbs more times than I can count, and feel like they're dialed in... But I haven't rechecked the clearances since I adjusted them the first time.

Thanks,
Aaron

 

What you notice, right away, when the Valves are a problem, ... is more of a "Performance" problem ... which contributes to an economy problem ... but, usually with some very distinct problems that are hard not to notice.

If the bike has become tough to start, is losing power or has started to backfire and smoke you might valve troubles that will be revealed in a Compression Test.

But, there are some sneaky ways to lose gas.

A perforated Petcock Vacuum Diaphragm will let the Engine draw gas from the Fuel Supply and burn it in which ever Hole's Manifold the Petcock Vacuum Line is attached too.

If the Carbs have sticking Diaphragm Pistons that's going to cost you some Fuel.
Look and see what the Plugs look like.
If they look like somebody colored them with a Black Magic Marker, ... you ain't going to want to hear it ... but, you might not have layed-it-up well enough too avoid having to pull the Carbs and Clean them.

If Crud clogged the Air Metering Ports on the Center Emulsion Tube, ...
Gas is getting sucked-up out of the Main Fuel Jet Supply without being atomized.
There's a lot of fuel that's not being burned efficiently reducing your MPG because you are sending unburned fuel out the tailpipes.

Check the Plugs.
Test the vacuum Line.
Compression Test.
Valve Clearances check / record.

 

So I should be able to just disconnect that line and cap the manifold nipple, set the petcock to prime (assuming everything is good with the floats/valves and they don't overflow!) and go down the road to rule that one in/out... right?

I will get on this soon. Recommendations on what type of feeler gauges to use? Last time I used a standard/imperial set and so the metric numbers were not the best. Do they make all-metric sets?

Thanks!
Aaron

 

Disconnect the Vacuum Line ...
Put it to your lips and take a drag.
Taste gas = Fail.

You can use the Standard Feeler Gauges.
Convert the Data to Metric using an online converter of the addendum in a Service Manual.

(.004 ~ .008) on the Intakes
(.006 ~ .008) on the Exhausts

 

lol, i like your vacuum test Rick

 

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.