Help me figure out my crappy fuel economy!

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