Chasing poor fuel economy, again. Looking for ideas

Let me start off with a quick description and a list of all the things I've done:

My XJ650 seems to get ~25MPG under all circumstances. Riding it hard, taking it easy. Puttering around in the city, or all freeway miles at ~55-60 MPH. Riding by myself, riding with my wife on the back. Hot weather (85*+) or cooler weather (55-60*). Bike runs "good" otherwise. Have a slightly high idle when hot but otherwise starts and runs predictably, and I just haven't made the time to fix the high idle.

Recent work done:
Top end rebuild, .5mm oversize pistons and rings. Complete head refurb (valve grind/lap/seal replacement)
Valves adjusted and rechecked after 500 and 1000 miles (after engine rebuild)
New pilot and main fuel jets (#41 / #112) - one size up, recommended by Len
Carbs cleaned. As in, you could drink wine from them, because they went to church
Checked vacuum diaphragms in carbs
New spark plugs
Float bowl fuel levels set to 3mm +/- 1mm
Colortuned and synced
Checked front and rear brakes to make sure they're not dragging; both wheels spin freely
Checked tire pressure and adjusted as needed
Checked air filter (stock, not pods)
Replaced inline fuel filter
Verified accuracy of speedometer/odometer using both GPS and known distances

I have been told everything from "it's an old bike, that's not too bad considering the age" to "your larger jets are what's causing the lower fuel economy". I don't buy either of those. Have excellent compression on all four cylinders and the bike runs strong. I'm looking for anything I might have missed here...

My next thought is to replace the needles on the CV slides, but I'm waiting to hear what Len thinks about this. I hate throwing parts at a problem without knowing it's the cause but I'm not sure there's a good way to 'measure' the needles.

Thanks for any new ideas!

 

Fit some k&n pod filters, sorted

 

one size up jets use more fuel, why did you go that route?
this will consume more fuel
needles what does the hole in the asperator tube look like

what do the spark plugs look like?

are you sure your enrichment plungers are closed and sealing?

compresion test./what are the numbers?

you may need to do another sync and color tune if you did it when first starting bike after rebuild

how long do you warm the bike up before riding?

fix the high idle. how did you sync and color tune it with the end result being a high idle?
it may just be a combination of small things

 

Agree it may be more than one small issue. High idle being the obvious one of them - it's still fuelling on overrun.

 

Sounds like my XJ700N when I got it, but I was getting a little over 30 mpg. Doing all you did to the carbs and valves got me up to about 35 mpg. After using a colortune to reset the pilot screws I now get up to 42 mpg on the road—65mph and under, and about 39-40 in town. My method is to use the colortune and close the pilot screws until I see a white flame (lean), then open them until the white just disappears. Then install new plugs and take a 20-25 mile ride a pull the plugs to “read” them. I am looking for a light brown or gray color on the insulator. If the insulator is still all white, I open the pilot screw about the width of a dime and take another ride. I repeat this process over maybe a week or so until all the plugs have a nice light brown color. On my bike the pilot screws are set between 1 3/4 and slightly over 2. The colortune plug is only used to initially set pilot screws at idle—after that it’s “ ride and read.” The new plugs make much easier to read the plugs. If your bike is all stock I don’t know why you would need anything bigger than the stock jet.

 

Thanks for the responses. The reason for going one size up on the pilot and main fuel jets were based on Len's suggestion, I had at least one pilot jet that a PO had visibly enlarged (don't know how/why, just the PO factor) so I ordered a new set of jets so I had a known starting point. He said they are jetted lean from the factory for emissions reasons, and recommended going up one size.

Keep in mind that we are talking .01mm and .02mm larger jets here - I don't think that's the sole reason I'm getting nearly half the fuel economy the bike is rated at. I could understand 1-3 MPG maybe...

The high idle is because when I swapped jets I did a real quick colortune and sync and started riding it again but haven't done it again as it required... but every motorcycle enthusiast or mechanic I've talked to (I'm lucky in that I have a lot of gearhead friends, both shadetree and professional) has mentioned they can't see these settings having such a large effect on fuel economy. I've specifically done an all-highway trip and been light on the throttle and kept it under 60MPH to see if I could eke out even 30MPG, with no luck. So while I agree I need to dial in the sync to fix the high idle, and maybe slightly tweak the idle screws, I was looking for something "bigger' that would cause such a low MPG that I hadn't thought of, not necessarily carb related.

I checked compression and the numbers were all between 148 and 156 after the rebuild and about 500 miles. So I know the rings are seated and things are good there, though I haven't done a leakdown test (I don't think this is necessary?)

Enrichment plungers are definitely closing properly, if I open the choke just a little the plungers lift and the bike responds immediately (runs worse when adding enrichmen when hot).

tabaka, I would be stoked if I got 35 MPG! That would mean about 100 miles on a tank instead of ~65 before hitting reserve which would be a gigantic improvement.

I will RESYNC and RECOLORTUNE, then check my plug color, and report back. I just have limited time to work on it and ride these days, between my job and most days now being too hot to ride, so I was looking for other ideas that maybe don't involve riding or running the bike in my garage.

 

Take a close look at your emulsion tubes and the slide needles. It wouldn't be the first time that emulsion tubes have 'egged' holes, allowing a lot of excess gas go through

 

Thanks Dave, I assume you mean the large hole that the slide needle moves through up top (in the carb throat) vs. the tiny holes in the tube itself, right? I will inspect that when I pull the carbs off again later this week.

 

A lot of folks think the pilot system is only in use at idle and that the main jet takes over as soon as you add throttle. While it is true that the main jet becomes the primary factor as the throttle is opened, the pilot circuit is always supplying fuel, so if it is supplying too much fuel at idle because the pilot screws are open too much and/or the jet is too large it will effect the amount of fuel being supplied throughout the throttle range. In fact, I have read that up to about 1/4 throttle the pilot system is the primary fuel source. So, I still think that is a large part of your problem. In any case I think it is worth your effort to try the stock jets.

 

Yes

 

are the numbers on the TCI correct for that model?

 

Crossed a similar bridge a couple of years ago with my '80' XJ650 maxim. Bike ran very good, but the best mileage I could get was 30 - 35 mpg, no matter how hard or easy I worked the throttle. I also noticed that all 4 spark plug insulators were blotchy, ie. had black spots and the plugs would soot up some. My carbs were church clean by this point in my bike revitalization process. So, I pulled the carbs and replaced all the needle jets and jet needles with new parts. (stock size Y-10 needle). Installed the carbs, did a quick sync check, filled the gas tank, set the petcock to run and took a long familiar ride. When my trip meter reached 85 miles, it kept running. Then 100 miles. Then 110 miles. At 117 miles, she stumbled, indicating time to hit reserve. Flipped the fuel valve to reserve, and went straight to the nearest gas pump and filled up. I was only able to squeeze in 2.4 gallons, or 49.5 mpg. Bout crapped in my pants! Before, 80 - 85 miles was it before reserve. I noticed absolutely no difference in how the bike was running either. This may not be your problem, but it sure was mine. I could not visually see any wear on the old parts either, and the old needles were clearly marked Y-10. I've ridden the bike 10K+ miles since this repair, and am still enjoying 47 - 51 mpg.
Good luck.

 

By the way, what color are your plugs?

 

Yes, I checked this earlier this year before I decided to rebuild the top end.

This is great info, and your pre-needle-swap description seems to mirror my situation. I feel like the bike is running strong and well (aside from the high idle, that I know I need to fix) but the MPG situation just isn't making any sense... I checked last night and I do have a spare set of needles that appear to be in good shape from my extra rack of carbs, I will be swapping those and seeing if it makes any difference.

They are tan to light grey, I do feel like they could use a little bit more fuel but I am not doing chops, just checking them after an extended ride and I pull into my garage (I have about a 1/2 mile low-speed route into my cul-de-sec and garage and usually it only idles for a few seconds after I pull into the garage but will try to go do some proper plug chops soon...) I'll try to get some pictures of the plugs after I swap the needles and get a ride in, but it might be a week or two before I get the chance.

 

By the way, what color are your plugs?

They are tan to light grey, I do feel like they could use a little bit more fuel but I am not doing chops, just checking them after an extended ride and I pull into my garage (I have about a 1/2 mile low-speed route into my cul-de-sec and garage and usually it only idles for a few seconds after I pull into the garage but will try to go do some proper plug chops soon...) I'll try to get some pictures of the plugs after I swap the needles and get a ride in, but it might be a week or two before I get the chance.[/QUOTE]

That seems to rule out running very rich, which was my problem. But I still do not like the idea of larger jets on a stock engine with stock air filter and exhaust.

 

35, I'd be piss%D if i only got that!!, and I DO rag both my XJ's......the fully faired one gets about 50, maybe 52 if I take it easy, the 1/2 faired about 46-49.....

No pods, plugs look light brownish.......little lighter on the fully faired..but I am chasing down an intake leak....

 

Just 1 more thing I would offer up - change both the jet needles (attached to the carb slide) and needle jets ( technically a called emulsion tube) in all 4 carbs. Keep the sets together (each carb, carb set)

 

OK well I have a little more news to report. I swapped needles, got it all tuned and synced and it was idling perfectly at 1100 RPM. Had time to go for a ride yesterday so rode to the Oregon coast, about 80 miles each way. Power was great at first but started noticeably decreasing a little after ~40-50 miles. I filled up in town, hand calculated about 28MPG (slightly better!), stopped for an hour and had some food, then started heading back home.

After just a few miles the bike was running like crap and down on power but there wasn't a good place to pull off the road, so I put a total of 15 miles on the full tank of gas until I could get to a rest stop. Bike was running on 3 cylinders, #3 exhaust pipe was cool, and it wouldn't idle. I pulled the #3 plug and it was oily grey and fouled. I swapped the plug for a clean one (I carry spares as this is not the first time that cylinder has fouled a plug) and tweaked the mixture screws just a little bit based on their color, one cylinder was a little lean and one was slightly more lean. I got it idling again, bit of smoke on the right side exhaust (likely from cyl 3) and puttered around the parking lot, tons of power, felt good. Rode home, filled up along the way (25MPG), and felt like by the time I reached home it was a little down on power again.

So here's my theory after these observations: cylinder 3 spark plug is getting oil-fouled and degrading performance slowly over about 50 miles. After about 50 miles it is fouled enough that it's not firing consistently and this is severely affecting mileage.

Originally, carb #3 was the one that had an enlarged pilot jet (thanks PO!), and I'm thinking that my oil rings never properly seated after the top end rebuild in cylinder 3 because of the extra fuel from the improper pilot jet. OR the machine shop screwed up and messed up one of the valve stem seals. Though the last time I checked the valve stems looked clean. So I'd order up a new set of oil rings and rehone that cylinder, and repeat the break-in process now that my fuel levels should be correct.

Even though I've feel like I've fixed the carb issues, that plug is still getting fouled from oil based on engine time since the last plug swap. This is why when I am tuning it, I've feel as if been chasing a moving target. So I need to figure out why that plug is getting fouled so quickly, but in the interim I'll put in fresh plugs, tune the bike on the spot, then go for a ride, bring a few extra plugs, change them every ~20 miles, and see if I get substantially better fuel economy. If so, then the eternally fouling plug is the culprit.

Any opinions on my theory?

 

It stands up, almost. The ring wear might be more dramatic than you think, it could be all the rings, piston and bore...
Best strip it and measure ring end gaps, as well as ring widths and side clearances. You might be lucky.
I don't know that a fouled plug would reduce your mileage that much though, without you hearing or feeling a misfire?

 

Oil fouling almost certainly points to rings and/or valve seals. Bummer. I can see how that might cause such poor mileage. When I got my 700 it was getting a little less than 30 and that was just from overly rich and dirty carbs.

 

Well... Went for several rides over the long weekend, and even changing the plug on cyl 3 every 15-20 miles with a spare clean plug I'm still getting similar fuel economy. With a fresh plug, bike feels great, gobs of power even with a passenger, but I can definitely feel performance degrade after a while. At one point in my ride yesterday (~30 miles on the plug) I felt one cylinder drop out, so rode to a safe spot to change the plug, bike wouldn't even idle. Swapped it - instant improvement, smooth idling.

So, I guess I'm pulling this back apart. I'll inspect valve stem seals through the intake/exhaust ports, and if those look good, pull it apart and check rings.

But I still don't understand why, even if I replace the plug as soon as I feel like it's not running great, my fuel economy doesn't improve?

 

Because it only does 30 miles before it degrades...
Dunno why but I felt that deserved to be sung in chorus?

 

What I mean is, if I know the plug is fouled at ~30 miles (which should negatively affect fuel economy), so I swap plugs every 15 miles "proactively" as a test, shouldn't my fuel economy improve, if the plug fouling is in fact the cause of my poor fuel economy?

This obviously isn't a long-term solution, just a test to see if the oil fouling of the spark plug is directly causing the poor fuel economy. If I'm burning a little oil and that's fouling a plug but that is not impacting my fuel economy until that plug stops firing completely, then I have two distinct and separate problems - bad oil control rings or valve stem seal, AND some fueling/drag issue.

That is one thing I'm trying to figure out now. Carbs have been gone through several times with parts replaced that "seemed" fine but I shouldn't keep throwing money at the carbs if that's not the cause of my low fuel economy. Likewise if the oil contamination is not affecting my fuel economy, I'll just ride the bike the rest of the season and swap the plugs when needed so I can tear into the motor in the winter months so I don't miss out on riding time... but it sure would be nice to not have to refuel every 60 miles.

 

Have you a mate you can ride with?
If so, select a suitable slope, ride up parallel, simultaneously drop both bikes out of gear and see what happens.
This gets you an answer on the rolling drag, with some assumptions on clothing, tyre pressures, er dunno what else?

 

When you tear the carbs apart, can you tell us which jets are in which locations please?

 

Certainly. I will try to get some pictures. Going off memory, though - in the float bowl, center, taller tube = copper washer and main jet (#112). Closer to the outside of the float bowl, no washer, pilot jet (#41).

Emulsion tube, no markings, but tiny holes are all clear/clean and top is not noticeable elongated.
Needles - seated fully in plastic cap; spring and plastic threaded fastener in slide are in good condition and snug. Originally had marked Y-10 needles, the spares I had from another set of HSC32 carbs are unlabeled but look "similar" and do not appear worn.
Air jets - I will have to pull the carbs apart again to verify this. I am fairly certain that these are correct, as I've noted several times the mistake in the Haynes manual and paid close attention to which jet goes where - one #50 and one #195

I will pull it all apart and verify one more time to make sure I'm not an idiot. This would be the... 4th or 5th time I've checked these this year?