Engine needs a refresh - make that a big bore!

[Syscrush]

OK, yesterday I was able to run a near-ideal test of fuel consumption, and the results are absurd.

I rode to my parents' place and back at steady highway speeds, 5000-5500 RPM. It was pretty windy, I had a 20+ kph side-headwind on the way there, and 30+ kph side-tailwind coming home. Here are the stats in both metric and US:

• Typical speed: 115 kph | 70 mph
• Total distance: 290 km | 180 mi
• Avg distance to reserve: 120 km | 75 mi
• Total fuel consumed: 34.5 L | 9.1 gal US
• Avg fuel consumption: 11.9 L/100 km | 20 mpg US
• Reference distance to reserve: 190 km | 120 mi
• Reference estimated fuel consumption: 7.55 L/100 km 31.4 mpg US

It looks like yesterday I was hitting reserve after consuming about 14.5 L | and riding. On my trip from Toronto to Montreal years ago, I was typically hitting reserve at 180 - 200 km. I didn't track fuel consumption on that trip, but I estimated today on the assumption that I was consuming the same 14.5L when hitting reserve on that trip as I did yesterday. Note that the gearing now is a little bit taller than it was on the Montreal trip, so I'm consistently turning a couple hundred RPM less than I did on this trip. Also, I wasn't goofing around trying to keep up with my semi-crazy cousin on his VFR yesterday.

To me, it seems clear that either something is very wrong with the jetting, or the bike is somehow leaking fuel but only while running. The plugs were checked at the 1000 km service after the rebuild and supposedly looked OK.

Overall it starts and runs well, it feels quite good through almost the entire rev range, from very light steady state cruising throttle to winding it on harder. 7000 RPM in 3rd at WFO is an absolute treat. The bike pulls just fine from 1200 RPM even in 5th gear (which I've obviously only done as an experiment, that's not part of normal riding).

Obviously I'm going to talk to my mechanic about this, but if anyone here has thoughts to share, I'm happy to hear them.

[daves79x]

You need to find out exactly what jets are in the carbs now. Since you are using the stock airbox, jetting should not need to have been changed much for your set-up to work fine. The #35 pressed-in jets maybe could go to #38, but no larger. The primary mains (I think these are '79 carbs?) could go from #65 to 68. The stock secondary mains should have been plenty rich enough at #98, but no higher than 100. The slide needles could be shimmed .020 inch also. This is assuming stock needles and not aftermarket DynoJet stuff. Those minor changes should have your bike running very well, with the stock airbox. Exhaust makes little difference in jetting.

I still suspect that your carbs were not completely cleaned and assembled correctly in the first place, then just jetted way up on the mains to compensate. Your set-up as I outlined should net you well over 30mpg riding as you described. The carbs still need serious work.

Dave

[Syscrush]

I still suspect that your carbs were not completely cleaned and assembled correctly in the first place, then just jetted way up on the mains to compensate. Your set-up as I outlined should net you well over 30mpg riding as you described. The carbs still need serious work.

I have thought for years that you are right, but hoped that locals could get me fixed up. I used to complain a bit about the 30 mpg I got on the Toronto-MTL-Toronto trip, but if I could get back there now I'd be pretty happy.

[Syscrush]

My choke cable is not working very reliably, so it occurred to me that the choke might have been stuck partially on all year - at least during the trip last week where I measured the fuel consumption. I dropped by the shop today to see if we could determine if that was the case, and it really doesn't look like it is. When the choke lever is pushed all the way home, the choke mechanism on the carbs appears to move all the way down to where it should be.

While we were at it, the #1 and #6 plugs were pulled. I expected them to be covered in black soot, but they looked and smelled decent. We ran it for about a minute at 4500 RPM while the mechanic put a laser thermometer on each of the 6 header pipes multiple times and they all measured the same temperature each time. We also pulled the oil filler cap to check for fuel in the oil, and that also checked out OK.

There is a bit of black smoke out the muffler when I quickly open the throttle under no load, which I attribute to the accelerator pump. I don't see black smoke (or any other color) under any other circumstances. I don't like seeing the black smoke, but it doesn't seem like an obvious/definite sign of bad jetting given that it only happens when snapping the throttle open.

[FalldownPhil]

20mpg = something very wrong !! Aren't you on imperial gallons ?
Phil

[cross]

Something is definitely wrong. My reserve kicks in at around 120 miles of 75mph hwy cruising. I also never fill gas up to the neck. Usually around 4 gallons from when reserve kicks in.
Perhaps it's a bit better due to the fairing, not sure

[Syscrush]

20mpg = something very wrong !! Aren't you on imperial gallons ?
Phil

We're on liters - haven't used Imperial gallons for 30 years or so. I gave L/100km and miles per US gallon above as they're the 2 most common fuel consumption units.

Thanks for weighing in.

[Mouse]

We're on liters - haven't used Imperial gallons for 30 years or so.

Well thanks .... for making me feel old.
I still can't "see" how good/bad LTR/100km is. Still have to convert it the Imp mpg to know my fuel economy.

[EMS]

It is not an easy conversion. One system denotes the amount of fuel used per a defined distance and the other the distance achieved per a defined amount of fuel.
To get close, you can use this trick:

Divide the number 235 by the number for fuel consumption you have and you have a close number for the other.

Example: You have 30 mpg. 235 ./. 30 = 7.8 thus 7.8l/100km
0r: You have 12l/100km 235 ./.12 = 19.6mpg

It is pretty close. We have been using this when calculating fuel consumption for our racecars on different tracks and discussing this with the engine tuners.
This is based on 3.785 liters per 1 U.S. Gallon

[Syscrush]

I still can't "see" how good/bad LTR/100km is. Still have to convert it the Imp mpg to know my fuel economy.

Same here. The switch happened before I started driving, but to me MPG makes so much more sense. The only reason I have any sense of L/100km is because that's the unit my Mazda2 reports fuel consumption in. Even so, I don't have an internalized feel for it like I do for MPG.

[Syscrush]

Divide the number 235 by the number for fuel consumption you have and you have a close number for the other.

That IS a handy trick!