MPG as an indicator of proper tune?

So just recently did a trip around Northern California, back roads and a lot of off road, and over the course of four fill-ups, I averaged about 41.5MPG, all really close to each other. Each time was mixed off-road, curvy back roads, and 75mph highway driving.

My question is: Is this a pretty good indication of tune?

I feel like I'm running a bit rich, as it gets kinda lumpy at WOT, and when I put on little shorty mufflers, it pulls evenly throughout. I'd rather be a tad rich any day.

Any thoughts?

A couple gratuitous pics:
Usal beach


Camping on the lost coast


Dirty bike

 

I was consistently holding around 48-50 with my 650 maxim. If you're getting lower 40's, I'd say you're running a bit on the rich side. Other things that can influence that--valve clearances, carb cleanliness, jets, air filter, etc.....

dave

 

I have to ask, where did you find those handguards for your seca? I have an 81 xj650 and i've been searching for a pair exactly like those.

 

Everything(valves, air filter, carbs churched, etc.) is up to snuff. I went to green on my colortune, so that could be it. Would a dime size rotation in on the screws help out? It seems like I should be messing with jets if it's through a day of mixed driving.

The shorty pipes give me 3 more mpg...

 

Those are bark buster bar-end mounted. They might not be as hardcore in a wreck, but the bars I have wouldn't allow for the two-point bark busters, and the fitment is hard to guess...

 

Two point mounted brushguards have one distinct disadvantage: during a crash they can trap your hand and make your wrist end up all floppy.

 

For the sort of riding you describe, I think that your mpg is about right. The mpg you get also depends on how strong your right wrist is. Panniers are not the most aerodynamic things you can fit to a bike; they might be lowering the mpg a bit. Do you usually run without them?

 

You could do a dedicated test - carry a gallon, ride until the bike wants to go on "Reserve", pull over and put that one gallon in, set your trip and ride at a steady 60 MPH. See if you get 50 miles.

Everything from hills, off-roading, idle time, fueling error will affect "gas mileage" figures. 41.5 sounds good considering.

 

Nice pics bro,

The wind blown trees, water and camp remind me of here except for well, warm

 

I was pretty happy with 41.5. The kit was about 40 lbs with the panniers, rack, food, camping gear and a rotopax 1-gallon. Yeah the panniers, although pretty small, have a pretty noticeable effect at 75mph when it's windy.

Back to the original question...should I be fidgeting a bit with my mixture, or leave it be?

 

I'd leave it alone. If the shorty pipes make it run better, put them on.

 

Mixture screws only affect it it idle anyway..... that's why its called the IDLE mixture screw. Once you crack the throttle the jets transfer to the main jet
Dave

 

What Dave said.

Plug chops are a better indication of ignition under load and in the proper rpm range.

 

Plug chops look pretty good all around, that's why the (somewhat) low mpg threw me off.

I ride my bike much more than driving my car at this point, so I get particular about the little things.

In general, she runs like a champ. Rotella 20W-50 every 1500, and valves every 5000.

 

That has not been my experience. I started with my pilot screws in a rich position and have been steadily leaning them to get my plugs the right color. My mpg has increased substantially.

The pilot system does not shut down when running above idle, but continues to feed fuel, and maybe even more with more vacuum demand, but as speed increases the main jets become more and more dominant. One source I read states that the pilot system is the main fuel supply from idle to 1/4 throttle. When I'm cruising at 65 or so my throttle is not opened more than 1/4 th. So maybe that is why it has been so effective in my case.

If the pilot system wasn't involved at high speed, then if you did a plug chop on new plugs and they were sooty or black, (with carbs, sync, and valves clean and in specs) then the only way to correct it would be a smaller main jet. Instead, we adjust the pilot screw.

At least that's the way I understand these CV carbs to work.

 

You understand correctly. The pilot circuit is always providing fuel. The main supplements the pilot so the mixture does not get leaned out as the engine pulls more air in at larger throttle openings.

 

tabaka45, this is most interesting. Since my bike is running with a 750 engine and 650 carbs, I've tried to get the mixture right without spending a small fortune on parts. I've drilled out the pilot air jet to get the thing running well at small throttle openings. (Cries of horror all around, but there's two more sets of carbs that came with the bike if it all goes wrong) It was obviously running very rich and I did wonder if I could improve the mpg by getting the pilot mixture right.

However, there's a complication; low compression caused by water damage in the bores. This probably explains the low figure of 35 mpg, or 29 mpg to an American gallon. The head and barrels are sitting on the workbench.

I understood that the pilot system has less and less effect as the throttle opens, then at some point around a ¼ throttle it stops contributing, by which time the main jet and needle are taking over. I may have read it on this forum, but I'm not sure. I know that I found the same info in a couple of different places on the 'net.

 

The mains never completely "take over" the pilot circuit is still part of the equation even at WOT.

 

Hey lacucaracha, so I was getting almost 50 mpg on my 82 Maxim after I picked her up, and gave some initial TLC, that includes valves in spec, oil and plugs.

Three years later, after a complete carb clean, floats, sync, wheel bearings, brakes, new tires, fresh oil, floats again this year, sync, and new plugs, tune, an added trunk, and good gas, I now get about 44 average.

However, Veronika rides like a wild stallion compared to that first year, and I push her like I am in the KY derby every day.

 

That's what I said. :wink:

 

Even at WOT the "slider piston" is regulating a controlled amount of vacuum, and this vac will continue to pull SOME fuel thru the Pilot and transfer ports.

 

Hey guys, I think the pilot circuit is always active even at WOT

 

3 more MPG and I could actually go to work and back on one tank Unfortunately my weight prohibits that lol

300km/tank would be awesome, last tank was 275km on 16.7L (6L/100 or 47 mpg)

 

the pilot circuit only supplies enough fuel to idle with the butterflies open a crack. so compared to the amount of fuel supplied from the needle and main jet at wide open, it's kind of insignificant. when we set the idle mixture we set air, or amount of vacuum. when the throttle plates open up, vacuum goes down so even less gas goes through the idle circuit.
but i t never goes completely away, it just gets less important.
thats my story and i'm sticking to it