MPG (spinoff) carb jetting

[Guest]

So I'm starting my thread here about bike gas mileage after rightfully being chided by Steve about not thinking my CV carbs needed to be jetted in tc862011's MPGs thread.

You are under the impression that you don't have to worry about jetting because you have CV-type carbs?

How stock is your 850? Stock airbox or pods? Stock exhaust or a header?

Armed now with a little more information - here is the data:

Stock carbs (and jets)
Stock engine (no mods)
New boots (box & engine side)
New o-rings (boots and carbs)
Valves adjusted
Carbs synched
Carbs cleaned (dipped 2x and ultrasonically 4x) using Nessism's tutorial
Floats adjusted and verified
Bench synched
Mercury Manometer synched
Stock airbox
Stock exhaust
Mean elevation 4500'

If the throttle is kept 25-33% then I can squeeze 28-30 mpg (4.5 - 5 k rpm in 5th gear) if i go 50% to WOT then it tanks to 20-22 mpg.

[Riding Again]

Performed one set of plug chops, WOT

Warmed the bike up with a 15 minute ride on the interstate.
Chop run was a hill that went up 150 ft over 1 mile.
Changed the existing plugs to new plugs at the bottom of the hill.
Went through the gears quickly and settled into 5th for about 1/2 - 3/4 mile, trying to keep the throttle wide open with each gear change.
At the end point, I hit the kill switch and pulled in the clutch and moved to the side of the road and started pulling plugs.

3, 4, 1, 2 - left to right.




Normal riding plugs look good except for #4 which looks like it spent its time in a coal mine (black, fluffy).

Would one carb be that much different from the others with the same jetting?
I feel like

[hampshirehog]

You must have a fuel leak to get petrol consumption that low on those throttle settings. Does your oil smell of petrol at all?

[sk843143]

Check ur petcock...... Vac line and seal

[Riding Again]

You must have a fuel leak to get petrol consumption that low on those throttle settings. Does your oil smell of petrol at all?

No fuel smell at all in the oil. No increase in the oil level. I DO smell a little in the exhaust--difficult to tell from the regular exhaust, except my wife notices it on the clothes and jacket (motorcycle smell)

Check ur petcock...... Vac line and seal

Brand new fuel and vac lines (METRIC not inches), new seal on the petcock and it doesn't leak when still. No sign of leaks using talc either.

[Steve]

Performed one set of plug chops, WOT

3, 4, 1, 2 - left to right.

OK, why are you showing us a picture of your NEW plugs?

If those are your plugs that just took you up the hill, they are SERIOUSLY lean.

I have no idea why your mileage would be so bad with jetting that lean, unless it simply needs to run two intake cycles to get enough fuel to burn ONCE.

I am interested in seeing what your plug chops will be for the other circuits, since it's actually quite hard to ride around on the main jets all the time.

Brand new fuel and vac lines (METRIC not inches), ...

Isn't that wonderful stuff?

.

[Big T]

That's terrible gas mileage - I don't think I've ever gone below 35 mpg and that's hauling thru 2-3 gear twistys

Since your bike is stock, it was set up to run lean (thanks EPA). At 4400', it should have balanced out to run nicely

There must be a leak, somewhere

[Riding Again]

OK, why are you showing us a picture of your NEW plugs?

If those are your plugs that just took you up the hill, they are SERIOUSLY lean.

I have no idea why your mileage would be so bad with jetting that lean, unless it simply needs to run two intake cycles to get enough fuel to burn ONCE.

Those plugs had at most 2 minutes of runtime on them. The double intake stroke theory is interesting. I have noticed the last couple of days with the throttle held constant at about 40% the bike will “surge” and then “drag”. Last night when I let off the throttle, I heard some “bubbling” backfires from the tailpipe. I figure these are pieces to the puzzle, but don’t know where they fit.

I am interested in seeing what your plug chops will be for the other circuits, since it's actually quite hard to ride around on the main jets all the time.

Ya, rarely do I get into WOT unless slabbing and trying to keep with traffic in headwinds [Its amazing that cagers want to do 80 in a construction zone]!

I began with WOT chops based on advice from this link from Bass Cliff's site. http://www.factorypro.com/tech_tunin...m_engines.html

Haven’t had time to do the next set, I was guessing I need to run the bike about where I’m normally cruising. But I think I should re-read the info from that link.

Guidance anyone?

Brand new fuel and vac lines (METRIC not inches)

Isn't that wonderful stuff?

Got it from an import/foreign auto parts store. Was hoping for a thinner wall, but its flexible enough and most important – THE RIGHT SIZE!

Since your bike is stock, it was set up to run lean (thanks EPA). At 4400', it should have balanced out to run nicely

There must be a leak, somewhere.

It echos Steve’s thoughts, but after replacing both sets of boots and o-rings; and making sure the airbox is tight and the filter is snug to the box (foam filter, not paper), how do you check? Plenty of info from the site that the traditional WD-40 spray method isn’t reliable. Also, if I pull the choke out to “richen” the mixture, the bike stumbles, indicating I believe, that it’s not too lean (WOT or 50% WOT).

[sk843143]

Check your ign sys......timing .....aval spark....dumb ques here....but your exhaust isn't plugged it's it

[Steve]

... the last couple of days with the throttle held constant at about 40% ...

I see your references to throttle percentage. Are you guessing or have you actually marked the throttle?

Here is one easy way to do it:


There are marks for IDLE and FULL, then HALF, 1//4 and 1/8. Those are the suggested settings for doing plug chops.

It's easy enough to measure for the marks, just mark the idle and full points, then keep splitting in half, keeping to the upper section each time.

.

[Riding Again]

AS DESCRIBED IN THE ABOVE POSTS:

I followed the markings halfway . Closed and WOT and attempted to make a mark halfway (without measuring-hey it was 5:30 am, with a sharpie, on black).

So the percentage was closer to an estimate.

TODAY
When I finished placing an actual guide


I found that my estimated 50 was closer to 30.

[Steve]

I found that my estimated 50 was closer to 30.

That is why it is highly recommended to actually do the markings.

Now that you know how far you have the throttle open:
Idle to 1/8 will be your pilot circuit
1/8 to about half is your needle circuit
Half to WOT is your main circuit.

Do plug chops for each circuit, note that it takes a bit longer to get proper color on the lower circuits. Fifth gear at 1/4 throttle works better to hold a lower speed than second or third gear. Remember that it is throttle opening, not engine speed, that determines the circuit in use. Try to find a slight uphill section where you can hold the lower throttle openings for a minute or two.

.