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Carburetor Tuning for MPG

4K views 48 replies 10 participants last post by  JPT 
#1 ·
So, I need a little help, I am not quite sure where to go as my mpg is way too low. I have the following:

2009 VTX 1300R
Batwing Fairing
Speedo adjusted matches GPS at all speeds
Roadhouse Classic 2:1
Arlen Ness Big Sucker (very open)
Factory Pro Jet Kit
Choke Cable removed, Ace hardware spring

Current Carb Settings:
Fast/Main Jet 220
Slow/Pilot Jet 58
Needle 4.5 (E clip at 4 with a #4 washer under)
Glen’s garage A/F screw at 2 from lightly seated

Tests
2nd gear from 45 mph through governor - pulls strong, no noticeable issues

4th gear 50 mph hard throttle - no bogging or choking or hesitation, pulls strong

Cold start requires throttle, RPM’s are low until warm. Will sputter (pop, or backfire) on a hot start.

MPG 25-28. Granted we likely still are on winter formula gas, but I think I should still be getting in the mid 30’s at least.

Thanks!
 
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#5 · (Edited)
my opinion..

the plugs are glazed... then covered with Rich fuel burn..
get new plugs and try again


order of investigation.
check for air leaks at the engine to intake and intake to carb. small cracks.. or LOOSE clamps.
check the vacuum diaphragm for cracks... very little cracks..
check the spark Timing...
check valve adjustment
check camshaft timing.

you are only a little rich... not 25mpg rich..
.
unless that thing attached to the throttle is a switch.. on/off/on/off....


side note.. Honda considers the vacuum diaphragm to be retired after 6 years... racers replace them every year.
its rubber.. heat/cold cycles... fuel vapors.. for a part that NEEDS to be very flexible and sealed.


if stiff and or leaking air.. you will NEVER get Full Throttle...
its a vacuum controlled carburetor
 
#6 ·
my opinion..

the plugs are glazed... then covered with Rich fuel burn..
get new plugs and try again


order of investigation.
check for air leaks at the engine to intake and intake to carb. small cracks.. or LOOSE clamps.
check the vacuum diaphragm for cracks... very little cracks..
check the spark Timing...
check valve adjustment
check camshaft timing.

you are only a little rich... not 25mpg rich..
.
unless that thing attached to the throttle is a switch.. on/off/on/off....

I will replace the plugs, cheap enough. What is the best way to check for leaks?


side note.. Honda considers the vacuum diaphragm to be retired after 6 years... racers replace them every year.
its rubber.. heat/cold cycles... fuel vapors.. for a part that NEEDS to be very flexible and sealed.


if stiff and or leaking air.. you will NEVER get Full Throttle...
its a vacuum controlled carburetor
The diaphragm is very soft and flexible, I did not see any cracks. I plan on lowering the needles to 3 1/2 when I get home tonight, and if that doesn't play well, I'll raise it up to 4. When I have the diaphragm out, I will double check for cracks.

Right now, it runs amazingly well at all throttle positions (except on initial start), just this mph is horrible...
 
#7 ·
Also note that my odometer/mpg is off by about -4%. So in reality I am getting closer to 26-27 mpg. Still not an acceptable range, but more accurate for this discussion (and anyone having similar issues).
 
#8 ·
.
hold diaphragm up to a bright light.. a light pull and see if light pass thru..
rotate 90 degrees and do it again... till 360 degrees are done.
 
#9 ·
Will do. Since I have to change the needle position, it will be a good time to check.

I was also double checking it by reaching through the air port with the diaphragm cap. I lifted up the bottom of the diaphragm by the needle to see if there was pressure. There was a good amount of pressure, so I assumed no tears or leaks. But obviously this is not a very scientific approach.
 
#10 ·
I wasn't having problems but replaced the diaphragm. It was flexible and soft. However, when I held it up to the light a very small pin hole was in it. Glad I replaced before trouble.

BTW: I always get between 35 and 40 mpg running interstate speeds (honest 70 & 80) with a full load of touring gear unless I am fighting a wind, then it is still above 30.

Mine is pure stock except for a K & N air cleaner.
 
#11 · (Edited)
I get about 165 miles before reserve, so approx mid 40's pretty consistently. Cobra swept, BAK, FP kit 215, 4th clip and maybe 2 turns or less out. Choke needed on cold starts as it should be, runs great.
Slide diaphram is available aftermarket for lot less than what Honda wants for a replacement slide.
Tune to run at its best, not for mileage.
Mileage will follow on a well tuned bike.
 
#14 ·
Thanks.

Did you try a 220 main? I noticed a considerable difference between 215 and 220 doing the 45 mph in 2nd gear slow roll of throttle up to governor. It was much slower, felt like I was running out of gas on my way up.

We basically have the same intake, I would assume you might get more acceleration with 220.
 
#12 ·
So, I spent some time this evening trying a few different needle positions. No cracks or holes in the diaphragm.

I started with 3 1/2, it wouldn’t accept any throttle without stalling.

I then when to 4, still like crap.

When back to 4 1/2 still crap. Then I realized that the E clip was loose on the needle (would slide up and down within the slot), allowing it wobble. So I went back to 4 with a new E clip. It then ran perfectly:

1) 45 mph in 2nd with smooth increase in throttle it smoothly accelerated hard all the way to governor.

2) 50 mph test in 4th gear with hard throttle, it resulted in smooth acceleration.

3) Idle around 900k blipping the throttle seemed to accept the fuel by dropping right back to 900k without any hanging or dipping.

Current settings (45 degrees, with 87 octane):
220 Main Jet
58 Pilot Jet
4th from blunt end on needle (with a NEW tight E clip)
2 turns out on the Glen’s Garage A/F screw

I will run this for a tank or two, depending on gas mileage I will then go back to 3 1/2 (making sure the E clip is tight) and see if I can squeeze mpg without loosing midrange.
 
#15 ·
So, I rode to work today... There were a couple issues that I noticed:

Mid to High RPM's the bike performed perfectly.

Low (off the light, or when too high of a gear) it would stumble, but if I downshifted to a higher RPM it would perform perfectly. I'm not sure how to describe if it is Lean or Rich. I'm assuming lean because of the heavy breather intake, 58 pilot and 2 turns for A/F screw.

I'm not quite sure where to go now, I assume try to open up the A/F screw to up to 3 turns, if it continues, I need a 60 pilot/slow jet?

When I get home I will check spark plugs, but last night when I checked the plugs after my ride, the white ceramic part looked perfect (didn't look rich or lean).
 
#22 ·
I read that. Since this requires a running bike, how are people doing this with the tank off? If I do find a way to get fuel to the carb line, will the bike run with the 2 green harness for the tank electronics disconnected?

Is there some sort of container that I can use to get fuel to the carb line?

Thanks
 
#23 · (Edited)
Lift up the rear of the fuel tank, put a 2x4 block to hold it up. Remove the fuel tank vent hose first..
You are spraying something in the manifold to cylinder area.
Do it outdoors...

Yes the speedo cluster can be disconnected.
I broke my retainer tabs on the Green connectors so have tape holding connectors together.
A better way is to remove the 3 screws at the seat end of the cluster, remove fuel cap, lift cluster rear slightly and push forward.
The cluster will come off and you can tie to the HB.
There is a small rubber piece that may slide off. RTV or something to keep it in place.

A good thing to do on all the side cover grommets.
Some people have pushed out the side cover grommets.
A little silicone or spit on the pins when putting the side covers back on...

Regarding MPG - what is your riding style??
WOT often?? I know you have mentioned the Rev Limiter a few times..
I rarely do that since I went into Grandpa mode....
 
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#24 ·
Lift up the rear of the fuel tank, put a 2x4 block to hold it up. Remove the fuel tank vent hose first..
You are spraying something in the manifold to cylinder area.
Do it outdoors...

Yes the speedo cluster can be disconnected.
I broke my retainer tabs on the Green connectors so have tape holding connectors together.
A better way is to remove the 3 screws at the seat end of the cluster, remove fuel cap, lift cluster rear slightly and push forward.
The cluster will come off and you can tie to the HB.
There is a small rubber piece that may slide off. RTV or something to keep it in place.

A good thing to do on all the side cover grommets.
Some people have pushed out the side cover grommets.
A little silicone or spit on the pins when putting the side covers back on...
Thanks. I saw someone used some type of container filled with fuel and a hose going down to the gas line to the carb. This way they could run the bike without the tank in place.

Regarding MPG - what is your riding style??
WOT often?? I know you have mentioned the Rev Limiter a few times..
I rarely do that since I went into Grandpa mode....
I do give it some, I am coming from the sport bike world, but have calmed down a lot. I have only hit the rev limiter for testing the main jet, as per the carb tuning instructions (2nd gear from 40 mph slow acceleration to rev limiter). If one feels a lost of power before the rev limiter, the main jet needs to be bigger, if no difference go back one (lowest with most pull).

My commute is 35 miles, 30 of which are in the HOV lane on a single highway (5th gear around 55-80 mph). I have had to warm up the bike longer than usual, as it is still cold.
 
#28 ·
Ok. So I have played some more.

I tried 215/58/5th needle, I got maybe 27-28 mpg. About 1-2 mpg better than with 220/58/4.5.

I went back to 220 but tried 60 with 4.5 needle (haven't filled up yet).

220 definitely pulls stronger (than 215) in the higher RPMs, the 60 definitely pulls stronger (than 58) in the lower RPMs, and my bike seems to like the needle at 4.5 best.

So, right now 220/60/4.5 the bike pulls very hard in every gear, in 5th I don't really have to downshift for uphill overtaking.

Now the bad, when I close the A/F screw at around 1k RPM (glen's garage) the bike's RPM only slows down, it does not stumble. Does this mean that the 60 is too large?

Thanks.
 
#33 ·
The O2 sensor plugs into the gauge, has a harness that you wire to power and ground.

Zip tie it all up to the bars and go for a ride .

It will tell you exactly where your at rich, lean, or whatever jet change, or whatever you need to get the most from your engine.

Take it back off , put it on the shelf until you make changes to something else and repeat .

Now your not going to get horsepower numbers , but you will get the most out of the engine you can get.

No dyno guy running up your engine past the redline to see what it will do.

You can do this in your driveway easy enough by yourself and be just as accurate .
 
#34 ·
This does seem like a good option. I think that it would end up costing more money though. I would need to get an O2 bung welded in, plus the cost of the gauge.

I decoded to go back to the 58 slow jet, and try 4 instead of 4.5 needle, so now:
220/58/4 needle/2 (right around 2)

I went for a ride, it rides and meets all the check boxes from the Carb guide. When I get a chance to put some serious miles on it, I will see if the mileage is better. I got 24 mpg today with 220/60/4.5 needle, so clearly that is the wrong direction. But I do have to say, there was a TON of power in all ranges with that setup.
 
#35 ·
Update:

So, with the 220 Main/58 Slow/4th needle, I went for a long ride today. It was about 70 degrees, so warm up was done while moving on the highway. I did not drive like a hooligan, I did not hit traffic. Here are my stats:

I went 96.8 miles (uncorrected so -4% of actual) then filled the tank back up with 3.173 gallons. Corrected that is 100.7 miles or 31.7 mpg.

I am not sure if it was still with winter blend gas, but I’m not sure how much else I can tweak without going the dyno tune route.
 
#38 ·
I took it easy for this 100 mile test... Making my max about 31.7 mpg. I still think I should be getting 5-10 more mpg. This would get me 20+ miles per tank, I think that is a big deal, not for $ but for having to stop on long rides.

I would like to be able to get around 140 miles before reserve, instead of <120.
 
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