Carb tuning/jetting questions

My Questions:

My basic question is should I go to a smaller idle jet?

Second question, why would MPG would decrease when the mixture screw setting is smaller (tighter)?

My bike:

2000 FXST

42.5K Miles

Stage I, Vance & Hines exhaust / K&N air-cleaner

S&S 510G cams

My situation:

My MPG on this bike has always been poor at about 30, regardless of urban, highway, whatever.

I just rejetted the carb with a stage 1 kit from CV Performance as MPG has recently dropped.

I rejetted as follows on the basis of CV Performance’s recommendations:

Idle Jet – replaced a #46 with a #48
Main Jet – replaced a #200 with a #195
Mixture Screw – installed an EZ Mixture screw

I did 4 test rides on the same 50 mile circuit:

1. MPG = 33.3, mixture screw at 1.75 turns.
2. MPG = 33.3, mixture screw at 1.5 turns.
3. MPG = 31.25, mixture screw at 1 turn.
4. MPG = 28, mixture screw at .75.

All fuel fills done at the same pump at the same station.

The last test run has me concerned because MPG has decreased with a tighter mixture screw setting. However, the bike seemed to run better than any of the previous tests. On the first two runs, it seemed as if there was low speed surging. The bike ran well at speeds of 60+ on all four runs.

The other issue is that I know that my tank will register full at different levels at different times, even when filled at the same pump. So, the mileage for the last ride may be something of an anomaly as the tank may have accepted more gas at this filling.

I plan to take another test run at the current mixture screw setting. If the MPG is the same or worse, I’ll try a setting of 1.25 and see what happens.

But, I would appreciate any thoughts on whether the idle jet is too high and the counter-intuitive behavior of mixture screw setting.

 

Your slow and main jets are too big. Probably better off with the 46 slow, and a 185 maybe even 180 main.

At .75 of a turn open it sounds like you're way lean on the idle mixture. First pull the jet all the way out and make sure the tip is not even a little bit deformed. I've never used an EZ screw. Just to be sure you may want to eyeball it next to your stock jet to make sure the needle has the same taper. I recommend LIGHTLY bottoming the jet and then backing out 1.5 turns. Typically the right mix is going to be .5 turn plus or minus.

My guess is that with the idle mixture all but closed you are staying on the choke longer and opening the throttle to get on the needle sooner and that is hurting your mileage. 510's are pretty mild cams and shouldn't affect your MPG. Even your high end is very low for your bike. Unless you are one gargantuan, tire smoking dude you should be averaging around 40.

 

Thank you, DK. That makes sense.

I’ll try a 46/185, and 1.5.

Mileage has always been low (~30) since stage 1 was done at 2.5K. But, other than MPG it runs well.

I am neither big nor abusive. But, I have added weight to the bike in the form of cast wheels instead of spokes, and a 150 rear instead of a 130 rear. While a possible factor in the equation, I doubt that wheels and tire solely account for a 25% drop in mileage (40 vs. 30). I have asked techs at dealerships and indy shops to look into it, but to no avail.

I would be happy with solid 30’s. High 30’s would be fantastic.

 

You should be able to average at least 40 on that bike. We're shooting in the dark a little bit. If the changes I've suggested don't do it for you, I'd suggest finding a car garage with an old fashioned exhaust gas analyzer. This will tell you right away where you're at on mixture. Also, none of our discussion has touched the needle. I assume that you swapped for an aftermarket one when your bike was tuned. Some of these have a very radical taper. They do richen up the mid range but suck gas. If you do have an aftermarket one, most of them specify the clip in the third slot. You may try dropping the needle a notch, or if there is a shim under it taking the shim out. Rule of thumb is a notch is a whole step and a shim is a half step.

 

Thanks again DK.

I am almost certain that the needle is aftermarket.

I made a mistake in my first post. The bike had a 45/200 before I made changes. I have a 46/190 set, and changed to that. I'll post an update with the change to 46/190.

 

if some testing was done with the oem fuel screw, then you changed to an aftermarket extended fuel screw, there's a good chance the tapered end of the aftermarket fuel screw is a bit off from the oem.

that may explain the fuel mileage getting worse with it opened less.

or, you may not be measuring mpg accurately.

should be able to verify correct pilot/slow jet size by:

get bike to operating temp
set fuel screw to 2 turns out
set idle just low enough to keep bike running
slowly turn fuel screw in to seated position.

if bike does not stall, pilot jet is too large.
if bike stalls out after turning fuel screw in only slightly, pilot is too small.

edit: don't think pilot jets come in a 46 - prob. a 45.

 

Same 50 mile circuit with the same pump returned 38 with 46/190 at 1.5. Bike idled and ran well.

I have no complaints with that MPG. But, I'll tinker some more if there is yet more potential. I do want to keep this setting for a bit to establish a more solid baseline.

Thank you for the advice on the idle tuning.

All of the tests noted above were conducted with the aftermarket fuel screw.

JP Cycles does list a 46, CVP also tagged the baggy (for the jet) a 46.

The scientific method is easier in theory than in practice.

Anyhow, I think that the tricky part of calculating MPG is trying to control for the various driving and filling factors. I assume that a large data set helps control some of those factors.

 

I am resurrecting this thread as I need to appeal for some more information or opinions. I fear that it is time to go to a professional mechanic.

Before I do that, this is my latest question:

Despite an improvement in mileage by running smaller/leaner jets, my spark plugs are fouling. Any thoughts on what could be causing fouled plugs with leaner jets and improved mileage?

Is the idle jet still too large? I tend to think not as I am now running the stock size, 45. Also, I was running a 45 previously. Having said that, the bike will not stall when idling and the mixture screw is completely seated. So, maybe 45 is too large?

Alternatively, I am beginning to suspect that the problem is not the carburetor, and would appreciate feedback as to other possible causes. Am I possibly experiencing a bad coil or faulty wires?

The fouled plugs manifest themselves by poor drivability, poor idle and loss of power (especially at higher speeds, 60+). Prior to the jetting change – while running with a much richer main jet – I did not experience these symptoms of fouled plugs.

I am a novice at reading plugs, but these plugs appear to me to be fouled by carbon, not oil. I do not think that I have a problem with piston rings as I do not believe I am burning any oil.

With idle/main jets of 46/190, I returned an average of 35 mpg and the old plugs with 7,500 miles exhibited fouling after 200 miles of the jetting change; 45/190, average of 35 mpg and fouled plugs at 450 miles; 45/185 average of 36 mpg and fouled plugs at 264 miles and again at 100 miles.

The fact that fouled plugs are occurring at more frequent intervals at leaner settings indicates to me that the problem of fouling may not be related to the carburetor.

Some other notes:

- I gapped all replacement plugs according to the manual at .040 inch.
- Most of my riding is at speeds of 60 – 70.
- I have been adjusting the mixture screw to determine the effect on fouling and mileage. I have found that a leaner setting reduces mileage and the fouling persists.
- I do not think my idle is high.

Again, I appreciate any thoughts on what may be the problem.

 

Check for cracks in one or both of your spark plug cables. Lower than required firing voltage in either cylinder will definitely cause plugs to foul. See if swapping wires with a buddy does anything for you.

 

DK, thanks.

The cables are bad. Visually they are okay. However, I tested them with an ohm meter. The results are as follows:

Front Cylinder cable: 1,120K. Top limit per manual is 4,375.

Rear Cylinder cable: 460K. Top limit per manual is 11,230.

Both cables seems to be significantly out of spec.

Another question: should I replace the coil too?

Of course, I do not want to throw money away, but I wonder if the bad wires can impact the coil or vice versa?

Thanks, again!