Pilot jets, mixture screws and MPG

[Capn Ron]

So I went from a very stock setup in the carbs including the 37.5 pilot jets.  I regularly got 43-47 MPG over the last 26,000 miles.  My biggest issue was cold starts and idle anomalies.  It took quite a bit of choke to start the bike...was a pig to warm up...and would stall until it was *really* warm about two miles down the road.  Add to that, when coming off the throttle at cruising speeds, the RPM's would drop well below idle before returning to 1,100 RPM...sometimes stalling out.

SO...On the recommendation of those in the know, I did one single change:  Swapped the 37.5 pilot jets for 40 pilot jets.  Starts GREAT now with no choke at all!  Problem is, I'm now struggling to get anything over 35 MPG no matter how I try!    Rode 250 miles yesterday with the Monkey and SimiEd.  Same ride...similar styles of riding.  Mark got 44.6MPG, Ed, with his punched out engine got 38.4MPG and I got 33MPG!!!    That pesky RPM drop below idle is gone, but has now been replaced by an RPM "hang" at around 2,000 RPM...THEN it drops to idle.

Reading as much as I can here.  I have read SIX different takes on all this.

1.  The RPM dip below idle and then back up to 1,100 RPM is a lean condition
2.  The RPM dip below idle and then back up to 1,100 RPM is a rich condition
3.  The RPM "hang" at 2,000 RPM is a lean condition
4.  The RPM "hang" at 2,000 RPM is a rich condition
5.  Turning the idle mixture screws OUT will lean the mixture
6.  Turning the idle mixture screws OUT will richen the mixture

WTF?  Seems as if not everyone agrees on what causes what or how to cure it.

Okay...Follow me here and PLEASE correct me as I go as I'm trying to understand this better.  I can only assume that making the ONE change...37.5's to 40 pilot jets richened the idle mixture...bigger hole=more fuel.  Since my symptoms went from the "Below idle RPM and then recovery" to the "RPM 'hang'", #1 and #4 above must be true.    I think I got that right?

Now...I checked each idle mixture screw to document their original positions.  They were all around 4 to 4.5 turns out.  I TIGHTENED them by 1/2 turn and test rode the bike.  I'd pull over to the side of the road and tightened them another full turn.  I pulled over again and tightened them ANOTHER full turn.  In each case, the 2,000 RPM "hang" got worse.  Given that, I'm assuming that tightening the idle mixture screws RICHENS the mixture?  Did I get that right?

I pulled over again and backed the screws OUT to where they started and then backed each out ANOTHER 1 1/2 turns.  The 2,000 RPM hang went away...and I would occasionally experience the "Below idle RPM and then recovery"...  That seems to me that loosening the idle mixture screw will LEAN the mixture?  Did I get that one right?

Since It seemed like the RPMs would dip...and occasionally stall...I may have gone too far.  I turned all the mixture screws IN a 1/2 turn.  This puts them now at 1 full turn OUT from their original settings...and my conclusion is that this is leaner than it was yesterday.

I need to go on a long ride (maybe tomorrow) to get a fix on fuel mileage and ridability.

I really don't want to spend my life chasing carburetor adjustments and will go back to the 37.5's if this doesn't fix the MPG drop (and really poor range on a tank of fuel) I'm experiencing.

Any thoughts?  Anything I'm overlooking here?

[racerrad8]

Pilot screw adjustment and pilot jet sizing has a minimal affect on fuel mileage...

The tighter the idle screw setting the less of the a/f mixture of the pilot circuit is allowed into the carb throat.

So, your mileage issue probably have more to due with worn needles and elmusion tubes tgan anything.

I can tell you that both of the carbs you are comparing too were both built be my and all attention to detail is tended too during the build.

Randy - RPM

[movenon]

Here is a video that might help explain the air/fuel idle air screw adjustment for others.  An effort to ad to the thread. In to lean, out to enrichen.
Not an FJ but the same principle. 
http://www.youtube.com/watch?v=zm5mB3R8Ucw
George

[Firehawk068]

Ron,
From what I know about CV carbs, I'd say that #2, #3, and #6 are correct...............

Here's a helpful link for you. Check out #4 down the page.
http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

[FJ_Hooligan]

NO!  That is NOT the same principle as the FJ carbs.

Re-read what Randy posted.  The FJ Mikuni carbs have idle MIXTURE screws.  They do not meter fuel OR air, they meter fuel AND air at the same time.

Therefore, the "IN is leaner and OUT is richer" or vise versa DOES NOT APPLY.  What matters is that whatever idle symptom you're experiencing can be adjusted out.  The key term is ADJUSTED.  Does it really matter whether the adjustment is IN or OUT?  No!  Adjust the screws one way.  If the problem gets better, then keep going until it gets worse.  If the problem gets worse, then go the other way.  

It's that easy.  Don't get hung up on the lean/rich direction because that is dependent on the how your idle circuit is jetted.  And the end result doesn't care.  Stop over-thinking the problem and apply the easy solution (to idle screw tuning).

Finally, I agree with Randy.  If your mileage is that bad, it's a needle circuit problem, not the idle circuit.

[Firehawk068]

NO!  That is NOT the same principle as the FJ carbs.
Re-read what Randy posted.  The FJ Mikuni carbs have idle MIXTURE screws.  They do not meter fuel OR air, they meter fuel AND air at the same time.
Therefore, the "IN is leaner and OUT is richer" or vise versa DOES NOT APPLY.

NOT true..................

There is a set amount of air entering your engine through the carburetor, at idle.
When you turn the idle mixture screw out, you are introducing more of the fuel/air mixture into the airstream, and making it richer.

The video that George posted has it correct, and that is how the CV carbs on our FJs are tuned.

Don't try to confuse him with "Turn it one way, then the other until it is better"...........................He wants to know exactly what it does when you turn it in, or turn it out.

I am not trying to discount what You or Randy are saying..........................

Ron, How many miles on your current set of carbs?  Have you ever replaced the needles, or the needle jets (emulsion tubes)?

[Mike Ramos]

Ron,

I do not know the intricacies of carb tuning as do the gentleman from RPM and mr. happyhooligan, however below is an earlier post relating my experience when my mileage dropped off similar to yours.

I have not utilized the new needle as I am still using the original ones.  Hope this helps.



"Howdy,

I have not followed this tread closely so perhaps the following has already been addressed.  

My mileage had dropped into the 36 mpg range.  I spoke with Randy and he suggested that the emulsion tubes may have worn.  Holding the tubes up to the light, sure enough they were no longer round but elliptical in shape.  

After replacing the tubes my mileage has returned to normal levels. Usually about 42 mpg regardless of how I ride - I did have a wind aided freeway cruise in the 70 to low 70 mph range of 47 mpg.  

There is a needle specific to RPM that I may eventually try to see if fine tuning the mileage/performance is possible but I wanted replace only the emulsion tubes first to see if that was actually the problem.  If & when I install the new needles I will certainly post the results.

Ride safe,

Mike Ramos."

[Capn Ron]

Ron, How many miles on your current set of carbs?  Have you ever replaced the needles, or the needle jets (emulsion tubes)?

Hey Alan!

The carbs are original to the bike.  At 20,000 miles, I had them cleaned and rebuilt by WiredGeorge.  There were no changes required...he asked me about pods and exhaust...and as I'm still stock in both of those areas, I don't imagine he changed anything.  I bolted them up two years ago and all ran well.  There are now 46,000 miles on the bike.

[Capn Ron]

Pilot screw adjustment and pilot jet sizing has a minimal affect on fuel mileage...

The tighter the idle screw setting the less of the a/f mixture of the pilot circuit is allowed into the carb throat.

So, your mileage issue probably have more to due with worn needles and elmusion tubes tgan anything.

I can tell you that both of the carbs you are comparing too were both built be my and all attention to detail is tended too during the build.

Randy - RPM

Looks like I'll have to tear the carbs down (sure was trying to avoid this) and get a good look at the condition of the nozzle.  Does the needle typically wear at the same rate?  As in...you always replace them in sets?

[racerrad8]

Looks like I'll have to tear the carbs down (sure was trying to avoid this) and get a good look at the condition of the nozzle.  Does the needle typically wear at the same rate?  As in...you always replace them in sets?

Yes, all four carbs run the needle at the same level and they wear pretty evenly.

Randy - RPM

[Capn Ron]

Oh...yeah, I assumed all four emulsion tubes would wear about evenly.  I was asking if the needle and tube in each carburetor are typically replaced as a set?

[Capn Ron]

Is this an accurate representation of how the carbs on the FJ work?

CV Carburetor

[FJ_Hooligan]

Don't try to confuse him with "Turn it one way, then the other until it is better"...........................He wants to know exactly what it does when you turn it in, or turn it out.

Go back and read the original post.  There's no need to try to confuse anyone, the information that's already out there is contradictory and confusing enough.

I was trying to focus him on the solution rather than trying to straighten out whether in or out is lean or rich or RPM hanging is lean or rich. 

That video was about a fuel screw. The FJ carbs do not have a "fuel" screw.

[racerrad8]

Oh...yeah, I assumed all four emulsion tubes would wear about evenly.  I was asking if the needle and tube in each carburetor are typically replaced as a set?

Honestly, it really depends on the air filter type, confirmed sealing and maintenance intervals.

It also depends if someone cut the index tab from the plastic needle holder.

Mike's emulsion tubes were worn but the needle did not show any wear. Those were a new set of carbs he installed several years ago and many, many miles. He used RPM pod filters exclusively.

You'll need to inspect to know for sure.

Randy  - RPM

[Derek Young]

Hey Ron. I find it strange that the worn emulsion tubes would show up at exactly the same time you changed the pilot jets. When you changed out the pilot jets, did you fully disassemble the carbs or only remove the bowls. Is it possible that the needles are set higher than they were before? Perhaps the plastic washer is below the clip?

Derek