Lore of the #20 Pilot

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radare
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Lore of the #20 Pilot

Post by radare » Tue May 23, 2017 9:19 pm

I started this forum back on November 7th, 2011 as a replacement for an even older Seca II forum called Seca2.forumup.org. You can still see it if you tune in the Wayback machine.

In the days before XJRider, the lore of the #20 pilot echoed through back alleys and basement-level stairwells as the fix-all for a host of fueling issues. From well into the darkness you'd here the whispers: "Gather around my dear friends, I would like to tell you about a cure for what ails you, be it sniffles, scurvy, stomach ailments, eye sight, nervousness, gout, pneumonia, hard starting, lean mixtures, crankiness, tiredness or plum just sick of life... Yes, sir, a bottle of my magic elixir will fix whatever ails you; the #20 pilot jet".

New Seca II owners would stare into that darkness with widened eyes and imagine such a wonderful world, full of power, throttle response, amazing cold starts and 4k smoothness. An how easily this utopia would be attained with the simple replacement of a few jets. . . . .

The notion of the #20 pilot jet upgrade has been around as long as I've owned a Seca II. It's always been the thing you do, right out of the gate. But why? And do we really know that a well-tuned machine runs better with #20's than with #17.5s? Do we know of any ill affects?

Why are you digging up such things, Radare? The Seca II gods have spoken and the #20 pilot jet is their answer. Well, I don't know. But I have a set of carbs in my ultrasonic right now and I'm thinking about going back to #17.5's. Why? Why again, radare?

You see, my good and patient reader, each of my XJ's has exhibited an odd behavior when warm; they get a bog right off idle when the throttle is opened. Every one of them and all with #20 pilot jets.

Now here's the kicker for me: I live at 5,280' in elevation. At this elevation, there is only 83% of the air that is present at sea level. That means, I'll need about 83% of the fuel I'd need if the bike were on the beach. Does it really make sense for me to have these #20 pilot jets?

(Oh, and interesting fact: On that Mt. Evans ride I take, there is only a bit over 50% of the air at its peak than at sea level)

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Re: Lore of the #20 Pilot

Post by TonyKZ1 » Wed May 24, 2017 7:46 am

I bought and installed the #20 Pilot jets and didn't like how rich my bike was running, so I removed them and went back to the original ones. Mine starts up easily with just a little choke/enrichment and I'm off down the road a few minutes later after checking my turn signals, lights, etc. I then turn the choke off about the time I reach the highway.
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Re: Lore of the #20 Pilot

Post by MechanicCo » Wed May 24, 2017 8:29 am

Radare how does your bike run at altitude with the 20's.

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Re: Lore of the #20 Pilot

Post by radare » Wed May 24, 2017 8:47 am

MechanicCo wrote:
Wed May 24, 2017 8:29 am
Radare how does your bike run at altitude with the 20's.
I have #100 mains and #20 pilots right now. It runs great but has a little bit of hesitation (or maybe stumble, still working through it) when hot, right off idle.

My other XJ (Scrambler) has #102 mains and #20 pilots. The off-idle condition is more pronounced in that bike.

I may try #17.5 pilots and #100 mains and see how my Wideband looks.

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Re: Lore of the #20 Pilot

Post by TonyKZ1 » Wed May 24, 2017 9:11 am

Those wideband sensors and readout meter sound like the best bet for adjusting these 4 carbs to get the mixture just right.
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Re: Lore of the #20 Pilot

Post by radare » Wed May 24, 2017 9:43 am

TonyKZ1 wrote:
Wed May 24, 2017 9:11 am
Those wideband sensors and readout meter sound like the best bet for adjusting these 4 carbs to get the mixture just right.
Idle mixture, anyway. Once the butterfly opens, ability to adjust the mixture drops considerably.

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Re: Lore of the #20 Pilot

Post by radare » Wed May 24, 2017 9:54 am

This information is stickied on the Wall but I'll share some of it here as it pertains to the discussion at hand.

Taken from Ian Williams Tuning: http://www.iwt.com.au/mikunicarb.htm

"Once the jetting is set and the bike is running good, there are many factors that will change the performane of the engine. Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temperature drops, the engine will run leaner and more fuel will have to be added to compensate. When the air temperature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32deg Fahrenheit may run poorly when the temperature reaches 90deg Fahrenheit.

Altitude affects jetting since there are less air molecules as altitude increases. A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air.

Humidity is how much moister is in the air. As humidity increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases.

Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes. The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct correction factor is found. Using fig 8 as an example, the air temperature is 95deg Fahrenheit and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.

Image


Correction factors can also be used to find the correct settings for the needle jet, jet needle, and air screw. Use the chart from fig 9 and determine the correction factor. Then use the table below to determine what to do with the needle jet, jet needle, and air screw.

Image


Original article was written by, and has been reprinted with the permission of Doug Jenks. All rights reserved.
smallengprep@yahoo.com"

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Re: Lore of the #20 Pilot

Post by radare » Wed May 24, 2017 10:06 am

Looking at the preceding post, my elevation is right at 5200 here in Gunbarrel. Assuming an average riding air-temp of somewhere around 68F, this puts me in the following range:

Image

This would give me a correction factor of .93. According to the article (I need to verify its validity) but that would give me a main and pilot jet sizing as follows:

Main: 102.5*.93 = 95
Pilot: 17.5*.93 = 16.3.

The point to illustrate here is that the elevation at which I live reduces the sizing by 7%. Adding a #20 pilot at my elevation is probably not ideal and using a smaller jet is probably better. For me, I think a #100 and 17.5 combination might be good. I'll do some tests and see.

But using that info as well as some other Mikuni charts, I've come up with the following notion regarding jet sizes for the stock Seca II (XJ600). What are your thoughts on this diagram, based on experience?:

Image

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Re: Lore of the #20 Pilot

Post by SpeedRacerOnline » Thu May 25, 2017 2:13 pm

Well, that sounds sensible to me. I've read some of the comments here and there about not being quite happy with the #20 change, and was always surprised. Changing to #20 was a massive improvement for my Ruby in almost every way. That last chart explains it, though. I live in the almost completely flat farm country of SE Michigan. The elevation here in Caro is around 725-750 feet, and we're kind of a high point in our area. I explored a few places I frequently ride on WhatIsMyElevation.com, and most of my travels are between 500-800 feet above sea level. According to that last chart, my current #20/#102.5 setup is ideal, which matches my seat-of-the-pants sensor observations. Since there's very little elevation change anywhere in Michigan, that setup just works for me all around. It would be quite different nearly a mile higher up.
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Re: Lore of the #20 Pilot - now edited with MPG!

Post by sleekitwan » Tue May 30, 2017 2:15 am

Altitude probably is the thing right enough in ur special case?!

I fixed my seca 2 pr divvie not rideable over 60mph on cool nights by the pilot jet being upped the main jet and needle from a dynojet kit, but mostly iam less than 500 feet above sea level.

There is agreat tuning guide somewhere, i will root around, basically i started with the top end, making 'Rosie' work well flat out in sixth up a long hill near us. Had the needle clip in middle notch at this time.

Then ride it around mid range, this was ok. The tuning guide points out the pilot jet effect is a critically-important one, it feeds in fuel THROUGHOUT THE ENTIRE RPM RANGE.

So, we can mask or alter a section pf the rpm or throttle position, with the 'wrong' method of adjustment. Hence the need for a methodical approach like this tuning guide gave!

U could also start from the bottom end of rpm and throttle position. I did NOT do this because to my mind, that's where all the complex interactions are in such flux! IE i made sure the simplest section was sorted first, the top third or quarter pf rpm/throttle position.

Will rummage for the guide, i think it was for a guy who specialised in Amal carbs for speedway or something, and a diagram showed which rpm/throttle positions were mostly dependent on which jet or component of the carburettor, the guy knows his stuff.

Is your bike turn of the century like mine? Yamaha played it lean as hell with these last bikes on carbs before fuel injection.

EDIT - The bike I have, Rosie, had given me consistently up to 60 mpg when I first got it, but the bike would not rev reliably over 60mph speed or so, cutting out when the air was say a cool summer night. Literally, I discovered, it was just running at that rpm and climatic conditions (I said 'climatic', just two c's) it would gasp and stutter. Almost undriveable.

I double checked my notes - it has the DJ102 now (Dynojet 102) main jet allegedly not comparable numbers to Mikuni or Keihin. The pilot I have indeed upped to #20, from #17.5. The bike when in mid-sorting, gave 40mpg and still wouldn't pull on the motorway (interstate) properly high up. ROSIE NOW GIVES 60MPG (Imperial gallon almost exactly 50mpg US) as normal, and runs properly, almost faultless I'd say, although warm-up is frankly still a crock, I hate riding it until 5 minutes have elapsed. The best I got fully laden and on flattish roads was 75mpg.

75mpg?! (62.5mpg US) You could have knocked me over with a diaphragm spring. I double-checked. I also have discovered how the figures I get where we reside are absolute 'pish'. (2 million people in a 1500-square mile area, the most densely populated part of the biggest county in England). In riding near home, Rosie only hits 62.5 mpg (imperial) or so. Go to Scotland or even the lakes, she's up to 65mpg immediately, often 67mpg (56mpg US).

None of this riding, is trying to be economical, is the great thing. I recall my old BMW 4-cylinder K100 (Roadster). 1000cc and very solid and nice. However, I never saw 46mpg no matter what the conditions or road types. 44 to 45 mpg, that was it. It lacked a 6th gear for a start, but it only had 8 valves, and despite fuel injection, I guess it just was not that efficient a bike.

The main other difference of course, between the Beemer and Rosie, is the mass. 250kg versus 200 or so? Once, I dropped it trying to turn in a gravelly car park. Two nice tradesmen guys in a van helped me at my asking. After they stopped laughing. Ha-bloody-ha.
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