Author Topic: What's Your Info??? - Carburetor Jetting 101  (Read 2185 times)

PilotSniper

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What's Your Info??? - Carburetor Jetting 101
« on: January 31, 2006, 09:15:43 PM »
I recently started taking a really, really good look at my carburetor's jetting and the relationship of jetting to EGT's. I'll get into the "why" on another thread, at a later date, which will bring my quest for information full-circle.

When I first purchased my Keihin PWK39, I did so because it was the next logical step towards the completion of the transformation of my Pilot from stock to modified. I performed the necessary modification to the throttle cable and the intake boot, installed it and forgot that it was even there. Yeah, not too smart, but I wasn't well versed in "carburetorese", so leaving it alone was the best option at the time. Just the other day, though, I decided to print up some great articles on jetting and read them over and over. Lots of experimentation, time, reading, a jetting kit and some nightly, after-work trips to the Everglades for test runs and I've got a good feel for what I'm doing with my carburetor settings (for the first time in my life).

Knowing that temperature, altitude and different setups on different machines play a huge role in which pilot jet, main jet, air screw turns, needle and needle clip position each one of you may have, I'm not looking for a "one size fits all" answer, but just a list containing your particular information with a small story behind it, if possible.

For Keihin PWK39 owners only, I'd like to see what you have installed, what modifications you have performed on your Pilots and, most importantly, what EGT readings you get at a particular throttle setting.

For example, I'm running a #58 pilot jet with the air screw at 1.5 turns out, a DGH needle with the clip in the #3 position and a #185 main jet. I'm trying to keep my EGT's under control, but I still get a spike of roughly 1,250 degrees that I haven't been able to control at just over 1/4 throttle position (which equates to roughly 5,800rpm's).

I read that aluminum (as in the material that the piston is made of) reaches its melting point at 1,220 degrees, so it's safe to say that everyone would want to run their EGT readings below this mark, right? What are your thoughts on this?

What is the highest EGT reading that you are comfortable with on your Pilot?

Also, isn't an accurate EGT reading better for jetting than the old plug-chop method? What's your opinion on this?

Please feel free to add your observations as you see fit. Please remember, this is a learning exercise for myself and anyone else who reads this, so let those creative ideas flow!!! (Secret Squirrel Stuff is also welcome here! :wink: )

Thanks in advance!!!
:)
I started out with nothing and I still have most of it left!  :shock:

ludedude

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What's Your Info??? - Carburetor Jetting 101
« Reply #1 on: January 31, 2006, 09:30:58 PM »
The most important thing to remember is...

Everyone's EGT readings are going to vary. The EGT readings you are getting are not necessarily what the temperature is at the piston, so a 1250? reading on your gauge may be fine for you...and it may melt someone else's setup.

Your readings...are just that, your readings. They will vary from others becuase of several factors, the largest being sensor location, and how far it is inseted into the pipe.

An EGT should only be used as a monitoring tool. You should jet according to plug chops, making note of what readings come with what plug colors. When you determine that 1300? is a nice tan plug, then you jet from there using your EGT readings.

I've run 4 differet pipes now, I think mine all come in around the same...I always try to put the sensor in the same location. I have mine EGT set to trigger the reverse limiter at 1300?....lights start flashuiig at 1275?, display (trigger)flashes at 1299?, at 1300? the limiter kicks in.

One thing I did find, was I could not get rid of a rough spot (not sure on the EGT readings) with my 37.5mm carb. Adding the dial-a-jet kit cleared that right up and made the whole rpm range run through smoother

Hope that helps some

PilotSniper

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What's Your Info??? - Carburetor Jetting 101
« Reply #2 on: January 31, 2006, 09:36:28 PM »
Great info, B!!! Thanks!!!

Keep the info coming y'all!!!
I started out with nothing and I still have most of it left!  :shock:

ludedude

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What's Your Info??? - Carburetor Jetting 101
« Reply #3 on: January 31, 2006, 09:41:44 PM »
I like to jet a little leaner on the top end than in the 1/2 to 3/4 range. Not to the point of on the edge, but a good crisp sounding exhaust, tan plug. then a little fatter from 1/2-3/4 so that when you get off the throttle after a WOT run it has a bit of extra cooling after that WOT stint. You're never on WOT for very long, so you can afford to have it a bit leaner (not a white plug mind you!!!) to get that extra bit of performance, then let off and back into a bit of a safety net zone.

Moskito

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Re: What's Your Info??? - Carburetor Jetting 101
« Reply #4 on: February 01, 2006, 05:37:50 AM »
Quote from: "PilotSniper"
I read that aluminum (as in the material that the piston is made of) reaches its melting point at 1,220 degrees, so it's safe to say that everyone would want to run their EGT readings below this mark, right? What are your thoughts on this?

What is the highest EGT reading that you are comfortable with on your Pilot?


Wrong on that melting point.

Aluminum starts to melt closer to 1400 - 1425.  Around 1400 the edges of the piston will start to deform and they start letting go (leaving the crown) around 1425.

Jet so that on a long wide open run @ full throttle you see a peak of around 1275.  A spike of 1300 won't hurt anything either.

You probably want to jet so that a regular top end run (think long trail at Ocala) brings you into the 1250 - 1275 range.

When racing short course (on/off throttle - usually either full open or fully closed) peaks of 1325 are alright - but remember, its a SHORT race with little time of running WFO - short bursts are the key - you can go slightly leaner than you would on a long track or outdoor play.

Now, you also want to take into account that these settings are more race oriented.  For longevitie's sake, keep peak temps down closer to 1200 and the engine should be a happy camper.
Moskito - Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming \'WOW-What a Ride!\'

PilotSniper

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Re: What's Your Info??? - Carburetor Jetting 101
« Reply #5 on: February 01, 2006, 09:00:25 AM »
Quote from: "Moskito"
Quote from: "PilotSniper"
I read that aluminum (as in the material that the piston is made of) reaches its melting point at 1,220 degrees, so it's safe to say that everyone would want to run their EGT readings below this mark, right? What are your thoughts on this?

What is the highest EGT reading that you are comfortable with on your Pilot?


Wrong on that melting point.

Aluminum starts to melt closer to 1400 - 1425.  Around 1400 the edges of the piston will start to deform and they start letting go (leaving the crown) around 1425.

Jet so that on a long wide open run @ full throttle you see a peak of around 1275.  A spike of 1300 won't hurt anything either.

You probably want to jet so that a regular top end run (think long trail at Ocala) brings you into the 1250 - 1275 range.

When racing short course (on/off throttle - usually either full open or fully closed) peaks of 1325 are alright - but remember, its a SHORT race with little time of running WFO - short bursts are the key - you can go slightly leaner than you would on a long track or outdoor play.

Now, you also want to take into account that these settings are more race oriented.  For longevitie's sake, keep peak temps down closer to 1200 and the engine should be a happy camper.


Skeeter, if you know, are pistons made of aluminum or an aluminum alloy? The reason I ask is because I just double-checked the melting point and it came back the same as what I saw the other day. Look here:

http://www.chemicalelements.com/elements/al.html

It's definitely 1,220 degrees Fahrenheit, according to this site and similar periodic table websites.

I read another article online a while back written by someone that stressed this point and jetted so that his peak EGT reading would not go into the 1,200 degree mark, hence my confusion. After reading what your particular experience is in this matter, though, I feel much better about where I'm at with regards to my particular jetting and EGT temperatures.

Please keep the info coming fellas!!!

Thanks...
I started out with nothing and I still have most of it left!  :shock:

PilotSniper

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What's Your Info??? - Carburetor Jetting 101
« Reply #6 on: February 01, 2006, 09:37:51 AM »
Quote from: "ludedude"
I like to jet a little leaner on the top end than in the 1/2 to 3/4 range. Not to the point of on the edge, but a good crisp sounding exhaust, tan plug. then a little fatter from 1/2-3/4 so that when you get off the throttle after a WOT run it has a bit of extra cooling after that WOT stint. You're never on WOT for very long, so you can afford to have it a bit leaner (not a white plug mind you!!!) to get that extra bit of performance, then let off and back into a bit of a safety net zone.


B, I see your logic and it's and understandable point. Here's the problem, though, I do have opportunities here where I could go for miles and run at WOT. Just the other day, I was riding on what amounted to a 12-mile long straight-shot levy run at WOT at the Rotenberger Holey Lands.

http://www.sfwmd.gov/org/ema/envmon/wqm/holy/holyindex.html

Also, out in the planting fields of Homestead, we always ride the long access roads at WOT.

This brings up another interesting point: Should anyone be running their Pilot at WOT for long periods of time? Should you just back down to 3/4 throttle on long runs?

What are your thoughts???
I started out with nothing and I still have most of it left!  :shock:

Moskito

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Re: What's Your Info??? - Carburetor Jetting 101
« Reply #7 on: February 01, 2006, 04:15:27 PM »
Quote from: "PilotSniper"
Skeeter, if you know, are pistons made of aluminum or an aluminum alloy? The reason I ask is because I just double-checked the melting point and it came back the same as what I saw the other day. Look here:

http://www.chemicalelements.com/elements/al.html

It's definitely 1,220 degrees Fahrenheit, according to this site and similar periodic table websites.


Definitely not pure aluminum.  Yea, it melts quite early...  (funny you mentioned the periodic table and such - I was trying to find melting temps for aluminum alloys before I posted my reply).

From what I recall and understand, most pistons are high in silicon content aluminum along with other stuff in the alloy.

The alloys are designed to combat heat related issues, make the piston stronger, less maleable and such.

Here, from the Machinist's Handbook:
Aluminum alloys:

1000 Series - high corrosion resistance, high thermal and electrical conductivity, lowe mechanical properties and good workability.  Iron and Silicon are the main impurities (additions to make the alloy)

2000 Series - Copper is the prinicpal alloying element - heat treated the properties can exceed mild steel.  Not as good of corrosion resistance as most other aluminum alloys.  2024 is the best known and most widely used alloy in the aircraft industry.

3000 Series - Manganese (1.5%) is major alloying element - doesn't like to be heat treated.  3003 is great for forming things like a fuel tank and airboxes.  (just picked up a 4x8x.080 sheet to form the Dez's new airbox this afternoon)

4000 Series - Silicon is the major alloying element.

5000 Series - Magnesium is the alloying element.  Works better than Manganese - .8% Magnesium = 1.25% Manganese and you get a harder aluminum alloy.

6000 Series - Silicon and magnesium alloying elements.  Heat treats well.

7000 Series - Zinc with small amounts of magnesium.  copper and Chromium may be added.  7075 is most "famous" of the 7000 series alloys - great for stressed air frame structures.

Alloying elements may/can be:  (percentages are ranges found in different types of Aluminum Alloy)
Silicon - .1 to 13% (not found in all alloys)
Iron - .15 to 1.4% (not found in all alloys)
Copper - .05 to 10.7% (not found in all alloys)
Manganese - .03 to .6 % (not found in all alloys)
Magnesium - .03 to 10.6% (not found in all alloys)
Chromium - .2 to .6% (not found in all alloys)
Nickle - .05 to 3.0% (not found in all alloys)
Zinc - .1 to 7.0% (not found in all alloys)
Titanium - .04 to .3% (not found in all alloys)

Sorry I can't find heat ranges.  I've looked in a few of my old Mechanical Engineering books and can't find squat.
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PilotSniper

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What's Your Info??? - Carburetor Jetting 101
« Reply #8 on: February 01, 2006, 05:03:26 PM »
This is excellent info, bud!!! I had no idea that aluminum was "categorized" in such a way. I imagine that these pistons must be some sort of alloy specifically designed to withstand extreme temperatures, while remaining as light in weight as possible. This in itself tells me that 1,220 degrees is a fictitious threshold, which is excellent news! This makes your previous data even more valid.

Thanks again, Skeeter! I think it's time to let the cat out of the bag and start another educational thread about the dangers of ambient temperature fluctuations and their effects on EGT readings and jetting. That will help clarify my sudden interest in carburetor jetting and bring this whole thing full-circle... :oops:
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ludedude

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What's Your Info??? - Carburetor Jetting 101
« Reply #9 on: February 02, 2006, 07:07:15 AM »
Quote from: "PilotSniper"
Quote from: "ludedude"
I like to jet a little leaner on the top end than in the 1/2 to 3/4 range. Not to the point of on the edge, but a good crisp sounding exhaust, tan plug. then a little fatter from 1/2-3/4 so that when you get off the throttle after a WOT run it has a bit of extra cooling after that WOT stint. You're never on WOT for very long, so you can afford to have it a bit leaner (not a white plug mind you!!!) to get that extra bit of performance, then let off and back into a bit of a safety net zone.


B, I see your logic and it's and understandable point. Here's the problem, though, I do have opportunities here where I could go for miles and run at WOT. Just the other day, I was riding on what amounted to a 12-mile long straight-shot levy run at WOT at the Rotenberger Holey Lands.

http://www.sfwmd.gov/org/ema/envmon/wqm/holy/holyindex.html

Also, out in the planting fields of Homestead, we always ride the long access roads at WOT.

This brings up another interesting point: Should anyone be running their Pilot at WOT for long periods of time? Should you just back down to 3/4 throttle on long runs?

What are your thoughts???


Do you still have control of the choke with the stock choke control with that carb?

If so, here's what you do. On long WOT runs, pull the choke on :) It'll richen up the jetting for the long extended runs, yet leave your jetting crisp for shorter WOT bursts.

I use the choke on WOT to my main jetting. If you can let off WOT with the choke on and it doesn't burble, you're lean and in danger. If it as soon as you let off WOT with he choke on it responds poorly...you're in a safer zone.

Play with that and watch your EGT temps when you pull the choke on and off at WOT.