Tuning the idle jet with EGT

[Ray Lovestead]

In the past Al Nunley (where is Al?) mentioned that you should use EGT to tune the idle jet as well as the main.  Can anyone out there give me their secrets to tuning the idle circuit?  I’d like to use EGT, but I can’t figure out how I’d do that..

 

"Karting Expert Since 2015"

[Ray Lovestead]

No one out there knows this answer?  What happened to Al?

"Karting Expert Since 2015"

[John Savage]

Why on earth would you want to do that?

Jetting and Kart Set-up Software

http://www.rotaxjetting.com

[Paul Kish]

 

Ray,

I don’t know how to do it on your engine but I think the why of it, is in general a properly working idle leads to a more >’responsive'< engine.

I know for fact it’s important in multiple cylinder engines and I assume it is the same with a single cylinder engine.  Generally most any engine has an Idle, Intermediate(low speed) and High speed circuit.  Proper tuning of each, requires the preceding circuit to be in tune.

Brake, Insert 'arc', Turn, Accelerate

[Ray Lovestead]

wow.  I ask how to “tune the idle circuit using EGT” and I get:

1.  Why would you want to do that?

2.  It’s important and should be done.

OK, anyone else want to take a stab?

 

"Karting Expert Since 2015"

[John Savage]

Sorry Ray, but a racing engine is NOT a road car’s engine

Set the idle speed for a smooth idle and put all your effort into needle clip and mainjet adjustment in response to changes in air density.

Jetting and Kart Set-up Software

http://www.rotaxjetting.com

[Eric Davis]

Ray,

I have experimented with using EGT to tune bottom-ends by by logging patterns in EGT temperatures under consistent weather, oil, gas, engine setup, and other conditions. .  My preference is to tune the bottom-end of 2-stroke engines based on length of stable idle, throttle response, corner acceleration(s), and overall temperature.

Focus on selecting a good combination of idle jet (or inner/outer idle jet for Dellorto), air screw adjustment, and idle speed that allows the engine to idle around 2k for 5-10 seconds. Then, you should notice the engine starting to load up on fuel, and the idle speed should start to slow down.  When you blip the throttle to clear the engine, the engine should stumble, then clear.  If you notice a pattern where the engine idles for a long period of time (e.g. 20 – 30 seconds) and/or bogs on quick throttle blips, then your bottom-end is too lean (General Rule).

The easiest way to finalize your bottom-end adjustment is to ask the local top racers what their jetting and air/idle screw adjustments are, and ONLY change the air idle screw setting to identify the best setting for you.  For example, if the local racers are using a 60 outer idle on a Dellorto, with the Air Screw at 1.5 turns, start there.  Then, adjust your air idle screw inwards 1 full turn, blip/clean the throttle, then notice the difference.  Do the same process at 2.5 turns out.  You’ll definitely notice some changes, and be able to make a more educated decision for your environment, weather, engine, etc.

You can get a good understanding of the bottom-end, its’ principles, and tuning approaches by reading A. Graham Bell’s book “Two-Stroke Performance Tuning”.  It’s one of the best books on the market, and thoroughly explains quite a few key concepts.

Also, if Riley Will is lurking around here, he’s probably forgotten more than I know. :)

[Ray Lovestead]

Thanks for the great info Eric.

I’ve got Alexander Graham Bell’s ;) book around here somewhere.  I read it a lot.

I’ll write down your notes and try them at the track this weekend.

Ray

"Karting Expert Since 2015"

[Ray Lovestead]

p.s.  I contacted Al and he gave me more info on using the EGT for the bottom end.  I don’t know why he hasn’t come over to here..

 

Ray

 

"Karting Expert Since 2015"

[Walt Gifford]

I would pick several critical rpms and look at the egt at those rpms and see what changes happen with adjustments. Highest egt at the lowest rpm on track might not be the best for lap times if it starves for fuel when you punch it so, you may have to develop good numbers with the seat of the pants indicator.

Gif

[Rob Kozakowski]

Ray, would you care to share what Al told you?  And what engine / carb are you asking about?

The problem with your question is that it seems to be asking one question, half of which relates more to 1 style carb and the other half relating more to a different style carb.

Al has extensive knowledge of the Walbro, Tilly, etc carbs where you have needles that you can use to tune while you are driving.  In this case, you can definitely use the EGT to help tune the low speed needle “on the fly”.

When it comes to tuning an idle jet in a Dellorto or similar carb with jets that can be changed, like a Dellorto, John Savage has a lot of knowledge, and I’d agree with him.  Find a smooth idle (EGT not very important here), and focus mostly on needle/clip/atomizer and mainjet, and adjust to changes in air density.

[Ray Lovestead]

Sorry I lost sight of this thread Rob.  Here is Al’s response to my question about the small circuit tuning:

 

People have a misunderstanding for the reason the low speed number on the EGT being lower than the number on the top.  The EGT number is lower because of the greater heat and/or pressure, (the effects of the pipe) on the low speed verses the high speed.<br clear=”none” />Just like on the high speed, you want the EGT to read it’s highest when coming out of a tight turn.  If the temp goes down, you need to richen the mixture.  If it goes down further, it may be because the engine is too rich, so then you can lean it out and see what happen.  It’s a challenge I know, but it’s what you have to do.<br clear=”none” />Detonation occurs long before you can hear it, and when it occurs, it’s because of excessive heat and/or pressure.  <br clear=”none” />I have no experience with your carb, but I doubt, (a guess really) that the clip position will have much to do with the low speed.  Play with it and find out for sure.  Let me know, I always like to expand my knowledge base.<br clear=”none” />The EGT is very dependent on the ambient air conditions, you can’t just set it and forget it.  Get an air density gauge and compare it to your carb settings.  Over time I think you will see a pattern emerging between the two.  Low air density, less fuel, lower temp, and the opposite with high air density.  Engine compression is also something to keep a record of as the engine gets time on it.  Low compression will also affect EGT temps.  Low compression, low temps

"Karting Expert Since 2015"

[Rob Kozakowski]

Some good info from Al, however I think his lack of knowledge of the Dellorto and similar carbs is a bit off when he says he doubts the needle clip position has much impact on the low speed.

Ray, do a google search for Rotax carb tuning, and go to rotaxjetting dot com and do some reading – John Savage provides some good info on the Dellorto’s – it’s obvious that the needle clip position is very important for low to mid range performance.  Also, Tom Barth has some good info on his site, greenflagkarting dot com

While a “carb is a carb”, there are some practical differences in how most approach the tuning of a Walbro on a KT vs. a Dellorto on a Rotax.

The beauty of the Walbro is that it can be adjusted on the fly so you can reallly fine-tune your carb to ever-changing conditions.  While I doubt that anyone would ever dispute the value of the EGT for tuning these carbs, there are LOTS of drivers who can tune these carbs just as “effectively”, with experience, by the seat of the pants.  The benefit to this is you focus more on your driving and less on your gauges.  The downside is if you don’t know what you’re doing, you can easily go “too far” or the wrong way and stick the engine.  An air density gauge in this case will help you learn and predict where you might want to set things for different conditions, but you can get by just fine without.

The beauty of the Dellorto and similar carbs is that they can’t be tuned on the fly, so who really cares what your gauges are telling you in real time while you’re driving – especially now with capabilities of the Mychrons, etc to store other data for analysis after you’re off the track.  The downside is they are very complicated to fine-tune to extract maximum performance from them – especially idle to mid-range, which is progressively affected in different ranges by various pilot jets, air screws, selection of needle, needle clip position, emulsion tube, etc.  Fortunately, top end is much more simple because you’re really just talking main jet.   There is almost no disputing that an air density gauge or weather station is critical to getting maximum performance from these carbs.  The trouble is conditions can occasionally change faster than you can adjust for, so while you might be able to tune to a set of conditions before you hit the track, if those conditions change while on the warm up lap, you’re suddenly not getting maximum performance.  The counter argument to this is that it becomes so hard to tune for “maximum” performance, that most people will be happy enough to accept something “close enough”.  One of the articles on Tom Barth’s site summarizes these carbs best when he says something to the effect that these carbs really are meant for serious tuners, otherwise, just run the thing, read the plug and change the main jet.

Which goes back to John Savage’s advice of setting the idle speed for a smooth idle and focus on mainjet and needle clip position in response to changes in air density.

[Thomas Barth]

This how I do it (Dellorto).

Before installing carb, look from rear at slide….adjust just a little gap in slide…set idle/air to 2 – 2 1/2 turns out….install carb, start engine set idle at preferred idle speed…accelerate engine quickly…if there is hesitation set needle clip to top position (lean accel setting)…normally engine will accelerate cleanly when you get to this point. If not adjust idle/air screw until accel is clean. Note, the clip position is used to raise or lower needle for timing of fuel to engine when accelerating (does not alter mixture – just timing).

[Scot Smith]

Rodney I sent you a PM

[Oscar Aguilera]

havent been on here in a while…if you are tuning a non programmable ignition motor with a shifter type carb…ie mikuni,,,delorto…etc i was taught this method

 

my top end goal egt would be 1250 for me on my one motor…(i had another motor that liked 1300)  to play it safe go for 1200.

i would sit in the kart with the motor off and the aif filter off and try to hit 1/2 throttle with my foot while some one verified if i was at half throttle…i would train myself to hit this spot as consistently as possible…then 3/4 throttle…then 1/4 throttle.  verifying each time till i could get a consistent feel.

 

i would then go on the track and about just past half way down the straight…i would go from full throttle back to 1/4….look at the tach and remember the number.  you are looking for around 1050.  then on the next lap after hitting full throttle down the straight just past the halfway point…i would back off to 1/2 throttle…remember the temp…i was looking for 1100….at 3/4 throttle i was looking for 1150.  if your high end temp is higher you adjust your range accordingly…you are looking for 50 degree increments going backwards from your high end of the straight desired temp.

 

check the plug…learn to read it…verify…get a feel for the egt your motor likes with your particular gauge and you will be able to better tune your motor.  it takes time..the ability to read the gauge…and a few uninterrupted laps.  this helped when traveling or tuning the motor for changing weather conditions…which you always chase.

 

just my opinion

[Ray Lovestead]

Man that seems like a really difficult way to tune.  I don’t think I’d be able to get that to work for me.

How did you come up with the 50deg increment with respect to 1/4 throttle increments?

Ray

"Karting Expert Since 2015"

[Oscar Aguilera]

my old retired engine builder taught me how to do this…paul leary

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