Box Stock

Running Lean on Spec Jet

DynoDon said:
I just took a 212 down to the bone. It was an engine we confiscated after a prestigious "predator cup" race. The plug was a 3910 and as lean as a bone. 16 / 31 jetting
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Old story; a friend of mine, running under the Mike Manning banner, at a 2 cycle race in Southern California, where he had fast time and won all 3 heats, who I was sitting with in his pits, when Mike came over and said he wanted to check the plug. No problem, go ahead. When Mike saw the plug, his eyebrows shot up, his comment "this is way too lean, there's no color on the plug at all". Me and my friend just looked at each other and smiled. At my urging, early in my friends racing efforts, I taught him the care and feeding of the EGT. He learned his lesson well.
 
DynoDon said:
Lean is good
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"Lean is good"! So true! But there is a problem if you go "too lean". You get detonation, which is caused by excessive heat and/or pressure. Have you ever seen somebody pulling out of the pits with their foot to the floor. Sometimes, the clutch is working really good, then it locks up and the RPMs drop and the kart continues on its way. The centrifugal clutch is a pretty good device and when the power stays consistent, it works pretty good. But sometimes, if you're low-speed jet is a little too small, and you're standing on it and the clutch is holding the engine at peak torque, and the pressure and/or temperature gets a little too high, causing some detonation, the power drops very quickly. This causes the clutch to engage, dropping the RPM, then the detonation stops, and you go on your merry way. I 1st got a real firm understanding of this phenomenon at a 2 cycle race in California back in the early seventies. Coming onto the straight, with the centrifugal Burco clutch, the engine would pull real good and then the RPM would drop, the clutch would engage, and off I'd go. I kind of had a theory of what was happening, so I started opening the low-speed jet, just to test my theory, coming out of that turn. 3 or 4 more passes, every time opening the low-speed jet just a little more, and that RPM drop, when the clutch locked up, went away.
 
Lean is mean! But on a serious note, you can run a 4 stroke, low RPM engine like these lean and it will be fine all day. Do that once in a 2 stroke and you're going to stick it. Two totally different animals.
 
Close to lean, lean, very lean, too lean! A few points on the air density scale can easily take you from one to the other. If you had an EGT, it would be very easy to see the difference in the exhaust gas temperature between "close to lean" all the way to "too lean".
 
We've been watching air density lately here in Oklahoma.In august dry density was 4200ft, last night 2400ft ,i wasn't paying close attention but in January around 1000ft
 
I sometimes wonder how people can go out and spend $5000 on wheels and tires and won't spend $100 on an air density gauge.
RCJ, if converted to air density numbers, what would those altitude numbers you posted say?
 
i have the performance trends weather station in my dyno. here are some numbers from last week, baro 29.65,temp 92.4 ,dew point 74.7 .It converts these to- density altitude 2811 dry density altitude 3431.As you have noted it the past 1 jet number can be a big change ,so i am working on a data base.
 
Just for reference the observed hp at that density was 9.99 .when corrected to std race dyno of baro 29.92,60deg, 0 humity 10.91hp.That is with an AFR of 13.5 ,witch gave best power.
 
RCJ said:
Just for reference the observed hp at that density was 9.99 .when corrected to std race dyno of baro 29.92,60deg, 0 humity 10.91hp.That is with an AFR of 13.5 ,witch gave best power.
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Strange, using the latest SAE air density numbers, 29.235 and 77°, my calculations come out to 1.0 air density. Meaning, 9.99 HP would be corrected to 9.99 HP. I'll be looking into where the discrepancy is coming from. I'm a stickler for accuracy and when I see discrepancies like this, I immediately want to question my calculations.
 
I put your numbers, what you gave me, into my dyno sheet, and at 4000 RPM (just a guess, you didn't give me that number) and 1312 foot-pounds of torque, the calculated horsepower was 9.99. With your baro and temperature numbers, the correction factor was still 1.0.

Years ago, when I 1st developed my spreadsheet, I used the old SAE standard numbers, just as a test, and found the calculate horsepower, compared to the new numbers, was almost identical.
You might be interested to know that as the temperature goes up and down from the old 60° standard, the calculated horsepower numbers start to very the father you get, on either side, of that 60° temperature. That's the reason for the new standard. 77° "tends" to be more in the middle of the variation in temperature you might normally experience. I can't help but wonder if the writers of the software for dyno's understand this?
If my numbers are in error, I would sure like to find out where I made my mistakes.
 
lets start with a new dyno pull
observed hp 9.99 at 5500
corrected hp10.91 at 5500
correction factor 1.092
weather 89 degrees,29.49 baro,78.5 dew piont, at time of pull
air density 2838
dry density 3547
weather was corrected to 60degrees,29.92 baro ,0 humitiy to get the corrected number of 10.91
The SAE correction (77 degees) is a more accurate correction ,but the shop I used for years used the old factor so i just stuck with it.
 
if that's a stock out of the box engine your Dino is reading high. Peak horsepower on my dino water break is 4900 RPM and that number is only 8.9.
 
The details I gave are nothing hidden if you know your predator out of the box rules. 16/31 happens to be the stock jetting up or down (1) on each. You guys should both freshen up before you are so quick to criticize
 
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