Author Topic: Ambient Temperature And Its Effects On Jetting  (Read 1471 times)


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Ambient Temperature And Its Effects On Jetting
« on: February 01, 2006, 06:57:10 PM »
Well, I promised to update all of you as to why I had a suddent interest in learning about carburetor jetting, so here it is. As I mentioned before, I had purchased a Keihin PWK39 carburetor a while back as part of my Pilot modification project and installed it without learning the basics about jetting.

The normal ambient temperature here in Miami is roughly 80 degrees Fahrenheit. Taking this into consideration, my EGT readings would normally top out at around 1,325 at WOT runs. Yeah, I was running dangerously lean, but to me this was normal and since I didn't know any better, it became completely acceptable.

Fast forward to three weeks ago at the Rotenberger Holey Lands ride. Odykid, myself and a few friends decided to meet out in the sticks in the middle of the Everglades. That Sunday morning, something very strange was happening in South Florida. The ambient temperature was hovering in the mid-40's!!! Little did I know that I was about to take a brand new top end and commit suicide with it!!! I unloaded my Pilot from the trailer and proceeded to the long levy road at WOT. Like the brain surgeon that I am, I completely ignored the four EGT warning lights that came on and blinked violently.

[insert humor]
I had even activated the human voice option on my Digatron unit and it was yelling "Dickhead! Lay off the throttle!", but I didn't listen.
[/insert humor]

Even with all of the Digatron buzz surrounding me, I proceeded to keep the throttle pinned down all of the way. It was almost like if I was in some sort of trance, mesmerized by the natural beauty and serene tranquility of the undisturbed Florida Everglades. Inside my head, my thoughts were those of justification. I kept thinking "Yeah, the EGT lights are all blinking, but maybe that sensor is just bad". Well, as the reading on the big LCD made its way up to 1,392 degrees, I heard a very faint "pop" and then rolled to a complete stop. At this point, I literally laughed at my stupidity and began mentally sketching thoughts of what a melted piston actually looked like. I rode a grand total of 2 minutes that beautiful, cold morning. That equated to about 1 minute of ride time for every one hour of travel time. Fortunately, Odykid saved my ass and towed me back to camp. We had a blast anyways, but obviously, my Pilot played no part in this fun.

Upon getting home, I decided that there was no better time than the present to begin learning about carburetor jetting. I scoured the internet for information and read for hours. I printed up a couple of good articles, too. I read and re-read each one until I couldn't soak in anymore. Today, as a result of this detonating setback, I have a very good understanding of how a carburetor works. After rebuilding my top end again, I acquired a full set of pilot jets, main jets and needles and I have tinkered to my heart's content. Almost every evening, upon arriving home from work, I change one more setting, load up the Pilot on the trailer and take her for a spin on the access roads of the Homestead corn fields. Every day I acquire more and more knowledge related to cause and effect between one single carburetor tweak and its relationship to EGT readings, throttle response and engine crispness (or lack thereof).

I guess what I really want to pass on to the next person is simply this: LEARN TO JET YOUR CARBURETOR, NO MATTER WHAT!!! Like everything else, I thought it was something I could never learn. Once again, I proved myself wrong. You need to understand the how's and why's of your carburetor because if you don't, you'll end up just like I did!

Well, most importantly, that cold, irregular morning taught me that cold air is denser than my usual 80 degree air. What happened that morning is simple, my air/fuel mixture was thrown completely out of whack due to more air going into the combustion chamber. My normally cutting-edge lean condition at 80 degrees of ambient temperature quickly turned into an overly-lean, detonating situation that 40 degree morning. Here's what that whole mess ended up looking like when I took the head off:

By no means of the imagination am I done with this project yet. I'm still tinkering, daily if I can. I want to prove to myself that I can take a throttle range and completely control its EGT range by tweaking "this" or "that". It may be a little extreme, but it's just my way of learning.

Well, I hope you enjoyed my little fiasco. I know I did! :wink:
I started out with nothing and I still have most of it left!  :shock: