Maintenance & Technical > KX500 Original

On the subject of race gas

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delphipro5:
After more thought I am thinking maybe 100LL AV may be the answer. I can prob. get it locally and store and use it rather easily. I NEED to figure this out because I have my Carb. at Dick's racing and I NEED to specifiy which fuel I will be using so they can jet it accordingly. How would I go about telling them I plan on using AV fuel? Will they be cool with that? Also I am sending my head to Larry Wiechman to have his Comp. Re. done and also the head work, so I will need to specify with him also the fuel used so he can machine it accordingly. Down to the wire now, so I need to figure this out this week by wed.  :|

kaw rider:
after you get your carb back from Dick's you will have to still rejet it because the K5 jetting will change with mods to motor or how well your motor runs.

Hillclimb#42:
The octane numbers are a little wacky, delphi. When it comes to the octane numbers and oxygenated fuels, there are alot of things to know and understand. The lower octane number of the u4 is a little deceptive. Its oxygenated properties make that a very effecient fuel that has a tremendous burn rate. Its similar to adding a supercharger, in a way, because you are adding oxygen into the actual combustion. Its not exactly clear to me what fuel is required per what mods. It has been stated many times that it depends on what your motor has done to it, but I have never really got a straight answer on what formula or process takes you to which octane. I had originally thought that the better, highest octane would be the best performance. There is another school of thought, that you just run high enough octane to get the job done, so as not to waste money on something not adding to performance.
  My eventual take on it is, C12 and U4.4 are both awesome fuels. U4 would likely out perform c12 or pump gas, in any motor. The 100LL I have no idea about. There is some confusion in my mind, if a higher octane than 100-110 would be "needed", but guys that have dynos have posted that the Q16 (116+, oxygenated and leaded) shows even better numbers. In general, pump gas is crappy and getting worse. All fuels will be less potent after exposure, but its not a major ordeal, like it seems. Exposed race fuel that has evaporated some, will still be good fuel. Its just that leaving it to vent in your tank, seems silly after you think about it. VP sells and stores every fuel in a sealed metal can, because its the only way to deliver it fresh to you. If you are riding the next day or two, it would be fine in the tank, IMO, just no reason to leave it for a month, race fuel or pump gas.

Jetting varies so much, for someone to tell you exactly what jets that you need would be tough. Some things will make the motor need leaner jetting and others  fatter jetting. Thats a deep subject. I think for the most part you are over thinking it. 100-110 octane is not much different from pump gas. I doubt these or c12 needs jetted much, if at all, and the U4 is probably only up one on the main. The trick is to be ready with a couple sizes of jets and be ready to get knee deep in the carb. Maybe go up one to start and see how it runs on a WFO run. Check plug and really listen and feel how the power comes on, and what it says when tapped. It may take a few runs, but once set-up, it won't change all of the time.
  Snows melting...., its almost time for dirt slingin'... 8-)

delphipro5:
I think I have come to a conclusion.
 The U4.4 is ROUGHLY the same OCT. as the 100LL, yes it is oxy'd but for the most part they shoud both be able to be used with little to no increase in the compression, which means I don't need to go nuts on my dome. Even VP says that U4.4 is to be a direct replacement for pump gas with min. Carb changes ( +2) . And for the most part I THINK I will be using 100LL, IF I can get it at my local airport readily without issue. That I will be finding out early this week! So I don't think I need to freak out calling dicks and telling then to stop everything, I will just call them and tell them what it is I am using and they can get the carb. close. U4.4 and 100LL should be close on thier jetting and if not, no big deal I will just have to jet myself to dial in.
As far as the head work I will specify what it is I will be using and that I really don't want to increase the compression by much. Idealiy I would like to be able to build this to run 100LL, to 50/50 pump 91-100LL(mix), to U4.4 - with nothing more than jetting changes. Which should be doable to my understanding.

If I can come about 100LL with ease this week then that is what I will start with, and work from there. I never thought picking a fuel would be so tough! But they have made it that way with having junk pump gas!  :-(

kaw rider:
becareful with U4.4 and other vp fuels it doesnt like hot weather or it will vapor lock.

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