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racing fuel vs. pump 91 oct.

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Can you guys tell me what kind of racing fuel you guys are using? What advantages or disadvantages are there in using racing fuel. I ride all the local mx tracks, I have been using pump 91 and have had no problems, but I always see the racing fuel cans at the track.

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VP ultimate 4. Its a bit pricey but it is absolutely impossible not to notice the almost 2 HP gain.

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with U4 do you to rejet???

You should to get the most gains....if jetting was perfect on pump gas, you WILL need to go richer with U4. If jetting was a little too rich with pump gas, you might get lucky and get real close just by running U4.

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Yes but most can't tell if they were rich on 4-strokes.

U4 is amazing stuff though. It's pricey but it works.

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I have been using U-4 and U-2 in 2 strokes and 4 strokes for years now. On the track and on the dyno, jetting with pump is real close to the same jetting with the U series race fuels. All this gota go much "richer" because of the extra oxygen really doesn't pan out if you look at the differences in the fuels end point temps. Pump fuel can easily go in to the 400's while the U4/U2 EP temps are in the mid to high 200's. This difference's pretty much cancels out each other at far as jetting goes.

High EP temps = leaner AFR

Low EP temps= richer AFR

The temp that fuel evaporates plays a huge role in jetting. A high EP temp fuel like pump gas runs lean because the fuel remains in droplets until the temp is very high. Low EP fuels like the U-4 and U-2 and VP C-12, the fuel completely evaps at a very low temp and the AFR(air fuel ratio) is richer.

So a fuel that has not completely evaporated at the time of combustion ( high end temp), means you have more air than fuel(that will burn), or a lean AFR.

This why most riders just dump in the U-4 and ride,, with no more jetting change than a fuel screw adjustment.

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U4 required 2 sizes bigger main jet on my 450.no guessing,its obvious on the real time a/f meter. :)

most riders can dump it in and ride becuase their jetting was to rich to begin with.

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8.50 per gallon is less than I pay for it (dealer) so buy it. Watch if it is the bottom of a 55 gal drum that is setting a while as that stuff can be bad.

The cam will make more power than the U4 if that is why you asked that :)

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:):p

someone who understands :)

This makes me a bit mad...I understand what he said, and I understand fuel lean/rich stuff perfectly fine. So don't go spouting crap insinuating that the rest of us are dumb or ignorant.

What pisses me off is the holier than thou attitude, and the laziness of spending $8 per gallon on race fuel (or more) and not even checking the jetting...what a waste of time. If you aren't going to at least check to see where you are, just put plain pump gas into the damn thing and ride it.

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I have been using U-4 and U-2 in 2 strokes and 4 strokes for years now. On the track and on the dyno, jetting with pump is real close to the same jetting with the U series race fuels. All this gota go much "richer" because of the extra oxygen really doesn't pan out if you look at the differences in the fuels end point temps.

This why most riders just dump in the U-4 and ride,, with no more jetting change than a fuel screw adjustment.

Well then I guess that makes you the resident fuel expert. "Real close" and "just right" are not the same. The difference could be as much as 1hp if you are off even one main jet size. Why not test and make sure you get the most bang for your buck out of that expensive race fuel? The fact remains that we all ride in different climates, temps, altitudes, humidity, etc. Most riders are off in jetting because we all don't live in 72f, 30% humidity sea level environments.

Spending big bucks on race fuel and not checking jetting is a waste of time and money and the reason most riders don't ever check is because they are too lazy to invest and hour of time dialing jetting in.

And I never said "much richer" I said if the jetting was spot on with pump gas, you will need to be a bit richer with U4. That is not speculation.

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This makes me a bit mad...I understand what he said, and I understand fuel lean/rich stuff perfectly fine. So don't go spouting crap insinuating that the rest of us are dumb or ignorant.

What pisses me off is the holier than thou attitude, and the laziness of spending $8 per gallon on race fuel (or more) and not even checking the jetting...what a waste of time. If you aren't going to at least check to see where you are, just put plain pump gas into the damn thing and ride it.

Rap

Sorry nothing directed at you by any means, you seem to know your stuff. :)

My :) was in general as most guys just have hard time understanding why some guys can go from pump gas right to U4 and have it run well. Regardless of the facts or notes taken by others one must check his own jetting. These 4 poppers are a bit different to jet at least for me as I have the 2 strokes down to a fine science....

Mike

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Sorry if I blew off some steam...from work I get pretty uptight sometimes because I WAS uptight about work...the whole vicious circle thing.... :):)

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Same here about Rap, my goal here was not to throw stones, but help riders think outside the box a bit. There is just a whole lot more to know about fuel and jetting, than it has lots of "02" so you must have to make a big jet changes for the richer.

It really depends on the pump fuel used and its true EP temps and the jetting used for this pump. Ive seen in the summer, higher fuel EP's in the 500's for pump gas, in this case the U-4 and U-2 actually made the jetting a bit richer. The added 02 wasn't enough 02 to make up the difference for the lean 500 EP number of pump vs the low EP number of the VP fuel.

So for this reason always check jetting whenever new fuels are tested.

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I've got another race fuel jetting question. Hope I am not opening a can of worms :) . I run C-12 in my engine and the stuff burns exceptionally clean.So clean in fact it is not possible to get a plug reading other than seeing if my engine is detonating.Its not running lean I do not beleive....infact even with the clean plug I think it is rich ,but because I cannot get a plug reading with the C-12 how do I jet this thing properly....I cannot read the plug...burns to clean....do I just use seat of the pants approach or what to get optimal results?I started running the C-12 because I installed a high compression piston...I have a CRF 250x

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C12 is clean burning. It may leave a yellow / green tint on the plug. Mainly from the lead.

As I mentioned the 4 poppers are alittle harder in mybook to jet. I was told by more than one tuner these bike rather be on the rich side then on brink of lean disaster like a 2 stroke. An A/F set up in the Exhayst pipe would help but that is not pratical.

The WOT side of jetting is the tough part. Needle is trail and error for me as you need to try a few setup to get the power or respnse you like. the main jet, well I have found to keep going rich, the bike will tell you fat if you are rich. The bike will start to blubber and the plug will get black or grey. Once you find the blubber point drop back a jet size or two and that should be fine. Naturally how the bike pulls / feels will let you know if you are going to far lean.

A dyno is nice here but a nice long dirt road or field can provide a good test bed. Get a helper who can time you with a stop watch and listen to the bike. Start the watch at a point 1/4 or half way down the stretch then stop the watch at the end. that will take the rider out of the equation some. Keep good notes.

One thing about C12, it is very consistant and will require a minor Air Screw change or 1 main size in a 20-30 degree temperture change Humidity will not kill it.. U4 or the MR fuels are not that stable :) .

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Here's a good link on how to properly read a spark plug. Notice what is called the fuel ring down inside the plug on the insulator. This is what you want to look at. It's width, I normally find 2-3mm is good power and safe, any thinner than 1 mm and it's getting dangerously lean. It's best to have a magnifier or even cut the plug open to view this black ring. Also note the plugs tip will show timing. This test is only for wide open throttle, the other ranges must be done by ride and feel.

http://tsrsoftware.com/images/read_plugs-tsr_software.jpg

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So pretty much tune the low speed and needle on feel and the main from a jet or two back from blubering rich?Wil I be able to see the ring at the base of the plug when running c-12?

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