Jetting Changes for Oxygenated Fuel

[luvtolean]

Thanks to the great info from you folks, and some help from my new pal SFO, I am going to start running Philipps B35. I have an 03 CR250 that currently is jetted slightly fat with a 410 main, 32.5 pilot and the stock needle and clip position. Air screw is at 2 turns out.

What changes should I make switching from a 50/50 pump/C12 mix to the B35? I will be running Mobil MX2T at 32:1. I know the B35 will need richer jetting, but I'd like to make sure I'm rich enough I don't lock it up.

TIA :)

 

[SuzookKING]

I would go 2 to 3 sizes bigger on the main to start with and fine tune from there. When I run B35 or a blend of fuel with around 15% MTBE, I have to go around 2 sizes bigger on main than when running straight B32 (non-oxy).

 

[MikeS]

If you are a tad "Fat" now then 1 0r 2 sizes should be enough. I have run MR2 and P45 with stock jetting that is normally fat with pump or C12.

Happy Fueling :)

 

[Senior KX Rider]

I went 1 on the main and 1 notch on the needle with my bike to change from pump fuel to MR2

 

[cp380sx]

I'd suggest running the B35 with your current jetting, then make adjustments as required.

 

[bclapham]

here is a couple stupid questions......

how do you recognise a lean condition when running a high octane gas? the reason i ask is that on pump gas, if i am lean there isnt really any real bogging, but the bike will ping. So what will i see with the race gas since the higher octane will resisit the pinging even if its lean?

How easy is it to feel a rich or lean main jet? eg a 250 on most tracks wouldnt see that much wide open (well with me riding anyway), so how should i look to optimise the main circuit?

 

[luvtolean]

Well, one of the ways I have been told to check jetting is to do "plug chops".
In my Honda manual, it gives a chart showing where each jet is most effective. For example, at full throttle, the main is the most important circuit. Just off idle, the pilot jet is important, and the needle has some effect as well.

Using this information you can go out and run the bike in an open area at the particular throttle postion you are interested in. After running at a specific throttle position, you hit the kill switch to kill the motor, and coast to a stop. Then you pull out the spark plug and check it. Some people mark the throttle so they can easily know what throttle position they are at.

Of course, other lean signs are poor cold performance, and the pinging you mention.

I'm sure others here have better on track riding inputs than I, hopefully they'll respond.

 

[cp380sx]

Originally posted by bclapham
How easy is it to feel a rich or lean main jet? eg a 250 on most tracks wouldnt see that much wide open (well with me riding anyway), so how should i look to optimise the main circuit?

My KTM 380SX runs well on B35 with stock jetting as long as I'm riding a SX type motocross track. All I do is raise the needle a notch or two depending on the temp.

On high speed sand tracks I get some pinging at extended WOT which requires a larger main.

I run straight B35.

 

[MikeS]

You need to find a wide open area to test the main jet.

As far ping on race fuel. If you go way lean you will here it. My 300 knocks on C12 with the wrong needle.

If you ride MX for fun then most of your time is spend on the needle. If you are way rich you will have spooge city. If lean it will bog.

One thing about Phillips fuel when you get it dialed in with a good oil it burns CLEAN. I had a hard time doing plug readings with it. I had to go 3 mains bigger to get carbon to show. I went back to near stock jetting for 70 degree temps and then went leaner when it got hot with P42(P45?) 2 stroke fuel.

 

[Jaybird]

I'm finding that oxygenated fuel is much more fickle than other fuels. Seems the slightest change in condition makes a change necessary.

 

[Rich Rohrich]

Originally posted by Jaybird
I'm finding that oxygenated fuel is much more fickle than other fuels. Seems the slightest change in condition makes a change necessary.

I've personally found the better the overall combustion and scavenging characteristics of the engine the less this tends to be true. I'm convinced (purely speculative on my part) that OXY fuels react differently in the presence of spent exhaust gases in the chamber than non-OXY fuels. By that I mean, the characteristics that Jaybird is describing seem much more pronounced when scavenging is poor and there is a greater percentage of residual exhaust products trapped in the chamber, like when a pipes characteristics are mismatched to the engine, or the transfers don't scavenge as well as they could. It seems to me that given the same engine, non-OXY fuels like C12 or B32 are less likely to be as sensitive to this. INcreasing compression and cleaning up the transfers along with reducing transfer port area on my 2001 CR125 made the bike much more tolerant of a wide range of weather conditions on various OXY fuel blends including nitro-propane-2 , compared to before the mods.

I'm trying to figure out if there is a reasonable science based cause and effect relationship to all this, but at this point it's mostly just anecdotal ramblings on my part. :thumb:

 

[bclapham]

Rich: in some cases anecdotal evidence has played quite an important role in the progress of modern science, so lets assume you are right on this one (just for now! :thumb: )
so in my application, eric cleaned up the cylinder and reduced the area of the transfers with epoxy so if what you have observed stands true, then an oxygenated fuel is a good way to go? or not? Additionally, if i go this direction, it would make the race gas cylinder head mods for more compression that much more worth while? or not?

thirdly, how can we tell if a pipe is mismatched? its obvious if you have low end porting with a rev pipe, but how about the SST for example. this pipe is supposed to be a good allrounder but how do i know if it matches up with my low end porting?

sorry about all the questions, if it makes you feel any better i still have another ten thousand that i want to find the answers to at some point!

thanks

 

[Rich Rohrich]

Originally posted by bclapham
so in my application, eric cleaned up the cylinder and reduced the area of the transfers with epoxy so if what you have observed stands true, then an oxygenated fuel is a good way to go? or not? Additionally, if i go this direction, it would make the race gas cylinder head mods for more compression that much more worth while? or not?

YES - I believe that the good oxy-fuels tend to be the way to go, and if you are willing to commit to consistent race gas use BY ALL MEANS get Eric to do the race gas setup. Making MX bikes live on pump fuel is de-tuning IMHO, so put the good stuff in and take maximum advantage of the power available to you.

Pipes are a much trickier answer. Matching pipe wave tuning to the existing time-area specs of the engine involves some deadly serious math. Without access to some type of simulation software like Tom Turner's TSR Pipe software it's mosty cut and try work and leveraging the existing knowledge of tuner's like Eric, Wes Gilbert, and the handful of other sharp guys.

The complexity of ACCURATELY simulating pipe interactions is the one of the main reasons for the existence for ultra-sophisticated programs like Optimum's Virtual Two-Stroke to exist.