1996 Honda CR125 jetting 4500ft

[Lee237]

Any guys running at this altitude(4500ft)?
I'm currently running:
50 pilot
160 main
Std needle 2nd from top
Air screw 2 out.
Fmf fatty and powercore silencer.

Bike runs fine,just want to hear if I'm not to lean,ticking time bomb area lean.
Been looking at fmf and pro circuit specs but they all spec sea level jetting options.

Thanks.

 

[Rich Rohrich]

At the risk of being the bad guy I'll tell you the big secret of tuning that rarely gets told these days. Unless you learn to jet your own combination all are doing is guessing.


If you're interested in some specifics keep reading.

The reality is all those smart guys you read about in magazines and on the internet DON’T KNOW WHAT YOUR JETTING SHOULD BE and either will anyone out here. There is no magic combination of jetting that will work for all situations. Even two seemingly identical brand new engines from the same year and the same manufacturer will tend to have radically different jetting requirements. Engine tuning is a dynamic process just like suspension tuning. It needs to change with conditions, YOUR conditions on your engine.

I know right now you are thinking, but those guys I read about are ace tuners so they must know what jetting to use for my engine at 4500 ft elevation. Sorry but what they know is the jetting required for a specific engine they built with a specific combination of mods and maybe most important a specific type of fuel under a specific set of atmospheric conditions. Chances are good they have a notebook full of notes that will let them zero in on a jetting combination fairly quickly once they know what all the variables are. They don’t know all your variables so they don’t know what your jetting should be. Add in the fact that your bike is 22 years old and it's true mechanical state is unknown to anyone out here means that anyone who is offering advice is just guessing and trying to be polite.

The good news is there are some rules of thumb about tuning that you can use to help answer your question by examining some of the details of your setup.

Keep in mind that the following are generalized rules of thumb not carved in stone facts:

Engines tend to run richer at higher elevation than they do at sea level. If you use generally safe sea level specs, meaning they aren't razor sharp and on the lean side at sea level then you are likely to be on the safe side of lean as the elevation increases.

Oxygenated race fuels "tend" to run leaner than race fuels without added oxygen bearing component like MTBE, Ethanol, Methanol etc. If you run race fuel of any type look for something that has a 0% oxygen content. Fuels like VP C12 are easy to tune and provide a huge margin or error especially at higher elevations.

Oxygenated pump fuels "tend" to run leaner than pump fuels without added oxygen bearing component like MTBE, Ethanol, Methanol etc. If you are using standard pump premium try and find something that doesn't have any of the above listed additives. Boat yards and small airports tend to be a good source for clean alcohol free premium gas that your engine will love.

I hope this provides some help. Learning to do your own jetting will provide you a giant margin of safety and chances are good your bike will run much better than the rest because you invested the time to do it right.

Here's some additional information on the subject from Eric Gorr's award winning book:
Motorcross and Off-Road Motorcycle Performance Handbook

https://www.eric-gorr.com/files/pdfs/carb_cleaning_and_jetting.pdf

Carb Tuning - Page 4, By Eric Gorr

https://www.eric-gorr.com/files/pdfs/model_tuning_tips_honda_cr.pdf

 

[Lee237]

Thank you for the great reply,interesting read.
I'm going to play around with what Eric Gorr mentioned.

Cheers.