Jetting and the effects of altitude

Not sure if this has been discussed as a specific topic, but considering where I live it is a very big issue for us carbureted folks.  I would definitely like to hear from people with experience with dealing with the various effects that altitude has on jetting.  Reason I bring this up is the jetting in my bike in particular.  I have an 03 XR250R.  It has a Wiseco 277 kit in it, 10.5/1 compression.  The air box snorkel and exhaust baffle are removed.  If I remember correctly the stock main jet on this bike (49 states) is 132.  But, I am running a 125 with the above mods, the plug color is good (stock heat range), the bike runs strong, and it doesn't seem to run too hot.  Needle is raised one notch.  Here's the variable.  I live at 6k ft. elevation in NW NM, and my riding is done at that elevation on up to 12k.  I'd like to hear from others with similar mods at various altitudes to find out what you are running.  (The bike starts fairly easily, I feel I could tweak the low speed a bit, and plan on doing so.)

Edited by cjjeepercreeper

It sounds like your main is jetted for 12,000 ft. and needle for sea level.  What size pilot jet?

 

Have you tuned the fuel screw to confirm the pilot jet size? 

It sounds like your main is jetted for 12,000 ft. and needle for sea level.  What size pilot jet?

 

Have you tuned the fuel screw to confirm the pilot jet size? 

 

Pilot is stock, I'm the original owner so I'm pretty sure of that.  Like I said in the OP, that's something I'm going to play around with.  I know, the jetting sounds odd, but it works, mid is strong and so is WOT.  Plug color is light brown.  That was why I posted this, altitude seems to do odd things to carbs, as does humidity, it is very dry here.  If it were your bike would you try a bigger main and play with needle position?  Definitely looking for input on this.

I don't doubt your mid and WOT are strong.  They overlap and the richer needle is perhaps making up for a lean main.  A lean main runs very strong but can melt your piston.

 

First, ride it and use Mikuni's roll-off method to see if your main is too lean.  You want the main jetted for your lowest elevation.  I run a main a couple sizes richer for better cooling when I'm winding the motor in a sand wash. Don't conclude that the other jets should be extra rich especially the pilot.  

 

With the bike fully warm and at the lowest idle, adjust the fuel screw to see if the pilot jet is the correct size.  Make a note how many turns out the screw is.  I back the idle screw completely so the slide is all the way down and tune the bike to run there. Then I adjust the idle where I want it.   

Thanks.  I just called my local independent shop that we do all our business with, he has a 128 and a 130 main.  I'll probably pick them both up to see how it goes.  I'm going to leave the needle at one notch raised for now, we'll see what that does to the midrange with a slightly larger main.

128 it is.  Went down this morning and bought a 128 and 130, the bike runs great on the 128.  And yes, doing a roll off with the 125 it was symptomatic of too small a main jet.  The 128 cleared that up.  Altitude is a funny thing, that is still 2 sizes smaller than sea level stock.  The pilot is perfect at 1 3/4 turns.  Left the needle raised one notch.

Altitude is a funny thing, that is still 2 sizes smaller than sea level stock.  

 

But you increased displacement without upsizing the head, carb, and valves.  That gives you more velocity and vacuum which draws more fuel.

 

At this point, I would try the 130 with the needle one notch leaner.  The needle really affects gas mileage and richening the main has an affect on the needle where they overlap.  

Most of the Honda Service manuals have a page on jetting for altitude and temperature. Besides the previous advice and what you have discovered here is some more info:

What I do as altitude changes:

I've found stock jetting works OK up to 5000ft on a warm day with minor blubbering, and to avoid main jet changes can be fixed first by about 1/4 turn in on the mixture screw, I do the same for hot days near sea level. If that doesn't work then a leaner needle and richer idle mixture can get you thru the day;  move the needle clip up one groove (leaner) and turning the mixture screw OUT by 1/2 turn from stock (richer).  A main jet change can be used instead but I've found it easier to just ride or use one of the above.

 

Jetting for Altitude & Temp Instr.JPG

Jetting for Altitude & Temp.JPG

Most of the Honda Service manuals have a page on jetting for altitude and temperature. Besides the previous advice and what you have discovered here is some more info:

What I do as altitude changes:

I've found stock jetting works OK up to 5000ft on a warm day with minor blubbering, and to avoid main jet changes can be fixed first by about 1/4 turn in on the mixture screw, I do the same for hot days near sea level. If that doesn't work then a leaner needle and richer idle mixture can get you thru the day;  move the needle clip up one groove (leaner) and turning the mixture screw OUT by 1/2 turn from stock (richer).  A main jet change can be used instead but I've found it easier to just ride or use one of the above.

 

attachicon.gifJetting for Altitude & Temp Instr.JPG

attachicon.gifJetting for Altitude & Temp.JPG

 

According to the XR250R OEM service manual I'm probably pretty close to spot on.  If you go with what the graph indicates (pg. 5-13) then a stock XR250R at my altitude should be running a 118 main with the needle raised one notch.  I have the baffle and air box snorkel removed and am at 277cc, and the setting I've ended up with per what has been discussed above is a 128 main with the needle raised one notch.  I'm at a .90 correction factor (temp in mid 80s) which indicates the above noted changes.  It should be noted that that is what the book says, as BR has indicated the real world may be different, but it looks like in my case the book is pretty spot on.

Yeah, with the 277 you're in uncharted territory and you need to use jetting skills to dial it in.

Yeah, with the 277 you're in uncharted territory and you need to use jetting skills to dial it in.

I tried the 130 last night, definitely too rich with the needle raised. Dropped it to stock, helped a bit but seemed a bit rich on wot. A 128 and 130 are so close even temperature would probably make a difference. Once it gets hot out I may mess with it further. I'm really curious how it will do this summer at 10-12k on the Dolores, CO trails.

Btw, I owe you an apology, my XR started 1st kick today, lol.

Edited by cjjeepercreeper

It takes some patience but it's pretty satisfying when you get it dialed in so well.  Congratulations and no apologies necessary.  

 

If you have good compression, spark, and the proper air/fuel mixture, there's no reason for it to need more than one kick. 

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