Temperature Effecting Jetting

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1/5/2019 9:21 AM

Hey guys, I'm sure theres no simple way to calculate this but reading online I'm confused by all the crazy scientific equations. Wondering if there is a generic starting point for me...

If I ride my 125 at a track and love the jetting but it's 70-80f outside, then I go to that same track but temps could be 38-50f now during the winter.. Is there a generic starting point where you'd go, "1 larger main jet" or "drop the clip 1 line."

Reading the equations I feel like this
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1/5/2019 9:54 AM

A drop in temperature from say 70 to 40° F doesn’t require a drastic change in jetting. 1 to 2 sizes richer jets should be all you need, since cold air is more oxygen dense than warm air. IMO, even if you are meticulous about performance you don’t need a scientific formula for this.

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1/5/2019 10:02 AM

I'm more or less just worried I'm going to run too lean with the temperature change.

If I go 1 larger main jet with a -20 or 30f degree temp drop is still a safe bet?

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1/5/2019 10:18 AM

A lot of the older Honda CR 125 owners manuals had a good graft that covered this in detail. You may want to search for one of them.

Paw Paw

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1/5/2019 11:57 AM

I spent 2 yrs studying this phenomenon and when we were on 2 strokes I had my jetting figured out for 30-50 degrees then 50-80 degrees then for 80+. And honestly it was only I think a change of the main jet by one size smaller and bigger and needle adjustment, we never rode below 30. We always ran a little richer pilot so it had some fuel after a wide open run, corner then wide open. I would sometimes adjust air/fuel screw too.

I do sometimes miss the 2 strokes because we would beat so many modded bikes because they had no idea how to tune their bike lol. But I don't miss messing around with jetting either lol

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1/5/2019 4:29 PM

CarlinoJoeVideo wrote:

I'm more or less just worried I'm going to run too lean with the temperature change.

If I go 1 larger main jet with a -20 or ...more

Yes.

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1/5/2019 5:25 PM

CarlinoJoeVideo wrote:

I'm more or less just worried I'm going to run too lean with the temperature change.

If I go 1 larger main jet with a -20 or ...more

You are at a higher risk running as lean as you are. You need more fuel to match the denser air.

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The older I get, the faster I was.

1/5/2019 10:42 PM

For a 40* temp swing (going cooler), just go up one main jet size, and possibly a half clip richer on the needle. air screw adjustment should take care of the low speed (pilot). I have an '18 150sx and this is all I had to do.

One important thing I started doing was really pay attention to how my bike ran when it was jetted perfectly (70-80 *F). I paid close attention to the sound and how the engine responded just revving the engine. I have that sound committed to memory so it makes jetting a little easier when the temp changes drastically. I really don't need to rejet unless it falls below 55*F or hotter than 100*F. The airscrew cleans it up pretty well. I am running a Keihin on my '18 which I feel is a little more stable jetting-wise.

The biggest thing I notice in the winter is that tracks don't dry out as fast and the added load from mud or moist tracks can put you over the edge with jetting. So it's not just the cold, it's also the added load of riding a track that is now more damp/wet as opposed to riding the same track in the summer that is a little more hard packed and puts less load on the bike.

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1/6/2019 6:35 AM

Cool, yeah I agree. I went up 1 main jet which is also the “sand” setting with the JD Jet kit. I’m also on the keihin carb.

Thanks guys!

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1/6/2019 7:45 AM

I always error on the side of too rich, then work my way down....last spring I had my 250sx jetted pretty good for Florida sand tracks @ 75-80 degrees...came back up north & raced 2 early races at southwick where temps were 35-40 degree"s....only took 1 main jet richer to compensate ....

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1/6/2019 9:05 AM

Maybe a bit overkill, but we use a Raceair for Stock Moto Honda shifter kart. Stock Moto uses a Keihin PWK 38. The Raceair has jetting for Keihin, Mikuni, and Dellorto.

https://computech.com/product/raceair-pro-weather-station/

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1/6/2019 9:40 PM

Regarding the stock TMX carb but you can at least see how drastic the changes are for elevation/temp deltas.

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1/7/2019 7:00 AM

CarlinoJoeVideo wrote:

I'm more or less just worried I'm going to run too lean with the temperature change.

If I go 1 larger main jet with a -20 or ...more

One larger main is about it, and maybe a air screw adjustment...

Personally that's not enough of a temperature jump for me to change any jetting...

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