A forum for owners of Yamaha TTR250 trail and enduro bikes!

TTRfan · Super Guru

Help! Pilot jet differences?

Lin · Senior Member

I think your only way of finding out for sure is put the carb on a bike and try it. From the pic it is clear there are a different number and different location of emulsion holes, and the distance from the seating flange to the tip is shorter. But it may not make any noticeable difference. Or it may introduce a horrible flat spot....

We have both 50 and 52.5 pilot jets and they are exactly the same dimensionally (apart from the metering hole) so I'm wondering exactly what carb that jet on the left is manufactured for.

TTRfan · Super Guru

Lin wrote:
I think your only way of finding out for sure is put the carb on a bike and try it. From the pic it is clear there are a different number and different location of emulsion holes, and the distance from the seating flange to the tip is shorter. But it may not make any noticeable difference. Or it may introduce a horrible flat spot....

We have both 50 and 52.5 pilot jets and they are exactly the same dimensionally (apart from the metering hole) so I'm wondering exactly what carb that jet on the left is manufactured for.

Thanks for the reply Lin.

I have been thinking about it and it is likely that the difference in length is such that it will cause issues with the pilot screw. It just won't screw in far enough on the short jet to be able to adjust it properly.

I got two carbs from a customer today. Both from TTR250s. One had the long pilot and the other the short one. I bought a new spare 52.5 pilot jet using the part number on the online fiche and got the short jet so it is definitely for the TTR250.

Can I ask how much difference you found between the 50 and 52.5 jets please?

Brian

Lin · Senior Member

Firstly, we are operating in high ambient temps - around 30~35C so for a start we have lower air density than you will have in the UK. We also operate from 500ft asl to over 3000ft asl so again that complicates a direct comparison.

However, I did find the 50 to provide a crisper initial edge to the roll-on at low/medium rpm, as one might expect in our conditions. But reading some USA based forums others find the 52.5 better, citing the bigger pilot overcoming flat spots, stalling etc. I am guessing they are operating in cold temp/high air density. As well fuel differences may account for some of it.

Don't think you will affect the mixture screw as it has it's own private discharge orifice, unrelated to the pilot jet oriface.

-- Edited by Lin on Wednesday 7th of December 2011 10:25:51 PM

TTRfan · Super Guru

Lin wrote:
Firstly, we are operating in high ambient temps - around 30~35C so for a start we have lower air density than you will have in the UK. We also operate from 500ft asl to over 3000ft asl so again that complicates a direct comparison.

However, I did find the 50 to provide a crisper initial edge to the roll-on at low/medium rpm, as one might expect in our conditions. But reading some USA based forums others find the 52.5 better, citing the bigger pilot overcoming flat spots, stalling etc. I am guessing they are operating in cold temp/high air density. As well fuel differences may account for some of it.
Don't think you will affect the mixture screw as it has it's own private discharge orifice, unrelated to the pilot jet oriface.

Thanks again for that Lin. Our UK air is very dense at the moment so I think I might try it with the 52.5 and the smaller main jet and see how we go.

You are of course correct about the mixture screw - I think I had a senior moment disbelief

Brian

Lin · Senior Member

Just of note: The Teikei carb seems to get into it's main jet quite quickly - sooner than one would expect if compared to other carbs, and especially cv type carbs. I changed from a 140 main to a 145 (just a suck-it-and-see exercise of no particular technical worth....) and almost immediately after initial roll open the engine bogged, a fraction more throttle and it cut completely (remember 35degC) I'm guessing the carb's asymmetric venturi shape allows a high airspeed (low pressure) over the needle jet at low rpms and the main jet size provides mixture calibration very early on in the piece. Maybe that's why there are no alternative needles listed.... as far as I know. I may be wrong. Has any other member experienced this?

Senior moments briefly occur when the brain is de-fragmenting years and years of stored wisdom.....

TTRfan · Super Guru

Good news biggrin

I decided to try the carb with the "wrong" length pilot jet in and it runs absolutely fine - no problems at all at low revs. Started well and ticks over nicely when warm.

Brian

Lin · Senior Member

Great! Are you going to do a back-to-back 50/52.5 comparison test?

TTRfan · Super Guru

Lin wrote:
Great! Are you going to do a back-to-back 50/52.5 comparison test?

I probably should have but was pushed for time. Everything I do these days seems to take twice as long as I think it should