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

Firstly my 325 Thumper Racing Open Enduro:

This bike is running standard header, Motad end can, which breathes a bit better than a standard pipe. Twinair filter, and an oversized snorkel on the airbox. Open Enduro carb so old size emulsifier tube V95.
I began with Pilot 50, Main 147 and the needle clip in the 2nd slot down from the top, and found on a dyno that it was running a rich air/fuel ratio between 10.5 and 12 through most of the rev range so I fiddled around with jets until ending up with 48/140 and the needle still in position 2. I have done some 8000 miles on this engine, including hot desert and hot mountain riding, she hasn't gone bang, doesn't kill spark plugs, and returns 65-70mpg.

Numbers-wise, AFR at tickover 11ish, 12-13 in the mid range and 11 at wide open throttle (WOT) (which is maybe a bit rich, but aids cooling when she's working hard on the motorway)

On my 325 Thumper Racing engine I am running a 137 main jet, V95 emulsion tube and a 50 pilot jet.

I am running a standard header pipe (to clear the oil cooler), Motad exhaust and the airbox is as standard with a Twin Air filter.

I had some issues on cold starting so went up from 48 to 50 on the pilot but haven't started it from cold often enough to see if it improved matters.

Once warm or off idle it runs really well on this jetting. It will run smoothly at constant throttle opening across the range with no stumbling. It doesn't bog and accelerates brilliantly. Great fun to ride 

Brian

mossproof wrote:

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Is that with shorty header and Motad pipe, standard airbox?
48/140 or 50/137 I guess do a similar job.
Thanks for adding to the info-mine

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I am running a standard header pipe (to clear the oil cooler), Motad exhaust and the airbox is as standard with a Twin Air filter. I have edited my post to include that information - thanks for the reminder Simon 

The needle clip is in the middle notch.

I think I may have found the cause of my cold starting problem (choke circuit not working) and, if I have, I might go back to a 48 pilot jet.

So, I'm starting out setting the carb on the 350. I've done 25km on it and checked the plug. No colour change. I have a feeling it's a bit lean.
I haven't changed anything on the carb. Tomorrow I want to play a bit with the air screw. What do you guys have it set to?

I haven't 'opened' the bike up yet.  I've been riding her gently until I've done a few hundred km.  I heard that it's important to do this.  

I would check your carb float height. (Clear tube on the drain, looped up the side of the carb. When you open the drain screw the fuel level inside will be the same as that in the tube. With bike upright not on sidestand)

Could try running with airbox lid off - if it goes better it was running too rich before.

Have you done a plug chop at wot? Ride flat-out, then pull in the clutch and simultaneously cut the engine then check colour of plug. Should give you an idea of what's going on inside at wot.

Well my TTR is only a 263cc 2mm over does that qualify as a big bore?

Anyway I may have found a compatible 80 mm piston so hopefully 300 cc soon. Anyway just bought this Keihin PWK carb.
Will see how it all goes, was only about $60 AUD.  I'll let you all know if it works.

Kaos wrote:

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Well my TTR is only a 263cc 2mm over does that qualify as a big bore?

Anyway I may have found a compatible 80 mm piston so hopefully 300 cc soon. Anyway just bought this Keihin PWK carb.
Will see how it all goes, was only about $60 AUD.  I'll let you all know if it works.

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 Qualifies as a big bore, yes, but the thread was aimed at the TK carb jetting setups

I'm interested to hear how it goes though, and awaiting Gadget's Mikuni results too...

If you're resleeving the barrel take care to get the liner positioned just right - otherwise the cracked barrel issues of the Thumper Racing 325 will occur. To mitigate the chances of leaks, get your machinist to slop plenty of green loctite round the sleeve before pressing it in. Worked for me on my 325.

OMG!

Lucky you have Trish around, you defo need to get an AFR reading on your bike ASAP, it must be super lean.

I just realised that the TM33 on TTR350 info from me never made it to this page... So here it is:

I use this one: www.mikunioz.com/shop/yamaha-ttr-250-mikuni-tm33-8012-pumper-carb-kit/ (the red frame bike in the pics is actually my bike)

There is a photo of a head bracket that has white X's on it.... That's what needs to be cut off. But I had spares, so I just used two of the left sided brackets.

The airbox boot- not as tight as the TK carb, but not sloppy. It clamps in nicely. The carb came with a 127.5 main fitted- I put it up to the 137.5 that was supplied in the kit.... But I'm in Australia where the kit comes from.

Whatever pilot came in the TM33, I left there. The main that it was installed with was a 127.5. The way that I originally read the ad for the TM33, was that it came with a few jets either way of what is in it..... The jets provided as extras were 137.5, 140, 142.5. I opted for the 137.5 which got the WOT AFR at about 12.5 (near enough for me). Then looking through the rest of the rev range, I found that it was pretty good everywhere but 3/4 throttle, where it took a dive into being too rich for me at 10.5. There was a distinct stumble at that range when I stopped looking too much at the meter. I lowered the needle 1 clip position (clip now at second from top) which seemed to smooth that out.


Kaos had teed up to get my follow up dyno run done yesterday (and fine tuning)... but unfortunately **** was happening and I couldn't make it- I even had the bike ready to go.

Kaos was still able to get his 263 done. What was interesting to note, was that his 263cc TTR had a 2.5hp gain changing from Teiki to Keihin 36mm carb.