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Post Info TOPIC: Race tuning a TTR250 engine


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Race tuning a TTR250 engine
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Most of us use our TTRs for trail. track or road and therefore the requirements are very different to those of a race-tuned engine.

I have been contacted by a kart racer who is using a TTR250 donk in his kart and has asked about tuning it to the max.

This was my reply:

Regarding the carb, the standard TTR carb that you have fitted is a pumper in that it has a round slide with an accelerator pump. This will shoot a spray of fuel every time you hit the throttle. This raw fuel helps the engine to rev up faster so that the engine does not have a spot where is wants more fuel but can't draw it yet from the jets. It can be problematical jetting replacement carbs but if you join the TTR250 forum and do a search you will see that some owners have fitted a Mikuni and given suggested jetting.

There used to be a guy in the Midlands that would get extra oomph out of TTRs by boring the carb but I can't find any contact details. He was called Phil Manning but hasn't advertised for years. Phil would also bore out the exhaust port to give more bhp and quicker pick up.

The usual way of getting more power from a 4-stroke engine is to increase the compression but I am not aware of anyone that has done this. It is worth checking out the CRs quoted by the manufacturers of replacement TTR pistons such as Wossner and Wiseco. It might be possible to skim the head using some Plastigauge to make sure that the piston does not make contact with the valves after the work has been done.

The head could be gas flowed but some tuners warn that making the inlet tract too smooth can make the mixture flow too smooth and thus the petrol and air don't mix as well as they would if there was turbulence.

Depending on how deep your pockets are, it is possible to get hot cams from the States - see http://www.webcamshafts.com/mobile/motorcycle/yamaha/yamaha_ttr_250_(99-06)_dohc_4v.html although I haven't had any feedback from anyone that has tried them so it might be a bit of a chance. Hot cams make an engine less tractable for road or trail use and are only suitable, in my opinion, for race applications but that is what you are doing with your TTR engine so probably OK.

Race engines are usually fitted with light flywheels to enhance pick up with less weight to get moving when you crack the throttle. The TTR flywheel is really heavy so there might be scope for lightening it. Again, I haven't heard of anyone doing this as most TTRs are used offroad where extra weight means improved tractability - something you don't need on the race track.

Further gains could be had by running a total loss ignition system and taking the charging coils out of the equation. Weight could be reduced with a smaller battery and taking the rectifier/regulator and associated wiring and connectors out of the circuit."

Has anyone tuned their TTR to the max and can give some first-hand advice please? Or do you know anyone that has?

What have I missed?

Brian 

 



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Can he run E85 ethanol fuel or av gas ?
The E85 will run a lot cooler so he should be able to lean the jetting out and get a few more HP .
That is if its available and or allowed in the classes he races


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Super Guru

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I know I am resurrecting a really old thread but am interested to know:

1. Has anyone heard of or have contact details for Phil Manning in the Midlands?

2. Does anyone have any experience of boring the TTR carb?

3. Has anyone tried the hot cams?

Brian



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Bump . id be interested to know how to do it myself. I doubt boring ot honeing a couple mm would be that hard but think its kinda pointless untill the head is ported and exahest is getting away fase enough to get any benifit from it it seems like one of the last things on the list to dran the last few 0.1 of a horspow out . would a compleat carb swap be an alternative ?

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Brian,

Some interesting feedback on the thread "fitting a Mikuni TM 32 / 33"  Info from "Bruns" on page two of that thread .  He bored his TTR tekei carb.  And later compared with the Mikuni 32 mm .  So for a RACE ENGINE, the Mikuni should be better, for just a budget upgrade the carb boring seemed worthwhile.

 

FYI see below.



"Thought I would add a little contribution to some carb mods I have tried. Bikes are 2003 ttr250 and 1994 ttr250 with staintune exhaust with silencer endcap in and hole in airbox side plus ground headers. Jetting specs may be off as Aussie fuel is in my testing a lot richer than other countries and I'm at sea level

On the blue one I found substantial response in low to mid by boring the carb oval to get rid of the teardrop shape, bending the pumper to come on right off idle and tsper off after 1/3rd throttle and the following tk jets

main 142

pilot 48

needle 1 richer

With this combo I ran 14 47 gearing and had wheel lifting grunt.



White one I have installed a 32mikuni flatslide non-pumper .

First i tried the xr280 baseline jetting of 250 main, 45 pilot 6dh2 needle q2 emulsion tube, it was flat and fluffy with a small promise of power

I can't remember exactly what I ended up with but will have a look if anyone else wants to do it

From memory it was:

main 190

pilot 22.5

needle 6dh7 leanest clip

q-0 emulsion

This combo transformed the bike, more down low than the bored carb and way more in the mid and top, very responsive fast revving throttle feel and hard to bog out plus never stutters on rough ground, basically a very wide very linear power curve. FWIW i went 32mm non pumper over 33pumper because in my experience pumpers over fuel to make up for too large a bore size, on old school tech bikes a well jetted smaller bore flatslide gives better low mid response with a tiny sacrifice up top to the 33mm.



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