RX125 Tuning - Jetting

[Muzzab]

After talking this over with a couple of people, the main question I got asked was why are you starting with the jetting so start off with? The could see the attraction (of trying to simply get more in to the combustion chamber), but felt that an expansion chamber would make a much bigger and immediate improvement than changing the jets. I noticed a couple of good deals on TM for suitable donors using 1/2 a 250 2-stroke exhaust.

The next questions was has he cleaned up his ports to warrant changing the jets? As it is worth spending sometime with a demel type tool and simply cleaning up the casting first, trying it out with the needle set at different levels. Or in other words taking one step at a time, and saving your money for later when you have identified where to best to go next.

If its any help this is the process i am following with my bucket (an RX125 engine in a RG50 rolling chassis).

Identified engine and frame I wanted/could afford/had access to

The fun of fitting something into something that shouldn't, including riding position, fairing, etc

Sorted out suitable exhaust expansion chamber

Worked out gearing (sprocket size(s)) suitable for where I am racing.

Sorted out tyres i am happy with/have access to.

Got the bike to run reliable

Had fun with some paint.

and am now working on porting, needle height, etc

and then if I need to jetting.

Good points made....But who said I was starting with jetting? the bike has a chamber on it, the gearing seems right , it is reliable, it has a custom paint job, I don't want to pull it apart just yet to look at ports (but this will happen), so having an experiment with the jetting was where I was at. I have no idea if the 125 jet in it is right for it (it could be perfect). But rather than buy jets I don't need, I was looking for a hint as to whether a 125 was at the large end of things. But understand that bikes are all different, so will start with a couple of bigger jets and see what happens.

Cheers
Muzza

[gav]

well, its not a million miles away is it, runs pretty good already. Only way to tell would be to change up one size and try, plug chop would be good too.

[Muzzab]

well, its not a million miles away is it, runs pretty good already. Only way to tell would be to change up one size and try, plug chop would be good too.

Cheers Gav, I'm sure you're right, half the fun is having a tinker. Got the kill switch wired in yesterday, so all set for the Greymouth Street Races now.

Muzza

[SHELRACING]

Yep I agree with B4me and all4a50s.

The jetting is probably the last thing to pay attention to. Be happy with the rest of the setup, Carb, Air filter ( or not) Chamber, porting, ignition etc etc.

Beware tho incorrect ignition timing can cause severe engine damage, so can incorrect mixture better to start slightly rich rather than lean.

[F5 Dave]

Arrgh, now I agree with no one. [edit except Gav. . . mostly]

I'm not trying to be rude, but please people, stop spouting potentially damaging 'advice'.

You should check the jetting (& ignition timing) whenever you make a significant change.

People are taking off airboxes & putting on chambers & not even considering that the jets will need optimising for the new required fuel rate.

People on Gokart tracks are protected a bit as unlike Ruapuna you aren't full open for ages down the backstraight. But if you are jetted lean it will seize solid. Some engines are tetchy, some are more resilient, but lean enough will ruin an engine.

Ok my two bits of advice;
first check that can be done in the pits (esp with carb swaps) is take the mainjet out & start the bike. The bike should die when full throttle applied. If it revs & revs on wide then the needle is restricting flow upstream of the mainjet. Not common, but not unheard of.

2nd is that I have built two engines running the same carb ignition & pipe & ran a 240 main in one & got down to a 130 in the other. Same engine but one modified with a different inlet style. Made a heck of a difference. Another chap ran a similar inlet & was using 280s.
Sometimes you will need to go down & sometimes up. Bigger is not the only way because you 'need more fuel'. That is ignoring the requirements of the engine.

[Muzzab]

Arrgh, now I agree with no one. [edit except Gav. . . mostly]

I'm not trying to be rude, but please people, stop spouting potentially damaging 'advice'.

You should check the jetting (& ignition timing) whenever you make a significant change.

People are taking off airboxes & putting on chambers & not even considering that the jets will need optimising for the new required fuel rate.

People on Gokart tracks are protected a bit as unlike Ruapuna you aren't full open for ages down the backstraight. But if you are jetted lean it will seize solid. Some engines are tetchy, some are more resilient, but lean enough will ruin an engine.

Ok my two bits of advice;
first check that can be done in the pits (esp with carb swaps) is take the mainjet out & start the bike. The bike should die when full throttle applied. If it revs & revs on wide then the needle is restricting flow upstream of the mainjet. Not common, but not unheard of.

2nd is that I have built two engines running the same carb ignition & pipe & ran a 240 main in one & got down to a 130 in the other. Same engine but one modified with a different inlet style. Made a heck of a difference. Another chap ran a similar inlet & was using 280s.
Sometimes you will need to go down & sometimes up. Bigger is not the only way because you 'need more fuel'. That is ignoring the requirements of the engine.

Thanks again....you have answered my question, that a 125 main jet is not large in the scope of things. I want to make the bike run rich rather than lean for the reasons you mention. I'm new to this and want to learn what I should notice by changing jets etc, hopefully one day I'll go out on the practice laps and come back in knowing that for "todays" conditions I need to go up or down a jet size, Cheers Muzza

[Buckets4Me]

just a quick note

had my 2 bikes a JC on sunday for a dyno run and tune
fist bike th gp125 didn't need much of anything
started with the timing and found we had it spot on
then the jetting and had to drop it 1 jet size (had raised it years ago as I thought it was leaning out at pukekohe)

then got on to the 50
I new it was lean and the timing was out
so started by retarding the timing and it lost power
advanced it and it gaind power so keeped going till it stoped getting more power and started to drop off again

then tried the jetting and found that what was wonce way lean was the best you could get not to lean and not to rich (for the room temp etc)

so now it's back to the track to see how it goes

and as another side note jetting sizes are 97.5 for the gp125
and about 107.5 for the 50

jetting and timing is where you should start and finish and go backto everytime you change anything

dont be cheap

[F5 Dave]

. . . jetting and timing is where you should start and finish and go backto everytime you change anything . . .

Now there is something I can agree with!!

[all4A50s]

Arrgh, now I agree with no one. [edit except Gav. . . mostly]

I'm not trying to be rude, but please people, stop spouting potentially damaging 'advice'.

You should check the jetting (& ignition timing) whenever you make a significant change.

People are taking off airboxes & putting on chambers & not even considering that the jets will need optimising for the new required fuel rate.

People on Gokart tracks are protected a bit as unlike Ruapuna you aren't full open for ages down the backstraight. But if are jetted lean it will seize solid. Some engines are tetchy, some are more resilient, but lean enough will ruin an engine.

you Ok my two bits of advice;
first check that can be done in the pits (esp with carb swaps) is take the mainjet out & start the bike. The bike should die when full throttle applied. If it revs & revs on wide then the needle is restricting flow upstream of the mainjet. Not common, but not unheard of.

2nd is that I have built two engines running the same carb ignition & pipe & ran a 240 main in one & got down to a 130 in the other. Same engine but one modified with a different inlet style. Made a heck of a difference. Another chap ran a similar inlet & was using 280s.
Sometimes you will need to go down & sometimes up. Bigger is not the only way because you 'need more fuel'. That is ignoring the requirements of the engine.

I was taught to do one thing at a time and changing the jetting should be done last. Identifying what the engine is actually doing (i,e flat spot at 1/2 of full throttle) first, then change the jets to try and rectify to what you want. I always try to remember that the companies have made/designed the beasts have already spend a considerable amount of money in development.

[Muzzab]

I was taught to do one thing at a time and changing the jetting should be done last. Identifying what the engine is actually doing (i,e flat spot at 1/2 of full throttle) first, then change the jets to try and rectify to what you want. I always try to remember that the companies have made/designed the beasts have already spend a considerable amount of money in development.

Fair enough, I'm still wanting to learn that if the bike goes 10kmph an hour faster one day over another and the only thing that has changed is the weather, then I can consider going up or down a jet size to tune it for that days conditions. Cheers Muzza

[Buckets4Me]

I was taught to do one thing at a time and changing the jetting should be done last. Identifying what the engine is actually doing (i,e flat spot at 1/2 of full throttle) first, then change the jets to try and rectify to what you want. I always try to remember that the companies have made/designed the beasts have already spend a considerable amount of money in development.

I was taught that you change one thing then jet it to suit

if you change how much petrol it uses you had better be ready to change how much petrol it gets

ie put a pipe on it that makes more power at xyz revs then it had better have more petrol as well or you go lean (BANG) and you are starting again

I always try to remember that the companies have made/designed the beasts have already spend a considerable amount of money in development.
and thewy should be right as rain till you change something
\then you need to makesure it's all still right ie jet it for the pipe
jet it for the carb
jet it for the weather
the more power you make the more it HAS to be RIGHT !!!
you jet it everytime you change anything so that you get the best out of that change

there is no point in putting on a better chamber if you are going to flood the bike with gass
or a bigger carb and stave it of petrol


and if you leave it till last then you end up like me with an engine you dont know where to start from and is only make half what it could (and runs like a dog)

[AndrewStroud]

Fair enough, I'm still wanting to learn that if the bike goes 10kmph an hour faster one day over another and the only thing that has changed is the weather, then I can consider going up or down a jet size to tune it for that days conditions. Cheers Muzza

Interesting, I just Googled tuning an RX 125 and this thread came up :-) On Thursday my son rode a bucket at Mt Wellington with a RX 125 standard motor and std exhaust but had a K&N filter. I rode it and it didn't run that well. It went flat with the throttle wide open but went better when I backed off the throttle a bit. This usually indicates it's lean. It was pretty flat mide range as well so I lifted the needle max by dropping 2 clips and put in a 135 main jet and it went heaps better. AS

[gav]

Really Andrew, an FXR150 is what your boy needs, then you could race it down here in the MotoFXR class! A Brother Suzuki FXR150, done in black and white, awesome!

[Muzzab]

Interesting, I just Googled tuning an RX 125 and this thread came up :-) On Thursday my son rode a bucket at Mt Wellington with a RX 125 standard motor and std exhaust but had a K&N filter. I rode it and it didn't run that well. It went flat with the throttle wide open but went better when I backed off the throttle a bit. This usually indicates it's lean. It was pretty flat mide range as well so I lifted the needle max by dropping 2 clips and put in a 135 main jet and it went heaps better. AS

Good stuff, just out of curiosity what size jet was in it to start with? I was happy with how my RX went today, I think it likes warm sunny days.
Cheers Muzza

[AndrewStroud]

Good stuff, just out of curiosity what size jet was in it to start with? I was happy with how my RX went today, I think it likes warm sunny days.
Cheers Muzza

It started with a 125 which I assume is standard. I suppose it was lean from having a k&n and also through the oil pump being disconnected. Apparently when u run oil with the gas the jets get a fine coating of oil so need to be a fraction bigger. I modified an RD 400F when I was about 16yrs. I spent some time at school reading Graham A Bell's book on tuning 2 strokes, then got right into it with some porting, plainned the head, designed and made chambers. So it's testing the memory a bit but I think I'd quite enjoy getting some more power out of an RX motor. Might look at the squish next as that makes a fair difference. I'd expect u could take off 0.7mm or so. I don't mind the standard pipe at this stage because it's quite.