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The TTR250 FAQ web pages -> TTR250 -> Carb stuff -> Some observations on jetting
Post Info TOPIC: Some observations on jetting
mossproof


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Some observations on jetting
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Been fiddling around with the jetting on my 325 recently as I've got the loan of an Air/fuel ratio meter and header pipe with lambda sensor boss. (Oh, and I've got no wife/kids/life biggrin )

The 325 runs nicely on 50 pilot/147main jets, needle clip 1 notch above middle. Motad exhaust, Twinair filter and mild opening of the airbox snorkle. It would seem to be a bit rich though, judging by the top of the piston and the plug, so as my other project bike has 48/142 jets in the carb, I thought I'd have a play...

Here's the AFR meter:

afr meter 004.JPG

3 light bars in the lean zone, 1 at stochiometric (the theoretical ideal, but I think you want to be above rather than below it to be safe), and 6 bars in the rich zone.

Results:

1: 50/147: Tickover - 4 bars over Sto, running mid up to high revs inc wide open - around 4 bars above Sto.

2: 50/142: Tickover - 4 bars over Sto, mid to high revs - 3 bars over.

3: 48/142: Tickover - 3 bars over, mid to high revs - 2 bars over.

Clearly reducing the main jet size reduced richness. Interesting that above small throttle openings, there wasn't much discernable difference whether it was partial or wide open. Adjusting the needle might alter that, but that's for another quiet night in! Also note that I was watching the road as well as the lights whilst clinging on for grim death  at 70mph so it was a little blurry!

Reducing the pilot obviously leaned off tickover a bit, but also had an effect at other throttle positions. I guess that just because it is less important above idle, it still counts as part of the total quantity of fuel going into the inlet?

My gut feeling was that the engine was perkiest in number 2, but I can't be sure. Wheelies just felt slightly easier! Oh for a rolling road...

My experience with the fuel screw remains unconclusive - starting from 2 turns out, engine note doesn't seem to change until half a turn from fully closed, then stalls at quarter. Going the other way from 2 turns, engine note/speed doesn't seem to change at all until the screw's nearly right out (6 turns ish?) and it stalls again. I just leave it at 2.5 turns. (Yes all the holes are clear - airline plus carb cleaner plus running with fuel system cleaner - the whole carb's spotless (at least on the inside)

Reducing some of the richness is obviously desireable for fuel consumption and clean burning. I'll probably run 48/142 for a while. There was no popping from the exhaust and the header wasn't glowing, so I think I trust the gauge.

Ride safe everyone (ie don't thrash about the lanes staring at little red lights on the handlebarsno)

Simon.



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TTRfan


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Excellent work Simon - good to see the fuel analyser kit works biggrin

I am not sure why but I am also of the opinion that the 325 runs rich on the standard jetting. I might go down on the main jet and perhaps pilot to see if it makes a difference.

Regarding the fuel screw I have found the same as you. Once you have screwed it out a turn or so it seems to make no difference confuse

Oh for a friendly dyno man nearby - as you say hmm

Brian



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ttboof


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I dont get into the jetting discusion  as its a bit hard to explain but the reason the 325 runs rich on standard jeting is fairly simple due to the increased airflow through the same orifice there is more of a pressure drop at the venruri increasing the fuel flow. To some degree carbys are self adjusing . based on air flow 

a completly different case with changes in altitude they realy struggle, 



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mossproof


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It's alright mate, I've got a degree in mechanical engineering, words like Bernouli and venturi are familiar to me, although I can't remember the equations What I'm guessing is that the increase in venturi effect is proportional to the square of the airflow past the orifice rather than directly proportional, but I can't be bothered to google it!
All I wanted to do was add some empirical data to the vagaries of plug chopping and what counts as grey and what counts as black deposits on the plug.
S.

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ttboof


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All good i have a similar bacground 

i like the  sensor though they give the info you need. Also good that it backs up the bigger capacity may no mean biger jets 

ive done several different big bore installs over the last 30 years never used an exhaust sensor a good way to get a quick result. 



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mossproof


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I'm now tempted to buy my own AFR meter as it was so effective in giving me the info I wanted at any throttle opening in real-time. They're not cheap though - I think it's the lambda sensor that's the expensive bit. Self heating sensor at the exhaust outlet would be more versatile than welding a threaded boss to the header of every different bike you want to test, but then the price goes up again - too much for a hobby.
It would seem that bigger capacity in fact requires smaller jets.
I agree it's a touchy subject. We lost Jarrah from this forum because of some perceived (on his part) disagreement over jetting (if a comment he made on another forum is to be believed) a shame as he was knowlegable and helpful. At least the AFR meter takes out the guesswork and assumptions and leaves you with real evidence.
Now, on to a 350 ttr...
S.

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mossproof


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Well I splashed out on my own AFR meter - couldn't help myselfno

I also put a 140 main jet in, so now the carb's running 48/140 with VO5 emulsifier (Open Enduro spec) and needle clip one notch above the central one.

Fuel screw strangely seems to have slightly more effect and is currently at 1.5 turns.

Tickover/idle is approx 11:1 (quite rich), midrange at the sort of throttle opening I saunter round the lanes on between 12 and 13:1, and WOT at 70mph 10.5 to 11:1

This translates to 3 bars over sto., 2 bars over, and 2 flickering to 3, on the k&n type meter I started with. (That's for your benefit Brian so you have some figures to compare with the lights on your meter. There is also some pdf file stuff on the internet on k&n afr meter graphs and such, although I don't know if your's is k&n?)

I've read that an AFR of13.2:1 is "ideal" for steady state power delivery and fuel consumption, and 12.7 to 12.9:1 for best acceleration, but that these figures will not be the same for all engines, and only a dyno can really sort out the ideal settings. I'm satisfied that it's not as rich as it was, and is still safe zone. Haven't got a 137 main to try. I suspect a 137 with the needle clip dropped 2 notches would maintain the mid range figure and lean out the WOT, but I've got bored of the carb in, carb out, carb in game for a while.

Think I've bored everyone enough on this subject. Time to just get on and ride it!

Cheers all,

Simon.



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TTRfan


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All good stuff Simon!

I have a 50/147 combo in the new 325 and realise that it is too rich. Doesn't need choke to start and just a bit of choke when running makes it misfire. 

Not too worried at the moment as running it in but will try a 137 main and see what I have by way of pilot jets and hope I got a 48!

Thanks for all your hard work.

If you need a spare header pipe for your sensor just let me know and I will wing one your way foc.

Brian

 



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