Carb Jetting

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Okay I've been talking to a few people regarding the TZR125 engine.

Now I am running a 180 main jet with the clip on the top needle notch (leanest setting).
Running the oil pump too.

And it is burning ever so slightly sooty on the plug.

Now I've been told from one person that I may need to up to a 200 main jet.

And I've also been told from a TZR125 enthusiast that my jetting is far too big. And I need to down jet to a 120~

The thing I cant get my head around is, Surely if its only slightly sooty on a 180 main jet. If I dropped to say, a 150 main, And reset the needle. Providing I can get it burning the right colour, Does that mean it is right?

I thought that if the plug is burning a "coffee brown" then its correct regardless of jet size etc?

Reason I ask is because its very boggy at low RPM. Power vavle open or closed really.

Also when I fit this new manifold, It is 4mm bigger in internal diameter. Will that make it run richer or leaner?

Ben

alains · World Chat Champion Joined: 08 Jul 2004 Karma :

fit a new plug and after 30-50 miles look the color with this chart
https://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
____________________
the worst day fishing is better than the best day working

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

I've done that, And its running black/sooty.

Ben

Timmeh · World Chat Champion Joined: 01 Nov 2006 Karma :

Id' say your main jet is massively OTT.

130-145 would be ballpark.
____________________
GSXR400 x2 | '94 RVF400 | '93 TZR125 4DL (again)
20:22:30 BLUEX5: i would love to be forced to undergo a**l plugging with different sizes
20:48:18 Temeluchus: comp you hunk of smouldering homos3x you

Si1000 · Two Stroke Sniffer Joined: 03 Nov 2008 Karma :

That main jet is hugely oversized. Get back down to 140's and reset the needle to position 3 or thereabouts. Also - make sure the oil pump is operating correctly as well as powervalve - both can and do effect burn.
Si

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Powervalve is pinned open as I have no Servo.

Down to a 150 main jet with clip on 1st notch (Top / lean)

Still a bit bogey but seems to be more powerful.

Will put the 140 jet in when I can be bothered and run it on the 3rd notch to see what happens.

Ben

alains · World Chat Champion Joined: 08 Jul 2004 Karma :

the best jetting for my NSR125R is main 138,idle 42,power 65 , needle set on 2nd notch from top (5 notches on the needle)
the carb is a 28PHBH if your carb is the same i suggest you use the same jetting and control your plug after 30 miles
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the worst day fishing is better than the best day working

binge · Emo Kiddy Joined: 03 Jul 2004 Karma :

Mikuni VM28 is what carb I have (I think).

I am running an open style Foam filter, And a Race exhaust. No baffles etc.

Ben

alains · World Chat Champion Joined: 08 Jul 2004 Karma :

in this case as main jets are step 10 you should buy 150,160 then try the smallest , check plug then adjust if necessary . look the size of the pilot jet and if you have difficulties to start when cold fit a size over
and look here
https://www.mikuni.com/pdf/vmmanual.pdf
____________________
the worst day fishing is better than the best day working