Some Help with Jetting? [Archive] - Bay Area Two Stroke


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05-14-2007, 01:06 PM
Hey Guys,

Curious if anyone here can help me clean up some jetting on my bike:

Location (bay area, California)

GPR50 with a 72cc SP kit squish at 0.9mm
Arrow Exhaust
Mikuni 24mm carb with Intake
Carbon Reeds

Carb Setup Now:
2 turns out (air screw)
#3 spot on needle

I've done a plug chop on the bike with the 175 main and the 180 main and the plug is a good brown and the plug is NOT wet.

However around 6-7K the bike sounds like a helicopter. This helicopter sound is elevated when using the larger main jet.

My question is how do you get rid of this rich-ness at 6-7K-ish?
And can you guys with similar setups post your jetting for reference?

05-14-2007, 01:55 PM
Put the needle clip at the top of the needle. That solved all my issues with the mid range bogging or helicopter as you call it. Other wise it is exactly the same set up as I run but I have a richer jet due to my location.

05-16-2007, 07:21 PM

I did exactly as you stated and put the needle to the top. That pretty much eliminated the helicopter and the bike revs pretty well all the way through to 13K. I did two plug chops, one with a 180main and another with a 185main. In both cases, the plug was pretty white. The very very base of the white area was dark black, but the area around the eletrode was pretty white (albeit less white with the 185main). I would assume this is a pretty good inditiation the bike is running lean?

I'm going to order some bigger jets. It seems leaning out the midrange (needle) has also effected the WOT portion.

Does this sound about right?

Again, i would love to see what others are running on their 72cc derbis with the 24mm mikuni.


05-16-2007, 11:38 PM
thats why i dont run carbs, i just have a main jet on a rubber hose from the tank.

05-17-2007, 06:04 AM
Fwiw, I am running a 160 main on the 24mm f/s Mikuni. Altho' I have the mixture running a little on the fat side.....smokes like crazy :p But, that's only because I haven't had the time to take it to a wide open area to optimally dial in the carb.

Oh! and the motor runs strong through all stages of throttle :D and that's w/ the OEM cyclinder and or 72 SP cylinder ;)

I have yet verified the needle position...will check when I get home.

05-17-2007, 01:50 PM
Thanks Daren,

you are running a 160 main on both stock and the 72sp cyilnder? Also what exactly does the air mixture screw do anyway? Team Calimari is very firm that the screw should be 2 turns out and not touched with.

-- danny

05-18-2007, 05:11 AM
Thanks Daren,

you are running a 160 main on both stock and the 72sp cyilnder? Also what exactly does the air mixture screw do anyway? Team Calimari is very firm that the screw should be 2 turns out and not touched with.

-- danny

When I installed the motor into mmy bike, it had the 50cc cylinder and the 24mm w/ the 160 Main.

I recently changed to the 72cc cylinder, and am still running the same carb set-up. Altho' I did have to adjust the air/fuel screw, as it was ran a little lean initially at WOT. So I richened it up some, by turning it in a 1/4 turn. I can't remember how many turns out it is, but I am sure it's close to 1 1/2, or there about.

TCR is knowledgable about set-ups. However, you can verifywith a plug chop on your own.

Air/Fuel screw, I believe, allows the amount of fuel from the float bowl thru the throat of the carb, then mixed with air, then thru the reed cage, and so on.

If you're still having problems or not confident of how to tune your GPR, Marcus and or AJ can get you machine dialed in ;) Btw, glad to see you got a smoker and hope you can join us on Mini Rides :)

05-18-2007, 09:03 AM
I have a 180 main jet on a stock 50cc, i see that a 180 is being used on 72 kits, should i drop to a 175? I do feel that I'm not quite getting the RPM up to 10,000, it holds back around 9,000,

05-18-2007, 09:04 AM
Check you plug. It should be light brown in color. If it is dark then yes if it is white then no.

05-18-2007, 11:52 PM
Alright Guys,

This pic below is of the carb with needle clip on very top running a 185 main. as you can see it's pretty white.

this pic below shows the same thing with 195 main with the needle clip at the 4th one up.

and again with some better light:

I don't seem to be able to get it to a tan/brown, but it would seem this blackish color is safe for now?


05-19-2007, 09:02 AM
Yeah that second one will treat you right. Is it bogging at all or running nicely?

05-19-2007, 06:37 PM

Yah, the bike is running pretty good, it revs very well up to 13K or so. It starts to die out on power right as it hits about 12.5K ish however. I think this has to do with the pipe i have on there now.