Yep, the GX-3R's are a little underated by most. I run one for non points club days (to save the MG for bigger events) and on a tight to average sized track they are awesome. They have pretty good top end, but not quite up to the 7 Port. However down low and mid range this motor is strong and smooth.
Well worth a try.

im experiencing something with my go engine (.21 5 port turbo, not the gx serie) and i want to know what can cause this. first ,the engine has about 3.5 gal and the pinch is excellent. i dont know why, but the idle is irregular. coming from few laps on the track, it idles slightly high (for 3-4 sec) then slows down to normal idle. but then after few seconds, goes up again for few seconds, and down again, and it keeps this pattern. the tuning seems right on and temps are good.one of the guy at the track told me to add another head shim (but instructions sheet for the engine says its shimmed for 25%-30% fuel). im currently using 30% fuel (byron) with stock shimming. and im using os p3 plugs. its the first time i see this kind of behavior from a nitro engine. anybody have an idea of what can cause this?

I use an O'Donnell 97T plug in my 5-port, running Byron's 30%, like it better then the OS P3. Try one if you can.

Check your tune via the Massive guide (search the thread).

Like kael said, switch to an OD 97T (med) plug. With stock shimming and 30% the ultra hot P3 might be a bit much.
I run stock shimming (and 97T) with 30%, but different fuel brands and mixes may dictate otherwise.
Change the plug to a 97T (or an equivalent medium temp plug) and add .1 or .2 of head shims and see if it settles down for you.

Change the HSN and LSN seals (Part# OR-0023) and the seal around the base of the carb (Part# OR-0004).

Also, proper deck height on a stock GX engine (not positive about previous gen) is set w/ 2 copper shims.

talking of orings, i find the go orings quite cheap and fragile. is it possible to find orings from another brand that fits go carbs?

the older ones tend to degrade. Please try the new nitrile ones. Nitrile is a synthetic rubber that is used in everything from petroleum pump seals to hypoallergenic medical gloves. It's quite stronger and more resilient than natural rubber or latex.

what he said!!!!!!!!!!

I think you will find there are 2 x thin .1mm brass shims, plus 1 x thicker .3mm alloy shim in the stock GX motors. This is the same as the previous Gen 5.5, Gen 5 etc motors.
Quite often the alloy shim sticks to the top of the sleeve and gives the impression of there only being 2 x brass shims in there, but if you look closely you will find the alloy shim there too.

So the engine ran fine for three and a half gallons of fuel,
then started the strange idle? I don't think any shim change
is going to fix your problem. Have you checked the front bearing,
fuel tank and lid, fuel lines, carb needles for leaks. I had a small
peice of h.s.n. o-ring floating around inside a carb one time.
Blow some fuel into the carb with th h.s.n. removed. I bet
you'll see some small chunks-o-shat float out the top of the
h.s.n. housing.

perhaps you should read the first part of my post, regarding the seals.

I read what you posted Runne. I was relating a
problem I had with a GO carb a few years ago
with the older style o-rings. Gave me a fit, even
after I replaced the o-ring. I was in a hurry and
didn't clean the carb out when I replaced the o-ring.
Since then I have had many people give me there, as
they called them "P.O.S. GO carbs". All I had to do to
them was douch them out good and replace the shatty
o-rings to make them work properly again.
I have ran Novarossi, O.S. carbs on my GO engines,
even found out that a MR Ninja on-road carb with a
9mm venturi kicks ars on these engines.

very interesting. have you tried the GX carb with that same venturi, or does it fit?

I may have to check out one of the GX carbs soon.
One of my truggys got t-boned a couple of weeks
ago. The carb on my .25 took the hit and the neck
now has the death wiggle. It still runs fine but I won't
run it any more until the carb is replaced. It took a lickin
and kept on tickin, can't say that for the body though.

we've got plenty of carb bodies in stock...