there must be something terribly wrong with your Torque converter setup or your kart in general.
the murray explorer came for All I know with that EXACT same setup..
an engine that turns 3600 rpms,
a series 30 Torque converter
a 6:1 gear ratio (10 tooth TC out and sixty on the axle)
And with it's 18" rear wheels that means it does 36'ish mph!
As you can see.. it's far from ten'ish speeds
Even if your engine isn't accelerating properly
say it only barely revs above engagement speeds (say 2500 rpms)
if the TC would work properly it should get you up to 22'ish mph
(as it would with a simple cent clutch)
Taken that... it sounds to me, like your Torque converter is stuck in low gear
(i.e. it doesn't change it's internal reduction)
which in turn would mean it's reducing the speed by a factor of 2.7 and you only end up with 13'ish mph..
See why I think it's the Torque Converter?
So, what next?
Pop up the rear of your kart (put it on SOLID stands),
remove the TC cover,
put your wife/kid/friend in the front seat and have him/her rev up the engine.
Observe the Torque Converter, is the belt moving properly up and down the sheaves of the two pulleys?
Does the belt reach the outer edge of the Driven pulley?
If so, you must replace your belt, since it's likely too worn out to do the same under load.
if not, post pictures of the Cam of your driven,
check that the spring is not pretzeled up and the six rubber buttons are still in place.
(if the cam slipped past these buttons it can be ruined and locked)
If that's okay, remove the bell of the driver pulley and check the garter springs and weights,
remove the belt and measure it's top width (3/4" or replace with original Comet belt or designated series 30 Torque converter belt! [NO automotive V-belt crap!!!])
And since the belt is off.. try to move the outer sheave of the driver, it must move nicely towards the engine, until it almost touches the fixed sheave,
else you will need to clean the TC and do full maintenance.
(clean all sheaves baby smooth dry lube only and JUST on the weights [a hair on the small clover hub doesn't harm])
Post pictures in case you are unsure,
we can identify the parts you need, and things you should work on from there.
Spare parts (weights, springs, buttons) can be found on
mfgsupply.com or
gokartsupply.com
'sid