Mini ROK carb help!

[Bolt (Doug Bolt)]

Hi everyone!
We race in the Mini class with both the IAME Swift and the Vortex Mini ROK engines. Recently, ROK changed the rules and are making us change from the Delortto carb to the Tillotson carb. Our engine runs great on the delortto and poorly on the Tillotson! It floods easily which makes it hard to start, then tuning the needles has been anything but normal. We started with the same basic settings as the Swift, half turn out on high and one and a half on the low, but realized that’s not gonna work. It’s slow to respond to the throttle and bogs out of the corners. With more high, like 1-1/14, it’s much better. We feel like it now needs more fuel on the bottom end but that makes the flooding worse. ( I had heard the vortex made this move to improve starting!)
So, has anyone made this carb change and what ballpark needle settings are you working with?
Here’s some numbers, everything else is the same, tires, fuel, gearing….
Max Rpm 14,080. W Tillotson, 15,100 Delortto
EGT 1142 Tillotson 1190 Del
Needle pops off at 16, holds forever at 10 psi
It was a brand new carb out of the box and over the past few weekends we’ve had it taken apart, cleaned reset and all new gaskets and diaphragm replaced by three different and well experienced Kart team owners!!
It’s very discouraging and expensive! Today I was told by one of the team owners that it’s probably a defective carb right out of the box……what do you all think?

[GregF (Greg Fine)]

Maybe this will help.

@GregF

The following in taken from E.C. Birt. A carburetor guru that is well known for his work with Tillotson Carbs. It gives good tuning instructions on how to find optimal settings for the HL and HR carbs. It can also be found on the ECcarburetors website at the bottom of the page under E. C. Birt Archives:

eccarburetors.com

EC%20speed%20secrets%2093.pdf

12.39 MB

“Your Tillotson carb earned the reputation of being “user
friendly” because if you got anywhere close to the above
information you carb will work for you. Track side there are a
few simple steps to follow on finding the right tune of your
carb. 1) Set the low side needle at 1 1/4 turns out from bottom
and the high side needle at 1 turn.
Now you’re ready to start the engine. Set the kart on the
starting grid and have it pointed at the race track. Pointed at
the privy or someone’s pick-up just could lead to major problems.
Never get into the kart without being dressed to race. Put
yourself in the kart, hit the throttle a couple of times and watch
the linkage of the carb. Make sure that it is going from full
throttle and have your handler put the starter into the starter
nut and spin the motor over. Watch the fuel come up the fuel
line. As soon as it disappears into the carb, count to two and hit
the throttle. The motor should fire.
lf the clutch is set up right, you’re still in the pits. lf not,
you’re on your way to the track. Now you know why you didn’t
point your kart at the privy. Hold the brake on and throttle the
engine a few times to get the clutch to hook up and move out
onto the track. lf the motor is loady, give it a chance to clear
out by itself. lf it won’t, you may have to take a touch of the low
side away. Turn the needle in a little bit at a time until the motor
cleans up. As soon as the motor cleans up brings the low side
needle back out where it was. Give the kart a couple of laps to
warm up. Once the motor is warmed up, come down to parade
lap speed then stand on the throttle. lf the motor bogs, it could
be on the lean side. Open the low side needle lust a touch.
Keep coming out with the low side needle in steps of 1/8 turn till
the motor pulls hard from the parade lap speed.
Now you’re ready to find the tune for the high side. On the
longest straightaway, bring the high side needle out till it goes
rich in the middle. This rich sound will be a splattering sound
or irregular firing. Better known as 4-cycling. Once you tind
this spot, start taking the high side in till it runs clean all the
way to the end of the straightaway and just 4-cycles at the
point where you’re going to lift for the turn. Once you have
found this spot, take the high side needle in lust a touch more.
Now go back to parade lap speed and make sure it pulls off the
bottom clean with no bogging.
Next check the feel ol the motor as you just come off the turn
after a high speed straightaway flat footed. lf the motor 4-
cycles here, the low side is just a touch on the rich side. Take a
little away from it. (Turn the needle in.) Watch your temperaturegauge. Anything over 350” F is starting to get too hot. lf you
don’t have a temperature gauge on your kart and the stickers
start to melt off the side of your helmet, that is also a good sign
that the motor is running on the hot side.
Once you are comfortable that the motor is tuned right,
come back into the pits and check the position of the needles.
lf you find that the high side needle is out more than the low
side needle you missed it somewhere out on the track. When
you’re tuned at the optimum, the low side needles will be out
just a touch more than the high side. Once you have found this
place for the needles you can start f ine lining the tune by the
plug color and temperature of the motor. Plug color will be a
light tan to brown color. Motor temp 250* to 350* F max on a
control. A lot of people tell me that a TS-40 or US-820 has to
run 500" plus to make good power. Try 375’ and put your stop
watch on it. Just maybe 500" is not the hot tip after all.
ODDS AND ENDS;
Never mix up any more fuel than you need for race day.
Dump left over fuel into your race hauler. Never use left-over
fuel for parts wash.
Service carb after every race meet. When pumps get
stretched, replace them. Stretched pumps make for a richer
running carburelor.
On real hot day set the pop-off pressure a bit higher. This
will let you run the needles out f urther and help coolthe motor
off without being too rich on the top end.
Cool weather, good air, high A.D. Set the pop-off just a
touch lower. Carb can stand to be a touch richer.
When setting pop-off pressure never have carb dry. Wet
everythng with WD-40 for safety factors.
When putting in new gaskets, wet them with your 2-cycle oil.
This way, if you have to pulla track side service you won’t have
gaskets stick and tear on you.
Pop-off pressure must pop then drop a touch and hold. lf it
doesn’t hold carb will not perform right. Will always be on the
rich side. Cure: reclean needle and clean seat with Q-tip (cotton swab) and
wD-40.
Always carry a spare cap, fuel screen and cork with you. lf
carb starts acting lean, pull cap and check screen. lf trash is in
it, replace with brass screen and hope that none of it gets to the
needle, causing it to leak.
Torque carb pump plates like you would a cylinder head.
Every other one till tight. Any leaks here will cause lots of
tuning problems. lf lap times are not fast, it’s always easy to
blame the problem on the carb. But just maybe you will find
that faster lap time by tuning the driver or the chassis.
Keep your fuel dry and have a good time at the races."

• E.C. Birt

(Minor edits for points of reference, i.e Q-tip = Swab & Degrees in F not C)

It was more for the Tag Senior engine, but the tuning explanation is spot on.

[Bolt (Doug Bolt)]

Thanks for sending that article along Greg!

[Fergie0829 (Ron Ferguson)]

As we are testing for Rok Vegas in mini I would say you need to take 5 to 8 minutes out of the high and give it way more on the bottom. Do not try and apply baseline Swift settings this motor. It won’t work.

There is no standard 1 1/4 and 1/2 aka 30 minutes for any Swift motor. They all are a little different. We race one nationally that likes 148 on the low even when it is summer in New Castle and our teammates motor likes it at 136. You should be tuning more with your data, high and low EGTs and low corner speeds and if you can run a lamda sensor to see if you are actually rich or lean it will help you immesly. It also helps make sure it is not the squishy bit in the sear.

Carbs are generally not defective especially if you have put a new kit in it. You can always run it on you swift and see hiw it performs.

[Bolt (Doug Bolt)]

Hi Ron,

Thanks for the reply even though I’ve not seen it until tonight! So, the story moved on with my dealer warranting the carb we bought. After that things absolutely fell into place. Tuning the ROK settled down but we still are learning. It is definitely different than the Swift….you’re right none of the settings cross over!
We have been able to get much better performance with this carb and the ROK now and we’re getting better.
I do have a question based on your reply, what egt temps are you shooting for on high end and low end? Also, what are you using for your data, Alfano or Mychron…I’ve heard there is a 50 degree difference between the two!

Lastly, are you Troy’s dad? If so we’ve met in orlando at the Benik tent.

Either way, thanks for your help!

Doug