Big block clone questions 420 440 460 390

1

1201

New Member
#1
#1
hi guys, im new here. Even though I dont own a mini bike(that will change one day - I love these things) I registered because there is a WEALTH of information on the clones on this site.

I ordered a 420cc motor and had them shave the heads to get me to about 12.5:1 compression. my plan is to run methanol. this is going in a racing kart btw

i went to the local kart shop to ask how to convert the stock carb to alky and he snickered. he said I need a tillotson, so now im in the market for one. im also ordering

my questions.

how much should it make with stock cam and alky carb and free flowing exhaust
will the stock cam rev to 6000 in this configuration?
is the stock rod good to 6000 or does that have to be changed out as well?
which tillotson , or which alky carb would you all recommend?

thanks all.
 
Boomstick

Boomstick

New Member
#2
#2
Welcome aboard.

First things first, you're going to need the billet rod & flywheel. After, then you can do the governor delete.


Don't even bother doing anything else unless you're going to get those two super, super critical parts.
 
1

1201

New Member
#3
#3
thanks for the feedback. i am ordering a billet flywheel tomorrow. i was hoping to use the stock rod though but if its not going to work and i have to order the billet rod, then i might as well go all out and get a racing cam as well.

what is the max reliable rpm for these engines? is 7500 safe?
 
Boomstick

Boomstick

New Member
#4
#4
You really need to buy & install the billet rod.

The cam profile is dependent on your intended usage. Are you going to do head work? Big valves? Big carb? etc.

There are a lot of little factors that play into builds.


7500rpm's is a realistic number for a 420 provided it's built correctly.
 
B

bandit 2026

New Member
#6
#6
You seem to be focused on rpm's. If you want a high revving motor stick with the 212 hemi, lots more aftermarket parts and they're cheeper. The 420 is a torque monster it builds power on the bottom end if geared right and has a tallish tire its stupid fast.
 
1

1201

New Member
#7
#7
bandit 2026 said:
You seem to be focused on rpm's. If you want a high revving motor stick with the 212 hemi, lots more aftermarket parts and they're cheeper. The 420 is a torque monster it builds power on the bottom end if geared right and has a tallish tire its stupid fast.
Click to expand...
Well, rpms is power so I'm only trying to figure out the Max reliable rpms.
 
Boomstick

Boomstick

New Member
#8
#8
1201 said:
Well, rpms is power so I'm only trying to figure out the Max reliable rpms.
Click to expand...
Not necessarily. If you happen to look at the charts dynamometers print off, you'll see that after a certain speed, that power & torque drop off... sometimes rather sharply.



Don't try to focus on the absolute MAXIMUM amount of power & engine speed you can get either. Focus on developing a broad, steady powerband that allows you to utilize a good majority of your power at any given time. This way, you're not lugging an engine all the way up until 8000rpm's & THEN it develops useable power. Likewise, you won't be building a 4000rpm torque monster that wheezes past 5000rpms'.

See where I'm going with this? It also depends a LOT on your running gear too. How tall are your tires? How wide are they/tread? Running a clutch or a TAV? How heavy is this vehicle? What is your gear-ratio? Do you want a top-speed queen that's slow getting there, or do you want a wheelie machine that does 30mph?
 
Boomstick

Boomstick

New Member
#11
#11
Cam choice, intake system, exhaust system, all of them play specific roles. It's not like you can just throw a bunch of mixmatched parts together & run a monster.


If you want to scream along at high-rpm's all of the time, you're going to really need the billet rod & flywheel, no questions asked. You can put a PILE of $$$ into the engine other than those two parts, and lose it all when it decides to pop.

Then, valvetrain. That has to be upgraded to meet the demands of a high-revving, hard-living engine. If your valvetrain fails, you'll probably drop a valve & destroy (again), everything.
 
1

1201

New Member
#12
#12
You're right. Btw I'm definitely getting upgraded rod and flywheel and valvetrain. Really not much to pay for peace of mind
 
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