Jetting for restrictors

Cash builds a good competitive engine, got nothing bad to say about them. To get those rpms you first got to get the kart free, low rpm drops does not necessarily mean the kart is free. I have seen karts losing 100 rpm or less get lapped. Work with your driver to instill patience, wheel control and smoothness. Make sure you are spot on with setup, tires and clutch (yes the clutch is huge) The Vortex single or double disc are the best for Jr's in my opinion. Hard hitting clutches cause to much hysteresis and as they tend to be either locked or unlocked can slow the engine with too much slippage. Whether thats right or wrong I don't care, it's my opinion. Yours may differ.

On the engine, I have never had a red plate pull past 6300 on my inertia dyno. I have a 500lbs wheel and they just can't move it. But that doesn't mean It won't do it on the track. A gear rule red plate is built differently than a non-gear rule red plate. With a gear rule you look for rpm, Without it you build for torque. You can have one or the other, but you can't have both, Again my opinion and you know what they say about opinions.
Ok thanks for the reply! Lots of good info here to think about. Wish I'd have known this sooner. I could have asked for the motor to be set up for gear rule/rpm, rather than torque. That's my bad I guess, you dont get what you dont ask for... With that being said, I've listed what I know about my engine, is there anything that I can change to help out? Timing? jetting? valve spring type?
 
Ok thanks for the reply! Lots of good info here to think about. Wish I'd have known this sooner. I could have asked for the motor to be set up for gear rule/rpm, rather than torque. That's my bad I guess, you dont get what you dont ask for... With that being said, I've listed what I know about my engine, is there anything that I can change to help out? Timing? jetting? valve spring type?
Nothing to do with engine but keep preaching, momentum, momentum, momentum, to the driver !!
 
Nothing to do with engine but keep preaching, momentum, momentum, momentum, to the driver !!
Always! My dad actually taught him his first lesson on momentum, when he first started racing a year ago, twirling a heavy bolt tied to a 2 ft long piece of rope. It was actually a good way for an 8 yr old to see it, rather than trying to explain it. I'd think you'd be pretty impressed with his momentum keeping ability, but there's always room for improvement!
 
Always! My dad actually taught him his first lesson on momentum, when he first started racing a year ago, twirling a heavy bolt tied to a 2 ft long piece of rope. It was actually a good way for an 8 yr old to see it, rather than trying to explain it. I'd think you'd be pretty impressed with his momentum keeping ability, but there's always room for improvement!
That's great !! Good luck !!
 
Something I will never understand: why the restricted engines need to run so rich. All my racing career I've had the understanding that engines run best when they're lean. Right on the edge of detonation. Even with two cycles (with oil in the fuel) the fast engines always have a clean plug. Why is this not true for the restricted engines? I'm not saying they don't run faster, I'm just asking "why"?
 
Something I will never understand: why the restricted engines need to run so rich. All my racing career I've had the understanding that engines run best when they're lean. Right on the edge of detonation. Even with two cycles (with oil in the fuel) the fast engines always have a clean plug. Why is this not true for the restricted engines? I'm not saying they don't run faster, I'm just asking "why"?
I dont think they are actually "running" rich , but more just "idling" rich. As a result of the plate and location of the hole in the plate, killing the fuel signal. Or at least that's how I've understood it
 
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Thats my take as well the fuel signal is so bad it barely draws fuel .
Big jets to compensate.
Interesting theory, but how come it runs overly rich when you put the bigger jetting in it?
Side note; there is no such thing as "draw" or "suck"! Fuel and air get pushed into the engine by the differential in pressure!
With the HPV 100, they just used a smaller carburetor for the juniors.
 
Likely because dyno and track time shows this to make best power in the usable range.

Compromises made elsewhere..

Lean and mean may not apply when airflow is restricted.
Rulez dictate carb choice and restrictor size.

Only so much air can flow at the available pressue. (1 atmosphere)
 
Interesting theory, but how come it runs overly rich when you put the bigger jetting in it?
Side note; there is no such thing as "draw" or "suck"! Fuel and air get pushed into the engine by the differential in pressure!
With the HPV 100, they just used a smaller carburetor for the juniors.
Draw, suck, push or blow - whichever way you want to put it, the plate kills the fuel signal so it needs bigger jetting so that its getting enough fuel on the top end to run. As far as running smaller carburetors, not legal in stock classes. The carbs may be worked differently by different builders to achieve their goal, but everyone is limited to the same 3 or 4 carbs and even all that has to meet tech requirements
 
I've had 02 sensors on the dyno, they are not rich with .039 or such jets. Not enough air velocity thru the venturi to create a large pressure drop to pull fuel thru the jet.Why would there be a gear rule that made you over rev the motor?
 
I've had 02 sensors on the dyno, they are not rich with .039 or such jets. Not enough air velocity thru the venturi to create a large pressure drop to pull fuel thru the jet.Why would there be a gear rule that made you over rev the motor?
Limiting them on gear is their way of trying to keep the red plates at a slower pace, since it is supposed to be a beginner/rookie class. But, like with everything racing, everyone is trying to go as fast as possible
 
I've had 02 sensors on the dyno, they are not rich with .039 or such jets. Not enough air velocity thru the venturi to create a large pressure drop to pull fuel thru the jet.Why would there be a gear rule that made you over rev the motor?
It was designed to slow them down. The thought was they would barely turn 5500 on the racetrack because of the tiny 3/8 inch restrictor, but they end up over 6400 when everything is right.
 
I've had 02 sensors on the dyno, they are not rich with .039 or such jets. Not enough air velocity thru the venturi to create a large pressure drop to pull fuel thru the jet.Why would there be a gear rule that made you over rev the motor?
Not to beat a dead horse, but in the carburetor, nothing gets pulled or sucked, the air and the fuel get pushed.
It would be interesting to see what the EGT would tell you about these restricted motors and whether they actually are, or are not, running rich on the top end with the big jets.
Just a theory, but maybe the restrictore is causing the mixture to go lean on the top end. An EGT would tell you immediately.
 
Limiting them on gear is their way of trying to keep the red plates at a slower pace, since it is supposed to be a beginner/rookie class. But, like with everything racing, everyone is trying to go as fast as possible
I didn't read every one of your posts with a critical eye, so I may have missed it.
Where are you finishing in the race ?
That's what's important.
Turning 5600 or 6400 does not matter either way if
you are finishing well.
In the past I've listened to other Karters tell me my RPM was too low and
to drop a couple of teeth. Turned more RPM. Finished worse. Lesson learned.
The goal is winning/finishing in the top 3. It's who crosses the finish line first,
not who turned the 'right' RPM>
 
I didn't read every one of your posts with a critical eye, so I may have missed it.
Where are you finishing in the race ?
That's what's important.
Turning 5600 or 6400 does not matter either way if
you are finishing well.
In the past I've listened to other Karters tell me my RPM was too low and
to drop a couple of teeth. Finished worse. Lesson learned.
Last race in jasper finished 2nd out of 8. But almost a straightaway behind the winner. Race went green with 0 cautions
 
Don't ever forget the driver on an oval circuit a driver that is slower by 1/10 of a sec in each end, after 12 laps is 1.2 seconds behind at the finish. Extra RPM, HP, tires don't make the difference, coaching does.
 
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