30 or 40 series Torque Converter on a 420cc engine

[cgavaller2]

Hey all. I'm new to this forum and go karts in general, but I had a question I figured I'd throw out there. I am in the process of making my first go kart, and picking out parts. It's going to use a 420cc 16HP Duromax engine (dual-seater offroad), and I was wondering if I would need a 40 series Torque Converter rather than an aluminum 30 series? I know these engines can put out serious power and I don't want anything to be under-specced, I just was wondering if the fully-steel construction was necessary for the engine of my size.

Thanks!

 

[madprofessor]

Absolutely you cannot use a 30 series asymmetrical CVT on that 420. You're stuck with a 40 series symmetrical, but you won't be burning it up with that motor because that's what it's built for.
You can order a 40 series as a complete unit with a backplate that everything mounts on ready to go, or as a separate parts group, ie; the driver, the driven, and the belt, where you build your own jackshaft to carry the driven.
Comet Drive Belts | Comet Torque Converter Belts | Go Kart Belts (gokartsupply.com)
Read the whole page on the link above, especially the big info at the bottom of the page. You'll see diagrams of how it all works, and your 40 series diagram is near the bottom, just above the red "500 Series Belts". You'll learn all you need to know about a CVT (Constant Variable Transmission), aka a TC (Torque Converter).

 

[cgavaller2]

Absolutely you cannot use a 30 series asymmetrical CVT on that 420. You're stuck with a 40 series symmetrical, but you won't be burning it up with that motor because that's what it's built for.
You can order a 40 series as a complete unit with a backplate that everything mounts on ready to go, or as a separate parts group, ie; the driver, the driven, and the belt, where you build your own jackshaft to carry the driven.

Would a unit like the one here suffice?

 

[madprofessor]

Yes, as long as your motor shaft is a 1", and good call on the ebay price advantage. Been getting my 30 series units there. Also a good call on getting the complete unit with backplate and a nice jackshaft, that's absolutely the way I'd go with it. Just keep the original packaging for 2 years, the length of the warranty. Gets rejected without it.
It doesn't say it, but I'm 99% certain that 10-tooth sprocket is for #40/#41 chain.

 

[bob58o]

With the backplate kits, I believe you need to mount the driver unit inboard, since you can’t mount the driven unit inboard.

These pictures show two possible installations of the driver unit. Only one way allows the belt to move in the same direction as the moveable sheaves move. Install incorrectly and as the front pulley pushes the belt closer to the engine, the back pulley pushes the belt further away from the engine and the belt gets crooked.

 

[Millwright]

Hey there, I'm new here as well. I'm also doing a build with a 420cc clone engine.
When I ordered my TC it came without the back plate so I ended up with a separate jack shaft.

I did the install exactly as the comet manual shows. In order to make it fit, I needed to order a longer belt than what was supplied. ( hasn't arrived yet ).

I went with a 13 tooth and 48 tooth set up.
I have 16.5" tires. The kart is a single seater but larger and heavier than typical. I'd guess about 225lbs, before I get on it.
It seams 6:1 is the typical set up, but I'm hoping I get by with the 3.7:1 I'm more interested in top end speed.

I'm about 2 weeks away from being able to do a test run.

Let us know what gearing and tires you are going with.

 

[madprofessor]

Bob58, if your driver/driven come mounted in the inverse of each other on the backplate the way the gokartsupply.com diagram shows they should be, you're good to go. If not, they might have sent you a 30 series instead. Wouldn't fit anyway, 3/4" shaft.
Millwright, I hope you used gokartsupply.com link above (post #2) for belt measurement, using center-to-center measure for correct 7.5" standard or 8.5" (44 Magnum) driven pulley. That O.C. measure (outside circumference) is way too easy to get wrong.

 

[Millwright]

Ya I used the chart in the comet manual. Various center distances to choose from.
+1/16" -0 tolerance on center to center. Or other way around lol.

I have play in the engine mounts, and can shim out my jack shaft pillow bearings if needed. Should be able to get it dialed in. If only I actually had a belt.
Canada stocks nothing! Getting products shipped across the border is time consuming and pricey. Covid BS is not helping.

 

[madprofessor]

My hopes for folks getting the right belts for their CVT's is mainly about the lack of consensus on just what kind of tension the belts should be under when they're not stretched out and are fitting correctly.
I personally don't know either. I know the machine shouldn't be creeping forward at idle, and I know it should never be able to throw the belt off. That's about it. Have my camera now to video what the pulleys do when the kart runs, but it's unclear if a problem with belt tension could be differentiated from a problem with the pulleys not acting right. Only running 30 series units with backplates right now, so no adjustments involved, only 1 correct size for them. 40 series with backplate? Not sure, but many sizes available, so chart's still the best bet.

 

[Millwright]

Driven pulley spring colour?

Red spring = clock wise
Green= ccw

With the rear pulley springs outboard, it's not just a matter of belt alignment. Even if you turn around the driver to correct the alignment issue, you still need to have the green reverse wound spring.
I've seen a number of pics with guys running the red spring mounted outboard. I guess it still must work to some extent?

 

[Denny]

Until the spring unwinds and comes apart.