What Happened to ThumperX (Four Stroke Shifter)?

Wondering if anyone knows what happened to Marcos Ambrose’s project called ThumperX. Pit bike based four stoke shifter.

I saw a little of it then, poof, never head of it again. Maybe Marcos stopped working with it when he moved back to Oz? I thought it was a pretty well thought out package. Here’s a overview with Marcos…

Here’s an onboard at GoPro (Before it was GoPro)

Fast forward seven years later… Does the concept of a gear bangin’ Briggs appeal to you?

Ps, if you have one, I’d like to buy it.

Sounds fabulous. Distinctive. The lift off/on is sweet sounding.

Billy Musgrave is going to offer his Factory Kart chassis with a Daytona 190, which is similar.


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Interesting. Good to know.

I feel like I saw a couple of these at Badger years ago. There were two dudes who ran them in the open class. They weren’t particularly fast, but sounded cool and seemed like they would be fun if you were into shifting but not destroying your body.

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I wonder if they were the Yamaha “exciter” shifters you saw? More upright engine like a CRF.

Noticed MRC are offering KTM’s fuel injected 250 two stroke. Runs about $8100 for the engine though alone. Which makes sense because KTM will not sell an engine by itself apparently.

Any 4-stroke engine making over 70 horsepower per liter is a hand grenade with the pin pulled on a kart. The limit of output for a 4-stroke is mechanical failure, the limit of output for a 2-stroke is gas flow.

THAT’S what they were. I remember now. Good call.

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I think the Swissauto with 28-32HP from a 250 has shown this to not be the case. OHC is a must, four valves even better at that power level though.

Not a great example, but I think it’s noteworthy that many production four stoke engines are around the 100HP/l mark on motorcycles and are very reliable when raced as stock too.

Also, Honda VTEC is 100hp/l+ and has shown to be tough when raced, if you can keep the head gasket on it.

In the other thread referencing this motor, the price for the engine package was quoted to ultimately be $3,000. I know how costly shifter motors can get, but even then, for a pit-bike derived engine I’m assuming is manufactured in China, that price seemed pretty high.

Whatever the cost may be, the recent discussion about these cheap, 4-stroke, pit-bike shifter engines hint at the much missed karting class the pit bike motor is supposed to emulate: the (2-stroke) 80 SHIFTER .

If the trends in recent years say anything, it’s that karting is “downsizing” with their motors. 125 TaGs used to be ubiquitous, now it’s the air-cooled 100cc TaG engine, along with the Briggs 206. Logic would suggest there is a market for a smaller, lower-powered, less expensive shifter engine as well.

Problem is, there isn’t many 80cc, or 85cc, shifter engines to be found. Honda used to dominate the moto-based 80 shifter class in numbers, just like they did with their CR125 moto, but Honda hasn’t made a 2-cycle 80/85cc motocross bike since 2007. And I suppose there are no manufacturers in China or elsewhere in Asia that build a cheap 2-stoke motocross-based engine either, or else someone would mounted one on a go-kart by now.

Yamaha still builds an 85cc motocross bike however, along with Kawasaki and KTM. I don’t know if any of these motors would work well on a kart, or if they would actually be “cheap”. But Yamaha changed the face of karting with their famous KT100, could they make it work with the YZ85? I just happen to have an old YZ85 engine I bought on ebay last year. and I want to bolt it on a kart and find out myself.

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I’m a KZ purist when it comes to shifter engines. I’ve always wanted a TPI 125 KZ motor (dedicated, not a MC swap in), but I doubt the market is large enough to buffer the price point vs development costs required develop & produce it.

I’ve heard - never verified - that TM has/had an EFI prototype, but karting has always evolved incrementally, so no surprise that carburetors are still the go to.

They’re already selling their EFI in their dirtbike range

the reason it isn’t in karts as it’s added complexity and expense. Carbs are still the best solution for high level racing.

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If that were factually correct, there wouldn’t be fuel injected motors in MX, MGP, F1, etc.

Well, it’s only factually correct if you ignore what I said in the previous sentence and extrapolate what I mean by ‘best solution’ to mean pure performance. That’s not what I mean. Also, almost all motorsport mandates against ‘best solutions’ for performance. Spec ECUs, non-movable aerp etc… so F1 and MotoGP aren’t necessarily representative of ‘best solutions’ either :slight_smile:

Complexity and cost is why we still use carbs in karting and EFI effectively outlawed or not part of spec-units (I am talking 2-stroke here by and large). As a solution to the problem of needing ‘a device to supply an internal combustion engine with air/fuel mixture’ in a racing context (which has to take into account secondary effects of regulatory changes like cost, complexity etc…) carbs are still the best. Their introduction would be a nuclear arms race (karting could barely cope with the introduction of power valves).

When I was doing all my research when we believed Rotax were developing a system, I was told that they only reason they would be introduced would because of government legislation. Maybe the tech is better now and more solid, but I was told that EFI is not something the manufacturers are overly keen on for the general racing kart market.

Moderna had a system iirc though.

@KartingIsLife looks like you’re getting your hands on a thumper injun?

Since you didn’t define the term, I had no basis to work with. I also did not specify performance, though you may have presumed it was implied by the examples I chose, which is reasonable, but not my point. I cited them as familiar, time-proven examples of successful implementation. That said, EFI does also offer gains in performance & efficiency.

I don’t buy the complexity arguments. This seems to be a cliche cruch in the kart world. I heard it from the start when TM introduced the 1st TaG-shifter engine (K9ES) in 2006: battery + starter & wiring harness is too complicated; it’ll break down. Except, that wasn’t the case. And it’s the same story with the KZ10ES. It holds up fine. EFI wouldn’t be any worse. Yes, there would be a fuel pump & ECU, but this is proven hardware. If anything, optimized fuel-air-oil ratios mean no more detonation worries. What’s more complicated to deal with: a dead fuel pump, or a blown & seized motor? And lest we forget, fiddling with carburetor jets vs plugging in an ECU (These can be firmware spec’ed, BTW) if/when it dies.

I don’t disagree on the cost issue, though. I attribute it primarily to a lack of effective economic scaling in the karting market. The numbers to mitigate the added costs & keep pricing reasonable probably just aren’t there.

Govt will intrude in karting when it runs out of higher profile heresies to crusade against, & I do think it’ll be coming at some point. Just don’t when.

Modena’s 2014 EFI prototype was vaporware, unfortunately. I was hoping it was going to kickstart a trend, but it vanished after the initial reveal.

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There was a 450 Honda powered Tony Kart shifter at GoPro last week on Thursday doing some testing. Guy said they had done some motor work on it and it made around 60hp. Sadly I was busy and didn’t take any pictures of it, looked like it really moved out.

They run 450s 4/ lumps in Superkarts - long and short circuit. Guys that run em seem happy enough

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The 450’s are a rough replacement for a 250cc two stroke. They are purse savage on sprint, and used in Superkarts in the US too.

Here’s one at Sonoma, at least I think it’s Sonoma.

For a while there was a thought (Not confirmed by SKUSA) that SKUSA might go CR250 to replace the CR125, in the end it was the 175SSE two stroke.

Different end of the spectrum to the Thumper though in terms of cost when new.

Reliable 4 stroke power isn’t difficult. NA numbers around 120 HP/liter are easily achieved, as is nearly double that with forced induction. Yes those are shy of what a 2 stroke can output in it’s tiny power window, but the area under the curve is much larger due to the torque a 4 stroke can make thanks to physical valves. Replace the mechanical link of the valves and the crankshaft with freevalve technology and that could climb to 300 HP and 221 ft lbs of torque per liter (strange mixed units but why not).

What complexity and cost? EFI is both simple and cheap to implement, this isn’t the early 80s… Plus its easier for most people to work with than carbs.

Regulation wise, I’ve found that stuck in the mud kart people abhor change - this is the real reason for the lack of change. But the environmental people are going to force this change sooner than later and it would be good to get ahead of the curve instead of being flattened by it when new 2-stroke engines are completely banned.

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