Robin EX21 Powered Yerf Dog 3203  Robin Go Kart Engine Swap

Robin EX21 Engine

This page covers some thoughts for engine swaps on go karts.  Specifically, this page focuses on a engine swap in a Yerf Dog 3203 go kart.  I know that most engine swaps are done in order to switch over to a much bigger engine. It's the old "BIGGER IS BETTER" and "THERE'S NO REPLACEMENT FOR ENGINE DISPLACEMENT" principles at work. In many cases, I do think these ideas do hold true. However, I also know that they are not true in all cases. 

I thought through the engine options on my Yerf Dog 3203 go kart and I decided to go a different route than the usual big engine swap. I wanted to go with a small engine that offered BIG ENGINE performance potential, and I didn't want to sacrifice reliability in the process.  So, I decided to switch over to an engine that is about the same size in terms of engine displacement, but to go with an engine that is a much better design than the stock Tecumseh 6.5 HP go kart engine. I wanted a powerplant that would have a good foundation to build upon, and one which would have unsurpassed reliability. After doing a lot of research, I settled on the Robin EX series of engines, and I bought a Robin EX21engine designed for applications such as go karts. The going rate for a Robin EX21 Deluxe engine (with oil safety switch and remote throttle linkage needed for go karts) is around $400.  It's not cheap, but it's an incredible value if you look closely at (and understand) it's superior design and construction.

The EX-21 is the 211cc model which is a little bit bigger than the Tecumseh 6.5 HP 195cc, but it's real close in terms of engine displacement. The Robin EX-21 is conservatively rated:  7.0 HP @ 4000 rpm AND 10.26 FT*LBS TORQUE @ 2500rpm.  The following dyno chart is in metric measurements, but the PS power corresponds real close to HP.

Robin EX-21 dyno chart

Robin makes a bigger EX engine - the EX27 - which is rated at 9 HP and also intended for applications such as go carts, but the higher cost and 1" diameter crankshaft output shaft turned me off from it. That leads me to another decision I made.  I decided that I wanted to stick with the stock 30 series go kart torque converter. Cost was one big reason, but also I believed the 30 series torque converter was capable of much more than just the 8 HP that Comet rates it at. I began hanging out on some of the go kart race forums to find out if any of the racers were playing with 3/4" diameter crank race engines on their fun go karts and projects. I ran across some people who were running powerful 3/4" crank engines with the 30 series converters. One guy shared that he was running a 30 series along with a modified Briggs Animal racing engine with 14+ HP. Now, I do believe that running higher HP through a 30 series requires proper gearing.  In my opinion, the stock Yerf 3203 go karts are geared way too tall and it might overload the torque converter with a high HP engine and a lot of load (like climbing a big hill with heavy go karts). So, I would expect that the right gearing will be necessary to make this work. In addition, I would expect that the torque converter will need to be calibrated for a higher output/higher revving small displacement go kart engine. Specifically, I expect that the driven unit spring will need to be changed to allow the TC to shift later and the engine to rev up higher. I did some research and bought a Yellow driven spring (Comet 216168A) to replace the stock green driven spring.

Let me share a little more detail about the Robin EX engine design. The Robin EX engines are true Over Head Cam design.  The EX OHC engines are not pushrod actuated OHV engines like most other engines out there. The Robin EX series has a chain driven cam that resides in the head.  Forged rocker arms ride on the cam and actuate the valves.  The EX engine reminds me of a high quality motorcycle engines. Some of the other specs on the EX engines are: forged aluminum connecting rod, forged steel crank, forged rocker arms, double ball bearing supported crank, and of course a cast iron cylinder. There are some other very nice features on the EX engines, but the ones listed are some of the big ones. Anyone that knows engine design knows that these are some VERY NICE features -especially for a small go kart engine.  Here's a cutaway diagram showing the Robin EX design...

Robin Subaru EX21 Engine

In contrast, the stock Tecumseh OHH65 go kart engine was truly a disappointment to me in terms of it's design. Sure, once modified it can crank out some good power, but the aluminum bore Tecumseh go kart engine is truly a throw a way engine design.  It has an aluminum cylinder bore, no "real" bearings to support the crankshaft, plastic cam... these sort of features are not supportive of long engine life. In most cases, these go kart engines are not used enough for this to be a big problem. For the occasional, recreational user this go kart engine might last for many years and give satisfactory performance. So, I'm not saying that this engine can not/will not serve it's owner well - as long as it's well maintained and as long as it's not run a high number of hours. Figure an average max life span of around 150 hours for an engine design like this. That could span many years if it's seldom used - just depends on the owner and how much they use it.

Here's an interesting quote I ran across while researching engines for the swap...

"Don't you want to finish what you start? With Robin-Subaru engines you will. Rated number one by independent dynamometer tests. Independent tests have shown that some cool bore aluminum engines (even the best known brands) have a short life cycle of only 150 hours running time before they start to self destruct. Even on their higher end engines, test results show them consuming oil at 1000 hours of running time, but not the king of the engine test, Robin-Subaru. Robin-Subaru engines from American Power Equipment beat all the competition in engines tests. Their engine dynamometer tests recorded over 2500 hours of testing, with no oil burn and no loss in horsepower."

The Robin engines are extremely high quality engines designed to last a LONG time. Robin is rated #1 in terms of small engine quality/reliability.  Tecumseh are at the very bottom of the list. No offense to those that have Briggs or Tecumseh engines. I just bought a new lawnmower with a Tecumseh. I bought the Yerf Dog 3203 go kart with the Tecumseh engine. I've used equipment with Briggs too, and all these have engines ran pretty good and served their purpose well. But, my background is Mechanical Engineering and I've worked on engines most of my life, and the Robin engines truly have a superior design.

I swapped the engine in and needed to make a few modifications to get things to fit. The Robin crank is a little shorter in length.  As a result, if the torque converter is installed just as it was setup on the Tecumseh go kart engine, there is not enough crankshaft to safely secure the outer portion of the torque converter driver assembly.  So, I had the crank spacer shortened at a shop (on a lathe) by about .200".   That exposed more crank at the end and the torque converter driver assembly fit well.  Then, I needed to re-align the torque converter pulleys so they were lined up again.  With the shorter crank spacer, the driver pulley was now moved closer to the engine, so I needed to offset the driven pulley the same amount to keep things line up.  I removed a washer from the jackshaft assembly (0.100" thick) and I also had the jackshaft sprocket hub turned down by .050" per side (for a total of .100").  That moved the driven pulley over the .200" to match the driver pulley.  That pretty much took care of the torque converter.  Then, I soldered the end of the throttle cable (to keep it from fraying), and I cut off the cast end that was meant for the Tecumseh go kart engine.  I could then insert the cable in the Robin throttle linkage. By the way, the "Deluxe" EX engines come with the necessary spring loaded throttle linkage for go karts. The "Value" EX engines will need to have some parts added to get the spring loaded linkage for go kart applications.

So, how does the Robin engine work? It works great!  Sure, in factory form it's not going to given anyone whiplash or pull wheelies!  But, everything about this engine communicates quality. When I started the engine for the very first time, it settled down into a smooth idle almost immediately. My tach showed a nice, low idle of around 1200rpm. Very smooth and rock solid idle. In contrast, my Tecumseh OHH65 was cold blooded and wasn't the easiest to start. Then, once it was running the Tecumseh did not want to idle well. Idle speeds were unstable, and would change frequently. It would usually idle too low when cold (and want to stall), and then once things heated up it would want to idle too high. I usually kept the idle up around 1800rpm (when warm) on the Tecumseh to prevent it from stalling. Even stuff like the exhaust note on the Robin engine is more pleasant. When the engine runs, it sounds like a quality piece of machinery. The Robin engine reminds me of motorcycle engines I've worked on in the past.

The first time I took the go kart out with the Robin engine (stock, governed), I was impressed with the torque of the EX21. With the governor shutting off the fun at 3600rpm, the engine was not going to break any top speed records; however, the seat of the pants pulling power of the stock EX21 felt very good. It is probably because of the torque of the EX engines. The EX21 is rated (conservatively) at 10+ft/lbs of peak torque at around 2500RPM. That's quite a bit of thump for such a mildly tuned, stock engine.  Seat of the pants of the stock Robin EX21 engine felt like my hot-rodded Tecumseh 6.5HP go kart engine, so I was impressed with a good starting point on the Robin engine.  In comparison, before I sold my Tecumseh OHH65, I had done many of the usual modifications to increase performance. I modified the carb for full throttle, ported the intake manifold, removed the governor, advanced the ignition timing, etc... The Tecumseh did run strong and it ran very well overall. It was just the design of the engine that turned me off. I didn't want to spend a lot more time or money modifying one of those disposable Tecumseh go cart engines, and then have a pile of scrap metal in a couple years. In contrast, any mods I make to the Robin engine should serve me well for MANY years longer because of the superior engine design.

Now, in terms of aftermarket parts for the Robin engines, it's almost 0 right now. However, that is slowly beginning to change. Robin is in the early stages of a Motorsports program. I know that they are developing a World Formula engine for the go kart race guys. The amazing thing is that most of the WF race engine is just off the shelf Robin parts/technology. It's a testimony to the superior design of the Robin EX engines.  Probably the biggest difference in the Robin World Formula engine will be the cam. Stock EX engines come with a unique single lobe cam that is set up to keep the EPA happy with low emissions. Not the best for HP!  This is a "weak" point in the stock EX engine design... in terms of the ability to hot rod these engines. Even so, I decided that it would be adequate for my off road go kart application. Not like I want/need to rev it up to 10,000rpm anyway!  6,000rpm MAX would be fine for me. For racing though, a new cam setup has been developed for the Robin EX engines. For those that don't know, World Formula engines are around 200cc and must fit without a set of guidelines set forth by a race governing body.  These World Formula engines crank out around 15-20HP on pump gas! An example of this is the new Briggs World Formula engine. Not cheap though at around $1000. Not sure what the Robin WF race engine will sell at. But anyway, the point is that as Robin gets more involved in racing of go karts, I'd expect more aftermarket performance parts to be available. Then again, if Robin backs out of racing, there could still be next to nothing in terms of aftermarket support.  Either way is OK with me. I think the stock EX21 engine has the potential for 10HP (and a nice amount of torque) just by freeing up the intake/air cleaner and exhaust side, removing the governor action, and maybe messing with ignition timing some.  It shouldn't take much to make this engine into a very strong performer, and also have a go kart engine with incredible reliability and longevity. 


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