Go Cart Torque Converter & Sprocket Changes

One of the first things I noticed after buying one of the Yerf Dog 3203 go carts was that it REALLY disliked any sort of climbing.  When new and unmodified, the go cart was so pathetic that it would wimper at the slightest incline or hill.  One of the first times I took our 3203 go cart out, I tried to drive over some small little "hills" (more like big bumps).  The elevation rise was probably only around 2 feet and the go cart could barely make it over without bogging and nearly stalling the engine.  It was really bad!  A 3 year old could probably peddle a tricycle over the bumps with more authority than the 3203 go cars were capable of mustering up!  One of the first things I did was to adjust the torque converter by moving the stock green driven spring from the #2 hole position to the #3 position.  This increased the pre-load on the driven torsional springs and delays the upshifting of the go carts torque converter.  Basically, it keeps the torque converter in a "lower gear" longer which improves low speed power.  At the same time, this can also lower the top end speed a little as it takes longer for the torque converter to shift up to higher ratios.  I was glad to trade off some top end power to improve the anemic low end power of my go cart.  In addition to adjusting the torque converter, I also made some modifications to the stock Tecumseh 6.5 HP engine that comes in the 3203 go carts.  The combination of increased power and adjusted torque converter made the go cart much more fun to drive.  No longer was it intimidated by small hills or climbs.  Even so, it was still not as good as I wanted.

Eventually, I sold the stock Tecumseh go cart engine and replaced it with a high quality Robin EX21 engine (see Go Kart Engine Swap page for more details on that).  While only rated at 7 HP from the factory, the Robin EX21 engine was very torquey for such a small engine - conservatively rated at 10.26 FT*LBS TORQUE at a low 2500RPM.  When I installed the Robin engine, I could feel the increased torque as it moved the heavy go cart around very nicely.  The go cart also had more grunt to climb up hills.  The only problem I noticed was that the torque converter belt would sometimes slip under heavy loads such as climbing a steep incline (especially when starting the climb from a complete stop).  I felt that the Yerf Dog 3203 go carts were geared too high with a 10T drive and 60T driven sprocket... at least it is geared too high for good low end climbing grunt with a lot of weight to carry.  The higher overall gearing put a lot of strain on the torque converter belt and this resulted in the belt slipping and the go cart lurching at times when climbing.  I decided that I wanted to experiment with a lower overall gear ratio and so I began to look for different sprocket options.  I decided to focus on the drive sprocket because I didn't want to go with a sprocket bigger than a 60T on the driven side.  The 60T sprocket was already big enough (and sits down low enough) to concern me that it could easily get hit by rocks or other obstructions on very rough terrain.  I found some 8T and 9T drive sprockets made by Asuza, but they were meant to work with both #40/41 chain.  As a result, the teeth were thin so they could work with the narrower #41 chain.  The Asuza #40/41 go cart sprockets looked more like saw blades.  I wanted as wide a sprocket as I could find so that it would have more load bearing area.  Fortunately, I found a #40 ONLY 9T sprocket from Comet (part # 204370A).  This sprocket had wider teeth for #40 chain, and I felt comfortable that the quality would be good since it was a Comet part.  Unfortunately, it was not inexpensive and cost over $30+ with shipping. 

Comet 9T Sprocket and Yerf Dog 10T Go Karts Sprockets

I also decided to try to recalibrate the torque converter even more.  I found out that Comet makes a yellow driven springs for the 30 series go cart torque converters.  This spring is stiffer and is supposed to delay upshifting of the torque converter even more to favor low end power.  Again, I ordered this Comet Yellow Driven Spring (part # 011190A).   I don't remember exact cost right now, but I think it was around $10 or so.  I won't go into all the details of  taking apart the 30 series driven torque converter unit, but it was a bit of a pain.  First my cheap Made in China snap ring pliers broke when trying to remove the large external snap ring.  Then I bought a new pair of high quality snap rings pliers from Home Depot.  These were strong enough, but because of the way the outer cam piece of the torque converter driven unit was made, it was still difficult to remove the snap ring.  In addition, the outer cam piece was also "frozen" onto the shaft and I had a hard time to coax it off even after the snap ring was gone. 

Comet Go Cart Torque Converter Yellow Driven Spring

After finally getting the snap ring off, the fun was just beginning when I tried to put the unit back together after installing the stiffer yellow spring.  With the stiffer spring, it was a real trick to try and rotate the cam assembly, push the driven unit back together, and then install the external snap ring back on.  I spent at least 30 minutes fighting with this - trying to reassemble the unit.  But finally with the help of a vice and some other gymnastics, I was able to get it back together.  I decided that I would try the stiffer yellow spring in the #3 position (which gives maximum preload), and so this also made it harder to twist and reassemble the unit with just 2 hands.  Another set of hands would have been very helpful so that one person could hold the preloaded assembly together while the other put the external snap ring back on.  In my case, some extra persistence and perspiration took care of task without an extra person.  Even so, I'd recommend an extra set of hands while reassembling the unit with a stiffer spring (on the stiffest #3 setting).

Comet Yellow Spring Installed in Driven Unit

So, what was the result of all this?  At first, before I switched over to the 9T sprocket, I took the go cart out for a spin with just the new yellow driven torque converter spring.  The go cart did shift up slower with the added preload of the stiffer driven spring.  It wasn't as big of a difference as I expected.  Even so, the go cart seemed to tackle the steep climbs better.  However, I still noticed some lurching and signs of the belt slippage on steeper climbs.  I was eager to see what would happen when switching to the 9T drive sprocket and lowering my overall sprocket ratio by around 10%.  When I did this, I immediately noticed a bigger difference.  Just cruising around at lower speeds and light throttle, the engine sounded much less loaded down.  The engine seemed to rev up quicker - as expected. 

By the way, at this point I had already overridden the engine governor so that was no longer cutting off the power at higher RPM's.  The real test was to try and do a little climbing.... The place where I drive most often is a nearby vacant field that I can get to by just driving the go cart from the house.  It is a very LARGE open area and has a variety of terrain.  It has some long straights, some sharp turns around obstacles like deep drainage ditches, a raised plateau section where fill dirt has been brought in, and finally the best of all... the "pit"!  The pit is a large water retention pond about the size of a football field.  It is probably about 10-12 feet deep.  The sides of the "pit" are fairly steep and provide some fun, short climbs.  In different areas along the pit, there are different grades to choose from when dropping in or climbing out.  I use the bottom of the pit as a sort of circle track where I can race around in a oval pattern.  It used to be that I would not dare even think about going into the pit with the mostly stock Yerf Dog 3203 go cart.  I knew that these stock go carts could barely climb over a small incline, so I didn't dare try to drop into the pit and then be stranded and not be able to get out!  The go cart would have had to be towed out for sure!  By the way, in case you're wondering... this water retention pond is bone dry most of the time - obviously, if I'm driving around in it!  After installing the torquey EX21 engine and recalibrating the torque converter, the go cart would then easily climb out of the pit starting from a dead stop (no running start).  However, I noticed the engine loading down a little and the belt slipping at times because of the high overall gear ratio.  It was clear that the go carts power was adequate to tackle even steeper/longer hills, but the belt would probably protest even more. 

After switching over to the 9T sprocket, the go cart now climbs up steeper grades without any hesitation.  The go karts engine just churns away at around 3000 RPM and it can easily climb the steep inclines with me and one of my children with no signs of belt slippage.  It is especially fun to come up to the side of the pit and face straight into a steeper incline, stop, let the engine idle down to around 1300 RPM, and then just punch it and feel the go cart easily tractor up and out.  The way the engine sounds (with no noticeable drop in engine RPM) and now that the belt shows no sign of slipping, the go cart should really be able to climb some nice size hills.  The 9T sprocket may have lowered the top speed a little, but it's hard to tell...  It doesn't seem significant to me.  The 3203 go carts were never going to break any land speed records as is, so I don't really notice much difference.  In fact, the 9T sprocket may not have lost me any top speed compared to the 10T since the tach shows the engine revving up a little higher under almost all conditions.  Since the engine governor is not cutting power at a lower engine RPM, there is not necessarily a loss in top speed since the engine can just wind up a little higher with the 9T sprocket.  My overall engine speeds are still usually under 4000RPM.  I am still trying to bring up the max engine speeds to more like 5000-6000 RPM since the EX21 engine is more than capable of spinning up that high (and higher).  So, I am thinking that I might experiment with a different set of driver springs for the go carts torque converter and see how that affects the overall combination.  In theory, that should primarily affect the engagement speed (stall) of the driver unit - like engaging a clutch on a car with a manual transmission at a higher RPM.  However, since both the driver and driven units work together, I am wondering how a higher stall will affect the upshifting characteristics of the whole torque converter.  There's one way to find out, and I plan to try it!  In any case, I still have some more tinkering to do.  For now, the go cart is a lot of fun to drive as is.  It is nothing like the stock Yerf Dog 3203 go carts.  Finally, my "Dog" has a little more bite to go along with the bark!  I only wish I had some steeper, longer hills where I can take advantage of the additional low end grunt released by these changes.  Soon a trailer will be arriving and I'll have a chance to take our Yerf Dog 3203 go cart to more places.     

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