Yerf Dog GoCart Howhit GY6 150cc Mods

Yerf Dog 3206 & 3209 GoCart GY6 Engine Upgrades

The Howhit GY6 based engine found in the Yerf Dog 3206 or 3209 gocarts is like most any other internal combustion engine - in that it can benefit from some well directed modifications.  This is true of just about any mass produced engine which have compromises that are driven because of manufacturing restraints (like time and cost).  These compromises usually limit efficiency and performance potential of an engine.  Basically, increasing air flow through an engine is the goal.  More air flow along with the appropriate amount of fuel will result in more power producing potential. 

One of the ways of increasing air flow in the Howhit gy6 engine is to port the cylinder head.  I will not attempt to give instructions on all the specifics of the "how to" of porting.  Just note that careless or improper porting can HURT performance.  Even worse, porting done improperly can not only hurt performance, but result in damage to the engine.  All that to say, if you aren't familiar with porting, then the best way to learn is to research, read up on the subject , watch someone who knows what they are doing, and get some hands on experience yourself.  Just don't experiment with a cylinder head that you can't afford to scrap.  When porting a cylinder head, you have to be very careful not to damage the valve seat sealing surface.  If you make a nick there, then it will at least require regrinding of the seat and if the gash is deep enough, grinding may not do it.  In that case, a new valve seat may need to be installed or a whole new cylinder head purchased.  In either case, it's not cheap.   

The picture below shows the ported intake port on my gocarts Howhit GY6 engine head.  The photo is not very clear, but you can see that the intake port has been enlarged and the valve guide "knot" (the hump in the port where the valve guide protrudes) has been shaped more to a dull knife edge shape to help enhance airflow around the guide knot.  Later, after this picture was taken, I did some additional porting on the intake port to better match the intake manifold which I also ported.  One of the most powerful tools to use during porting is the finger!  The finger can feel the shape of the port and help find spots that need to be worked on.  A smooth air flow path is the goal.  No abrupt changes in the flow path - especially where a passage restricts down to a smaller size.  This is bad.  If there is a mismatch in ports necessary, then it should be done so that air flow goes into a bigger diameter.  In other words, you don't want the airflow along the surface of a passage to slam into an abrupt "wall" (or edge) as it transitions to another passage - like the transition of air flowing through an intake manifold and then entering the intake port in the head.

Howhit GY6 GoCart Engine Ported Intake Port

On the exhaust port, I opened up the port much more to match the big tube header I planned to use.  I also took the head to a machine shop and had them cut the exhaust gasket seat (counterbore) bigger so that I could use a bigger exhaust gasket.  I found a larger gasket at the Honda dealer that was intended for different small displacement Honda motorcycle engines.  The ID was a little over 1" on the new gasket.  This was a good match to my exhaust port and the ID of my header which were both around 1" ID.  The stock exhaust gasket is small and the inner diameter was a bottleneck on my particularly ported GY6 engine. 

Howhit GY6 Engine Ported Exhaust Port

I decided to get rid of the stock airbox/air filter system and go with a high flow Uni foam filter.  I ordered the Uni filter for 1-3/4" with a 15degree bend (Uni Part #: UP4182AST).  I made an intake tube out of 1-3/4" 6061 T6 aluminum tubing (.065" wall thickness) and bought some 1-5/8" radiator hose and cut a straight section for the connector that slipped over the carburetor mouth.  I also put in a 1/4" hose barb fitting (from hardware store) to go to the breather/oil separator system from the Howhit engine.  Below is a picture of the intake tube on my gocart.  I ordered my aluminum tubing on the internet through OnlineMetals .  It would probably also be possible to go to a local muffler shop and get a short piece of steel exhaust pipe - if someone wanted to do it that way, it would probably be cheaper.  Just be sure you secure your gocarts intake tube to help support the added weight - otherwise the intake manifold will take all the stress (including the weight of the carburetor).  The overmolded rubber section of the intake manifold will probably be more likely to crack if the carb is not also supported from behind.  On my gocart, I zip tied the intake tube up against the gocarts rear subframe cross member that is above it.  The radiator hose helps prevent metal-metal contract on the frame tube and probably also dampens vibrations a little.

Just a quick note about the breather and oil separator system on the Howhit GY6 based engines.  It might be tempting to just do away with the oil separator and run the hose from the valve cover directly to the intake tube.  Doing this may result in a large amount of oil being sucked into the intake over time.  Under high RPM, the engine can be filled with an oil mist.  This oil mist will just travel through the breather hose on the valve cover and into the intake.  This oil will then be burned in the combustion chamber.   Oil in the combustion process can leave carbon deposits and make a mess over a long period of time.  In addition, oil in the combustion chamber can lead to pre-ignition and detonation under certain engine operating conditions.  The oil separator on the GY6 helps remove the oil droplets from the air that is vented through the breather hose before the air enters the intake tube.  I cut off the oil separator from the front of the stock Howhit engine air box assembly and mounted it back on the engine by itself.  Just to give you an example of how much oil can be trapped by the oil separator, once after I took my gocart for around 30 minutes of wide open throttle driving on some country dirt roads, about 1" of oil collected in the oil separator drain tube.  May not seem like much, but with a lot of high RPM engine running over a long period of time, there could be a lot of oil mist vented through the breather system. 

Homemade GY6 Intake Tube for Uni Filter

Another modification I made to my GY6 engine was to deck the top of the cylinder block by .020" to boost the compression slightly.  On my particular engine, I measured around .030" clearance between the top of the piston and the top of cylinder head at TDC.  To be safe, I just decided to shave off .020" off the top of the cylinder block.  Removing material from the top of the cylinder block can boost compression more than milling off an equal amount from the cylinder head because it is possible to remove more volume by taking a slice off the full cylindrical shape of the cylinder block.  When doing something like this, you just need to make sure that you know exactly what the clearances and piston pop up are on your specific engine.  Decking the block too much can be catastrophic because the piston can hit the head if done improperly!  Also, you have to be aware of valve to piston clearances when boosting compression.  If in doubt, better to leave extra clearance, or don't mess with it at all!

The picture below shows the finished product of some of these modifications on my gocart.  You may also notice that I ran the gear box vent tube into a fuel filter and put a hose pointing down in back.  I had the filter laying around and so I decided to just put it on there.  There is not real air flow in/out of the gocarts gear box, but I just didn't want to have the vent tube hanging disconnected and prone to collecting dirt or water.  The filter and routing the hose downward in back just help ensure that no garbage makes it's way through the vent tube and into the gear box. 

GY6 High Flow Intake System

The other thing that I wanted to do was to make sure that the torque converter received a good amount of cooling air.  The stock Howhit GY6 based engine has a tiny piece of foam in the rubber boot at the front of the torque converter.  This can plug up quickly while driving the gocart and can restrict air flow to the torque converter.  Less airflow means that the torque converter can get hotter (especially since the TC is enclosed) and this extra heat could be hard on the belt and other components.  So, to make life easier on the torque converter, I installed another Uni filter just for the intake of the torque converter, but with NO OIL!  Obviously, an oiled filter could bring oil into torque converter, and that's the last thing you want (an oily mess and belt slippage)!  I removed the stock Howhit torque converter plastic filter insert, and used a short piece of 1-1/2" aluminum tubing that I inserted into the rubber boot and zip tied it.  The Uni filter (1-1/2" with 15degree) clamped onto the aluminum tubing.  Part # on this filter was UP4152AST.  I also zip tied the Uni filter against one of the gocarts engine frame tubes so that it would not sag or bounce around and get too close to the hot exhaust header. 

Uni Filter (no oil) for GY6 Torque Converter

In case you haven't seen the inside of the Howhit GY6 engine torque converter, the driver unit has an impeller cast into it and this provides the airflow through the torque converter to help keep things cooler (see picture below).  The torque converter vents out the back of the case through a passage cast into the die cast cover. 

Howhit GY6 Engine Torque Converter

After making these modifications and some other adjustments, I took my gocart out for a spin on some country dirt roads.  The engine was noticeably stronger - especially in the upper RPM range.  The gocarts GY6 engine now just liked to rev up and it felt faster.  On my stock 3206, the gocart used to rev up more slowly, and it felt really choked off and slowly accelerated to higher RPM's.  It took a very long time (and a very long straight stretch) to rev up to the 8000-9000RPM range.  The GY6 engine did not seem happy at higher RPM's.  It seemed to really be struggling to maintain high RPM's.  Any slight uphill would really bog the engine and drop the engine RPM a lot.  To summarize it in one sentence, the stock Howhit gocart engine felt very restricted and choked off at higher engine RPM's. 

This is no surprise.... look at the stock exhaust system with the tiny primary tube header coming off the head.  Look at the tiny outlet on the muffler.  If you pull the engine apart and know what to look for, you can see some serious port mismatch between the intake manifold and head.  The ports are also pretty bad in stock form.  So, eliminating all these bottlenecks will definitely free up the GY6 engine to breathe better at higher RPM's and produce more power.

Now, my gocart revs up freely and has reached 10,000+RPM quickly and easily on long straight stretches.  It doesn't feel like it's choked off and struggling to rev up.  It revs up smoothly and much quicker than before.  It does not feel restricted like the stock engine feels.  Realistically, we are still taking about a small 150cc engine on a big, heavy gocart.  So, while the improvements were very nice, this is still not a 100HP sand rail that can throw up big roosts of dirt while pinning you to the seat and accelerating to 100+mph!  In comparison, these gocarts are mild in performance (stock rating 7.8HP) and relatively slow (compared to other high powered vehicles).  Even so, they can be a lot of fun.  On my gocart, I'd estimate peak power has now increased 2HP or so and the powerband is much stronger at higher RPM's.  Approximately 2HP increase may not sound like much, but 7.8HP base power to begin with is not much either!  Looking at it as a percentage increase helps to see things in proper perspective.  So, maybe we are talking about 10 or 11HP peak power, but it feels like more than this.  These modifications have helped unleash some more power (and more fun) out of this tiny GY6 based 150cc Howhit engine.  As I've modified my engine to favor more top end power, I have noticed that low RPM power has not really improved.  With my current modifications, the gocart really comes to life at around 4000RPM and then accelerates stronger to high RPM's.  Things like big tube headers and high flow exhausts usually favor higher RPM power and can sometimes take the edge off low end power.  If someone just wants more power under 4000RPM, then this may not be the way to go.  For me, overall my gocart is now much more fun to drive.  I like the way the power band feels now... of course I could always use some more POWER - all across the powerband!

When engine modifications are done to increase airflow through a carbureted engine, it is usually necessary to re-jet the carburetor to richen up the mixture to match the increased airflow.  Failure to do so can lead to an overly lean condition and potential engine damage. 

A quick comment about the factory rev limit on the Howhit GY6 engine.  In the past, my gocart seemed to be bouncing off a rev limit/ignition cut around 9200-9300RPM.  However, now I am seeing 10,000+RPM.  So, I do not know for sure what is going on here.  The ignition cut/rev limit would be programmed into the CDI module and is not something that can be changed or adjusted, so I don't know how I have seen 10,000+RPM - unless the rev limit is really not in the 9000RPM range.  Whatever the reason, let's just say that I hope my Howhit engine continues to rev up to 10,000+RPM!   

There are definitely benefits to be found with just "bolt ons" like an intake and exhaust upgrades.  These can make nice improvements in performance.  However, bottlenecks in the cylinder head ports will ultimately limit power potential.  Only by removing ALL the bottlenecks, can the engine realize it's full potential.  The principle of "a chain is only as strong as it's weakest link" applies here.  All "links" in the chain must be addressed to maximize the power potential of an engine.  This is a fun little engine and it can really scream when set up properly!  The Yerf Dog 3209 and 3206 gocarts can be a lot of fun even with a stock Howhit engine.  The Howhit GY6 based engine in stock form sure beats the cheap and underpowered engines found on most kids gocarts!

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