Introduction: Honda GX25, Part 2: Porting and Mods

I have broken the process of working on this engine down:

- Air Box

- Carby

- Intake Manifold

- Intake

- Exhaust

- Muffler

I try to work following the flow of air ie start where the air comes in, working downstream to where the air (exhaust) is expelled.

What you need:

- Dremel or similar rotatory tool (I've done it with a die grinder but they are big and not suited for the tight spaces of this engine). Assorted bits to go with it and sanding abrasives. I made sanding rolls by cutting strips of sandpaper putting glue on the backing and making tight rolls. Note: if you make your own use fabric backed paper where possible as it will last longer.

- Drill press and/or hand drill and various drills. The drill press will get much better and neater results.

- Files

- Verniers

- Ruler

-Pen and paper


Step 1: Porting

Porting is scary for the first time but only because it’s seen as a bit of a dark art. The reality is that it’s really easy. It’s really easy to stuff up if your not careful too. I think the trick to porting is matching your goals/purpose with the physical limitations of the engines and how far your willing to push the limits.

TIP: Dont be in a hurry to stuff it up!

A mild port, just taking off casting marks and rounding a few edges is not a big deal. Just be gentle and careful and do a little slowly. If your trying to get every last bit of flow out of it, you really need to think about it and do a lot of measuring.

So what is porting: making an engine flow air as optimally as possible. This doesn’t always mean bigger. In this engine on the intake side it’s possible to have 10.5mm opening on some things 11 or 12mm on others and 5 or 6 restriction changes along the way. You are better off having a slightly smaller passage if it is consistent the whole way. This will effect air velocity and fuel atomisation and heaps of other stuff I don’t understand.

So aim for smooth, gradual radius’ and consistent diameters. The downstream side should always be equal to or slightly larger the the opening before it.

I’ve included some measurements before work starts. I think this is important to plan what your going to do.



Intake: 9mm. Beware the thin wall on top of the intake is only 2mm but looks much thicker

Exhaust: 9.5mm tapering out to 11mm

Intake manifold:

Carb side: 9.3mm

Engine side: 8.6mm

Center restriction: 6.5mm (nearly half the filter side of the carb of 12mm!!)


Filter side: 12mm

Engine side: 9mm

Rotating barrel: 8.5mm


Engine side: 12mm with raised lip

Arrestor opening: 11mm x 7.5mm

Step 2: Air Box Porting

This engine does not have much of an intake/air filter box that can be modified. The only thing you can do is make sure the filter is clean and oiled. I did ream out the opening under the choke a fair bit. It will effect the full choke a little, but I rarely use it and never on full choke.

The air box has an Oring on the engine side. If you take to much out, the seal wont have a seating surface and there will be a 2mm gap around the edge of the choke. If you are doing the carby mod then the Oring is not required so grind away.

Step 3: Carby

I would recommend not modifying the carb. If you increase the size of the venturi, the velocity of air will decrease and might not suck pull the fuel up as well. I might do an instructible on it as i have a few laying around now. The engine side of the carb can be ported slightly at the opening to match the intake manifold. Just use porting abrasives or rolled up wet and dry.

Just be aware of mods so they do not effect the pump on the carby. It uses pulses from the intake to operate the pump. This is fed via the small hole near the venturi. You can see the track on the gasket. Care must be taken on the manifold as well.

Step 4: Cylinder Porting

The next logical step should have been the Intake manifold. Due to the very small amount of material available on the top of the intake, I drilled this first so I could match the manifold to it. The Manifold has a fair amount of material so the passage can be offset without much drama.

So 3 parts to porting a cylinder: Intake, Exhaust and Combustion chamber. The cylinder on these engines is closed or blind so accessing the chamber to do anything of consequence is extremely difficult. I did not bother doing any work to the chamber.

WARNING: if you do polish the combustion chamber, ensure you do not hit the cylinder walls or the valve seats (leave the valves in if you do) with grinders/sanders/polishing wheels. You will end up with a ruined engine. Little will be gained from this work, so I wouldn't bother!

The drawing shows roughly what the cross section looks like. The red scribble is where i removed material. The sharp corners just above the valve seats need to be done by feel. If using a grinding stone you will feel when the edge is gone as you grind that lip away. Do it from the port side. The tool shaft should prevent you from heating the valve seat that way.

Work from inside out with the port and hardest to reach to the easiest. If you put a mirror finish on the port then go back in and try and flow the area above the valve seat, you'll just ruin your work. You also leave a little extra metal to safe guard against accidents and you can clean them up as you work your way out.

Step 5: Intake

Intake ported out to 11mm. Note the first pic, the piece moved and I took a bit off the side. This was rounded and matched to manifold and wont effect flow.

While discussing orientation of drills etc, I will reference as if the cylinder were sitting upright on the bench with the top being the top and bottom the bottom.

Place the head into the vice of the drill press. Line the edge of the drill bit with the top of the intake port. We are aiming to not alter the height of the port but will lower and widen it by several mm. Slightly raise the cylinder so the drill will drill on a sight downhill. Only about a 1mm or so, just to reduce the corner the air must negotiate entering the valve area. Dont need to though just dont remove material from the roof of the intake. It looks really thin.

Once your happy with the position of the drill, start drill. Just a bit at a time and make sure its tracking where you want it.

TIP: If you want to make sure alignment is spot on, take the biggest drill bit that will fit into the intake and insert to the press chuck. Plunge the drill bit into the intake (without spinning). Tighten your peice in the vice. The port is now aligned with the spindle. Insert the 11mm bit slightly offset to avoid drill the top of the intake, Job done.

Keep drilling bit by bit. I found cutting lube helped. I drilled right down until i removed a small ount of material from the valve guide, just to round it over a bit. Dont push too hard or you will either jamm up the bit against the valve guide or take a big ding out of it. Keep checking your progress in the intake as well as looking into the cylinder.

Once the hole is drilled, start using the dremel to smooth out the burrs. Then use sanding rolls and abrasives to smooth and finish the port. The general consensus is dont polish the intake as it assist with fuel atomisation.

Step 6: Intake Manifold

Huge gains here. The stock has a restriction in the middle of 6-6.5mm in diameter. On this engine I bored it out to 11mm which represents a gain of nearly 240% cross section area. That is alot of extra flow. You could go more with really good machining and tools, even CNC but at home with a drill press and Dremel, 11mm is a solid number without major problems.

Since we lowered the port opening on the intake, we must line the manifold up as well. You could always drill the manifold out centered and then blend into the intake part but this would result in another whoop in the path which I aimed to avoid.

Put a 10.5mm bit in the press. Drill from airbox side to engine side. Its easier to align with the intake than removing material from the carby. Drill out the center of the manifold (If you worried about centering, insert the largest bit that will fit and align the manifold that way).

Insert the manifold into the intake. Inspect the alignment of the holes. You should see a lip and need to lower the exit of the manifold.Regardless we need to chase the area with the most offest against the intake.

Insert 11mm drill to the press. Adjust the manifold in the vice. Try not to move the center of the hole on the airbox side of the manifold. You just want the drill to exit slightly offset on the engine side and matched to the intake.

Insert the manifold back into the intake. There might be very slight misalgnment and thats fine. Use an abrasive roll to sand and match the ports. Remove material from the intake first as its metal and harder to remove. In the 1st photo, in the top left you can see the intake is proud. I ground that to match the manifold. Pretty much directly opposite i needed to match the manifold to the intake.

NOTE: Make sure you are removing material from where the two pieces and nowhere else.

Next we need to match the carby and manifold. With the two bolts in place, slide the carby into position. Let the carby hang down on the bolts. This will ensure the port lines up perfectly everytime.

Note where material need to be removed. On the carby, I only removed material about 3mm into the venturi and angled as required. If you kept the hole on the manifold centered (airbox side) then you shouldnt need to remove much.

The 2nd photo shows everything perfectly lined up and ported. The pic doesnt make it look as good as it is.

Once you have finished, you need to remove any swarf from the passage (airbox side). In the third pic I am pointing to the bit of fluff that needs removal. This will prevent your carby pumping fuel.

Last step is to open up your gasket. DONT DRILL IT!! It will ruin it. Use a punch or even a dremel. Be careful. They are relatively expensive and rip easily. A really sharp knife might do it. The better you match it the better the flow. You need to note the passage for the carby pump and not wreck that. This can be fiddly.

Step 7: Exhaust

Similar to the intake there is not a whole heap of meat but we can still port out to 13mm. Just check your drill is tracking center on the exhast. Again cutting compound seemed to help. Drill until the bit removes a small amount from the valve guide.

Once ported, blend in all the edges. Remove the edges from the shoulders of the valve guide and make them a bit domed shape. Leave atleast 0.5mm from the middle of the guide untouched. Otherwise you might create a burr and damage the valve or the area might be too thin and heat up too much causing a failure.

Step 8: Exhaust Mod

The muffler is very restrictive. If you choose to leave this muffler on some basic modding can help it breath or alternatively you can make up your own contraption.

The exhaust opening on the engine side of this muffler has a raised lip that protrudes into the exhaust manifold. This can be be removed and the hole enlarged to 13mm. You can go larger however nothing will be gained.

Next remove the spark arrest that is held in by two x screws. Depending on how adventurous you feel depends on how you proceed at this point. The more you open it up the louder it will be. I wear ear protection so it doesn’t bother me.

There are plenty of rules such as 80% of port area for your exhaust. In this case of 13mm it’s about 11.5mm in total. You do need a little back pressure for best results.

Choose a drill size (about 6mm) and drill several holes within the area of the spark arrestor housing. It will be the soothed area on the muffler. In the 2nd last pic, pretty much anywhere inside the sooted area.

The arrestor you can enlarge. You can either leave the mesh in place and have a smaller opening or remove and have a larger exhaust hole.

NOTE: some areas require by law to have an arrestor, so stick to drilling holes if that is the case.

You can spray your muffler with exhaust paint to finish it off.

Hang the gasket (heat shield) on the exhaust studs fitted to the engine. Match the gasket to the exhaust. Then match the gasket to the muffler in a similar manner. Line the top of the bolt holes up on both pieces and match from there. This will ensure alignment when fitted to the engine.

Step 9: Carby Upgrade

The standard carb in this engine is a major restriction. The air filter side opening of the Venturi is 12mm, the engine side is 9mm and the rotating barrel is 8.5mm. Carbs work on air flow sucking fuel up so the velocity of air is important but the difference of nearly 4mm is a lot. If you choose to stick with the standard carb you could try drilling it out but I wouldn’t. Put up a destructible if you do it.

The Walbro WYK194 carby is an upgrade and has a Venturi of 10.5mm.

I have used an AliExpress. Erosion (CG330 Carburetor fit for brush cutter cg260 TL26 TU26 TL33 grass trimmer 1E34F 1E36F engine 26cc 33cc weed eater carb 11mm) and it works well. Genuine is pretty pricey and the generic was $14 usd delivered.

It’s a direct bolt on. This carb also has a high and low fuel adjustment which is great for tuning. I will cover setting it up in Part 3 Reassembly.