Modern Buddy

After much researching and stockpiling parts, I took the plunge and modded my cylinder head. I did a port, polish and decked the head 25 thousandths of an inch. Decking the head increases the compression ration of the engine. I also installed a cylinder head temperature sensor.

TL;DR: It made a big difference. The engine feels more powerful and sounds more loud and pissed off. (both good things from my perspective)

I ported the head using small sanding tools and a dremel. I put some duct tape over the valve seats to protect against accidental damage.

Intake port before:


Exhaust port before:


Intake port after:


Exhaust port after:


The porting really opened up the head. In stock form, the ports had lots of square and bumpy parts inside. I managed to smooth things out quite a bit.

I also smoothed out any sharp edges in the combustion chamber. This helps avoid detonation caused by increased compression. I left the valves in place when I did this to avoid nicking the valve seats.

Combustion chamber before:


Combustion chamber after:


To deck the head, I used an overhead mill with fly cutting attachment. I also cut a small pocket to mount the a cylinder head temp sensor.

To ensure a good sealing surface, I sanded the machined side of the head on a flat surface with 150 grit sandpaper.

I didn't take a compression reading before this project. After installing the head, I was happy to get a reading of 225psi. I was also a bit worried about detonation.

For my first fire, I put in a 132 main and 45 pilot jet. This was up from my previous 127 main and 38 pilot.

It fired right up! It was also backfiring like a machine gun I believe this was from running rich, as when I swapped in lower sized pilot jets, it decreased. I’m still tuning the main jet. It backfires when I let off the throttle at certain times. From what I could tell, it wasn't detonating.

The difference in power is very noticeable when riding. The engine has more power and quicker response across all RPM ranges.

Definitely a fun project!

I'm running a similar setup. Mine is happy with a 120 main and a 38 pilot. It could probably stand to have the pilot bumped to a 40 though. It runs strong.

I'm going to try a 121 main jet tonight and see how that works.

With the increased compression, are you now using premium gas? Also, it may be possible that the backfiring is crackling from a lean condition; sounds like backfiring but isn't.

Interesting mod. I'll follow along, I am interested to see how it goes.

I'm using premium gas. However, it's really more like 89. I filled the tank with premium (92), but the hose was full of 87. So I know I'm running diluted premium.

I'm planning on getting a gas can to keep at home to fill the scoot. That way I know I'll be running premium.

Is there any distinction between the sounds of lean crackling and backfiring?

I've been watching my head temp gauge when I got to WOT. It hasn't spiked up. I've taken this as an indication that I'm not running lean.

With the 121 jet installed, there was less backfiring when I let off. Instead of a series of loud noises, it was a single less loud noise.

With the scoot on its center stand and holding WOT for several seconds, there was still a series of backfires when releasing the throttle. There was also a noticeable flame shooting from the exhaust. It may not be running well, but I think that's pretty damn cool!

There is another type of backfire which happens in the muffler and can cause the described symptoms of flames out the exhaust. This can be very dangerous for your muffler, as the increased pressure in the expansion chamber can pop the seams and cause failure of the muffler.

I don't have an expansion chamber, I have a 4 stroke with Prima exhaust.

I think flames from the exhaust would indicate running rich.

Usually backfiring on deceleration is do to a lean condition. The charge is too lean and doesn't burn then ignites in the exhaust which is usually hotter than the head.

Check that your in and out gaskets are properly sealing.

If you are running too rich, you will get a gurgling sound when you lay on the throttle.


-duo

Turns out I had a small exhaust leak. It's running much better now that I've got a new exhaust gasket in place.

Roofaloof, did you know the prima pipe has a silencer core just before the muffler can. I cut mine apart and removed it. Also my airbox is modded. I removed the snorkel tube. I removed the interior baffle. I drilled out the drain on the bottom and I currently have 5 half inch holes drilled at the rear of the box. I use tape to cover them and adjust the tune on cool days. I'm running a wide band oxygen sensor to watch the tune. Taping just one hole can adjust the afr by a full point. It's very sensitive.

Roofaloof I can't see the after pictures. How can I view them?

They're not displaying correctly here for some reason.

If you copy and paste everything in between the into your web browser, it'll show the picture.