Squirrel Power: Making our Suzuki FA50 Road Worthy

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With our moped now safe to ride, we turned our attention to making it more comfortable.  Our bike had no exhaust when we bought it and our ears would start ringing after only a few blocks.  While it is possible to find a used muffler for the FA50, they are ludicrously overpriced.  They are also designed to restrict power which isn't cool.  Instead we decided to get clever and try out an internet rumor we saw for the FA50.

One of the best ways of adding power to a 2-stroke bike is to add an expansion chamber to the exhaust.  An expansion chamber is a carefully tuned pipe that helps prevent intake blow through.  You see, a 2-stroke engine has no camshaft or valves, instead using ports cut into the cylinder walls to allow intake air in, and to let exhaust gas out.  Because of this, there is a lot of overlap and the engine will actually blow some of the intake charge right out the tailpipe!  An expansion chamber uses harmonics to push that intake charge back in.  It’s a bit weird to visualize, so instead, check out this animation that shows how it works.

The internet theory goes like this: 50cc engines are all basically the same inside, so if you can find a cheap aftermarket expansion chamber exhaust, cut off the expansion chamber and muffler, weld it onto the factory exhaust pipe, you can make your own exhaust for half the price!  This was a really tempting option and we poked around the internet to find parts.  We also found a local muffler shop to do the cutting and welding parts for us (while I absolutely have the skills to perform this job, I do not actually have the tools to do so).

A company called MLM does make direct fit exhausts for the FA50, which you can buy from Treatland if you’re so inclined.  Those custom, handmade exhausts cost $200 each, or as much as we paid for this entire bike.  This exhaust was less than $40 and has both an expansion chamber and a muffler.  

The mounting flange is too small for our engine and the angle of the pipe is way wrong, but the expansion chamber and muffler are all we really need.

To make make up the rest, we got extremely lucky and found this stock muffler online.  It is obviously very used, but finding a decent FA50 muffler for an affordable price is impossible.  Because they are so rare, even rusty old pipes like this go for up to $60!  We were able to deal down the seller on this one for much less.  We can chop off the old muffler and weld on our new expansion chamber and muffler.  By using a stock muffler for parts, we can also steal the support bracket so that our new exhaust won’t vibrate itself to bits.

Both exhausts use crush-bent tubes, however the Suzuki pipe is slightly larger.  It also has gentler bends, so the crush effect isn’t as severe.  Even with someone else doing the work, this would be far cheaper than the MLM pipe.  

For kicks and giggles, we slapped on the stock exhaust and went for a short ride.  The backpressure of the exhaust caused us to lose 2 MPH of top speed.  A lot of that can be attributed to the muffler itself, which is an old, poor flowing design.  It’s also likely the muffler was clogged with carbon deposits caused by 2-stroke oil blow-through.  Clogged mufflers are a common problem in FA50s and cleaning the muffler is an easy way of bringing back some power in an old bike.  Since we’re going to bin this muffler, we didn’t bother.

The length of the pipe is perfect, but the expansion chamber seems a bit small, even for a 50cc bike (check out the chamber on our ICG kart, which has a 125cc engine and get an idea of the difference).  Judging by the pictures we saw of the MLM muffler, we thought our expansion chamber wasn’t all that much smaller, so we thought we might be fine.  The science was solid(ish), but the reality was not.  With our custom ghettofabbed exhaust bolted on, our speed was no better than it was with the stock exhaust and in fact our acceleration was slightly worse.  A worthy, but ultimately failed attempt at being clever and cheap.  While the muffler looks adequate from the outside, internally the pipe is much narrower and it is likely causing a lot of backpressure, preventing our engine from breathing properly.  We also found out later that our expansion chamber is indeed way too small and is probably causing some issues.  We'll be keeping an eye on Treatland's sales and pounce on the MLM exhaust when we can.

So, that's myth busted.  Just buy the right exhaust the first time and be happy, don't bother with trying to make your own.  You can't believe every rumor you read!  Now that we've actually put some miles on this little bike we want more power and a bit more speed.  Currently, we are hitting the rev limit at 28 and even in the city, we want more headroom.  Some gearing tweaks should fix this.  We also need more torque as any sort of inclined start requires a good kick to get us going.  There are a few engine kits available, one a completely drop in 60 cc kit and another that goes to 70, which will require a litle bit of fab work.  One very cheap mod is to chuck the head onto a mill and lop off 1mm of deck height to bump the compression ratio from 6.5:1 to a bit more than 7:1.  The MLM exhaust is on our radar as well.  Finally, with more speed we will require more rear shock.  The factory shock is pretty worn out and with any more speed will require a stiffer spring, better damping, and a bit more ride height to prevent us from bottoming or being flung off over bumps.