Drilling into muffler cup

This seems funny to me.
((((I don't mean to offend any one here))))
I spent an entire day building a pipe that ran significantly quieter while adding a small amount of climbing ability to the bike.


It did restrict the top end a bit but i figured that if I want to go faster than 30 mph I'd ride my Bobber.



The whole reason ( for me ) that I built and ride a motorized bicycle is for the smile factor .
The ones on my face and the ones on just about every person I meet while riding the machine.
And I just love to build unusual toys.
But it was surly not for the speed .
If you feel the need for speed, and you are building a motorized bicycle to satisfy that need.

You have brought a knife to a gun fight....

I will admit that search for more efficiency and reliability have led me to alter the engine.
I have added a Delorto clone , I have an expansion chamber that I will run if I can quiet it down properly with a silencer, and am in the midst of some minor porting of the intake , exhaust and cylinder.
So far I have noticed some very small gains.
And am very excited about them.
Speed wasn't one of them.

I share the belief that if we garner to much attention by making loud and annoying machines just to gain a few mph. We will be regulated out of existence.....

The beauty of these machines is in there simplicity and efficiency, not there speed or raw power...... That's my rant for the day....
Happy trails all....

I have played around with modifying the exhaust system on the stock China girl. The best thing I every did was cut it open, take the some what of a cat out of it take me out original baffle out of it make one with bigger holes take the original exhaust cap ,drill the one hole bigger weld a bigger size pipe that comes out not to far and there you go its a tad bit louder put the perfomace gains are better. You will also need to up jet or change c clip down one as it will run a little leaner this all can be achieved by a ****y welder ,grinder, and sheet metal. Good luck.

I don't have a CG but I do have a 4-stroke. I can tell you from my rc car 2-stroke days, removing the stock muffler did make it more powerful, but then I added a tuned pipe and got almost double the power with less noise. A 2-stroke relies on exhaust back pressure to keep the intake charge in the cylinder, because both intake and exhaust ports are open at the same time. Without back pressure you will have to burn way more gas to get the same power.


On my bike, which is a 4-stroke, removing the muffler and putting a straight pipe was the difference between being able, and not being able, to do burnouts on pavement. I had to richen the mixture significantly but it sure has a lot more power. And it is far louder. When I punch it it will make bystanders jump.

My plan is to put a more powerful engine (think double hp) and run it with the quietest muffler I can find.

The easiest way to quiet a 2 stroke and still run a very free flowing exhaust is by capping off the end and running a side bleed off the belly section of the expansion chamber, this will usually cut the noise by more than half, then use a moto cross silencer and it should run as quiet as stock if not quieter. The biggest drawback to a side bleed pipe is making it all fit the bike and still look good...

That's a nice looking bobber... haven't seen an SU carb on one in a long time... was thinking about the same carb for my sportster but I ended up getting a 36mm Edelbrock performer carb for it instead. I like it but they discontinued it several years ago and it's hard to find the right jetting rods for it so I may need to switch to a 45 or 48mm mikuni when I put the Buell firebolt heads and a thunderheader on it as I don't think that little 36mm Edelbrock will feed it enough after those upgrades. A well tuned SU would work and look really well too if I can find one...

Today i took the end cap off my muffler to clean out all the oil, and i took it for a quick run to see what it sounded like, and i was definitely feeling an increase in torque and the engine just generally ran much better. I am wondering if this has negative effects on the engine, but i have an expansion chamber in the mail so would it be alright to do this every now and then, or will it immediately hurt my engine? It is ear-ringing loud, but it really doesnt matter because i live far away from any major road or any houses.

It won't hurt your engine as long as you got some kind of a pipe on there... what can hurt it is running no pipe at all or a really short pipe...

As far as noise is concerned, drilling extra holes in the end cap will be considerably quieter than riding without it installed but will offer up some decent performance.

Mike B said:

Yes, I think it is even a federal offense to modify a factory stock exhaust system.

But I get Harleys with straight pipes loud enough to rattle the windows go by my house every day.

I hate them. I am going to start writing letters demanding the existing laws be enforced.

I would like their bikes to be impounded, they be made to reinstall a factory exhaust system and pay a nice fat fine or do a little jail time or some community service.

No regards for anyone but themselves have they.

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How do i dislike your comment? I dont see a dislike button...

geoldr said:

On my youtube one member of this forum commented saying that if I drill holes in the muffler cup it will increase power in in the straights. Is this true? And how do I do that? Just drill straight into the muffler from the opening?

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Dont know why everyone is freaking out about this subject. I run an huge zeda expansion chamber connected to the stock muffler. I cut off the crappy restrictive pipe thats welded inside the cap. Then drilled (6) tiny holes with a 1/16" drill bit around the outlet cap. And (1) hole at the bottom so any excess oil can drain. Sounds great, has suffient back pressure, got that 2 stroke crackle, breathes ALOT better and not 2 obnoxious sounding. You can ALWAYS use JB weld after, or sheetmetal screws to fill the holes to tune it to your liking. Plus you can get another cap to experiment with if you mess it up. Thats whats great about this hobby is actually doing it yourself. Have fun with it.

geoldr said:

On my youtube one member of this forum commented saying that if I drill holes in the muffler cup it will increase power in in the straights. Is this true? And how do I do that? Just drill straight into the muffler from the opening?

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1 way 2 find out!

that cup reverses flow to direct exhaust back up into muffler body where the noise baffles are - it then escapes down the narrow tube that sticks out bottom - it will get noisier, and gas mileage may go down as lack of back pressure allows fuel to escape thru exhaust port - OTOH, the hole can be easily plugged with a bolt & nut if you don't like it

geoldr said:

On my youtube one member of this forum commented saying that if I drill holes in the muffler cup it will increase power in in the straights. Is this true? And how do I do that? Just drill straight into the muffler from the opening?

Click to expand...

2 stroke engines live and die on the exhaust pipe.
Changing the exhaust is the best you can do, not drilling holes.