Bill,

Thanks for the feedback on the carb.

The engine I have is a single needle carb (stamped WT675 on the body). This one is going to be replaced.

Where did you get your prop hub?

Thanks

Tim,

By the way that's a great looking engine.
Did you make the prop hub?
Who made your muffler?

Thanks

Foamcut, I have a 28cc mac that I used a 24# carb on the 27,28# carb would not transition and gave me the same top numbers as the 24 carb did, I just trying to say don't get to hung up on the 11 mm carbs everyone talks about some engines can't handle it. I used the stock carb to get running and to check my glow fuel mix then just went up sizes in carbs till I found the one that ran best.


FWIW my mac spins a 18x6 @7400 on gas/glow

cdale03,

That's great feedback on the carb. It sounds like I should be looking for a #24 (24/64, .375", or 9.5mm) carb.


Thanks for that insight.

cdale03,

I just ordered a WT419 carb which has a 9.53 mm venturi and 2 needles.

Was your exhaust modified? What muffler were you using?


Thanks

Stock muffler just taken apart and gutted, two 1/2 inch holes drilled out the bottom and tubes welded on.

Bill,

According to the Walbro website your WT732 carb is a 10.5mm venturi...good to know....

Thanks

WA-167-1 is a common carb to use on these conversions and I have run it successfully on 25cc through to 31cc engines. There is a list of 11.11mm carbs somewhere here in the conversion section that should be suitable, your job will be to pick what you think will suit you. Still, a WA-167-1 is a good choice...

foamcut: Here is the prop adapter I made...simple but it works. Capt,n

Thanks for your input Raven.

I'm going to start with a 9.5mm Walbro WT419.
After that I'll move up to a 11.1mm and finally try a 12.7mm.
It is likely that these modifications tend to work in concert with one another rather than influence performance independently.
By the time I get to the largest carbs I will have modified the port timing as well.

This is going to take a little while.

captainjohn,

That's a great looking conversion you've created. You've got more fin area on yours than I do. My cylinder looks much like an Echo.
After cutting the blower shroud (2 oz roughly) I determined that I really won't save any weight by trimming the fins though.

Thanks

I have been recently flying with a gutted muffler. Previously I had a 25cc Homelite spinning a 16x8 prop at 8000 rpm using a pitts muffler. Now using a 30cc engine with the same prop I can only achieve 7300 rpm with a gutted stock muffler. My conclusion is that that unless you gut the muffler entirely you will be unlikely to achieve the same rpms as would occur with a suitable pitts muffler.

I will be able to confirm this on the weekend when I will run the 30cc engine with the same pitts muffler as used on the 25cc. The only difference shall be a spacer, which was required to clear the magneto bracket (my first mistake on the 25cc engine). I fully expect the peak rpms to easily be 8000rpm or higher, this will prove that a properly matched pitts muffler will always exceed a modified stock motor.

All the same, good luck with your conversion. Let us know how it goes.

Raven,

That's good feedback. Please let us know your test results.

Ken Lambert will be making my mufflers. In the "What a Deal" thread on this engine Lambert's mufflers generated significant increases over the stock set up.

In another thread a 16 to 1 ratio was quoted as being sufficient to minimize noise without restricting power. Using that ratio the muffler for a 25cc engine would need to be 400cc. I won't be able to test that theory until I get it on a stand and devise a way to change the main muffler canister (some kind of header / chamber design like a Pitts).

Check out e--- item#160072326908

Oldtimer,

That's the ad from the guy I bought mine from. He's been super nice to work with and as a result I've bought several more engines (including some 4 strokes) from him.
That option is certainly cheaper than any of the Home Depot or lawn equipment stores.

Foamcut,

The mufflers from Ken are 1” x 2” x 2 1/2” outside or .750 x 1.750 x 2.125 inside. The volume is 46cc. And yes the mufflers are not particularly quiet.

The original muffler without packing is slightly more than twice the volume of Ken’s muffler. A 16 to 1 ratio would be hard to achieve. I believe a ratio of 4 to 8 would be achievable and quieter but don’t have the material or equipment to experiment. 1 1/2” by 3” steel tubing would provide a starting point for experimentation.

Bill

Bill,

I'm curious what the outlet tube diameter is on the Lambert mufflers. In a prior thread I believe you were getting an 800 RPM increase from the stock version.
I'm wondering if the larger volume design (gutted stock muffler) would benefit from larger outlets.

Thanks

Bill,

I scanned a page from the August 2000 copy of the AMA magazine. In that article the author describes a 2 chamber design. he seems to indicate that the ratios between the chamber length as being critical to the design. I was unable to save a copy of the scan (pdf format) in my gallery. Let me know if you'd like a copy emailed to you. This might be information tha could be incorporated into a two-part Pitts design.....one more avenue to explore...

Foamcut,

Ken’s muffler is good for a sizable RPM increase. There are two outlets and the ID is .580” each. I thought the stock muffler needed outlet pipes but don’t have brazing equipment.

Bill

Foamcut,

I owned a Yamaha motorcycle dealership during the 70’s and watched many very skilled people experiment with 2 stroke exhaust systems.

The thermodynamics of muffler design for 2 strokes is very complicated and subject to much experimentation. The stock muffler appeared to have two chambers forcing the exhaust gas to travel further than it appeared.

Bill

Interesting, I shall measure my Pitts style muffler and find out it's volume. It's your typical rectangular muffler with no baffles or chambering and a simple dual pipe outlet. I do know that it makes a significant difference to peak rpm but not the exact measure. This weekend should provide an opportunity for some measurements to quantify that.

Check some of the other forum sites. Some fellas were using the new Bud Aluminum Beer Bottles for fabricatiing exhausts for gassers. Maybe Flying Giants site.

16 oz gets pretty close to your 16 to 1 on a 25cc!

Mike,

That's really good feedback. If you find a particular thread on the topic please post a link.

Thanks

Foamcut,

The interest in muffler design is high but I don’t have the necessary equipment to build various designs for experimentation. Measured the engine exhaust bolt pattern at 32mm. For all useful purposes 1.250”.

From the ‘What a Deal’ thread and the following mufflers are a close fit:
Poulan 25
G-23
Evolution 26

I would like to know the effect of larger volume mufflers is anyone experiments.

Bill


Seems that you sent the ice to the east coast.

Maybe the exhaust used on a MVVS 26 would work good. Or one like it! Does that give you any ideas!!! Capt,n