25cc 2-stroke conversion
 

While I'm waiting on my 26cc 4 stroke to arrive I've started on a 25cc 2 stroke.
I'll try to upload pics as the project progresses but I've had pretty dismal luck getting any pic into these forums.
This power head was apparently manufactured in Taiwan. The pics below represent the unit before disassembly.
Instead of a throttle cable it operates on a lever that apparently gives you a speed range between a tortoise and a hare.

RE: 25cc 2-stroke conversion
 

OK it looks like the pics are working now.
Below are the next set of pics from the disassembly sequence.
With the housings stripped the unit weighs in at 3 lbs 14 oz.

RE: 25cc 2-stroke conversion
 

The intake components appear to be the most restrictive portion of the unit's potential.
The carb is a China made WT675. The "20" in the casting throat indicates a venturi size of .3125" (20/64).
If you look in the attached pics the venturi is seriously obstructed by the butterfly shaft as well as the screw holding the butterfly to the shaft.
Additionally this carb only has a low end needle. Can anyone explain how a single needle carb should be tuned?
As well if anyone can suggest an alternate carb I'd like to hear suggestions. I'm wondering if a WT644 would be too large for a gas / glow conversion of 25cc?

RE: 25cc 2-stroke conversion
 

The manifold (aka carb mounting block) has a bore diameter of 0.375". This is slightly larger than the carb venturi (necessary criteria to get the venturi to function) but will need to be enlarged depending on the alternate carb used. At present this seems way too small.

RE: 25cc 2-stroke conversion
 

Next I removed the muffler which weighed in at almost 8 oz. I thought I'd be smart and peel open the crimped edges of the muffler, gut it, and then braze everything back together. After a significant investment in time prying and twisting I got the housing apart only to find a second slightly smaller muffler inside. They were apparently going for a dual chamber approach. Each of the two chambers weighs in at nearly 4 oz and has the same exhaust port opening. This means there are 2 mufflers to work with. I may go with a larger design to reduce back pressure and noise.

RE: 25cc 2-stroke conversion
 

The flywheel and coil together weigh in at almost 15 oz so they get removed. The flywheel has 2 tapped holes in it that could be used to remove it from the shaft (these are really tight) or could be used as a part of the prop adapter by coring the flywheel.

RE: 25cc 2-stroke conversion
 

After cutting the excess shroud from the engine casing, removing the cylinder, and splitting the case you can see that the conrod ends are supported by needle bearings while the crank is supported by caged ball bearings. The connecting rod appears to be plated with copper. Anyone have any idea what that indicates?
The case, cylinder, crank assembly, and carb together weigh in at almost 2 lbs.

RE: 25cc 2-stroke conversion
 

The exhaust port looks to be plenty large. The intake port looks large enough as well. I may still have to entertain the urge to further widen and polish both ports at some point. The carb pulse port is directly below the intake.

The piston is a 2 ringed pinned design. These rings do not have a square cross section so I may get a set of Bowman rings.

In general I like what I see.

If one or more of the more experienced members here could describe the method used to check port timing I would greatly appreciate it.

RE: 25cc 2-stroke conversion
 

The major diameter of the crankshaft output end is 0.474". The threaded section measures 0.310" peak to peak on the threads. With the thread gauge I have the threads lined up with the 25G blade reasonably well and even better with the 1.0. Since this set of gauges is marked as "metric and standard" I'm guessing the 25G was the standard and the 1.0 metric. My knowledge of threads is limited so I measured the length of the threaded portion at .590" and counted 15 threads over that length. This gives me 25.4237 TPI wich given a reasonable margin of error corresponds to my thread gauge measurements. I'm guessing this is a metric thread of M8 x 1. Somebody more knowledgable please feel free to comment.

Now I need to find somone who makes a prop hub to fit this application.

RE: 25cc 2-stroke conversion
 

Who makes the engine? Looks good in the pictures. I have done many of these smaller engines and tried many carbs. You need a 2 needle carb so you can adjust both low and high ends. I have used a WT-434 Walbro. They have 24/64 venturi and a 9.53mm bore. Any bigger on the carb and you simply have too much fuel unless you want to scream the motor. I prop and carb the motors I do to run about 7200 or so. Seems to be the best combination here where I am at, sea level and humid. All the carbs have the shaft and plate so I wouldn't worry about it. These carbs are great and in so many applications you wouldn't believe it. Check this link for a hub. Don't know if it is tapered or not but it is a start. Not that simple to just buy a hub as the dimensions are critical. Sounds like you plan to use an electronic ignition. I use CH
Here is what my conversions look like and the parts I make.

http://cgi.ebay.com

Direct link to a specific item on Ebay removed by moderator . . . .

RE: 25cc 2-stroke conversion
 

Here are the external views of the exhaust and intake ports.

RE: 25cc 2-stroke conversion
 

Take a look here:

http://testfly.0catch.com/homelite/e...lindermod.html

RE: 25cc 2-stroke conversion
 

Here are some pics of the shortblock prior to trimming any excess material.
Let me know if you recognize this unit. There were no brand markings on anything but the carb and flywheel.

Thanks

RE: 25cc 2-stroke conversion
 

Tim,

Thanks for that input.
I'm going for the gas / glow conversion here so I'll need a prop hub similar to the ones intended for electronic ignitons (ie no flywheel) but without the magnet.
I appreciate your tips and suggestions!

RE: 25cc 2-stroke conversion
 

Flipflop,

Thanks for that link. I think I can figure out the timing based on that information.

RE: 25cc 2-stroke conversion
 

I am using a 16 x8 prop on two 25cc engines and get 8100 and 8200 RPM. Exhaust is modified to 155 and 157 degrees. Muffler is changed and carb is stock.

Bill

RE: 25cc 2-stroke conversion
 

Bill,

Thanks for that input. What carb are you running on your engine?

RE: 25cc 2-stroke conversion
 

foamcut: Your engine seems to be the exact one I have on a Mac leaf blower. It is found on Mac and Yardman leaf blowers ect. Go to the Yardman web-site and you can view a parts break-down. Also read thread....What a deal. Regards Capt,n

RE: 25cc 2-stroke conversion
 

captinjohn,

Thanks for that feedback...I'll check out that thread.

RE: 25cc 2-stroke conversion
 

OK it looks like captainjohn called it right..this is the same engine in the "What a deal thread". It looks like much has already been done on this design.

Anyway this thing looks like it does have some really great potential. It's light but will likely need some porting changes.
The carb definitely needs to be replaced. Hopefully Ken Lambert is still making mufflers for it. So far I haven't found a prop adapter appropriate for gas / glow (ie no flywheel).

Anyway I would recommend that anyone interested in this conversion now go to the "What a deal" thread to learn more about the potential of this engine.

RE: 25cc 2-stroke conversion
 

where did you buy this engine???

RE: 25cc 2-stroke conversion
 

Tune that carb by discarding it. A single needle carb will lean out when the motor is loaded with a prop. Been there, done that.

RE: 25cc 2-stroke conversion
 

The carb is the one that came stock with the engine in the blower. The numbers on the carb are:

Body
WT732
535

Throttle plate
34-121


Bill

RE: 25cc 2-stroke conversion
 

foamcut,

Don’t throw the magneto coil away. I have a dead coil on one of my engines.

Bill

RE: 25cc 2-stroke conversion
 

Raven,

Thanks for that feedback. It looks like I need to locate a better carb.

Anyone have suggestions for a 2 needle carb with a at least a 10mm venturi?
The WT-434 has been suggested but I haven't located a new one yet.

Thanks

RE: 25cc 2-stroke conversion
 

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

RE: 25cc 2-stroke conversion
 

Tim,

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

Thanks

RE: 25cc 2-stroke conversion
 

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 :D

RE: 25cc 2-stroke conversion
 

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.

RE: 25cc 2-stroke conversion
 

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

RE: 25cc 2-stroke conversion
 

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

RE: 25cc 2-stroke conversion
 

Bill,

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

Thanks

RE: 25cc 2-stroke conversion
 

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...

RE: 25cc 2-stroke conversion
 

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

RE: 25cc 2-stroke conversion
 

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.

RE: 25cc 2-stroke conversion
 

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

RE: 25cc 2-stroke conversion
 

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.

RE: 25cc 2-stroke conversion
 

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).

RE: 25cc 2-stroke conversion
 

Check out e--- item#160072326908

RE: 25cc 2-stroke conversion
 

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.