40cc Engine Tuning

[superfluus]

I've got a 55 inch scarab that has a 40cc backpack blower engine in it. My first question is how are carbs usually set up (how many turns?), and instead of a muffler i am just running a 3/4 straight pipe, does this change the way things should be set up?

 

[wavejumper]

There are a lot of variables in your engine. the first thing to do would be get an owners manual for the engine usually the places where they are sold is a good source for these.

Second you have to adjust for the pipe. I am assuming you have just a piece of copper or brass tubing as a pipe. That will cause the engine to run lean. If it is a two stroke engine the easiest way to tune it is by replacing the jets. if it is a four stroke engine play with the needle settings.

I work for a Honda dealer here in Anchorage so I know a little about these but not a lot. The easiest way is to go to a power equipment store that services the engine you have, and they could tell you everything you ever wanted to know.

 

[Superfluus]

It is a 2 stroke...but what are the "jets" ?

 

[wavejumper]

the jets are small open screws in the carbureator ( much like the needle settings on a nitro boat opens for the fuel to be let in.) Only jets can be changed rather than turned to allow more fuel in to the carb. an example is if I have a 170 stock jet (lets say on a motorcycle) and I put a pipe on it I would probably have to go to a 175 or 180 size jet.

If you are looking for it on the carb. take the float bowl off and it will be the only screw that takes a flat head only screw driver.

 

[superfluus]

so, do i have to buy a new carb, or can i find just the jets? I think you're right tho, the engie seems kind of starved for fuel

 

[Propjockey]

You should be able to buy just the jets from a dealer. Try something like...

http://www.mfgsupply.com/m/c/index.html

Something else to think about...two strokers love tuned pipes.

 

[superfluus]

thanks for all your help...where is a good place to buy a tuned pipe? I've seen some for smaller engines, but nothing that was made for the larger 40cc. Could i use a pipe that was made for like a 25cc?

 

[Mike T.]

What brand is the engine? You can sometimes find an owner's manual online at the manufacturer's website. Second, what brand is the carb? If it's a Walbro or a Zama with a butterfly-type throttle, it would have adjustable needle valves. There would be a tiny H (high) by one and L (low) by the other. (Exception: Some California engines have one or both jets non-adjustable. They would be drilled into the carb body and cannot be changed except by drilling out.) As a general rule, Walbros run 1-1/2 turns open on the high needle and 1-1/2 turns on the low. Another good general rule is a total of three turns, since the low bleeds over onto the high and vice-vera. If it has a Walbro with a drum-type throttle, its mixture control setup is a little different but is still adjustable. I've not fooled with one of those. Go to Jim's RC Boat Dock Message Board and there will be someone there with answers on the drum throttle Walbro. Marine Specialties has sold some larger gas engines and would have pipe advice for you, although it's my bet that any gas pipe that you can get to fit would work, since most will work on the 30cc Homelites and 42cc Zenoahs. If the engine is stock, a stock cannister muffler will still give you about 90% of the performance that you could expect with a tuned pipe anyway, and be a lot cheaper and a lot less trouble.

 

[superfluus]

the engine is an Echo that is about 15 years old (came out of a massive backpack blower). Everything on the engine is stock...so i'm looking to upgrade the carb and pipe. Right now it just has a long 3/4 piece of straight pipe (about 18 inches) on it.

 

[superfluus]

Just pulled the carb out... it is a walbro

View attachment 529

 

[superfluus]

this is a picture of the engine

View attachment 530

 

[Mike T]

In the top picture, the two screws with the springs under them at the front left corner of the picture are the mixture screws. The one closest the air intake is the high speed. That looks like it has a 1/2" venturi. In gas RC racing, the most popular 1/2" carb is the Walbro WT-157. It sells new for about $50. There is another 1/2" Walbro, I think it's a WT-644 that comes stock on the new G-260 and G-231 Zenoahs. It has a choke on it. A lot of guys replace them with the the 257 so the 644 is being resold new or nearly new for around $25. Check with Midwest engines, he might have some of those to sell. On the 257, the needles set at Low, 2-1/4 turns, High, 7/8 turn. The model number is stamped on the side of the carb body opposite the side where the needles are. Bad things often happen when you bore out a Walbro carb's venturi; I don't recommend that. Echos are well built and durable engines that don't tend to be as fast as the Zenoah but are great to play with. Does yours get enough air to cool it without a fan and shroud? I guess in the blower installation, the air going to the blower was drawn over the cylinder fins. I would think if you did any mods to it, you would need to go to water cooling.

 

[Mike T]

One other thing - that tube barb fitting on the top of the one-screw cover (the pumper diaphram cover) is for crankcase pressure pulse. There should be a tube connected from that to a fitting on the crankcase. It passes crankcase pressure pulses to the fuel pump diaphram in the carb. If that's not connected, it will run poorly and stall after a minute or so on the water.