As I got through my new junk pile I am finding things I don't understand. Are engines that use these odd double barrel over/under type carbs of any use? The engine is light. It is brand new. Would making a manifold to use both intakes and a norma carb work? How about I cover the top intake? What is that thing for anyway?

I was told at the repair shop that the one manifold lets in air only on deceleration

I would avoid that repair shop in the future, go find one with a tech thats knows something about walbros...

Scota4570
Those carbs are designed to work with engines using the stratified charge principal. The stratified charge principal is a way of reducing unburned hydrocarbons from exiting engine. During the end of the exhaust phase and prior to the transfer phase, plain air is let into the engine the create a wedge of air with no fuel into cylinder to go between exhaust gas and incoming air fuel mixture coming from transfer ports a few degrees later, so it dosen't go out the exhaust port.

Redmax uses this approach and its explained on their website.

You could block off top hole.I personaly would find another engine.

Ditto.

Yep...

I do have to say however, that my Poulan 42cc chainsaw with the dual barrel carb runs quite well and has good power.

AV8TOR

Mine appears to be the engine on the REDMAX site. I ran the thing out of curiosity. It starts easy. The top end is non existant. There is no way to get enough gas to open it up. It leans out regardless of the carb setting. Too bad it seems like quality construction and I have two of them. Redmax is/was Zenoah. I hope somebody doesn't get the idea to force this "technowledgy" on model engines.

Like avi8tor said, he has a good one. But it seems a whole new learning curve tuning and handling these SC engines.

Well for one thing, they are designed and set to run as lean as possible without burning themselves up. If/when you take off the air filter and/or add some sort of a free breathing exhaust, they are going to lean out, perhaps (probably?) more than the stock carb can be richened up to compensate. Since one is basically stuck with the carb, if that is the case all one could do is try to modify the carb internally to allow it to flow more fuel. The other alternative is to block off the secondary intake port, and run a "normal" carb. I really don't know how this would affect overall power with the SC design.

I think someone here in the forum did just that with a Poulan however. You might search for that thread for more info.

AV8TOR

I'm going to make a new intake mainifold and try a normal carb. Perhaps if the scaving port is fed fuel air and the regular port is fed the same the mix will be more pure. That is to say less exhaust mixed with the new charge. That could conteract a factor that limits 2-strokes? Dunno, I'll see. Maybe flame will come out the exhaust.! IF it doesn't work I can block the top port.

I hope this kind of nonesense is not going to become the norm. Such exersizes in low emmision foolishness will never make any meaningfull difference in air quality or other enviornmental concearns. "Every little bit" does not help. Sometimes it is just a failure to do a viald cost benefit anlaysis.

The stratified porting background has been described in great detail in USA patents. Can't remember which ones, but a search will reveal them.
If you want to deal with the engines and their design, it might be a good idea to read those publications first.. They probably are here to stay.

So, make an intake minifold that forks the trail, install a 11MM walbro with the pump. You get:

5700RPM 20x8 Power point
5900RPM 18x10 APC
6400RPM 18X8 Power Point
8000RPM 16x8 APC
9900RPM 16x4 APC

It idles down real slow. All the above with with the factory exhaust. IT only added a couple hundred rpm with off. The factory one is VERY quiet. The literature someone posted for me to link to said something about a glass pack type thing. I probably won't fly it anytime soon but it was an interesting exersize.

I am for new ideas, stratified charge in this case, if it has some real adavantage. They may or may not show some improvement in performance or economy, we'll see. If the only reason is so that less hydrocarbons are released in the atmosphere, I think it is government blackmail. The "enviornmental" power brokers are mandating things that make no practical sense. The idea that small 2-strokes are of any real threat to air quality is absurd. There are not enough of them around to make any measureable difference. So these idealogs in the government mandate how something will be made and they themselves could not make or design anyting. Show me any governmnet designed car or whatever that is any good. They are buerocrats not engineers. Most of them have no real skills or depth of knowledge. The result is they are hurting ligitiment businesses and putting people out of work. Maybe that is their intent in the first place.

This engine peaked my intrest again. It has a ton of compression. I bent up to pop off lever on the original double barrel carb. It works fine. I guess that is the key to these carbs with no high speed needle, just bend the pop off lever to meter more fuel.

Interesting appect of this engine, the stroke us 26mm and the bore is 38mm. Not close to square.

Is there anything to be done about the agressive return spring built in to the Walbro barrel type carbs? ....other than a beefie servo.

Pleease don't start a Camp fuel flame war. This one does audibly ping on Camp fuel. It sounds like the muffler is loose or there is a piece of junk rattling around in the muffler. It must have to do with the stratified charge business.

16x6 8600
17x8 7800
18x8 6700
20x8 5700

The ping is from the low octane camp fuel. People at higher altitude often get away with it, but not in real powerful engines.
Agressive spring? Just wind a selfmade spring out of a ball point pen spring or thin piece of piano wire, or maybe guitar string. Not too hard to do.