tango28charlie

Hello all,

I plan on making a velocity stack to put on my Honda GX25. I am thinking about having a piece of tubing curve 90 degrees as it comes off the carb, with a trumpet shaped inlet end. Do you conversion gurus think it would be better for the inlet to face forward into the airstream (I am not using a cowl on the airplane) to take advantage of the ram air effect from the propwash and forward motion of the plane, or should the inlet face backward away from the prop? Or do you think it would be better not to curve the stack at all and just have it come straight out of the carb?
Thanks,
Charlie

av8tor1977

My Feedback: (6)

In testing on cars, no significant benefit was realized from using ram air until over 100 mph. Further, it can really upset carburetion. They say the only way to pull it off is to make a box that encloses the whole carburetor, and run the ram air to that. I think you might encounter similar problems in your application. Having said that, if it were me I would probably try it anyway just to see. [>:]

Good luck,
AV8TOR

BillS

Don’t know about four strokes but:

We modify two stroke engines in several ways fundamental to carburetor operation.

The most serious is the removal of the air filter and air box (plenum chamber). Two stroke engines are always pulsing air at the carburetor inlet. The prop wash and air flow across the carb inlet carries a large portion of the fuel charge away. The fuel charge is not carried away in chainsaw configuration. On a high humidity night shining car lights on an idling engine will allow one to see the halo (looks like a smoke ring) being forced out and back in the carb. It is an interesting phenomenon to watch. Much of the fuel charge is lost to the air stream.

A velocity stack helped my engine, which is also open to the air stream. Also an air filter or plenum chamber probably would have helped.

Engines with cowls probably don’t loose as much charge. You will probably fine the velocity stack needs to point straight out.

w8ye

My Feedback: (16)

After you make your modification, and if you have trouble getting the mixture set right for different engine speeds, you will then know you are experiencing what av8tor is talking about.

Enjoy,

Jim

tango28charlie

I can say that there is no backflow pulse through the carburetor like on a 2-stroke. The only problem is that the Bernoulli effect from the propwash acting on the open inlet of the carburetor is effectively reducing the air pressure there. The engine gains about 1,000 rpm when I put my hand in front of the carb inlet at WOT. I think from everybody's comments I'm going to bag the velocity stack idea for now and put the air cleaner back on. Thanks for the input.

Charlie

Antique

My Feedback: (4)

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Antique

My Feedback: (4)

you might not have that with a carb that could be adjusted better for high speed...

tango28charlie

Yes, the stock carb is a problem because it has no mixture screws and a cam that adjusts the mixture as the throttle is advanced. I am trying to get a Walbro WYK-16 to put on it.
Charlie