av8tor1977
Posts: 2697
Joined: 5/3/2003
From: ASH FORK,
AZ, USA
Status: offline
|
It rather sounds like you may be a touch too rich on the idle, but it's hard to be sure. Usually if you are too lean and get a transition stumble, it will quit instead of burbling for a bit and then throttling up. But it really is hard to trouble shoot these fine tuning problems without actually hearing the engine as Captain John said.
One thing I do when tuning, is to get the engine all warmed up and tuned to where I think it is about right, and then I clean it out with a 20 second run at high throttle. Then I slowly idle it down to it's lowest reliable idle, and just listen to it. If while idling, the speed remains the same, the idle mixture is either very close to correct or possibly slightly lean. Conversely, if after idling for a while it gradually starts slowing down, the idle mixture is rich. (What is happening here is that with the rich idle, fuel accumulates in the crankcase and the mixture gets richer and richer; slowing the engine down. Then when you throttle up, all that accumulated fuel in the crankcase gets "scooped up" into the flow and the engine becomes really rich until it clears out.)
Now, having said all that, Walbro admits in their tech literature that for a good transition, an engine will often have to be tuned with a slightly richer than optimum idle setting. The bottom line is that these carbs just don't like the throttle slammed open fast with a prop load on them. If you think about it, in their designed use on a weedeater, chain saw, etc., they are revved up before the load is introduced in most cases. In our applications, the load (propeller) is ALWAYS there, or applied. There are some Walbro carbs with accelerator pumps that help with the transition problem, but they are in the larger sizes that won't help in your case.
So, what can you do in your case? First, keep trying to get the mixture optimized. Second, if you have a computer radio, you can program some exponential into the throttle channel to slow down the throttle response a bit. I often use this trick on my gas planes. If you don't have this option, you will just have to refrain from "slamming" the throttle open from idle.
There is a third option that I hesitate to recommend. If you look into the carb near the throttle butterfly, you can see the little holes in the venturi where the idle and transition fuel exits. If you take a small jewelers file and file a tiny "V" slot in the throttle butterfly aligned with those holes in the carb venturi, you can often improve idle and throttle response. The problem is, especially on a small engine, if you get the "V" too large, you will not be able to adjust the idle low enough....
Good luck and keep at it,
AV8TOR
_____________________________
If it's not scary, it's NOWHERE NEAR powerful enough!!
|
Printable Version
All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Engine Conversions >> RE: gas fuel with glo plug no ignition 
RE: gas fuel with glo plug no i... - 
