dogshome

I have been flying my 180FS conversion some more (WT38 carb fitted) and have noticed that a much lower idle is possible by leaning the bottom end needle. This results in a poor transition/cut on opening up. To get a reliable idle and transition, the low end is really rather too rich.

Can I make any adjustment to the throttle plate or pop-off pressure to richen the mid-range? Top and bottom have plenty of adjustment as it is.

av8tor1977

My Feedback: (6)

What you need, expeciially on a four stroke, is one of the Walbro carbs with an accelerator pump. Someone posted a list of them here in the forum once.

Just recently, (again), I have been dreaming of trying to modify a Walbro to have a midrange adjustment. Too many projects; don't know if I'll ever get around to trying it....

AV8TOR

dogshome

Got pump [&:]

bluejets

Changes in pop-off pressure only changes adjustment needle sensitivity. Increase pressure, needles need less adjustment to effect a change and visa-versa.
Usually set to about 2lb but difficult set precisely without the right guage.

dogshome

Ok thanks for that [8D]

I've seen modifications by adding cut-outs in the throttle plate just above the jets, I guess that is just for the very first few mm of travel?

Also got several small drills, so opening up a transition hole/jet is a possibility?


Other ideas? []

tkg

My Feedback: (1)

On a Walbro the idle needle adjusts the idle and midrange. About all you can do is dismember the carb and remove the Welch plugs and make the midrange jet bigger. This would make the mid range rich even after you lean out the low speed.
How much???? well thats why they sell bags of welch plugs.

av8tor1977

My Feedback: (6)

Yep, and carb throttle butterfly "cutouts" work great, but they actually lean the midrange, not richen it...

AV8TOR

dogshome

Allegedly, the higher flow directly over the jet RICHENS the idle/pickup mixture, rather than the air passing around the circumfernece of the throttle plate. Not entirely convinced.... [&:] My problem is at 1/4 throttle.

The amount of richness required in the mid is in the 1/4 to 1/3 of a turn range (so quite a lot). That is with the low end set at a very best idle. I've got several carbs, so drilling and welching and possible FUBAR are not a problem
Where do I drill???

av8tor1977

My Feedback: (6)

Right. With the notch in the butterfly, the fuel for idle is drawn more directly over the idle orifice(s) instead of around the circumference of the butterfly as you said. This more effectively draws fuel out of the idle circuit and makes for a more homogenous mixture, allowing one to lean the idle mixture adjustment screw. This often leads to a nicer idle, but has the side effect of leaning out the midrange; the opposite of what you are hoping to do.

As far as where to drill, that depends on what you want to do. You could take the welch plug out and drill out the hole which is closest to the middle of the carb to try to richen the midrange. If you want to try to make an adjustable midrange circuit, you will have to study the carb and see where/how a person could accomplish that.

For now at least, what I do is setup both the geometry of my throttle linkage, and use exponential on my throttle channel, to slow down the opening of the throttle to avoid such a quick opening of the throttle. This allows me to run a somewhat leaner idle mix without the engine "falling on it's face" during throttle up transition. When "jazzing" an engine on the test stand by hand, you are almost always opening the throttle quicker than a servo setup does anyway.

AV8TOR

tkg

My Feedback: (1)

From a place far away and long ago, involving Amal, SU and Weber carbs, I have a set of tapered micro reamers. So much better than drills.
Remove the low speed needle, squirt carb cleaner in and see which jets flow. Remove the welch plug and enlarge the hole closest to the center.

dogshome

Got it. 3 jets. I loosely put a pin through so you can see where they are aimed. They seem easily accessible to drill. The middle and top tie up with where she is lean. I'll let you know what happens....

av8tor1977

My Feedback: (6)

Nice pics. It will be interesting to see how it works.

Good luck,
AV8TOR

dogshome

The carb in the pics is an old (and well-worn) one. before I whip the plug out of the good ones(s), do you think it might be worth closing up the cutout in the butterfly? It's right over the centre idle jet.

av8tor1977

My Feedback: (6)

Well, that should richen the midrange some, though you might lose some idle mixture quality. It's always worth a try if you are willing to experiment. Wouldn't take much to solder up that notch and dress it round and smooth. I would probably use Sta-Brite silver solder for that.

How about analyzing the acclerator pump circuit and see if there isn't a way to get a bigger accelerator pump shot?

One other thing.... I don't remember the details of your project. What size carb on what size engine? A too big carb will be really hard to get to transition well, especially on a four stroke. Note also that a four stroke generally needs a somewhat smaller carb than the equivalent size two stroke.

AV8TOR

tkg

My Feedback: (1)

That looks to be a factory cutout, I'd leave it alone. It maybe there to fix another problem.

Nosedragger

The solution is simple, don't idle it down that low.

To go to a real low idle and maintain throttle response you should go to a smaller venturi size.If doing that takes off rpm from the the top end then you will just have to live with the higher idle.
or switch to a wyk type of carb they always work better than butterfly type on four strokes.

tkg

My Feedback: (1)

Ever get results???

dogshome

Not.... yet []

dogshome

Soldered up the butterfly cutout and that seems to have solved it. Some tweaking of the needles (leaner on bottom end, strangely) and it revs up and down well. The Edge has had a few flights today.