Scale Wendell Hostetler 30% Piper Cherokee Glass Fuselage Build.

[Snoopy1]

The spiral diaphragm kit arrived. Comes with new diaphragm, level and setting gauge, the gauge drives the cost up, you can buy the kit without the gauge. Unlike other Walbro carbs, the lever is set .055" above gasket surface. Look closely on the bottom of the gauge under the WALBRO is the surface used to set the lever height. All I need now is a carb to put it in!

View attachment 123285

Can you please explain the spiral diaphragm kit, why and what are the benefits.

SORRY I BOTHERED YOU FOUND A VIDEO ON IT LOOKS INTERESTING.

 

[TonyHallo]

From Google AI;
"
A Walbro spiral diaphragm kit's primary benefit is its enhanced durability compared to traditional rubber diaphragms, offering resistance to fuel deterioration, ethanol damage, and extended lifespan, making it ideal for applications with modern fuels; however, potential downsides include potential compatibility issues with certain carburetor models and a potentially higher cost compared to standard diaphragm kits.

Key benefits of a Walbro spiral diaphragm kit:

• Increased longevity:
  The spiral design is less prone to cracking, tearing, or hardening over time, leading to a significantly longer lifespan compared to standard rubber diaphragms.
  
  
• Ethanol resistance:
  Spiral diaphragms are designed to withstand the effects of ethanol-blended fuels, which can degrade traditional rubber diaphragms quickly.
  
  
• Consistent fuel metering:
  The unique design provides more precise fuel delivery, reducing fluctuations in engine performance due to diaphragm wear.
  
  
• Improved reliability:
  By eliminating issues related to diaphragm deterioration, the spiral design can contribute to more reliable engine operation.
  

Potential disadvantages of a Walbro spiral diaphragm kit:

• Compatibility concerns:
  Not all carburetors are designed to accommodate a spiral diaphragm, so checking compatibility before purchase is crucial.
  
  
• Cost:
  Spiral diaphragm kits may be more expensive than standard replacement diaphragms.
  
  
• Installation complexity:
  Depending on the carburetor design, installing a spiral diaphragm may require more technical knowledge compared to a standard diaphragm"
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• 
  Now Tony's spin on why, this is pure conjecture on my part. The diaphragm is almost invisible and the spiral weights almost nothing, note the Special note on top of the instructions. Years ago I had a GT80 in my One Design, the plane would always quit running in the bottom of an outside loop. The carburetor was mounted such that the diaphragm was facing up in level flight. The pressure on the top of the diaphragm is atmospheric, the pressure under the diaphragm was atmospheric less the weight of the diaphragm and any forces acting on it. When entering the final turn on the outside you are experiencing negative G's so in my case the pressure under the diaphragm was atmospheric plus two times the weight of the diaphragm plus centripetal force acting on the diaphragm due to the curvature of the flight path. Results were an overly rich condition, the engine would start on the first flip following the dead stick after landing of course.
  
  
• I did share these experiences with another fellow over at the giants, Kcorb, he had similar experiences with an possibly a MVVS twin, was a long time ago. He corrected his problem by rotating the carb 90 degrees if I recall correctly, I had planned to do the same to test the theory but elected to replace the GT80 with a ZDZ 80J instead. We had discussions with Pe Rivers about the problem and he said the MVVS engineers said that was not the problem, I think I suggested they get out of the office and do a little work at the flying field!
• The other reason I just wanted to give it a try. The kits are not available for all models. Based on the pro and cons above it's all a win, I ready paid and have it installed, I think I will change out the diaphragms on both Roto 85 FS as well.
  
  
  
  
  

 

[TonyHallo]

Sanding away on the cowl and finishing up the stabilator. Turned 3/4" dowel down to fit inside the cf tube on both ends, these were glued in with thin CA. The stabilator tips are fitted with 1/4" lite ply plates, an additional piece of 1/4" lite ply was glued over the socket area, a 4-40 bolt joins the socket to the tube on each end. The bolts pass through the tube into a threaded insert on the opposite side.

 

[TonyHallo]

Quick update on the carburetor modification. The clone carb I bought had a 17MM choke diameter, I found a 17 mm velocity stack Ebay for $8 so I bought it. The original carb has a 9 mm diameter velocity stack. Looking at the 17 mm velocity stack on the new carb with the choke open just looks silly. The venturi diameter is 9 mm on the new carb, the velocity stack has 4 times the area of the venturi! Morris Mini Motors does make a velocity stack with a 6 or 9 mm bore that bolts up to a standard Walbro carb, the cost is like $35 Canadian however the shipping is $40, maybe the conversion kit wasn't such a bad deal after all? But hey why have machine tools if you don't use them?
Have a piece of 2" aluminum on hand but decided to order a 12" piece of 1-5/8" from McMaster Carr, can't have too much stock on hand. Plan to whittle out a new velocity stack. Pretty sure the larger 17 mm stack will impact the throttle response. The throttle response on the Roto 85's is not good, I have a 1 second slow function on throttle opening that made the response better, closing has no slow function, additionally the throttle opening is set so that the RPM's slightly decrease at full throttle. Might use the shape in figure 9 below scaled down.