Prop effect on A4 Performance

[raleighm]

My Ericson 27 had a CDI Perfect Pitch prop installed by the previous owner. It was a 12 inch diameter X 6 pitch 2-blader. I had tuned up my engine and could still only achieve a maximum of 1600 RPM. I also noticed that when I looked into the carburetor throat with the flame arrestor removed, fuel would begin to splash around noticeably when I continued to open the throttle beyond the point where max RPM was reached (this occurred well before wide open throttle). When I backed off the throttle the fuel would disappear!

When I hauled the boat to paint the bottom last week, I pulled the prop and replaced it with the original bronze 10X6 2-blade prop that the previous owner had left with the boat. What a difference! I can now get up to 2400 RPM and notice less vibration due to greater tip clearance. The previous owner mentioned that he thought thrust in reverse was poor with the smaller prop but I did not notice any significant difference and there is less prop walk with the smaller prop. How much time do you spend in reverse anyway?

I would like to try the Indigo 3-blade prop but don't have and extra $270 lying around right now.

Anyone want a used CDI Perfect Pitch 12 inch prop? Mine's got to go!

[dtinder]

12X6 prop

My Ericson 27 had a CDI Perfect Pitch prop installed by the previous owner. It was a 12 inch diameter X 6 pitch 2-blader. I had tuned up my engine and could still only achieve a maximum of 1600 RPM. I also noticed that when I looked into the carburetor throat with the flame arrestor removed, fuel would begin to splash around noticeably when I continued to open the throttle beyond the point where max RPM was reached (this occurred well before wide open throttle). When I backed off the throttle the fuel would disappear!

When I hauled the boat to paint the bottom last week, I pulled the prop and replaced it with the original bronze 10X6 2-blade prop that the previous owner had left with the boat. What a difference! I can now get up to 2400 RPM and notice less vibration due to greater tip clearance. The previous owner mentioned that he thought thrust in reverse was poor with the smaller prop but I did not notice any significant difference and there is less prop walk with the smaller prop. How much time do you spend in reverse anyway?

I would like to try the Indigo 3-blade prop but don't have and extra $270 lying around right now.

Anyone want a used CDI Perfect Pitch 12 inch prop? Mine's got to go!

Your local prop shop can cut your 12" down to 10"---for about 35 bucks=_-

[Don Moyer]

Hello,

I really hate to inject any kind of negativity on this otherwise happy report, but the only time I've ever seen fuel droplets "dancing around" in the intake throat of a carburetor was when there was a rather severe restriction within the exhaust system which was limiting maximum RPM.

By most standards, a 10 X 6 two bladed prop is very small for a well running Atomic 4, and I would expect closer to 3000 RPM than 2400. I'm wondering if you see any fuel droplets splashing around in the intake throat when you reach max RPM with the small prop. If you do, it's very likely that an exhaust restriction is a limiting factor with the small prop as well.

Other indications of an exhaust blockage include a rather distinctive "gagging" sound coming from the carburetor during acceleration or when maximum RPM is reached, and water "batching" out of the exhaust at cruising power. Water batching out of the exhaust at idle is a normal condition, but at cruising RPM the water should be flowing out of the exhaust in a continuous stream.

If you wish to pursue the issue of exhaust back pressure, please give me a call and I'll try to put together a small test kit for you to measure your exhaust.

Regards,

Don