Water in Sutton Exhaust Shroud getting into Carb Heat Box?

Hey everyone I have a pretty weird issue happening with my cub. I have a 1976 PA-18 160 HP cub with a pretty new engine only about 250smoh which was done in January of 21’. Have not had any huge issues with it at all the engine itself has been great so far knock on wood.

I apologize for the long post in advance but I figured the details matter with this. If you care to read the whole thing I’m sure you may be intrigued as well.

For those of you from Alaska you probably know we’ve had a pretty rainy summer as of late. I usually fly the heck out of my cub when I’m not flying at my other pilot job. Anyways this July I didn’t fly it for about 2.5 weeks because of crappy weather with non stop rain and work getting in the way. When I finally got around to flying it again at the end of July I started it up and it had a huge backfire that got my attention. I didn’t do anything different than I usually do with the start. Primed it 2 times with my primer and cranked it over. It ran smooth afterwards so I didn’t think too much of it.

When I taxi’d over to do my run up everything was fine with the mag check, I have new bendix mags. Then I went to do the carb heat check at 1700-1800rpm ish and instantly as I pulled the carb heat the engine sputtered and quit. I was pretty surprised at first I tried to chock it up to maybe being too rich since it was a hot sunny day but I’ve flown my cub on lots of hot sunny days and never had any issues with it. I then restarted it and did the run up again and checked carb heat and it then dropped 400-500rpm and was just spitting and sputtering. Then it gradually smoothed out and ran okay. I shut it down and hopped out and popped the cowling.

I wiggled my carb filter to make sure it wasn’t loose, it was tight. I checked for blue stains on the carb itself, it was clean. Called my mechanic and I thought that maybe it was water in the carb somehow and he agreed that it was likely but without looking at it couldn’t say for sure. I started it back up and ran it up 2 more times and the carb heat would work normally and the engine would not quit. It ran normal like this never even happened. So I opted to fly it around the pattern and checked everything in the air and sure enough it worked as normal. No issues at all.

I flew it the next day again in the pattern and the first run up the carb heat dropped the RPM 400ish and it ran rough but didn’t kill the engine. Subsequent carb heat drops were normal after that and everything worked fine. Flew it and it worked great like it never even happened.

I had two hypotheses, one that water got into the carb heat box somehow, or two that I maybe had a primer leak that was causing it to run too rich to where carb heat would kill it but I wasn’t sure why it wouldn’t replicate in flight.

Mechanic and I pulled the carb air filter and it was fine. We also then pulled the scat tube coming from
the exhaust shroud that goes to the carb heat box and saw standing water drops in the hose. And we were pretty convinced that water was dripping down the baffling into the exhaust shroud and into that hose. Even though it’s angled up it does have a low spot or 2 in it to where water could sit. We blew out the water with compressed air. The carb heat box has drain holes of course but maybe enough water is coming through it just doesn’t matter.

My question is this: has anyone else had issues with their Sutton exhaust like this ? I have had this exhaust for 3 years now and probably flown it 550 hours with no issues until now. To be fair the plane used to live in a hangar but now stays outside. It’s also been a pretty rainy summer definitely more so than usual.

I know the Sutton exhaust heat shroud for the carb heat and cabin heat air is open ended on the back end of the left side at least (not sure about the right) and we did a little experiment and poured a small amount of water on the top of the left side of the baffling (#2 and #4 cylinder side) and watched it drop a bit down into the open end of the exhaust and make its way to where that scat tube connects. As with other Sutton exhaust applications there is the relocation of the air scoop for carb heat and cabin heat into the top of the #1 cylinder side baffling. So maybe water is leaking in from
there too. I know others that have the Sutton who have had no issues flying in this rain lately so I’m not sure what’s so special about mine. When comparing to a friends Sutton with a brand new engine his scat hose is perfectly aligned and has no low spots whatsoever except for the very inlet at the shroud. Maybe replacing the hose is the right answer ? Sorry for rambling just lots of thoughts here as to what the issue could be. It is still happening after rainy days and just happens for the first 1-2 applications of carb heat then runs normal.

Attached below are photos of a buddies Sutton exhaust, mine is the exact same except maybe my Scat tube is a little more weathered. You can see how the inlet for the carb heat is open and exposed with the shroud to where water could get in. But I would think it would also be able to get in at the relocated air scoop as well in the baffling. It’s been a week since we cleaned the tube out and today it quit on me again during run up after some good rain. As much as I’d like to say a solution would be to just buy a hangar or don’t fly it after it rains I think it’s obnoxious it happens in the first place and I don’t have warm fuzzies about ingesting water all the time on start.

If mine I'd put small drain holes in any low spots....metal shroud or SCAT tubing. Make sure the airbox drains are good to flow water. If it's still holding water think about a thin breathable engine cover to exclude heavy topside rain.

Gary

I've said this before and got flamed but that heat shroud design is problematic. My brother managed to get dirt in the end of the heat shroud and it destroyed a cylinder last year.

I have experienced the water issue when pulling carb heat; once in flight. Wind-blown rain finding its way into the shroud has been the cause. My solution has been to pull carb heat slowly, if conditions make me suspect the problem could arise.

My carb heat doesn't have ram air it just pulls through the shroud. The ram air goes to cabin heat. Maybe a small hole drilled in the bottom of the shroud to let any water drain out?

Crash said:

I've said this before and got flamed but that heat shroud design is problematic. My brother managed to get dirt in the end of the heat shroud and it destroyed a cylinder last year.

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Was going to comment the same- it’s not dirt; it’s the pk screws that hold the shroud together along the top. Most are missing 2-3 screws when they come in for annual. Problem is, they rattle along the lip until they fall off; if it happens to be the left side, they can go right into the open carb heat inlet and get sucked up the next time you pull carb heat. I put screens over the intakes now...

Piston after eating a screw or two...

Apologies to the OP about the hijack, but I think this is worth adding to the conversation...

Low point drains and inlet screens. Who certifies these without?

Gary

Crash said:

I've said this before and got flamed but that heat shroud design is problematic. My brother managed to get dirt in the end of the heat shroud and it destroyed a cylinder last year.

Click to expand...

How did he get dirt in the shroud? Dirt was in the cowl?

ak49flyer said:

Was going to comment the same- it’s not dirt; it’s the pk screws that hold the shroud together along the top. Most are missing 2-3 screws when they come in for annual. Problem is, they rattle along the lip until they fall off; if it happens to be the left side, they can go right into the open carb heat inlet and get sucked up the next time you pull carb heat. I put screens over the intakes now...

Click to expand...

Interesting, had a few come in with screws missing and they seemed to be in the most pita place to install them so I figured previous mechanics left them out. Have had a few that I had to crimp the Tinnerman nut or replace it cause it didn't bite. Never thought of a screw going into the shroud and getting sucked into the carb heat box. Looked at mine this morning and took some pictures for those unfamiliar.

Steve Pierce said:

How did he get dirt in the shroud? Dirt was in the cowl?

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The beach had lots of sticky mud with small rocks getting thrown into the bottom of the wing and back of the prop from the bushwheels. The theory from my brother and his mechanic is some of the small rocks (pea gravel size) got thrown into the back of the cowling cheek and made its way into the heat muff. Lots of little round dents on the piston and in the cylinder that got damaged.

Crash said:

The beach had lots of sticky mud with small rocks getting thrown into the bottom of the wing and back of the prop from the bushwheels. The theory from my brother and his mechanic is some of the small rocks (pea gravel size) got thrown into the back of the cowling cheek and made its way into the heat muff. Lots of little round dents on the piston and in the cylinder that got damaged.

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I get into mud and such but it tears up my prop. Never found any mud or sand inside my cowl. When I find it on the cowl it is usually bad cause I dinged my prop.

Sounds like the stock exhaust might work better for some. It is sealed up well. I like the access to the back of the engine and not having the heat on the firewall.

It sure is nice having all that extra room in the back of the cowling area that’s for sure. It does make me wonder with the design of these Sutton exhausts though how easily something can get into the carb heat box etc. I have since started using cowling plugs with all this rain we’ve been getting and haven’t had any issues. Definitely just a bandaid fix but it works