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US Engines 41cc, Gas Engine Rescue and Rehab

Joker 53150

Mmmmmmm, balsa.
Mentor
#1
There are plenty of threads on rescuing and rehabbing old planes, but is there any interest in a thread on rehabbing an old gas engine? I just picked up an old US Engines 41cc gasser that deserves a new chance at life. It needs a lot of TLC, but is a good way for people to get into mid-size gas engines fairly cheaply. Although I don't have a plane for it right now it'd be a fun & easy project to do so it's ready for the future.
 

TooJung2Die

Well-known member
#2
I'm interested. I like fixing small engines and watching YouTube videos of small engine restorations; everything from weed whackers to mopeds. 41cc is considered a mid-size model airplane engine? Wow. :eek: That's only 8cc smaller than a street legal scooter engine.
 

Joker 53150

Mmmmmmm, balsa.
Mentor
#5
Apparently, these engines are really good at being nose weight and not much else.
Their power-to-weight ratio is nowhere near as good as modern engines, but they run, and run, and run. They aren't necessarily a good choice in a plane that needs a light engine. I personally like the old-school engines with magneto ignitions over the new electronic ignitions. Yes, there is more weight, but there is no ignition module, or battery, or opti-kill switch needed. For me it's kind of like finding an older beat-up plane at a swap meet, there's a certain joy to saving a piece of RC history.
 

Joker 53150

Mmmmmmm, balsa.
Mentor
#6
Here's my subject, a well-used US Engines 41cc gasser. It's well used, but the price was dirt cheap. Including purchase price of the engine I'll probably have it running for less than $70. If I wanted to make my own carb mounting gaskets and re-use the carb isolator it'd be closer to $55. Some "optional" stuff will be done on it as well while I'm at it.

The first thing I did was to give it a general once-over. It's very dirty, but I can see the piston and don't see any scoring or other black-flags. Since the plug was installed I rotated the propshaft counterclockwise and verified that it has compression. I then pulled the plug and rotated it some more, listening and feeling for any issues. None were found.

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Checking out the carb there were a few issues. First, it's dirty as heck, the mounting bolts (well, a bolt and a screw) don't match, and the throttle linkage is bent/broken. I pulled the mounting hardware and confirmed the two pieces are the same thread pitch, so that's just a cosmetic issue. The broken linkage is no big deal, it's a homemade piece that will be replaced. The throttle itself works just fine, opening and closing the intake. It's also got a Walbro carb, so I know the rebuild kit is cheap.

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Here's a very important step in determining the health of the victim subject - does it spark? This is easier on a magneto engine as there are no extra bits of electronics or batteries needed. With the plug out and the plug boot/wire attached, ground the plug body against the engine and turn the flywheel/propshaft. FAST, where the magnet passes the magneto, and if all goes well you'll see a spark at the plug. Note that you've got to turn it fairly quickly to see a spark - faster spin = better spark. To do this on an electronic ignition engine you need to power the ignition module. This engine passed the spark test. Part of the rehab on this engine will include a new plug wire, plug, and plug boot. I'm not a fan of the pliers scars on the propshaft…. Those will be cleaned up a bit as the work happens.

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The engine came with an aftermarket Bennett muffler. They're loud enough to wake the dead, kind of negating the word "muffler". :) This is also a smoke muffler, although one side is capped with a bolt. It's also got a little leaking around the outlets. The plan is to clean it all up and re-seal the outlets to keep from making a mess on the plane it eventually powers.

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The magneto isn't the cleanest one I've ever seen, but it works. The terminals will be cleaned up and I'll need to do something about that corroded wire coming out the top. When inspecting an engine, take a BRIGHT light and shine it between the cooling fins! When I first looked at this engine I thought there were cracks in the head, but the light let me see it's all just casting marks. Note that a couple fins have slight bends, but no fins are broken and there is no sign of physical damage (is neglect physical damage?). The engine is missing the prop bolt and washer, but I've got spare parts to take care of that.

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Soooo, the engine is rough, no doubt about it. But it's cheap and it'll be a fun project. Now that I know it has a good foundation I'll make up a shopping list of "needs" and "wants". I'll also pull the carb and get that cleaning started, as I am pretty sure I've got a rebuild kit for it.
 

Joker 53150

Mmmmmmm, balsa.
Mentor
#7
I'm really committed to this one now. I have a new plug, prop bolt, prop washer, and the carb rebuild kit. I only needed to order a couple parts, a new plug wire & boot (the original pieces were serviceable, but they're cheap & easy to replace), a carb block, both carb block gaskets, and an exhaust gasket - $25 spent with B&B Specialties. That could have been cut down to about $10-15 if I wanted to make my own gaskets.

When the parts arrive I'll hit the hardware store and pick up some new carb mounting bolts, and I should then have everything needed to get it cleaned up and running. I'll also re-use the engine mount that came with one of my Zenoah G-38 engines for now - it should work if I drill new holes for this engine.
 

Joker 53150

Mmmmmmm, balsa.
Mentor
#8
Moving right along...

The carb was in pretty ugly shape, with a lot of caked on grime. It did come apart easily, and it's very obvious people have taken this carb apart a number of times over the years. That's not surprising, as a carb rebuild is quick, easy, cheap, and the gaskets inside get hard quickly with age. Using a fuel with ethanol kills the gaskets even more quickly.

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I pulled the magneto to clean it up and fix the corroded wire. Well, fix it as much as I could, at least. Hopefully the repair lasts and the corrosion was down into the coil - I guess I'll find out eventually. So the old wire was stripped back to the coil, cleaned up, fluxed, and tinned so I could graft a new piece of silicone wire to it. I then went back and cleaned up the terminals with a wire brush on my Dremel tool and for insurance I added a little JB Weld around the base of the terminal and the (hopefully) fixed wire.

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With the coil re-installed I verified that it still has spark. The carb was done soaking so I re-assembled it and put it back on the engine to wait for the new carb gaskets and spacer.

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I had a prop bolt for a US41 engine, but it didn't fit - the thread pitch is wrong. A little inspection finds that the prop shaft has a heli-coil insert installed, so whoever did it changed away from the stock pitch. I ran a tap into the prop shaft to clean up the threads and confirmed it's an M8x1.25 pitch, so now I'll need to find the right size bolt. The one in the pic is close, but is a bit loose.

Beyond getting the right prop bolt, my only real concern on making this engine run is the carb. They don't last forever and this one has certainly seen better days. Fingers are crossed the rebuild and cleaning is all it needs.

I'm also going to modify a Zenoah engine mount for the different bolt pattern on this engine so I can fire it up soon. If all goes well I'll get it cleaned up a bit.
 

Joker 53150

Mmmmmmm, balsa.
Mentor
#9
New carb and exhaust gaskets arrived today, along with a new 3D printed carb spacer. After a little research it looks like the 18x8 and 18x10 props are favored, and since I had an 18x10 the choice was easy. ;) A new prop bolt and old spinner complete the package. Well, that plus re-drilling a motor mount meant for a Zenoah G-38, that is. The engine had the remains of a carb linkage system installed that re-aligned the pushrod angles more favorably, so I replaced those old pieces with a bellcrank I've had for probably 40 years, from back at my first attempt at building in balsa.

The next order of business for this old girl is mounting it to my test stand and seeing if she'll fire! Tuning the carb will be an adventure, as the high & low needles are facing the prop and don't give much room for error. :eek: If everything works as planned I'll run it for a while to heat it up, then work on cleaning it and setting it aside for a future project. If it doesn't run, maybe it'll become a nice display piece for my desk.

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Joker 53150

Mmmmmmm, balsa.
Mentor
#10
I got bored so I decided to mount the 41 on my test bench and try firing it up. Long story short, SUCCESS! It pulled gas to the carb just as well as my other gassers this size and after priming the carb I gave it a few flips with the chicken stick. Well, it only popped once... Then I decided to try my big 24v Sullivan starter which can turn it over FAR faster than I can by hand. A quick pulse and it fired and ran for a few seconds. This happened a few times as I richened up the low end needle. Getting to the needle was fun, since the needles are on the side of the carb facing the prop, so I had to adjust it while the engine wasn't running. A couple tweaks later and it was idling well enough to take a quick video. It still needs a lot of tuning, but that'll wait till the weekend when my buddy is available to help.

 

Joker 53150

Mmmmmmm, balsa.
Mentor
#11
An interesting (to me) follow-up. The engine has had a couple hours to cool completely, and rotating it by hand now I can feel it's got noticeably more compression than before (it wasn't bad before, now it's just better). I was running premium gasoline (no ethanol) with Klotz synthetic oil at 40:1. Hopefully running it a few more times and getting some good heat cycles through it frees it all up a bit more so it's ready to sit and wait for a future project. ;)
 

Joker 53150

Mmmmmmm, balsa.
Mentor
#12
Playing with all these gas engines is paying off for me. Later this week I’m having a dead tree in my yard dropped, but I have to cut it up and dispose of it to save money. I’ve got three chainsaws, but none have been used for the last 7 or 8 years, so the carbs are all needing rebuilds due to the gaskets/diaphragms all getting old and brittle. Well, old engines like this US 41 are basically chainsaw engines without the chainsaw specific parts, so I should be able to fix the issue and save a bunch of money as well.

Last night I pulled the carb off the first saw and found it’s a typical Walbro, and I had all new gaskets on hand to do the rebuild. 30 minutes later and it fired right up and is running well again!

The carb on the second saw is a Zama, so none of my Walbro gaskets will work. I was going to order a rebuild kit until I saw how cheap complete replacement carbs are ($20-30). I went that route instead to save time.

The third saw I’m not going to do yet. It’s got a Walbro carb, but it’s a pain to work on and I just don’t need three saws for this job.

The biggest difference on the chainsaw carb is the fuel return and primer bulb. Well, that and all the other stuff that makes up a saw... I’m glad I can do this myself, it is far less expensive and faster being self sufficient in this case.