Brian Lynch

Interesting! @Webhead Joe may be able to give some info on those carbs. I've never had one. Didn't have any knowledge about that tool either until now.
Typically you won't need a choke if there's a primer button or bulb but going by the the decal, one would think the "trigger" is a throttle but...  Guess the trigger was the kill switch LoL
Yeah, they scream at 6300 rpm and without any load it will stumble a little.
Too bad you don't have the rest of it.

That’s really cool!  I have a similar model with the HU carb. I was able to find a carb kit on eBay.  I haven’t tried to run the engine yet. 

The Groomer string trimmers (weed wacker) were made in the late 1970's by Advanced Engine Products Incorporated (formerly known as O&R), this was towards the end of AEP (O&R) as the company was dissolved in 1978. Hopefully someone will find a complete trimmer one day.
 
As you've noticed the 13B427 is a constant speed engine using the Tillotson HU38A carb, this is pre-calibrated for constant speed operation only and has no fuel/speed adjustments. The added primer button is for easier starting and the extra tube to the crankcase is to pulse the fuel pump.
 
I don't recommend looking at a standard parts diagram for the Tillotson carbs as they show all the bits not used on the HU38A, the diagram below is from the book "Small Air-Cooled Engines" 16th edition from Intertec Publishing, which has a small section for O&R/AEP engines, I don't want to add the rest as the current version of this book is still available and I don't want copyright problems.

 
The air filter would have been on the end of the tubing that made up the pole & handle for these trimmers, the engine stop/kill switch would have been on this handle too. Some models had another tube with handle used as the gas tank and others had a gas tank attached to the starter housing.
 
The trigger mentioned on the decal is indeed for releasing more cutting line on the trimmer head, they called this a "Touch-n-go" trigger. Other models had a "Bounce-N-Feed" trimmer head, this version released more cutting line when lightly bumped on the ground.
 
David

I found one and got all excited but the owner wasn’t interested in selling it.

Did some looking around in my shop and located the other O&R engine I have. This came off a Groomer trimmer. See the pics. It runs wide open all the time. It was built with a Tillotson HU that has no throttle shaft. Never had one. No choke either, just the primer button. The only thing to impede air flow into the engine is a screw that holds the base and reed plate in place. Installed a new diaphragms in the carb. I had this hooked up to a remote tank to run it. It sounds like an irritated bumblebee!

Tried using Simple Green. It removed some debris but not all. Then on to vinegar. Nope, no results. Next was 2 psi of air pressure. Nope. Figured it was a lost cause. It got the drill. Back together and it's running. Started quite easily, maybe 6 pulls after pushing the primer button. Thanks to all for the help on this. My friend's eyes lit up when he saw it running. On to the next project...I think I've got a mid 60s vintage O&R around here somewhere...

Tried using Simple Green. It removed some debris but not all. Then on to vinegar. Nope, no results. Next was 2 psi of air pressure. Nope. Figured it was a lost cause. It got the drill. Back together and it's running. Started quite easily, maybe 6 pulls after pushing the primer button. Thanks to all for the help on this. My friend's eyes lit up when he saw it running. On to the next project...I think I've got a mid 60s vintage O&R around here somewhere...

Tried using Simple Green. It removed some debris but not all. Then on to vinegar. Nope, no results. Next was 2 psi of air pressure. Nope. Figured it was a lost cause. It got the drill. Back together and it's running. Started quite easily, maybe 6 pulls after pushing the primer button. Thanks to all for the help on this. My friend's eyes lit up when he saw it running. On to the next project...I think I've got a mid 60s vintage O&R around here somewhere...

It’s worth a try. I was surprised just how solid that old oil/gas can get. If you remove all the inside parts from the carb, especially gaskets and the little plunger with the rubber tip then you should be able to soak it in carb cleaner. I think I ended up using a needle or small paper clip to get in there and dig out all the gunk.

Someone more knowledgeable than me will likely respond but I think I had one of those once and it was just completely clogged with dried fuel/oil mix. It took a lot of poking with small tools and frequent spraying with B12 carb cleaner.  I don’t think the one I had had any screen or anything inside it but I’m not certain.