I recently purchased a trainer type plane with a converted engine at a swap meet. Upon taking my new pride and joy home I immediately went to fire up the engine. It sputtered and sputtered than it came life. Needless to say I was pleased! The engine however did not transition smoothly (it probably had not been fired up recently either!) but I thought I would run a little fuel thru it when BANG!!!!!! the magneto came off the engine housing and smacked into the prop. It did not look like a repair was possible because the mag was attached to the actual engine housing itself not bolted......
Just by chance I had to go to the local small engine repair shop to pick up my ailing weedwacker so I thought I would take along the engine for a "second opinion". The owner of the shop was amazed that weedwacker type engines were flying about the skies.... he immediately told me that my engine could not be repaired BUT he knew that this engine was a "Featherlight" engine and he had one in the junk pile on a weedwacker that had a problem with the long shaft..... but the engine was fine. Without encouragement he began to take apart the weedwacker, "wacking" off useless weight, and installing the machined stuff off my broken engine onto this "new" engine. Long story short, within 30 minutes I had an exact replacement.... I told him that I had questions about the Walbro carb with respect to high/low settings. This Walbro only has one screw adjustment on the top.... he thought that it was for the high setting.

Upon returning home I installed this "new" engine on the plane (EXACT fit..... all the mounting bolts lined up just fine!) I took the plane outside and it fired up on the first hit of the starter! I was back in business! Now here is my question...... The RPM's of this engine are 3200 low and 5990 high, swinging a Masterscrew 15x9 prop. I think the low is not low enough, and the high is not high enough. Turning the lone adjustment screw smoothed out the transition a little, but not much. There are not numbers on this Walbro carb that I can see.... I would send along a picture of the carb but I cannot find my docking station for my camera to get pictures into the computer.
Any suggestions??
Thanks!

Go back to your weed whacker repairman and get a carb Walbro "20" from a Homelite and put on your engine. Or a Zama from a Homelite if it has two needles
I'm assuming your engine is a "Weed Eater 21"
You want to turn around 7000 on the top end and 2500 on the low end

When I did my Ryobi and Homelite conversions I completed a long search of RCUniverse (and beyond) and settled on a Walbro WA-167. It will easily get a Ryobi or Homelite to 7000+ RPM provided you don't go crazy with oversized props (which some people are convinced MUST be fitted to every gasser).

OK, this is an example of underpropping but a Ryobi 31cc with a simple conversion (no porting etc) turns 8900RPM with a 16x8 MAS. Ideally, you want to keep under 8000RPM with a Ryobi or risk rod failures. An 18x6 or 18x8 prop should get you into a reliable rev range using a WA-167.

I only bench ran with the 16x8 as that was all I had available.....wouldn't recommend it for any extended running or flight.

I wouldn't go back to him on a bet, if a service man doesn't know what screw does what, hes not qualified to work on it.

I have several Walbro WA167's and a WT499 and they are the greatest for the 25-30cc engines

[quote]ORIGINAL: cattail

I recently purchased a trainer type plane with a converted engine at a swap meet. Upon taking my new pride and joy home I immediately went to fire up the engine. It sputtered and sputtered than it came life. Needless to say I was pleased! The engine however did not transition smoothly (it probably had not been fired up recently either!) but I thought I would run a little fuel thru it when BANG!!!!!! the magneto came off the engine housing and smacked into the prop. It did not look like a repair was possible because the mag was attached to the actual engine housing itself not bolted......
Just by chance I had to go to the local small engine repair shop to pick up my ailing weedwacker so I thought I would take along the engine for a ''second opinion''. The owner of the shop was amazed that weedwacker type engines were flying about the skies.... he immediately told me that my engine could not be repaired BUT he knew that this engine was a ''Featherlight'' engine and he had one in the junk pile on a weedwacker that had a problem with the long shaft..... but the engine was fine. Without encouragement he began to take apart the weedwacker, ''wacking'' off useless weight, and installing the machined stuff off my broken engine onto this ''new'' engine. Long story short, within 30 minutes I had an exact replacement.... I told him that I had questions about the Walbro carb with respect to high/low settings. Th

Upon returning home I installed this ''new'' engine on the plane (EXACT fit..... all the mounting bolts lined up just fine!) I took the plane outside and it fired up on the first hit of the starter! I was back in business! Now here is my question...... The RPM's of this engine are 3200 low and 5990 high, swinging a Masterscrew 15x9 prop. I think the low is not low enough, and the high is not high enough. Turning the lone adjustment screw smoothed out the transition a little, but not much. There are not numbers on this Walbro carb that I can see.... I would send along a picture of the carb but I cannot find my docking station for my camera to get pictures into the computer.
Any suggestions??
Thanks!
[/quot

Sorry, missed post, finger slipped, wasn't done yet.

The reason for the narrow rpm range is because the single needle carb is supposed to be better for the manufacturers.

No excessively high rpms so no excessive emissions,and you really don't need a low idle with a weedwacker.

The single screw blends the low and high speed fuel circuits together for a compromise.

The cure is the two needle carb.

Definitely get a two needle carb, specifically one with full adjustment ranges.

For example, the Zama carbs that come on many Homelites appear to have two needles covered by a plastic limiter. Even with the adjustment limiter cut off you'll find only one needle is really adjustable.

To get a full range of RPM you need a two needle carb with approximately 11+mm of venturi. A WA167 does this at 11.11mm.

A member in my club has put a very large Walbro on his Ryobi and it runs better than any other conversion I've seen. Very reliable setup with well over a 100 recorded flights. I intend to identify the carb model for myself and, so I can share it with others here.

How and WHEREcan we review and find which Walbro carbs have what sizes and venturi's? THe Walbro site is impossible to find specs on. Ihave the option to buy a cheap WT-532 but can't find specs or venturi sizes?

Someone on here posted a link to a walbro parts diagram. It had all the info. I will post the link once im done work. Unless someone else beats me to it.
One way to tell what size carb, is to look in the throat, and there should be a number stamped in there. It will be the venturi size in 64ths of an inch. Take that number and convert to mm's.
Im actually really surprised that the repair guy didnt know what the needle was on that carb. All the single needle carbs ive seen, had a preset low needle, and the adjustable one was the high speed needle. It should be stamped with an "H" near it.

Here you go.

http://wem.walbro.com/walbro/family2.asp

Adrian.

that one has a 13.49mm venturi

Wow...I think with all the low cost China made engines...it is cutting into this conversion fever!!! My as well pay a few $$$ more to get a running engine ready to go! [:-] Capt,n