The complete radio control model airplane FAQ.  


How do you convert a weed wacker or chainsaw engine into a model airplane engine? Are converted weedwacker engines any good? What are good airplanes for these string trimmer engines? (If you like this page, you might also like the links on building engines.)


Pros, Cons and Descriptions

Planes good for flying weedies

Converting the engine

    Poulan 46cc, 2.8ci NEW and updated, 2-02-02.

    McCulloch 32cc NEW

Installing the engine

Ignition Systems

Fuel Requirements

Conversion Mufflers

Conversion Props to use

Engine Hop ups

engine conversion links

How To pages

Sources for Engines

Hop Up information

Pre-converted engine suppliers and manufacturers

Additional Model Airplane engine suppliers



Pro/Cons and Descriptions

Turning string trimmer and chainsaw engines into model airplane engines has become quite popular lately. There is a sense of accomplishment that comes with completion, as well as the opportunity to save a few dollars. They all burn cheap fuel, and are very fuel efficient at that. You can often use as small as a 8-10oz tank for 15 minute flights.  Not only that, you might not even have to clean your plane off at the end of the day! Another benefit is the hassle-free factor--you go to the field with your plane, a glove, a tank of gas, and your transmitter. You don't need an electric starter for these engines (although it helps if you have one with a flywheel) and you don't need a glow plug or glow driver. 


Just keep in mind that the little utility engines are not going to be lightweight powerhouse engines. They often (but not always) have very poor power power/weight ratios--especially the cheaper brands, and the smaller models in particular. You'll have to go up to the 40cc engines, as a minimum, to get good power/weight ratios. If you compare some engines, you'll see that the 25cc Homelite trimmer that's been lightened and the Poulan 46cc chainsaw are within 2-3oz of each other! The Echo 44cc chainsaw weighs about the same, too, but these chainsaw engines have a LOT more power. On the other end of the spectrum, the 22.5cc Husqvarna trimmer engine weighs a full pound less than the three former engines. However, it's also a considerably weaker engine.


The little engines are good for light, floating planes. Cub-type planes are the best, especially for the smaller engines. Because they don't have a strong power/weight ratio, you will probably be disappointed in their performance if you try to put one  on an aerobatic plane. The 4*120 is a good intermediate plane, and flies very well with a 31cc Ryobi. 

I put a hopped up 25cc Homelite on a 67" Joss Stick, and the plane was not very fun to fly at all--it was way too heavy. There was plenty of power in the Homelite (APC 16x8 at over 8,000 rpm) and it would hover it at wide open throttle with a wide 16x6 prop, but at 10.6lbs, the Joss was just too heavy. After swapping out to an RCS 1.4 (APC 16x8 at 8500 with the small carb & stock muffler) the plane was about a pound lighter, and a lot more fun to fly. 

It is possible to put these littler engines on an aerobatic plane. Personally, I wouldn't put one in a plane that has fewer than about 1,000-1,100 sq". (The Joss Stick has 877.) The Stinger 1.20 has successfully been flown with the 30cc Homelite and 32cc McCulloch string trimmer engines. Those two engines weigh just about the same as the 25cc Homey, but they have more power. Just keep in mind that they will not have unlimited vertical. The BME 44, which is based on an Echo chainsaw engine, weighs a full pound less than the aforementioned engines (when they are run on flywheels), but the BME has probably twice the power or more. Unfortunately, the Echo engine also costs almost 6 times as much as a cheap Homelite! Also, my 25cc Homey has about the same power as a mediocre 1.20 glow fuel engine--and weighs nearly twice as much. I guess you get what you pay for, because it also cost about 1/2 as much. A middle of the road alternative is the 46cc Poulan, which is available for under $150 new but reconditioned. Further details are below.


Converting The Engine

    Poulan 46cc, 2.8ci

Converting just about any utility engine into a model airplane engine is easy. While some engine models are simple to convert, like the Ryobi, others require considerably more work, like the Homelite. The Ryobi is very popular to convert because there are basically two steps--remove everything that doesn't look like a model airplane engine, then add a prop hub and an engine mount. The Ryobi takes minimal cutting, which is limited to the plastic backplate. It's also a good idea to reduce the cooling fins on the flywheel to improve output, but care must be taken to balance it afterwards. Alternatively, you can use an electronic ignition, which eliminates the flywheel all together--you end up saving about 6-8oz by going this option. 

Ryobi string trimmers can frequently be found for $80 or less--brand new. They have the benefit of being slim, which works well for hiding it inside a cowl. The carb and muffler are both on the rear of the engine--which makes this engine considerably longer than the other brands, as well.

The Homelite, on the other hand, is popular for a different reason--it's in abundance, and can be found for about $50 new. They take a little more work than the Ryobi though. In addition to cutting the backplate and flywheel, you also need to remove a substantial amount of aluminum from the case--if you don't, the engine will weigh a ton! Take the time to do this, and you won't regret it. Remember, for every 3oz you leave on the engine, you also have to add one in the tail.

Other engines are simple to convert as well. The Stihl's and Husqvarna's require minimal cutting, and are pretty light too. I converted one Husky, and it was extra light even with the flywheel--unfortunately, it is on the lighter side of power, too. The 32cc McCulloch looks like a cross between the Homey and the Ryobi--side exhaust and carb like the Homelite, but with the quick converting bolt-together design of the Ryobi. The Mac has power that is comparable to the ST2300, so this engine is a little more lively than the other two.

A powerful alternative is the 46cc Poulan chainsaw, which is available for $190 new, or less if you get a reconditioned unit. This 2.8ci engine weighs very close to the same as the BME, but needs a little work to get it set up properly. The  carb mount needs to be bolted to the engine--it's just a friction fit rubber boot--and you'll also need a throttle linkage. I can't comment on the durability of this engine, as mine is brand new. However, initial tests show it to be a great alternative to the BME. Another drawback is the flywheel--you really need to start it with an electric starter, or else put a CH ignition on it. When I was hand starting mine, it backfired and broke the flywheel alignment key, which is merely aluminum die cast into the flywheel. Your best bet is the CH with the SyncroSpark--it starts much easier, and is rather smooth, too.

Installing the engine is just like any other engine. Some engines, like the Homelite 25cc string trimmer, can just be bolted to the firewall using bolts from inside the plane. Mounting is simplified with a mounting plate, although that is added weight and time. If you do make one, put the largest lightening hole that you can in the center of the plate--these things don't need added weight. Other engines, like the Poulan 2.8 chainsaw and some Stihl weedies, can be beam mounted like a standard glow engine. Engines like the Ryobi 28 & 31 are reed valve engines, and have the carb at the rear of the crankcase. These will need either a hole in the firewall to accomodate the carb, or else some standoffs mounting the engine.

To choke the engines, there are several different options. The most convenient is if there is a choke built into (usually the larger ones) added to the carb (like the Ryobi). If you have a Ryobi in a cowl, just add a control rod running from the choke to the outside of the cowl. You can mount the Ryobi either with an extended mount, like the suppliers sell, or you can bolt it right to the firewall, with the carb in the engine box. Just make sure you have plenty of breathing room to get air and allow any excess fuel to drip out. Most other engines have the carb on the side, so access is even easier.

You do have to take the ignition noise into consideration. If you use electronic ignition, keep the module, battery and switch as far forward as your CG will allow, and all other radio equipment as far back as possible. Make sure to use a resistor plug--they usually have an "R" in the name. 

Also, gas compatible fuel line must be used. Typically Tygon is common, but your lawn mower shop will have some good options too. Run your vent line to the bottom of the fuse. You don't want muffler pressure with these engines--the carbs have built in fuel pumps. For filling, you have several options. You can "T" off of the clunk line going to the carb, or you can add a third line to the tank. The third line should have the brass tube bent down so it reaches the front corner of the tank. The only disadvantage to that is the need to tip the plane forward to get the last drop of fuel out of a taildragger.

Ignition systems for small gas engines vary from brand to brand. The Ryobi, Homey and Mac's use a heavier flywheel and coil, so the ready to fly weight of those engines' ignitions are close to 18-19oz. When you are looking at saving nearly 1/2lb, and another half is moved from the nose to behind the firewall, that can be a significant boost in moving your CG around. The little Husky that I converted, on the other hand, had a flywheel and coil that was very light, at about 13.6oz ready to go with prop hub. When you consider that the CH ignition is about 10.5oz with the prop hub, ignition and battery, it may not pay off to spend the $120 to upgrade for only a 3oz savings, but the easier starting and smoother idle may make it worthwhile. 

Electronic ignitions have other advantages, too, besides just weight savings. For starters, they are the simplest installation, because you don't have to modify the flywheel. Also, they will fire the plug at the appropriate time, regardless of engine speed, which makes starting the engine a LOT easier. In addition, the electronic ignitions can have the timing adjusted a lot easier--usually you just have to loosen one screw to make the adjustment. Not only that, but if you spend an extra $40 or so, you can get one with variable timing, which automatically advances the ignition timing. The CH Ignition version of this module is called the SyncroSpark. This makes hand starting even easier, because you don't have to worry about the engine backfiring on you. Even better, though, is that the bottom end of the engine rpm band will smooth out considerably, and it will idle slower. If your budget allows for the upgrade, I highly recommend the CH Ignitions--especially with the SyncroSpark, which have always given me good performance and value. There are other ignition manufacturer available, but this is the one that I'm most familiar with.

The flywheel and coil ignitions are very simple, and lower maintenance. They weigh more, which hurts the wing loading and also the wind-up rate of the engine. The flywheel ignitions require the magnet pass by the coil at a high speed before it will spark, which means you have to flip that prop FAST, or use an electric starter. Another big drawback to the flywheel is the fact that you usually have to cut the fins off of it before you use it. If you don't, then you'll be blowing unproductive air around, which will reduce your power...and remember, these engines need all the help they can get. Just make sure to balance the flywheel afterwards. If you are uncertain about how to modify it safely, then don't do it--I won't be held responsible for your mistakes. By all means, make certain only a qualified individual cuts on your flywheel. There are several links below that list sources for modifying your flywheel, or you can find a machinist in your neighborhood. Just don't call me up if something goes wrong. If you are in doubt, then go with a CH for maximum safety.

In a nutshell, here are the pros and cons of each system:
  Flywheel/magneto CH Ignition CH w/SyncroSpark
Cost low, ~$45 for hub & flywheel machining, through Carr Precision $120 complete $160
Effort Med.- or send FW out Low Low
Weight High, some are up to about 19oz, all out at the prop Lower, about 10oz, most of it behind the firewall Lower, about 10oz, most of it behind the firewall
Timing Fixed, usually 27-30 degrees Adjustable but fixed Adjustable and Variable
Starting Very Difficult to hand start, especially larger models--about 500rpm! Not bad, just needs a good firm flip Very Easy, needs practically just enough to get it past compression
Idle Ok Ok Great, smooth & Low
Maintenance Low Battery Pack to Charge  Battery Pack to Charge
Service Your local lawn mower guy CH has excellent service CH has excellent service

The Fuel requirement for conversion engines is very basic. You basically run the same fuel that is run in a weedwacker or chainsaw. Because they have low compression, your engine will probably run better with low octane fuel, in the 87-89 octane range. You can probably get away with higher octane fuel, but you won't see any benefits from it. I like Penzoil 2-stroke 50:1 for breaking in an engine, or Amsoil 50:1 for a well run engine. Lawnboy is also a popular brand. Just about any ashless 2-stroke oil will do. Do NOT run more oil than is necessary, and pay attention to the instructions that are on the oil jug. Ol' man Brison, before he retired, told me he could always tell which guys ran synthetic oil too rich, because they always had too much carbon build up. You can actually damage to your engine running the oil at a mixture rate that is richer than recommended. With enough carbon buildup, the ring will stick, causing blow-by. Also, if it's thick enough, it can fill in the clearance between your piston and cylinder head.

Mufflers have a large effect on small gas engine performance. You can build the Ultra Cheap Muffler, or see the links to read about Mousse Can Pipes. I bought a small hex unit from Dave Carr (see links below) and compared the performance to a flat box from Paragon Aero. The original outlet tubes on the Carr muffler were tiny, at 1/4" ID, (Dave has since changed this) whereas the Paragon outlet tubes were 1/2" ID--a HUGE difference. (Remember, flow has to do with cross-sectional area, so the 1/2 tube is 4 times the size.) The engine with the Carr muffler was a lot quieter, but I lost about 1,000 rpm! 

The Paragon muffler with the two 1/2" ID tubes had the best performance by far, but it was also much louder. I was curious about where the performance would be with the outlets somewhere in between, so I took a screwdriver handle and plugged one outlet on the Paragon muffler. There was no drop in RPM, but the sound level was dropped significantly. After that test, I made some restrictor inserts with 7/16" ID's and bolted them into the outlet tubes with a pair of 4-40 bolts.  True to the screwdriver test, the engine didn't lose any power, and it was significantly quieter. One benefit that I didn't anticipate, though, was the improvement in the midrange! With the larger outlet tubes, the engine had a nasty jump in the midrange, and it smoothed out considerably with the muffler modification.

The best commercial muffler I've found for these engines is the Bisson Custom Muffler (BCM) model. It's the largest, lightest, quietest, and strongest performing. It's also one of the most expensive, at about $40-45, but still cheaper than the Carr muffler. The latter has its advantages too--being hex shaped, you have a choice of three output configurations. The Jag muffler is round, and will rotate output directions, but I have never run one. The Paragon muffler isn't a bad unit for it's price, and probably my second choice because it's heavier than the BCM, and louder. I try to stay away from the cast aluminum mufflers, like the Jtec and B&B units, because they are heavier yet, and not as good looking. 

Propellers for small gas engines converted from weedwackers can vary in size. With my 25cc Homelite, I started with a 15x8 and 15x10 Zinger, and as it broke in I switched to a 16x8 APC. With a couple gallons through mine, I picked up well over 500rpm. The first time I took this engine to the field I had a 16x8 on it, and it was taching in the 7k range. I made a comment about switching to a 15" prop, and the local gas "guru" swore up and down that I had to keep the rpm to the 7k range. Oddly enough, when I returned with the next week with a 15x8, that guru commented on how the engine "woke up!" His song changed when I told him I dropped the prop size though. (Now it turns that same rpm with the larger prop.) Anyway, don't pay attention to those guys who swear you have to stay in the 7k range. These little gassers are similar in construction to the 1.20 and 1.40 glow engines, and can easily run in the 8k range, although I personally would keep it well under 9k, because I like thrust and not speed. From what I've been told, the Zenoah G23--which is just a converted weedwacker--was designed to develop peak power at about 10k. Boat racers often run their engines well above 15k...but not for long!

Anyway, prop selection depends on the size of the engine, and partly the condition of it. Although my Homey was broken in when I received it, I had disassembled the jug and cut on the ports, so the ring needed breaking in again. For a typical 25cc gasser, I'd recommend sticking with the 16" props, if you have the ground clearance. You could start with a 15" like I did, but it's not really worth the money.  Note that the 15x10 and 16x8 put about the same load on the engine, so you can choose the former if you like speed or need clearance, or the latter if you like better thrust. For the 30cc engines, you can easily swing an 16x8, 18x6, or even 18x8 or a 19x8, depending on the brand of prop and the specific engine. The Ryobi's typically run 18x8's at 7k. MA's and Zingers are popular, mostly because they are cheap, I think, but you'll get better performance out of a Menz or just about any other prop. For the larger engines like the Poulan 2.8, I like the Menz 20x8. That prop has a good combination of thrust and downline breaking, with plenty of top end speed.

APC props are very good for the smaller gassers. Because they have a poor power/weight ratio, they need every little bit of help. The cheap plastic Master Airscrew 16x8 props typically run a full 1,000 rpm slower than the APC's. You won't get nearly the performance out of the MA's, and they are only a couple dollars cheaper too. They suffer in both top end speed as well as thrust--the only benefits that the MA's have is the downline breaking. Zingers work too, but here again, not as well as the APC's. I haven't tried the TopFlite or Menz props with the smaller engines, but I fly the Menz 20x8 on my BME 44 powered Midwest Extra. I like the wide blade of the Menz because it gives good vertical performance, and good downline breaking, but I think the smaller engines benefit from the higher revving, thin APC prop. The MA Scimitar series may be another option--I know Dick Hanson runs them on his ZDZ 40, and this will help remove an ounce or two from the nose. (Those plastic props are heavy!) You may have to try several different props to get the performance you like. I prefer thrust and downline breaking to speed, but the 16x6 that I flew on my Homey didn't have enough speed, so you do need to balance the prop style to the engine. rcfaq

Engine Hop Ups

If you don't know how to do it, there are instructions for measuring the timing of your engine available. You should check your timing before cutting on the engine, because you may end up with scrap metal if you don't know where you are headed. Opening up the ports on an engine that already has radical timing can reduce your torque considerably.


Besides altering the port timing, you can boost the compression ratio. Other things to do are to stuff the crankcase--if there's a plastic bolt-on backplate that has a cup going into the case, make sure that it goes up almost to the crankshaft. You can mill the case so the backplate goes in further, or just bolt on a spacer to take up space. This will increase the pressure inside the crankcase, improving flow into the cylinder. Some engines also have casting things in the way, so you can grind them smoother so the air path is more streamlined. I've also notched the backplate, and filled it in with JB Weld so the notch didn't go through. That made flow to that one port better.


For the McCulloch 32cc, Terry Grant of CH Ignitions upgraded to a Walbro WT-9 carb, and added a free flowing CH muffler. For the 25cc Homelite, he sent the following: I removed .025" from the base of the cylinder, this raises the compression. This also lowers the exhaust port timing, so I cut a bevel on the piston going in about .125" and down about.035"  Exhaust port timing is 155-162 degrees. Milled the back of the crankcase .100" to stuff it. Flowed the cylinder intake ports down into the crankcase. Put a big blob of JB Weld epoxy on the rear cover so you can port the rear intake better into the crankcase. The rear cover is to thin to cut any ports into it so that is what the JB weld is for. I actually cut through the rear cover and into the JB. A bigger carb Walbro WT-9 (almost 4mm larger venturi), A very free flow "muffler", with 7/16" ID outlet pipes. [Note, this is the same size that I found works best with my Homey, too.] Results...My 25cc mule engine will turn a 16x8 APC at 8900-9000. but it is well broken in with 2 years of flying.

We'll be adding more as time allows. rcfaq

Weedwacker and String Trimmer Conversion LINKS

How To pages

Sources for Engines

Hop Up information

Pre-converted engine suppliers and manufacturers

Additional Model Airplane engine suppliers


For Engine Tuning, Ignition, Prop and Muffler sources, also see our Engine FAQ page!


How To pages & sources:

CH Ignitions--these have worked great for me!

How to measure the port timing of your engine

Carr Precision, Experimental and Model Aircraft


JAG Engines Home Page

Marty Hammersmith's Home Page

Convert Poulan Chainsaw to Model Airplane Engine

Convert Stihl Line Trimmer to Model Airplane Engine

Convert Ryobi Line Trimmer to Model Airplane Engine

Taurus Engines--radial mounts, prop hubs, vel. stacks, & services

Teaching a Weed Wacker to Fly

Son of Weed Wacker, with Hop-up tips

Small-Pogo Project

Weed Wacker


Sources for trimmer and chainsaw engines to convert, and misc. parts

Air Hobbies Accessories--Vel. stacks, smoke systems, fuel valves, exhaust tubing

Harbor Freight Tools - Home Page

Poulan engines at LawnStation.com

Engine Rings, custom made, about $4

Engine Rings, custom made, source 2:

For all of us modelers who need piston rings and cannot find them, try contacting Frank Bowman. He manufactures repro and current piston rings for our modeling needs. He can make standard and Dykes type rings. If he doesn't have the ring you need in stock, you send the piston and cylinder and he will make it then. prices are $7.50 up to $9.50. His work is of the finest quality. 

Happy landings, Zach 

Frank can be reached at fbowman@acs-online.net or 1-505-327-0696 ( 6pm to 9pm mst. weekdays) and the following address:
Frank Bowman
1211 N. Allen
Farmington, NM. 87401

Hop up information:

How to measure the port timing of your engine

CH Ignitions

Homelite Hopup spec's

Radio Control Boating - Homelite Hop-ups

Radio Control Boating Links

Troy's Boat Websites!

Marine Specialties - RC model boating products.

Warehouse Hobbies - Radio Controlled Model Boats

Homelite Engine Work
Degree Wheel



Pre-converted engine suppliers (Yes, believe it or not, most of the popular engines are converted string trimmers or chainsaws.)

Homelite conversions:

Carr Precision, Experimental and Model Aircraft

GCBM R-C Models

Taurus Engines

A J Engineering


Ryobi conversions:

Paragon Aero

JAG Engines Home Page

Carr Precision, Experimental and Model Aircraft


Sachs conversions:

Brison Aircraft

Air Hobbies

D&B Engines by Ridge Machine

Fox Manufacturing

FP Engines

J&A Engines

Taurus Engines

Precision Eagle engines


Misc. Engines: (Echo, McCulloch, Weedeater, Honda, Husqvarna etc.)

BME Baker Model Engines--Echo

Cheetah Engines, by Reid--an improved US Engine

M& R Gas Engines - Echo

Precision Eagle engines-- discontinued?


Quadra Aerrow. Engines

Taurus Engines--Husky

US Engines Home Page

US Engines new page


Additional model airplane engines (Just so they don't feel left out!):

Cactus Aviation

Desert Aircraft -- Contacting DA

Desert Sky Model Aviation (raptor)

Erickson Motors - Home

FP Engines

Jett Engineering

Just Engines (Aeromodelling Engines Spares Accessories) &amp; Aeromodelisme &amp; Aeromodeling

K&B model airplane engines

Laser Engines

Leo engines


Magnum engines

Model Engine Company Of America - MECOA HP RJL 

O.S. Engines

OS's New EFI Engines

R-C Airplane Engine Central

RC Showcase

RCV Engines

Saito Engines at Horizon

Super Tigre

Tartan Engines

Thunder Tiger Engines

Webra Motor

YS Performance Specialties

ZDZ 40 RV-L from RC Showcase

ZDZ ... TOPMODEL - ZDZ engines


Revised: February 02, 2002 .

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