Hi. I'm about to break-in a .40 engine. I know I need to run it rich and gradually lean it out, but at what speed(s) do I run the engine at during these break-in runs? Also, I know the actual differences between the two types of engines, but what it is meant by running an engine "2-stroke" or "4-stroke"?

Thanks in advance.

Speed= Full Throttle

A specificly desinged four stroke engine (as a begginer you wont need to worry those yet) will ONLY four stroke. i.e. firing ever other compression stroke. They make a "purring" sound as they run and are distinguishable by the rods up front.

A two stroke engine (like yours) is desinged to fire every compression stroke. HOWEVER---- They can "four stroke" when run at very rich settings. HOWEVER---- Modern (abc) two stroke engines (probably like yours) SHOULD NOT BE PURPOSLY RUN FOUR STROKING RICH. IT CAUSES LOTS AND LOTS OF PREMATURE WEAR ON THE PISTON AND CYLINDER!!! When braking your engine in, ajust the needle valve so it makes a nice whine while running at full throttle.

A 2 stroke engine is said to be 4 stroking when the mixture is richened enough that it begins to fire only every second cycle. It literally doesn't fire on one downstroke but fires on the next somewhat like a true 4 stroke engine. The exhaust note changes from the usual high pitched scream to a growl. With careful adjusment of the needle most engines can sit right on the transition point and will continuously switch between the two modes.

This transition point is where the running in mixture should be based. If an engine has rings then the transition point is the leanest it should be set, in other words it must be 4 stroking all the time. For an ABx (which yours is almost certain to be) then the correct setting is just barely leaner than the transition point so it's always 2 stroking although I like to hear the odd misfire. It's adviseable to use a small prop for running in to keep the revs up when running rich and to take some of the load off the con rod (which needs to be run in as well as the piston/liner). A 9x6 would be a good size for a 40 engine.

Always use a fuel that's got at least 20% oil and half of that should be castor for running in. Nitro content doesn't matter because it has little affect on the running temp of the engine. There are a number of ways used to finally run them in (especially ABx) but in general aim for about 40 minutes total while very slowly leaning it out but NEVER going to peak revs.

All of this is basically the way both Jett and Rossi say to run in their engines and I figure they know what they're talking about

Read the instructions with the engine,and or use qualified help at the flying field if possible.

MasterSmasher, (or anyone else), I was just going to ask a similar question, specifically about an OS 46FX. I was breaking it in yesterday, opened the needle valve 2 turns to start, I was leaning it out until I got a nice clean whine, and occasionally backing up a bit, sort of a 'two steps forward, one step back" approach. I ran it the last few minutes at a real clean whine, with white smoke still coming out the muffler, but when I got done, I realized I now only had the needle valve open 1/2 turn!!
Is it likely I screwed something up? It sure sounded nice, and there was no sign anything was wrong.....
Also, what to do next? Would it help to rerun it richer, or is it too late, just continue following OS's directions re. the first few flights?

Thanks!!

Hi uonlylivetwice,follow the directions,the turns of the needle valve is a reference or starting point,the main thing is to never run the engine lean. With the engine running wide open pinch the fuel line,the engine should speed up 300 to 500 rpm,if no tact you'll still hear the sound,if engine speeds up more it is to rich, if engine dies the engine is to lean,if engine does nothing that is the max safe speed,richen the engine for insurance,these settings are for an engine that is broken in,until then run the engine a tad rich,and use the instructions.

1/2 a turn does sound weird, but it is possibal. Make sure there is plenty of WHITE smoke comming out.

While my advice may not be popular, I have had success after following it. You need to run your engine lean enough to get it to two stroke but rich enough to keep the cylinder temperature down. To reduce the load on the engine, run with the smallest propeller recommended for the engine. What you want is to let the engine heat up so everything seals good and begins to wear but not too hot to distort the metal.

Thanks, everyone. I do have other question: Do I let the engine completely cool off between break-in runs?

Hi WDWings,no need to let the engine completely cool down between runs,but it will not hurt anything if you do.

The directions for my Magnum .46 XLS say 10 min between run times WDWings.

OS,Norvel,MDS,TSI,Super Tigre,RCS,Zenoah doesn't ask for any cool down time,or my spec sheets are different from the rest of the worlds.

Depends on what you mean about cool down. The time it takes to refuel the engine and have a short conversation is enough cool down time.

I can't see any reason to let most engines have a cool down. The exceptions are the ones with a cast iron piston because the cast iron needs heat cycling (I believe some of the Enyas still use this system and it's actually extremely good).

Let it cool down till you can grab it in your hand and hold on. Then restart the engine.