It seems to me a lot of the newer design planes are designed for heavy 4 stroke engines, because the ones I have are very tail heavy with lighter 2 stroke engines. I prefer the low cost and simplicity of the 2 strokes, but I have had to extend the engine mount 1-2 inches and move servos forward to get CG even close. Is there really that much difference in weight between the two? Or is it just the particular planes I have? Any other suggestions besides adding nose weight? No - I ain't buying no 4 strokes!

4 strokes are heavier than 2 strokes.. more parts!
Build in lots of tail lightness..

I agree 100%. Have run into the same problem frequently - kit manufacturers place the firewall away back to allow for the longer four stroke engine. The two-stroke majority are thus forced to find some way to accommodate a bad engine fit.

What the two-stroke owner frequently does is to use a long mount, which is naturally more flexible, and that may go into mechanical resonance with a high RPM two stroke. Flexibility in an engine mount is a two-edge sword. A huge amount of flex is good - as in soft mounts. A rigid mount transmits virtually all the engine vibration to the airframe, but this normally does not cause a lot or trouble.

A mount with just the wrong amount of flex is very bad - it can go into resonance at peak RPM, transmitting several times as much force to the airframe as a rigid mount would, and this is frequently not detected during ground running checks, because the engine unloads to 1,000 or more RPM in the air. An engine and mount combination that goes into resonance in the air shakes everything brutally. I managed to diagnose this problem by installing a prop with one inch smaller diameter than was being used for flight, whereupon the engine vibrated extremely violently at full RPM on the ground. The engine was reaching roughly the same RPM on the ground as it would have attained with the normal size prop in the air. When using plastic mounts, I think that it is best to place the engine well back on the mount. If the engine will be too far to the rear, move the firewall forward, or shorten the nose ahead of the firewall.

Four stroke engines are fine if you just love their sound; otherwise they are best avoided.

I never thought of the possible resonance problem. I have resorted to putting construction grade 2 X 4 behind the engine mount on my built up fuselage type planes to get the engine far enough forward. Not pretty, but has worked. I worry that the longer moment arm of the engine being so far forward might twist off the firewall. Thanks for your response. Now I know I'm not plane crazy. I will take this into consideration before I buy any new planes.

four strokes are chosen for aerobatics and 3D for their ability to swing big props. The theory is that in the downward segment of a loop the (feathering) prop can help slow down the aircraft - thus making the speed throughout the loop more even. which should help the pilot score points. Big props are also more efficient than small ones.

Pumped YS four strokes are chosen because they have a very high power to weight ratio (they are supercharged and love 30% nitro) and can operate at any attitude.

Last time I checked, a .91 4-stroke weighs less than a .92 2-stroke. I have a couple of examples of each. Here are the weights, with muffler:
Magnum .91FS: 22.5 oz
Magnum .70FS: 21.5
Magnum .91XLS: 26.5
OS .61/.91FX: 26.5
Saito .91 19.8
Saito .72 18.2
Every .46 size engine I have is in the 16.7-17.5 oz range. I find a Saito .72 or .91 is the perfect nose weight to add to a tail heavy 3D plane. For totally light weight, simplicity and on a hot twin, you can't beat a 2-stroke. For torque, lugging power and that sweet sound I like on some planes, you can't beat a 4-stroke.

" Last time I checked, a .91 4-stroke weighs less than a .92 2-stroke"
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"I find a Saito .72 or .91 is the perfect nose weight to add to a tail heavy 3D plane."
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So to get around the tail heavy problem, you use the lighter motor... ?