I'm hoping someone may have some thoughts for me on an engine issue I have....which in fact may not even be an issue with the engine...read on...

I have an Enya 1.20 4-stroke (not the '1.20R') in a Top Flite P47 and I have just gone completely through the engine and got everything working nice and smooth, even replaced 2 ailing bearings up front. The engine is hard mounted in the model and runs extremely well while holding it still on the ground. Peaks nice and has a lot of pull. I'm running a 15x6 APC Prop with Omega 4-stroke fuel and everything seems great. I have fuel running through a Perry Oscillating pump (as the tank is slightly above the carb when the plane is right way up) and an onboard glow for reliable idle. Like I said everything seems great on the ground, however the problem is when I start my take off run the engine doesn't seem to come back to that 'peak' and doesn't have that same authority. I'm not sure if it is the pump arrangement as I would think that running it up while holding it still would also result in issues if this were the case. I should also add that I have plenty of inlet and exit areas for airflow over the engine.

OK, now the part that isn't related to the engine...I think the plane is having physical difficulty coming up to speed that isn't engine related. When I first flew the airplane with the ailing motor it seemed tailheavy. I rebalanced and added some extra nose weight. It still seemed to require more down elevator and as it turns out it may be a slight incidence problem as adding further nose weight didn't change much. Slight down elevator is req'd to maintain straight and level flight. Now, adding to this I'm using Robart scale tires without any foam inserts and apart from not running totally smooth on the axels they seem very soft. What I picture happening is that as the plane starts to roll on our grass strip the tail wants to lift very quickly like the brakes are on due to (a) the wheels not being totall free running on the axels, (b) the soft wheels wanting to compress into the ground, (c) the added nose weight, and (d) the slight down elevator when the servo is at neutral to create trimmed straight and level flight.

So, do you think that this mechanical issue that the plane has with rollout on take off is not letting the engine really pull the plane? Like it's trying to overcome this added drag? Or do I have 2 totally separate issues here?

Sorry to be so long winded, but I wanted to cover as many bases as I could with the current configuration. Thanks for any constructive thoughts!!

Put some inserts in the wheels. You can get some similar foam at the local hardware store. Just make sure it's 3/4" diameter. Check the toe in or toe out of your wheels. To much ether way can cause a lot of drag on takeoff.
If you need down trim to fly you may have an engine thrust problem or an incidence problem. They can amount to the same thing. Check for to much up or down thrust. I believe it's supposed to be "0" degrees engine thrust on the .60 sized TF Thunderbolt. If the thrust line is correct the wing or tail is off. The wing can maybe shimmed a degree or two, but the tail. OOOps !!! You will need some incident meters for this. If you don't have any, see if you can borrow some. Two would be best.
Let us know what you find.
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I had the same engine problem with an old Satio 1.20 fs with a Perry Oscilating pump. It turned out that the pump, while the engine idled, the low end was set to rich. I leaned the low end and the trouble went away.

I agree that the trim problem sounds like a thrust or wing and not a balance problem.

Dru.

The 120 enya I used to have had an airbleed carb and was hard to set without any pump. Maybe the pump is just adding to the problem your having with the engine. Can you try it without a pump?

Well, I checked the incidence and it is indeed out. They are both supposed to be at +1deg and as it sits there is a +2deg difference in the wing than the tail. There really isn't any way to correct this as the plane sits so it's been trimmed out with some down elevator. The plane is ballanced dead nuts on and left that way. I added the foam inserts into the Robart scale wheels and got them running freely and put a 14x8 prop on the bird. Had 2 great flights yesterday without a problem and was amazed at the difference in how the plane rolls now with the wheels being denser. That's a total of 5 flights now on it, and now it's time to have some fun! [8D] Thanks all for your feedback!
I can't seem to figure out how to add some pictures or i'd share a couple...

If it has an airbleed carb and a pump that may be your problem. Pumps do not work well with an airbleed carb. You may have to use a regulator instead.

Hmm....what do you mean by an 'air bleed' carb? I am concerned about the tank being slightly higher in the plane than the carb...although I may be able to lower the tank a little. I figured I may get syphoning issues. What do you think?

Sean,

There are two basic types of low speed mixture adjustment, air bleed screws and needle valve.

An air bleed carb does not have a needle valve for the low speed mixture adjustment, it has an air bleed hole whose size is modified by an air bleed adjustment screw. Turning the air bleed screw one way or the other changes the amount of air that is allowed to bypass the throttle plate, thus changing the mixture, more air (bigger hole) is a leaner mixture.

HTH

OK, I understand now. I was told once that i should take a pin, put it in that hole and run the screw down to that pin, then remove the pin. this is where the engine should be. i think at the moment that opening may be slightly larger but it seems to be ok. How will the pump affect this opening? I am running a Perry Oscillating pump not a Perry Regulating pump. I am not using muffler pressure.