I have a custom built powered airplane and for now I'm trying to just get it to glide decently. It glides fairly well but at the end of most every flight, whether with or against the wind, the plane pitches up and then nose dives. The fuselage doesn't yet have a motor but it's ballasted to compensate.
I've made adjustments to the horizontal stabilizer but the problem persists.
Any suggestions? Thanx.

It sounds like it's stalling. Perhaps as it slows the centre of pressure moves toward the leading edge, causing the pitch up and stall.

It'doesn't have enough speed towards the end of the flight , causing it stall and pitch up and the nose diving to the ground . Doesn't matter it you added weight to the nose to compensate , once it lose's speed that's it . You will have to get your self a engine for it and go from there or throw it harder , or even from higher ground . Still though it is all about speed to keep it up in the air , then you can make changes to the flight controls for a smoother flight .

I would have also thought that apart from the above that maybe your ballast is too far forward of the centre of gravity?

Also as this is a scale moddeling site this question may be better if it was asked on a scale flying forum like RCMF

+1 on stalling, I don't fly model ones but real ones and the aircraft is definitely stalling, something that is always going to happen if you are trying to set an aircraft up to glide straight and level.

Weight in the nose will not help, in fact it will only make things worse as the aircraft is now heavier and will lose speed quicker. It will also move the centre of gravity forwards which is a bad thing. As a rule of thumb you want the CoG to be at the point where the lift vector is, this is usually the thickest part of the wing. Put a finger under each wing tip and the A/C should sit level, if it doesn't you need to adjust the weight distribution. Do the assembly instructions include a location for the CoG ?

What you need to do is attempt to get the aircraft trimmed to glide on an even glide slope. That glide slope will be determined by the aircraft maintaining the same speed but losing height.

I wouldn't change the angle of attack of the horizontal stabiliser because the less horizontal it is the less stabilising it is. You'll also find that as the aircraft speeds up the lift generated by the stabiliser will change which will destabilise the aircraft even more. You want the lift vector from the wing and stabiliser to be parallel i.e. both lifting straight up, not at an angle.

I'm guessing it is a remote control aircraft? I so most R/C controllers have a trim slider under the controls so what you need to do is adjust that control to give the A/C a little bit of constant positive or negative elevator. i.e. the elevator "neutral" position has it pointing slightly up or down.

Not sure on the exact process but I'd think you want to test glide the aircraft and if it stalls in the end of the glide give it a bit more down elevator. Try again, and keep putting on a bit more down elevator until the aircraft glides steadily to ground.

The trim of the aircraft will change in flight as fuel is used up but for model aircraft I don't think it will be too much of an issue so once you get a good trim you can sit on that for the day's flying. You can help this by mounting the fuel tank at the CoG or as close too as you can get.

Hope that helps?