well i jsut jetted my 400 up to a 155 main and i love the power gain i got but the only way itl run normal is without the air box lid on it so how dangerus is it to run without out it or is there another way i can go to get more air to my engine and keep the lid on right now im running a yoshimura slip on uni filter and the 155

No problem running with the lid off. Just keep the filter cleaned and oiled. Be careful in the water that you don't flood your airbox.
I would say most modded quads are running with the lid off or cut open.

i do ride in a lot of mud/mountain trails though so will that heart it in any way

You should be fine. Just make sure ot keep the filter clean and a clean outerwear on it also! I'm running no airbox at all right now, just have to make sure I dont go through any water too deep.

I ran a K&N/with outerwears on my warrior with the lid cut and never had any problems. The place we ride can be either extremely dusty or wet and muddy depending on when it rained. I went with a UNI on my 700 so I added a Pro-Tec lid and I like it alot. After 7 hours of riding on wet and muddy trails the filter looked as good as when I got there.

On my three larger quads, I run the EHS Racing lid, which has an Outerwears cover. It passes lots of air, yet give more protection than a wide open box!

so is it a bad idea or no cuz i cant aford to ruin anything on my engine

To sum it up, no it's not a bad idea as long as you take the necessary precautions and maintain your bike. If you tend to ride in deep water or mud bog, you probably shouldn't do it. If that's not the case, then have fun and make sure you clean and check your air filter after every ride to make sure it's not damaged or too saturated with water or other nasty stuff.

Most with modded sport quads dont run a lid you just have to take a few precautions.

I wont run without a lid except with these 2 set-ups

1) K&N with a outer wears cover

2) UNI with a Pro-tec lid or EHS lid

I ride in alot of wet and muddy conditions and have had no issues with either set-up. I recommend these 2 ways if ride in wet and muddy conditions.

I run the K&N extreme flow power lid i like it a lot.