Is there any power gain if you take off an airbox lid on a fourwheeler? Specifically my 1986 Fourtrax 250.

 

You gain nothing with just removing the lid. You also stand a good chance of burning up you engine by making it run too lean and over heating.
If you remove the lid you need to add at least a slip on muffler, different air filter and rejet the carb. These are your least expensive ways to gain power.

 

Most factory bikes are pig rich from the facotry so yes you will notice a diffrence without hurting anything.

 

Sorry to disagree with you Dragracer, but a Fourtrax 250 will run extremely lean with the lid removed.

 

Hey, you can believe w/e you want man.

Jed, take it off and tell us how it runs, and what color is the plug is....

 

Be aware that those old 250 cc motors were lucky if they hit 8 horsepower...so even if you remove the lid, put on a KN filter, pipe it and jet it...youd be lucky to get a 10% improvement...a whole .8 hp.
The increased risk of dirt and mud getting thru, isnt worth the posible gain....my 2 cents.

 

Man are you sure a 1986 250cc only puts out about 8 horsepower? I'm thinking it puts out at least 13.

 

I might be off a hp or two max.
Look at the figures for a modern 450 cc honda motor, like i have. Its got a whole 12 ponies , maxed out stock. This is almost twice the displacemant of the 86 250. First time i drove it, i figured it had lots of power, actually i still do, but its just 12 horse.

foreman hp graph

Then if I wanted to...i could add the modifactions(pipe, jet kit and KN filter) and maybe end up with just under 15, and be 500 bucks lighter.
These motors were built to last, and be dependable...but they werent built to win races.

 

Okay, I shouldn't have argued with you. I was only making assumptions but I was comparing it to the 400ex's engine not the 450cc foreman engine. The engine is dependable like you say though.

 

No problem..
The differences between the sport quad motors and the utility motors is big. The old 250 r motor had to have at least twice the output of the 4 stroker, just like todays 450r motor is at least twice the power of the foremans 450.
Building the hp, takes a team effort, you cant just do one thing, like remove the lid. Thats just one of the things. The exhaust needs to be freer flowing, and then the jetting needs to be bumped up to match the increased flow.
Most likely if you remove the lid, youll need to richen it up, because the cold starting and running will be poor.
Be aware of the symptoms of lean, and fix them if you encounter them, Heres a good article on jetting, and what the symptoms of too lean and too rich are.
jetting