What engine do you have?
250 single, 250 twin, or the 294 twin?
Just venturing a guess that you have the ultra common 250 single...that being said, they usually chug along pretty decent in the deep stuff. Not fast, but not really boggy. I have a 76 Elan 250 single that's been the boggy pain in the butt wonder for years, but I just cured it. I have been fighting with it's temperamental Tillitson "HR" carb. Boggy and just all around poor running.
There are 3 cures that I know of, the first two are fairly simple.
1 - upgrade to a Tillitson HR carb off of a later model Elan Deluxe with the 250 twin. This carb should have a fixed high speed jet, not a adjustable screw type high speed jet. And just to be on the safe side, toss a new carb kit at it and you should be cruising away great!
2 - The Mikuni upgrade. Use a Mikuni or Kehin carb off a newer small sled, or perhaps off an motorcycle. This works great of you want consistent reliability and performance. Trick is in getting it properly jetted.
3 - Engine swap time. Swap in a 340, 377, 399, 440, 640, etc. Takes a bit of work, but you shouldn't ever have a problem with boggy poor performance!
I did the Tillitson HR off a 250 twin to my ol 250 single and took it out for a rip night before last and was very impressed! I actually got stuck due to spinning out! Top end improved huge!
I'm doing a engine swap in my Elan Deluxe 250 twin. It's engine was siezed when I picked it up, so I figured, hey why not go a little crazy! Let's build it like Tim Allen would do...More Power!!!
That being said, just trying to figure if I should use a 399, or 440 Rotax, or a 500 Suzuki Spirit engine I have in my sled spare parts collection.....