i am having problem with my van. when i get on the high-way it spits and spuders and will only go about 35mph. i dont know what to change or check. i have put gas cleaner in it but it has not help. if anyone can tell mewhat i need to do please let me know. thank you
it does not have a smell it just does not get any speed when i'm on the high way but when i'm on side streets it does fine. when on the highway can go a few miles just fine and then it starts jumping and starts slowing down. if let off gas and then back on it goes a few mile and starts back up.

Can you give more specifics, i.e. "rotten egg smell", backfires through carb etc?

As silicon noted it is had to evaluate with limited information. Is the engine carbureted or fuel injected?