87 115 v4 2 pistons not firing?

[grizz5]

Here goes Im new to this forum and new to boating. A total rookie. Well I bought a 20 foot seacraft with a 1990 Yamaha 115 2 stroke V4. My problem is that at idle or low speeds its only firing on 2 cylinders. When I give it gas it seems to get on plane and be allright when Im running. When Im idling or going at slow speeds it is spitting gas out the exhuast. My guess was that the cylinders wernt firing and the gas was passing through and getting spit out the exhuast.I took the cover off and checked the coils and powerpacks its getting spark to all the spark plugs and they look allright but when you pull both right side coils off the plugs theres no difference in the engine so I guess there not firing but when I give it gas the boat gets on plane and goes fine and if only 2 cylinders were firing I dont think I could get on plane. I had a friend look at it and we played around with the mixture screws under the carbuerator cover. It didnt do anything. His guess was that the cylinders were getting to much gas and when the throttle was increased it blew through and fired the cylinders. If that makes any sense. Does anybody have any idea whats going on? Sorry Im pretty new to this if I ddint get any terminology right I appologize.

Thanks grizz

[boats.net]

Start with a compression check make sure compression is ok. Check fuel pump diaphrams for ruptured diaphrams.