Re: rough idle / stalling below 1000 rpm on a 97 force 120hp
There are many things that could cause your symptoms. I would try the least costly and easiest things first.
1. Try checking the compression. Should be around 125 LBS min. in each cylinder and approx. 10% from highest to lowest. Pressure reading is not as important as variance from lowest to highest. This will tell you what type of mechnical shape the rings and cylinders are in.
2. Take a look at the plugs. Do they look fouled? If so, do step #3 below.
3. Check for leaks in the small vacum line that goes from the fitting in the by-pass covers to check valves on the block. Make sure the check vales them-selves aren't plugged. They are one way check valves. This is part of the recirculation system and helps prevent the cylinders from loading up with excess fuel at low speeds.
4. Check the fuel pump. If you've never changed the fuel pump diaphram, it's probably a good idea to do so. About $20. It's difficult to visually determine if the diaphram is bad. It needs to be flexible and no tears or leaks. As they age, they tend to get hard and not flexible. They don't pump well at low speeds.
Next step would be to clean or maybe rebuild the carbs. I wouldn't do this until you've tried 1 - 4 though.
