Johnson GT200

[Don's AE21]

Have a 1989 Johnson GT200 that started running cool when run wide open. Next time out would come out of the hole slow when cold, then would clear up and run pretty good but temp staying low. Stopped motor and changed plugs then restart and runs great and temp comes back up to normal. Drive for hours (cruising at 1/2-3/4 throttle) perfectly, then open up and temp goes back down low(still running great though). Am going to change thermostats and head gaskets before summer. Can I get thinner (higher compression) head gaskets for this motor? Compression ratio is pathetically low in these motors I heard. Would still like to run regular fuel and have reliability

[boxcar]

The stock setup, if still functioning, routes cooling water through the thermostats at idle and low speeds to keep the temp up and when you open the throttle the 'stats are bypassed and the engine temp (hopefully) is reduced.

This function is controlled by a valve on the starboard side of the powerhead that is connected to the throttle linkage. The valve routes/vents water to/from the backside of a diaphragm that the 'stat is mounted to allowing it to either control temps or get out of the way.

Pop off one of the thermostat covers and you'll see how it works.

[Don's AE21]

This has never happened until the last two trips last fall. This was also the first time my plugs have fouled (probably from running cold). I have a temp gauge and pressure gauge.

[VELOCITY 3.6GT]

With the lovely creation of ethanol for our fuel i would leave the compression low. Any and all precautions should be taken to keep the motor from detonation. Higher compression is great for all out raceboats that get rebuilt often but it is not really worth the risk on an everyday boat that you hammer on every so often. The gains on higher compression are minimal from what i have seen.

[Don's AE21]

Thanx for the input. I think I'll leave it stock!

[THEFERMANATOR]

How cold was the water temps? I know at 50 degree water temps my engine will barely hold any heat when running, but warms up nicely at idle. Many of the people up north would get rid of the factory valve set-up and go to the 70HP T-stat covers and stats as they tended to hold more engine heat in as they require higher water pressure to bypass.