Re: Cold Running Motor…Revisited

[Don]

Yeah, I’m thinking there isn’t too much wrong with this engine, unfortunately. I think your best bet is to follow Frank’s recirc advice, and try to get the engine to heat up more. Unlike the 9.9/15 we were discussing in a past thread, this engine DOES have a thermostat bypass in the head. Are you SURE the thermostat is sealing correctly? Did you use a new 310058 rubber thermostat seal? Is the head surface under the thermostat flat/clean/not corroded/eroded allowing water to bypass the thermostat? I know there were bulletins back in the day about plugging the drilled bypass in the head, and that really did help em heat up and run better. Unfortunately, it seemed to me they ran a little too hot in the heat of the summer, but the temp would be great any other time of year. I can’t remember the part number of the rubber plug that was used, but I’m sure you could come up with something. Certainly easy enough to try, you just have to pop the thermostat cover off.
Like Franks says, these engines ran crappy at low speeds for the most part. That recirc hose that is rerouted to the top cylinder seemed to create uneven running/fuel mix conditions in the two cylinders. The usual adjustment scenario was having to lean out the carb to clean up a rich idle, but then there was a lean spit. So, you usually had to leave the engine raggedly rich at idle in order to avoid the sneeze. I have often wondered about ways to correct this situation. Frank’s cure surely would work. I have wondered about putting another bypass nipple on the bottom cover and splitting the recirc hose so it runs to both cylinders. Perhaps this would create more even conditions, or maybe just a mess because of the opposing pulses between cylinders. Never really got a chance to try any of this out, just left em rich and made sure they were warming up.
You could certainly "test" the recirc theory by unplugging the hose and capping off the bypass nipple. Then extend the recirc hose and route it overboard. Now run the engine, clean it out a bit at WOT, then let it idle and see if idle quality improves…Go ahead with Frank’s fix if temporarily draining overboard improves things.