140 rude question

[rwash91]

Does anyone know how far in the water a 78 evinrude should be or better yet , can be ? I have one on a boat and the water line when trimmed down all the way is just about to cover the exhaust relief holes. The reason I ask is when coming down from full throttle the engine will idle ok for a few minutes and then goes on to act rough and acts as if it is running on 3 cylinders. I know however that it is not because when I pull plug wires all 4 will make a change in the engine. Before anyone asks....compression is great,plugs are correct and most of the ignition has been replaced. The engine runs great except when it idles for more than a minute or 2. Almost as if it can't exhaust right or like the exhaust is clear full of water. Anyone have any thought's or expierienced anything like this? Also when I trim it up to get it out of the water a bit it helps with the idling !!

 

[Solittle]

Re: 140 rude question

The engine should be set so that the cavitation plate is at or just above the bottom of the hull. The exact setting depends on hull, weigh distribution, prop etc and is set by trial and error.

On the idle problem - - have you pulled the thermostats? Done a sync & link? Set idle speed?

 

[rwash91]

Re: 140 rude question

I have replaced the thermostats,and have even went as far as trying a buddies carbs that I know are good so that pretty much rule's out carbs. I need to redo the sync and link. I have done it but have also fooled around with moving the throttle cam a little here or there, so i need to get it back for sure. The idle speed is dependant on weather i set it as soon as i slow down or after it idles in the water for a few?? It has been frustrating for me.If I set it as soon as I slow down then it isn't high enough after a few minutes....if I set it after 3-4 minutes idling then it is to high right after slowing down.
I will check the cavitation plate setting tomorrow and also go over the sync and link and probably take her back out again this week after moving that engine up a few holes.
Thanks for the help........any others??

 

[emdsapmgr]

Re: 140 rude question

New water pump? Get an infrared heat gun and check the head temps, both at idle and full speed. The deeper the engine sets in the water, the lower the idle rpm's will be. Set the idle when the engine is lowest in the water. 650-700 in gear. 900-950 in neutral.

 

[Dhadley]

Re: 140 rude question

What you're describing is typical of large, heavy motors on small, narrow hulls. There are holes in the exhaust tuner that are idle relief holes. If they get plugged or go under water it will act just like you say.

Obviously moving the motor up will not only help the idle but it may well help the overall efficiency if the plate is below the bottom as Solittle asked about.

 

[rwash91]

Re: 140 rude question

Thanks for the input guys! I hope to get her out before weeks end with the engine up as far as I can get it and see if it helps the idling and also see if she'll still get up on plane. I'll report back with my findings as soon as I know...Thanks a bunch!!

 

[Dhadley]

Re: 140 rude question

What kind of boat is it?

 

[rwash91]

Re: 140 rude question

D ....
It's a 1968 fabuglass...ugly as all git out ! It is a model number 00-m It measures 18 1/2 feet long and 6 1/2 across the transom. I guess when new it was just a pleasure/ski boat. Now it's a catfishin boat ! We use it here on the ohio river. It is pretty much open in the middle making it as you guessed, light and narrow. Our bass boat just wasn't safe enough for use on the river at night in rough water.(we fish below the meldahl dam a lot ) This boat allows us to be "in" the boat not on it. It actually works great if we can get her lined out. Thanks for the interest.