Exhaust Relief Holes at Idle??

[4link1955]

I have a 1973 115 Evinrude that will sometimes try and load up at idle and at low RPM's in a "No-Wake" zone. I have heard of some people drilling out the idle relief holes on the back of the midsection where water and exhaust comes out when the boat is sitting still or at low speeds. I have the lower unit off (replacing waterpump impellor) and I looked up into the midsection but it looked like there was another chamber in between the exhaust tube and the outside face of the midsection. If I were to drill these holes do I drill all the way through both chambers right into the main exhaust tube? Also, what can I expect from the motor in doing this? Is it going to make it idle louder? How big do the holes need to be?<br /><br />Thanks in advance:<br />Jason

 

[Dhadley]

Re: Exhaust Relief Holes at Idle??

If your motor sits very low in the water it may help. But so could raising the motor which may improve overall performance. <br /><br />To be of any real help we need to know;<br />Top rpm's,<br />What spark plugs (be specific),<br />What oil and ratio,<br />What fuel and if its fresh.<br /><br />This is all assuming you are hittin' on all 4 at idle and the carbs are clean.<br /><br />Good luck!

 

[4link1955]

Re: Exhaust Relief Holes at Idle??

Dhadley,<br />The motor foes sit low in the water sitting still. I can't seem to get over 5000rpms out of this motor no matter what I do. I'm running the UL77V plugs in it. I have been using 87 octane fuel with a 50:1 mixture of Penzoil TCW-3. It's running very smooth a t idle but sometimes tries to load up if it runs through a long "No-Wake Zone". I have heard of some guys drilling those releif holes out into the exhaust tube for releif but was wondering on how to do it.<br /><br />Thanks:<br />Jason

 

[Dhadley]

Re: Exhaust Relief Holes at Idle??

OK. Good info. Everything looks real good except the top rpms. <br /><br />With your current prop, if you can use 80 to 100% of the trim range, consider raising the engine. That will increase top rpm (which you desperately need) and improve the hole shot as well. Beyond that it will reduce the water in the exhaust cavity (back pressure).<br /><br />Try this first. Good luck!