Merc 90 IL 6 Questions

Hi,and THX in advance if anyone can help me.. I recently bought a boat with the 90 in it...<br /><br />I cant seem to find a ser # anywhere, I looked where I have seen them before on Mers's (on the silver plate between the tighting bolts)Is there some where else it might be?<br />If there is no ser# what year approx. is the engine where the shroud is blue with black dots towards the front of the engine?<br /><br />Also what is a good RPM cruisin speed on that engine....it will max out at about 5,000...but thats prolly to much....so what would be a good average?<br /><br />And also what is a good fuel & oil mixture? 50 to 1?<br /><br />Thx again... Im new to boating.

Re: Merc 90 IL 6 Questions

That engine is from sometime in the 80's, I don't think the 90's came out until after 1983. I use about 40:1 or more oil in my inline 6. It was designed to use even more than 32:1, although todays oils are better. It should be getting to 6000 RPMs at WOT, if it isn't you need a different prop. If it's only getting 5k at WOT you will burn a piston. Don't be afraid to go to high RPMs, thats they way outboards are designed to operate. A good cruising speed is around 3000-4000 RPMs.

Re: Merc 90 IL 6 Questions

Thx for that info..but.. what is WOT?<br /><br />and..is there another place to find the ser #?<br /><br />THX again

Re: Merc 90 IL 6 Questions

WOT is Wide Open Throttle, "flooring it". I don't have any idea where the serial number would be...

Re: Merc 90 IL 6 Questions

Does anyone else have any idea....as to my original questions at the beginning of my post...<br /><br />THX

Re: Merc 90 IL 6 Questions

Hi WIrED,<br /><br />the inline 90 hp 6 cylinder as sold in Australia in 1979 had the same colour scheme as you have described on the cowling. The WOT rpm range as recommended by Mercury in their manual for the 90hp was 4,500 to 5,000 rpm, the 115hp was 5,000 to 5,500 and the 140hp was 5,300 to 5,800.<br /><br />It was my understanding if you exceed the suggested WOT rpm for the 90hp they tend to run lean and damage powerheads. For your information the manual also states the gasoline / oil ratio is 50:1 also from the manual - fuel recommendations (not really current)<br /> # regular leaded or premium gas with pump posted octane of 89 (RON 94) preferred<br /><br /> # or unleaded gas with a minimum pump posted octane of 86 (RON 90) was permitted<br /> <br />The serial number should have been on a plate attached to swivel bracket at the front of the motor below the cowling.<br /><br />Bob

Re: Merc 90 IL 6 Questions

does she have a distributor cap or is it an electronic ignition? If it is distributor the newest it could possibly be would be 1979. I have a 79 exactly as you have described yours. In 1979 they produced the first electronic ignition I6's. If you go to the Merc links and do a search for WOT RPM, you will find that most of the Merc Experts say 5500 rpms is ideal at WOT for your motor. If you are turning 5000 now drop 4degrees in pitch and you should be on the money. But you should only run your motor at 3/4 throttle making about 3500-4500 rpm's to help extend the life of your motor. Also when determining WOT RPM, make sure you trim your motor out for least resistance so you get a true reading. Motor all the way down against the stop will create a lot of drag, I have seen as much as 800rpm worth at WOT.

Re: Merc 90 IL 6 Questions

I have a 79 also<br /><br /> http://www.myfishingpictures.com/showphoto.php?photo=16468&sort=7&thecat=527&password= <br /><br />Has the same cowlings you discribed. 79 was indeed the changeover year from distrubiter - ADI. the leftover 78's were sold as 79's and have the distributer. I have mine propped at 6000 at sea level. Ran it in Idaho (Priest River to Sand Point) and WOT was about 5200 - 5400 and about 45 MPH (GPS) with a 19 pitch prop. I use Amsoil in the fuel and in the lower unit (In the lower unit gave me about 200 RPM's more to play with). Amsoil said that I could run it at 100:1, but I am running it at about 80:1.<br /><br />As far as octane ratings. I use my boat as a patrol boat for the Sea Fair hydor races and they fill our gas tanks with 100 or 110 octane. I don't have a choice but to use their fuel as I only have a 16 gallon tank and am on the water for at least a full 4 days without shutting it down for more than an hour or so from 0530 - 1600 and then again from 1800 - 2130. Motor runs soo smooth and at such a low idle on that fuel and we are limited to idle almost all of the time. <br /><br />There should be a serial # on the block, but as I understand it, it's behind the electronics.<br /><br />Hope this helps.<br /><br />Craig

Re: Merc 90 IL 6 Questions

Muchly appriciated....thx to all that responded...Things are clearer now....