1973 Mercruiser 888 timing ???

Doug Redinger

Seaman
Joined
May 22, 2005
Messages
73
I bought an old 1973 Larson Bowrider 186 with a Mercruiser 888 sterndrive. I was curious what the total overall timing advance should be. The manual I have for it shows 10 degrees BTDC at idle for the initial timing , but I wondered what the total timing should be at cruise RPM or WOT RPM. It is showing about 32 - 34 degrees of advance at about 2400 rpm now, so I think the timing is about right.

A second question about this old motor and outdrive if I may?? There are 2 fairly heavy wires (about 12 gauge or so) going to the top of the outdrive or gimbal area from inside the engine compartment. One of them is purple and the other is light blue. They both have bullet connectors on the ends inside the engine compartment, but they are not connected to anything. I have a wiring diagram for this motor, but the schematic is not real clear as to what purpose they serve!! Perhaps a optional mercathode system or something??? Any Ideas???? The trim sender already works on this boat so I don't think it would be that. Besides you wouldn't really need a 12 gauge wire for a sender would you???

Thanks in advance for all responses

Doug
 

ziggy

Admiral
Joined
Jun 30, 2004
Messages
7,473
Re: 1973 Mercruiser 888 timing ???

you must need a real service manual. get it in the adults only sticky at the top of the i/o fourm. #9, chose #1 i believe, confirm by serial number.

in looking at the wiring dia. i'd speculate the blue and purple wire coming out of the transom to be trim limit switch. which are blue and purple wires according to the wiring dia.

i must say, i don't see adv. specs in sm #1.. hum, i do see them in sm #2 though. so for a 74 888, looks like max adv. is 24-26* for the 888 and 233. max adv. occurs at 4k rpm. since that info is out of sm #2, i don't know if it would be the facts for a 73. so take that info with a grain of salt i'd think. have you set the dwell with a dwell meter + used a timing light to set the timing?
 

Doug Redinger

Seaman
Joined
May 22, 2005
Messages
73
Re: 1973 Mercruiser 888 timing ???

Thanks for the response Ziggy!! Yes I did set the dwell on the points to about 29 degrees, which is within the spec of 28-32 degrees. As far as the trim limit switch goes, just what does it do??

Thanks again
 

ziggy

Admiral
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Jun 30, 2004
Messages
7,473
Re: 1973 Mercruiser 888 timing ???

it limits the travel of the drive for trim while underway. you don't want to over trim as it's bad for ujoints. this limit switch prevents you from going to far up with your drive.

i don't know about your ford, but my 75 chevy has a three button trim switch on the dash. the way it's supposed to work is that pushing the bottom button only brings the drive all the way down. pushing the middle button trims the drive up, but limits the travel via the limit switch (which i speculate is disconnected on yours) to only raising the drive half way or so. when you push the middle button + the top button. it brings the drive all the way up for trailering.

on my L6. the limit wires breaking off is common. when this occurs, you just blow off fixing it since you have to remove the gimble ring to repair it, which is much effort. instead, i just use the trailer mode, which is using the middle button and top button together. making darn sure i don't raise the drive to far. i think there is a way to wire around the using of the two buttons too. making it so you ony have to use the middle button instead of two. i've not done this mod, but think i could find out here on iboats if i ask or searched for that question.. hope that helps..

hopefully someone will confirm the advance numbers i put up. if they're right, i'd think your over advanced. @ 2400 rpm. you should be between 18 and 22*... be careful. over advanced i believe can lead to detonation.... that would be bad..
 

WizeOne

Commander
Joined
Mar 23, 2008
Messages
2,097
Re: 1973 Mercruiser 888 timing ???

Initial advance for the 888 is 10 deg BTDC. Maximum distributor advance, at 4000 rpm is 26 Deg. The distributor advance is to be added to the initial advance making a total of 36 deg at 4000 rpm.

Adding the 10 deg intitial and the 23 deg distrib advance at 2400 rpm, you get 33 deg maximum total. The minimums are of course several deg's lower.
 

ziggy

Admiral
Joined
Jun 30, 2004
Messages
7,473
Re: 1973 Mercruiser 888 timing ???

The distributor advance is to be added to the initial advance
that's true, and i didn't take that into account... good thing you clarified that for the op...
 

Doug Redinger

Seaman
Joined
May 22, 2005
Messages
73
Re: 1973 Mercruiser 888 timing ???

Thank you guys for the great information. I think I should be okay on the timing issue. I didn't want to risk running the RPM up too high on the muffs. Next time I get out out on the lake I will bring my timing light along and test it at 4000 RPM.

Ziggy, I think you are correct on the trim limit switch wiring being bad in the gimbal. My trim control switch has 3 buttons also. The bottom button lowers the unit. The middle button by itself does nothing, but if I push the middle and top buttons at the same time then the outdrive will go up all the way if needed. I will definately be careful not to raise the outdrive too much while underway. I never really thought about having the u-joint at too severe an angle, but it now makes sense to meand I will be very careful!! In looking at some of the wiring next to the trim motor solenoid, it now appears that the previous owner wired the trim to bypass this trim limit switch. I am going to try to figure out what and where the problem is and see if I can fix it so the trim limit will work. However if I can't figure it out, I can still use the trim if I am careful.

Thanks again!!!
 
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