Mercruiser Tune Up

[Homer_Jay]

Just wondering if anyone (where are you BM) can give me a run down on how to give my 488 mercruiser a tune up.
It seems to have a problem with pre ignition. Runs fine but when you shut it down she just want to keep going.

[Ian1]

Running on is most likly caused by carbon deposits in the combustion chambers. Pull the plugs out and have a look at them. You will probably find they are very black and perhaps have a buildup of carbon on them. The carbon will be black and granular looking deposits. Unfortunatly it probably means the engine is getting a bit tired. Check the condition of the plugs first.
The carbon deposits will normaly cause some preignition pinging when the engine is under load. You might just get away with a new set of plugs.

Does the engine use any oil?

If the plugs are fouled then shout it a new set, take it out and give it a good hard run. Pull the plugs again a check the colour. They should be dry and dark charcoal greyish. If they are black and oily then its getting tired.

Cheers

Ian

[Homer_Jay]

Cheers Ian,
engine doesnt use any oil. its in pretty good condition really. But I will pull the plugs and have a look.

[BM]

Aaron,

Most common cause of a marine engine to run on is overheating (causing hotspots).

Can be caused by carbon deposits but less likely.

Can also be caused by overadvanced timing. Wheres your timing at? And how sloppy is your dizzy shaft?

How is your temp? Depending on the gauge, most of them sit about 1/4 gauge when up and running (producing power). Are you noticing the temp continually creeping up over 10-15 minutes at say 3000-4000rpm and getting to half gauge or higher?

While we are at it, how is your oil pressure? Any clunking from the drive unit when you turn the engine from lock to lock? (clunking indicates u-joint wear). Gear oil ok?

Sorry mate but I cannot recall precisely what work you had done to yours.

Cheers

[Homer_Jay]

Nick,

runs fine, no overheating, oil pressure is good.
Its only had the problem since it was serviced last (im sure you remember some of the problems i had with that guy)
So I am thinking that it is probably just the timing out a bit.
Where should the timing be set at on the 488?
The only other problenm i have is that after running for a while it wont return to idle. It will only drop back to about 1000 revs.
I think this might be a carby rebuild needed.

[Ian1]

If its only done it since the last service then yes I'd go with BM and check the timing. Maybe they replaced the points and got the timing wrong. If its got a Mallory distributor then there can be some confusion as to the correct points. If the wrong ones where fitted and then set up with a dwell meter the timing could be anywhere. Advanced or retarded timing can cause hot spots. I would definetly check the timing before you use it to much more. Also check that the timing is consistent with a timing light and not wandering around. If it wanders around then the bearings in the dizzy are probably worn.

Also check the plug type. Might have been hotter plugs fitted at the service?

As for the idle being high, check for disconnected or incorrectly fitted vacume lines at the carb or manifold etc. If the engine is sucking stray air the idle will come up to about 1000rpm but only when it warms up. A disconnected vacume line may also retard the timing on vacume advance distributors but I'm not sure the mercruisers use vacume advance. Did they have the carb apart at the service? maybe an air leak there.

Might give you somthing to look for anyway. I wouldn't jump into a carb rebuild untill you are sure everything else is good.

Cheers

Ian

[BM]

Aaron,

8 degrees timing, BPR6FS plugs gapped at 36 thou (0.9mm), idle rpm (in gear) 650-700 and max WOT of 4800rpm.

Point gap 22 thou and if replacing the points make sure you fit a new condenser as they had a habit of going bung... While your at it check the dizzy shaft for sideways play. This makes getting your timing correct difficult and cause fluctuations. If worn badly take it out and have a machine shop turn it down and fit some bushes.

Correct Ian, no vac advance units on merCruiser marine engines.

The idle issue, I 'd be making sure that there is not a linkage or cabling issue at play here. Check that the throttle arm is returning to the throttle stop when the remote control is in the nuetral position.

Make sure the carb is bolted down tightly. Get some flexible tubing say about 1/2 inch diameter and use it as a "mechanics stethoscope" to listen around the base of the carby and the carby itself. Any airleaks will be evident with it running.

What rpm does it idle at when cold? does the choke work or has it been disconnected?

Cheers

[Homer_Jay]

Once again, thanks for your help guys.

I have just been out and had a good look at it and I have found a bit of fuel leaking from around the top gasket on the carby. Dont think I will play around with it now, I might put it in to be done and get it all sorted out now.

Nick, your not coming up this way looking at houses in the next month or so are you? you could do a job while here and offset the cost of the airfares abit!
Maybe there is someone else that needs some work done too. Make it a profitable trip.
Just a thought.

[BM]

That has been on my mind Aaron!!! I am waiting on the final judgement of something on the go down here.

I would happily organise a trip to bris for nothing other than to do a few services over say a 5 day period and then return home. How's Gilbo's boat going? If I could arrange a few jons to make it worthwhile I would gladly pop up there.

Cheers

[Homer_Jay]

Well, thats a good offer. Just need to find a few for ya.

Not sure how Gilbo's going. I will have to msg him.