Tune-Up - neglected 91 5.7 Merc TB IV 4B Roch

tarm

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Jul 9, 2012
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3
New to posting but the info posted here has been my goto knowledge base for yrs.

Boat : 1991 JC Tritoon 26' Drive platform

Inboard/Stern Drive

Engine: MCM 5.7 Mercruiser Thunderbolt IV 4 Barrel Rochester
Engine Serial: 0D622206

Stern Drive: Alpha One
Transom Serial: 0D642254

Total Hours: 390. FRESH WATER ONLY

Dry storage 7 years

Boat was professionally Winterized before a long storage. Boat has only used Premium Ethonal Free gasoline.

The boat is a friends that I agreed to help with it. He had never done a basic tune up since it was new. So as you can imagine it needed it. Boat was missing and lossing power.

Fuel system was gone thru. Checked pressure. Changed Fuel water separator filter and the carb fuel inlet port filter. Fuel was purged and all fresh fuel.

Oil and filter was changed.

Spark aresstor replaced.

Checking the spark plug wires from the dist while running found what appears to be a bad spark plug. Wow what a suprise for 18 yr ignition system parts even if its only 350 hours.. While pulling each wire one at a time I was getting shocked on many just touching them. Again not too suprising wire insulation is cracked at jacket on some.

Replaced Distributor cap and rotor. Rotor was siezed on and had to be broken off. Cleaned up with emery cloth and greased prior to rotor install.

Have a new full wire set that is going on as I do the plugs.

Went to replace spark plugs. I now know why you see no newer pontoon boats with inboards. Outboard only is all you see. What a royal @/#&$!!. The engine is completely recess mounted into the well with no side access ports. The top of the carb is 5" below the top of the well. Maybe 3" clearance from the outside edge of the manifolds to box side wall. But I THINK I can get at them by pulling the 90 degree hose fittings on the pipes. So far I pulled the right side looking at it from the stren drive flywheel end. Combo of thinwall socket 360 flex adapter and hinge jointed socket wrench I think will get them.

I managed to pull the first one before I lost light. I guess that is number 8 cyl as its the closet one there on the right side with 1 being left side far end.

I had picked up NGK-BR6FS which equal the standard AC43T. But I got a suprise when I pulled the plug. It was a NGK-UR4 which of course is a resistor plug with ext tip and cross ref with the newer vortec engine ver of the 5.7 plugs. There is no question these are the plugs that came on this engine new from the boat manf. They also have the extended tip. I had always thought using these would or could create a clearance issue?

I then think I figured out why they may have been used. The longer body puts the nut face for the socket farther out which allows a easier fit of sockets to get past the manifold at that angle. I noticed I could not get the standard 43t/br6fs with the socket without further grinding the socket as it has to go further into the clearance area. This is something I have never seenbmentioned. The socket nut surface is further extended on the resistor ext tip plugs and more recessed on the standard plugs. At least in my situation it makes them easier to get a socket on becuase of manifold design. Anyways just an observation.

My other thought and its just a swag is if an extended tip does clear the piston it should give a cleaner combustion and a longer life to the plug. Nothing to write home about but at least in theory is should??

Any ideas or helpful hints for getting to the other plugs short of pulling the manifolds?

Anything else I should be looking to replace? The stern has been fully serviced and has fresh grease impeller etc

I replaced the tank anti-siphon valve fitting and the float/sending unit as the latter was not working .

One great upgrade he got it wiyh that everyone that sees iy loves is the upper drive plaform. But it has to add serious weight but man do people love using it for jumping and sunbathing. Basiclly adds 50% deck space to an already good size pontoon.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
The rotor is supposed to be tight, it's Loctited on. The right plugs (BR6FS) need to go in. Yes, you need to grind down the socket for it to fit under the manifold. Sounds like you need a new set of plug leads too.

If the carb wasn't pulled off, stripped down and very thoroughly cleaned out, it needs to be done. Doesn't matter how much stabilizer or 'winterizing' was done, after 7 years, it will be gummed up...

Do you know if the boat was stored with the drive trimmed down and the steering straight? If it wasn't or you don't know, then consider a bellows replacement too.

I hope the drive doesn't have grease in it, only Merc High Performance Gear OIL should be put in.. And only filled from the bottom, up... ;)

Chris....
 

tarm

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Jul 9, 2012
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Thanks Chris.

The carb was run dry and fuel bowl drained. But if I see any issues after this I will pull the carb and rebuild it. Already purchased the complete merc rochester 4b kit.

Thealpha one was correctly serviced. I used wrong term for description. My fault. It was down and straight as that is how I found it and owner had a pic when they parked it and it was in same position. Amazing the kind of damage that can be done when people do not get and ready service manual or take time to do things right.

What limited view I have pulling the 90s off the manifold looks clean with little wear.

I have brand new set of spark plug cables. I even pu a coil just in case.

On the spark plugs themselves. While I fully understand that merc clearly indicates the standard tip and shorter body without resistor i.e. 43t or br6fs why would this one come from factory with the ext tip resistor ur4 version. I have also seen doing a search on here before I posted looking for answers that others have found the same thing on there first plug change on new oem pre vortec versions?

Do you think the boat manf makes those changes vs merc themselves?

I know the good solider answer iis not to question and just follow oem standard but I like to understand the whys and function. After all tge engine ran great from new to 11 or so yrs on the original plugs. While not high on hours thats alot of winters stored for plugs to go thru. It would be cool if there was still a tech at merc from the carb pre vortec days that could give data/ opinion on it. As long as I can get a socket ground doen and can get it to fit in all the locations I will prob go standard as I have them already. If not then the longer ones will go in as no way Iam pulling the manifolds just to change plugs that personally I want to see done every yr as part of spring prep. That turns plug swap into a multiday job and more parts etc.

The only thing I can not be sure of on the stern drive is if it was offical merc hp stern oil. I will drain and refill end of this season in a month anyways so it will be handled.

Is there any maintanence or lubrication that is suppose to be done to the throttle lever. I notice his is rough and not smooth. Takes more force then I think it should to move especially inital movement?

Thanks for the help from you and anyone else.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
The BR6FS spark plug is a resistored plug (that's the 'R' in the number). It is possible the installer changed the plugs...

For throttle roughness, disconnect the cable at the carb and see if it's still rough/stiff. If so, you have a cable of control box problem.

Chris......
 
Last edited:

tarm

Recruit
Joined
Jul 9, 2012
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Ok will check that out. That also makes sense on the plugs. I am going with the factory spec ones as you recommeded.

Thanks again. Getting to number 1 and 2 plugs is proving to be a bear. I have popyee forearms and I just cant squeeze them in there that end. Need to find someone with long smaller arms. They are not over tq which is nice. They loosen with very little pressure almost too little IMO. No way to get a tq wrench in there so Im just going to snug them up once fully seated. Maybe 1/8 turn ? Dont want to overtighten but do mot want loosening and blow by either.
 

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Small children trained in changing spark plugs are excellent for situations like this (who said child labour was dead :D)...

Snugged up, then a 'bit'.

Chris...
 
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