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Author Topic:   Mercury 90 hp lower unit problem
baltica posted 10-16-2001 08:21 PM ET (US)   Profile for baltica   Send Email to baltica  
I am having trouble with my engine (a 1986 Merc 90), I think the lower unit is messed up. I just bought the boat this summer (a 16 foot 1970 Katama), and it ran fine the first time or two, but then the problems started.
The engine runs well in neutral, even at high rpm, but has trouble getting up to higher speeds when its in forward gear. Sometimes, it will not go above 7-8 mph, other times it maxes out at around 25 mph, well below its capacity. You can hear the transmission struggline to get it on.
So I pulled the plug on the lower unit, and out came stinking greyish oil- a real mess. Obviously water had seeped in. On top of this, the magnetic lower plug had some fine metal shavings on it (not lots, but enough to make me worried). I changed the lower unit oil, and ran the boat hoping this might help, but the problem is still there- she runs fine at trolling speeds but can't get above 20-24 mph on plane.
So my questions are-
1) is the lower unit finished, or is there hope that I might be able to fix it?
2) Can I fix it myself, or is this a job for an expert? I've checked, and the cost of a replacement lower unit is over $1000....
3) given the age of the engine (1985), should I be thinking another whole engine? Since I just just bought this boat (my first Whaler), I'm relunctant to blow a wad on expensive repairs if this engine's days are numbered...
I need advice- any suggestions?
Taylor posted 10-16-2001 09:00 PM ET (US)     Profile for Taylor  Send Email to Taylor     
There are experts here, I'm not one of them. My added value is about approach.

I don't know much about outboards. But ealier this summer I got a new 88 Merc 100, and paid way to much money to have someone else replace the tilt/trim pump. And I feel pretty dumb, because I'm begining to realize that outboards are not nearly as exotic as I thought they were.

If I can interpert from your email address (which is at, I might assume you are student. You really have to figure out if you have more money than time or vice versa. I'd guess vice versa. (Oh wait, isn't Duke private?! :)

Anway, my recommendation is that this is not rocket science, allocate some time and do it yourself. Replacing the whole lower unit by yourself at one plus kilobucks is way cheaper than a new engine.

I leave it to others to comment on if they think the lower unit is done for or not. I'm guessing yes, but heck, I'm no expert.

-Taylor (Whitman College, class of '78)

acseatsri posted 10-16-2001 09:26 PM ET (US)     Profile for acseatsri  Send Email to acseatsri     
Try EBAY for a used one. Most sellers are pretty honest. You should be able to get a used one for around 300.
baltica posted 10-16-2001 09:58 PM ET (US)     Profile for baltica  Send Email to baltica     
Thanks for the encouragement, Taylor. To answer your ??s, I'm on the Duke faculty (so I'm not rich). I prefer to fix the motor for free if possible so i will try and tear the l.u. down.
acseatsri's suggestion of e-bay is most excellent- there seem to be at least 2 lower units up for bidding this moment.
DJS posted 10-17-2001 01:36 PM ET (US)     Profile for DJS  Send Email to DJS     
Before you go too far, you should have the hub of your propeller checked to see if it is slipping.
A Li Volsi posted 10-17-2001 03:16 PM ET (US)     Profile for A Li Volsi  Send Email to A Li Volsi     
I had similar problems with my 1986 90hp Mercury. I had my mechanic check out the electronics and found that the engine has two black boxes called "ignitors" -one each for three cylinders-and found that one ignitor was bad and only firing two cylinders. He replaced it and it ran like a raped ape! I couldn't beleive how one cylinder effected the performance. Later that season the same thing happened and the other ignitor was found to be bad on one cylinder. He has told me that this item is the most common to go on mid eighties Mercurys and that the rest of the electronics are very hardy. The engine is a good design and worth repairing-its a very strong engine and will last indefinitly if maintained.
Bigshot posted 10-17-2001 05:01 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I have messed with the inlines TOOO much. You lower unit is fine. Check it periodically to see if it is getting milky again. Small shavings are fine, chunks are bad. Do a compression test first and make sure you have 6 good cyls. Then replace plugs with new. Check each cyl for spark by removing the boot while running. Any change in sound means plug is firing. No change can be two things. No spark or no gas. Each carb runs 2 cyls so usually if no gas then 2 cyls are dead. If 3 are dead(3 top or 3 bottom) then it is a power pack fix and you are done. If all cyls are sparking then it is usually carb time or weak spark. 3 things make any engine run, gas, spark, & compression. When running poorly, spray some carb cleaner or gas mixed with oil into each carb throat. If it bogs down, she is getting fuel. If it picks up or nothing changes you found a problem. If all carbs pick up it could be fuel pump, gas restriction, or air leak in line etc. If all that checks out then on to electronics. You have to disect this and work in parts. If you get down to electronics, they are non-refundable. You have a trigger, stator, coils, rectifier, regulator, and powerpacks. All of these can be tested but I would use a mechanic at that point, cheaper in the long run. If it ran good last time or so, do not mess with the timing. Everyone jumps on timing and screws it up. Outboards rarely need to be timed. Check to make sure you timing is advancing with the throttle. If that linkage popped, it could be that simple. The timing would be good at idle but needs to advance as RPM's increase. When running try and advance it manually and see if it makes a difference.
baltica posted 10-17-2001 09:09 PM ET (US)     Profile for baltica  Send Email to baltica     
thanks everyone for the input. I will check the spark and cylinder compression before going further. Those are at least a few things I can do.
I only assumed that it might be the lower unit because the motor roars so strongly when its in neutral, but does not open up all the way not when its underway. Instead it creeps at wake-breaking speed.
I'll look forward to any other input. You folks are better than the "car guys". Seriously. thanks, RV (aka Baltica)
baltica posted 10-23-2001 10:13 PM ET (US)     Profile for baltica  Send Email to baltica     
Can anyone with a Merc repair manual tell me what is considered the proper compression for an in-line 6 cylinder like the Merc 90 (1985 model)?
Bigshot posted 10-24-2001 10:53 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
No proper 3's per say. They should all be within 10% of each other. 125,120,130,122,118,125 etc. Reason being my guage might read 125lbs and yours might read 115. What are your readings?
baltica posted 11-01-2001 04:52 PM ET (US)     Profile for baltica  Send Email to baltica     
I had the local boat shop check the compression and spark- 150 psi for all cylinders, so that looks fine.
They surmised my carbs were shot, so I had them rebuild all three carbs. But the problem is still there- the boat still seems to choke up when trying to get out of the hole; once on plane, it still seems to get choked up at WOT like its not getting power. I'm lucky to get it up on plane at 15-20 mph.

My mechanic's current hypothesis is that the prop may be slipping when under load- this would trace the problem back to the lower unit, and might get me out on the water by this wknd. My questions now are:

1. Would a slipping prop cause this kind of power problem? What is the clutch mechanism like on the Merc 90?
2. If I replace the prop, what would you recommend for a Merc inline-6 90 HP with a 17' montauk. (We can't find any numbers on the old prop- I'll check the performance forum links in the meanwhile)

Bigshot posted 11-02-2001 10:42 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
You just have it fixed $50. Prop shop will see if it is spun or not. Do you get the RPMs or is it dogging. When you say you are doing 20 at WOT what rpm's are you doing? If more than 3k you have a spun hub.
baltica posted 11-03-2001 07:18 AM ET (US)     Profile for baltica  Send Email to baltica     
DJS was right - The prop hub was slipping, and the problem solved with a new prop hub. Thanks BigShot and everyone for your help! You all give much better advice than my mechanic (who insists I should consider a new engine)

one not-so-necessary carb rebuild job - $400
new hub + prop- $200
getting back into the fish - priceless

jimh posted 11-03-2001 01:08 PM ET (US)     Profile for jimh  Send Email to jimh     
Take you old prop with the spun hub to a prop-shop. For about $40-50 you can get the hub replaced. Then you'll have a spare prop.
Bigshot posted 11-05-2001 09:47 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Do yourself a favor Baltica, get a new mechanic. $400 for 3 carbs....Wowser! Most guys charge $50/carb + any parts, you got ripped.
where2 posted 11-05-2001 12:15 PM ET (US)     Profile for where2  Send Email to where2     
I have to agree that most local shops charge $40-50 to rehub a prop. That's why I almost spilled my coke when watching Ship-Shape one afternoon and hearing that General Propeller in Bradenton, FL will do a rehub for something like $25.

Speaking of them, check out the archives for the dinged props they get. They're hillarious!

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