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Author Topic:   Merc 100
Taylor posted 08-07-2001 01:23 PM ET (US)   Profile for Taylor   Send Email to Taylor  
Does anyone want to pass judgement on the reliability or foilbles of a 100 hp oil-injected Merc from 1988. Its on a Montauk, and I'm thinking about making an offer.
Bigshot posted 08-07-2001 01:41 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
More info. $$, condition, options, etc. Any motor is a good motor if maintained and not abused. The BEST used motor can last 10 minutes or 10 years, can never tell. Do a compression test and check lower unit oil. If all looks good and checkout run is good, your odds are as good as any.
JFM posted 08-07-2001 02:44 PM ET (US)     Profile for JFM  Send Email to JFM     
I think that if it is oil injected only that it is probably good. It doesn't look like merc or many other OB manufacturors make a 100 hp any more. They jump to a 115hp so you might have the most legal power you can have on a late model Montauk. Regards, Jay
JFM posted 08-07-2001 02:45 PM ET (US)     Profile for JFM  Send Email to JFM     
Sorry I thought it read 1998 merc.
Clark Roberts posted 08-08-2001 08:40 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Taylor, the 1988 to present (inline four cyl)100hp Merc is a real workhorse and is offered in the 100 hp and also 115 and 125hp versions. Max rated rpm is 5250 and should not be exceeded because of crankshaft harmonics and the very large pistons. They have "blind bore" construction with no head bolts, no exhaust baffle bolts etc. and are extremely rugged. If not overreved and if fed good gas and oil they will run for an undertermined time.. most likely 3000 hrs between rebuilds. These engines have four carbs and the bottom two have no idle jets so that below about 1700 rpm they idle on only two carbs.. there is a small orifice on the jetless carbs for oiling. I have owned a montauk with a 1988 merc 100 and it was a great performer (very fast although not much faster than my other 17's with 90 mercs).. I always felt that it was too heavy (at approx 360lbs) and shallow water running is what I do so not ideal motor for me. Also it used lots of gas compared to the 90's... If the Montauk is a great boat then don't let the 100hp heavy engine stop you. You can use it and then sell or trade it in on a 70hp or 90hp or how about a 60hp merc four stroker EFI.. that would be my ultimate choice. Anyway you asked about the 100hp Merc... be sure to check the compression, oil in lower unit etc.. good luck.. Clark... The Old Man and the Sea
Taylor posted 08-08-2001 06:07 PM ET (US)     Profile for Taylor  Send Email to Taylor     
I found not one but two of these '88 Montauk's with 100HP Mercs. Both look pretty good, one is much cleaner, and I think the hours are lower. I've made an offer. I'm having someone check if for me next week.

Thanks for the advice.. I was worried about the weight a little too, but maybe next year I could trade it on something lighter and quieter. On thing I noticed is the shop that's looking at it for me (I'm a outboard virgin) has a price jump between 90 and 100 for their 'standard' tuneup, so maybe smaller is better.

Bigshot posted 08-09-2001 09:27 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
NEXT WEEK! You don't want that boat do you? It will be GONE if it is a reasonable price.
Angryeel posted 08-09-2001 03:46 PM ET (US)     Profile for Angryeel  Send Email to Angryeel     
I also have a 90 merc that is about to go on a 17' sconnet, I could have got the 100 but a friend who is a merc mechanic told me there is the one difference related to the carb issue mentioned above. As I recall what he told me, it works like this: the 100 is a great motor and will last forever if taken care of and run the way it was meant to be run. The only caveat he gave me on this motor is that in the middle of the rpm range it is only running on two of the cylinders. This is hard for me to explain, but as I understood it, the 100hp is a great motor for trolling, as it will get great gas milliage, and a great motor for running wide open, or at least over 3k, in between those speeds it is not happy and will not run that well. I went with the 90 since I will be using it at mid speeds freequently. hope this helps some, or at least makes sense.
Clark Roberts posted 08-09-2001 05:26 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Angree, either you misunderstood your mechanic or he didn't explain it correctly to you... the engine only runs on the top two cyls at idle to about 1500-1800 rpm. due to the lack of idle jets on the lower tow carbs... and it idles nicely. As you increase the throttle and rpms go past about 1500-1800 the air rush through the lower carbs begins to suck (venturi effect) fuel through the high speed jets and bingo, it takes off like a breeze. There is no mid-range problem!!! In any case you made the correct choice in going with the 90hp, IMHO! The 90hp is the same engine as the 100-125 with the top cyl taken off and a lower ratio in lower unit (2.3:1).. and it's rated at 5500rpm instead of the 5250 for the 100-125.. the 90hp Merc is a great engine and should give you fine performance and outstanding fuel economy for a 2 stroker.. Happy Whalin'.. Clark
Taylor posted 08-13-2001 03:53 PM ET (US)     Profile for Taylor  Send Email to Taylor     
Just got back from the mechanic, compression looks good 140,140,135,137. Power trim motor is shot, and that looks like the expensive repair. Yikes!

Regarding the two cylinder idle, there is an accellerator pump that is triggered as you advance the throttle, and that kicks the intial burst of fuel into the lower two cyl. when you go above idle. The mechanic says it really takes off with a whoosh.

The same guy also says that a lot of mechanics have trouble getting the engine to run smoothly in the midrange, claiming the engine just runs rough. But if you do it right its a great engine.

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