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Author Topic:   Merc. 100HP on '98 Montauk - Questions
KDW posted 09-09-2003 07:56 PM ET (US)   Profile for KDW   Send Email to KDW  
I recently aquired a 1998 Montauk 17' with a 100 HP 2s Mercury 100. The engine runs very rough at 1800-1900 RPM, but seems to run fine above and below this range. Seems like a carburator issue to me. Anyone out there have any ideas or similar experiences?

Also, I'm not familiar much with Mercury engines. Is it normal to hear a little "grinding" noise when changing from neutral to forward and neutral to reverse?

Lastly, I wonder approximately what kind of speed I should expect at WOT if this boat is still rigged like from the factory? The motor is set at one hole up on transom. I'd comment on the prop size, but don't have the boat near right now - I just know it is a 3 blade SS and I get 4,800-4,900 RPM at WOT (Motor spec. is 4,750 - 5,250).

Any help is certainly appreciated.


ratherwhalering posted 09-09-2003 09:58 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
Everyone here knows I'm no mechanic...but here are some fuel related sugguestions.
Check the gas tanks for solid debris.
Check the pick-up tube (in the gas tank) for blockage.
Check any tank vents for blockage.
Check/replace the gas line for pinches, leaks or soft spots.
Check/replace the primer bulb for proper function.
Check all "quick connect" fittings for proper seal.
Run the engine on a portable tank, with concentrated Techron for cleaning the fuel system.
Clean/Replace any external fuel filters.
Replace external fuel lines

Under the engine coweling:
Check the in-line fuel filter for clogging.
Locate the fuel pump, check for flow, and check all connecting fittings and lines.

The last thing to do is to have the carbs serviced by your mechanic. Before you do that, you may want to run some valve-tec engine de-carb, and later some carb cleaner through it, just in case it is a simple fix.

A little grinding is normal on outboard engines, just don't prolong the agony with a slow shift. I was once told by a mechanic that it costs 25 cents every time you shift a boat into gear!

I'm not sure on speed.

Clark Roberts posted 09-10-2003 07:21 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
KDW, see "Disabled Cyl Cutout...on 115Merc" post above. Your engine idles on two cyls and changes to 4 cyl at about 1800 rpm... read post and that should explain it... you may, however, need complete fuel sys cleaning including those carbs.... Happy Whalin'... Clark... SCN
KDW posted 09-10-2003 08:20 AM ET (US)     Profile for KDW  Send Email to KDW     
Thanks RatherWhaling and Clark Roberts for the help.

I think the thread about the Mercs running on 2 then 4 cylinders may explain where my problems reside. This is useful information so I can ask the right questions at the shop.


jimh posted 09-10-2003 09:47 AM ET (US)     Profile for jimh  Send Email to jimh     

for info referred to above.

KDW posted 09-19-2003 08:04 AM ET (US)     Profile for KDW  Send Email to KDW     
Well, got my whaler back from the shop last Saturday and took her out Sunday. They said the carbs were cleaned and rebuilt and new spark plugs added.

It now runs worse than it did before I took it in, so I'll be taking it back....again. I don't think the Merc. 100HP was tested/adjusted on the lake, but only on a flush hose after the carbs were rebuilt. I certainly hope this problem can be fixed, because I'm beginning to worry that the engine roughness when changing from 2 to 4 cylinders will always be apparent.......but I just can't believe Mercury would ever design an engine with a flaw like this?

Another little problem is that after putting around a while on 2 cylinders, quite a bit of smoke can be seen when opening the throttle and running on 4 cylinders. I suppose this is due to a build-up of 2 cycle oil being deposited into the two cylinders that aren't firing at low engine RPMs?

Does anyone have good experiences with a '98 Merc. 100HP 2s or similar engine that would help rebuild my confidence?


jimh posted 09-19-2003 08:31 PM ET (US)     Profile for jimh  Send Email to jimh     
Getting good results from engine repairs depends a great deal on the person making the repairs. It is discouraging to hear of another case of a boater spending money for a "tune up" that does not improve performance. Dissatisfaction with engine service is a boating industry wide problem.

In your case, the 4-cylinder in-line Mercury 2-stroke engine, currently still made in 115 and 125 horsepower models, has been reported to be a bit cantankerous to get running smoothly.

If you will review the comments made by Clark Roberts in the other recent discussion on this topic, (see Hyperlink above), you will find that there is not a great deal of mechanical or electrical control of the change between 2-cylinder operation and 4-cylinder operation.

The spark is not switched off. It is just the lack of idle jets in the carburetor feeding the two cylinders that cuts them off from combustion. As for the extra smoke when the two cylinders begin firing, it may just be the doubling of the amount of fuel/oil being burned that accounts for that.

It has been mentioned that the in-line four cylinder design is an engine that will be prone to some vibration. Some manufacturers have added balancing weights to help smooth their 4-cylinder in-line engines. I do not know if your Mercury has these balancing weights.

Engine designers can predict the smoothness of a particular design by considering the number of cylinders, the engine cycle, and the amount of "v" if any. Clark has the numbers for this, and has explained this in a prior post. Sorry, I do not have a pointer to it.

KDW posted 09-19-2003 09:02 PM ET (US)     Profile for KDW  Send Email to KDW     
Thanks Jimh for the response and information.

I did read the good information by Clark Roberts and certainly respect and appreciate his expertise, and advice.

I guess a small bright spot for now is that I haven't paid for any repairs since the motor came with a 30 day warranty when I bought the boat and I took it back before the warranty ran out. However, the question may become what is "normal" and what isn't when it comes to any more repair work that will be covered.

I love the Whaler, but if improvements can't be made I'll probably have to decide if I can live with the vibration/roughness or if I need to pursue other avenues for the motor. Maybe I'll look into what it will take to make all cylinders run throughout the RPM range.

Thanks Again.


Barney posted 09-19-2003 09:12 PM ET (US)     Profile for Barney  Send Email to Barney     
You also mention grinding when going into gear. I've got a Mercury. Don't drag or baby it into gear. The manual might talk more about that. Sometimes it's hard to remember when going into or coming out of the dock. From my manual "Always shift outboard into gear with a quick motion". Good luck with the other stuff. Jim

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