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  Lower RPM...raise the engine and re-prop?

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Author Topic:   Lower RPM...raise the engine and re-prop?
ratherwhalering posted 07-16-2003 03:37 PM ET (US)   Profile for ratherwhalering   Send Email to ratherwhalering  
I have a 1987 Johnson 90 that is currently running at about 5000 RPM at wide open throttle (WOT) and properly trimmed up. It has been this way for some time, but since I rarely run at WOT, I really didn't pay much attention to it. The engine has decent compression in all cylinders (+ or- 119), although No. 3 is a little low (110). This is probably due to a worn ring, which was making noise and I recently "unstuck" with some Techron, ValveTec de-carb spray, and Yamaha Ring Free fuel additive. I am currently running a Michigan 13.25 x 17" aluminum propeller. (I just had a tune-up, and I noticed the engine boggs at mid-range, under load, on the initial hole shot, but then runs fine, although a little smokey at cold idle...maybe a carb adjustment?)

After a careful inspection, I realized the cavitation plate is basically even with the hull. I also noticed that there are only the two upper transom bolts installed. The lower bolts are not drilled or installed. The engine bracket rests flush on the transom, and at the top of its verticle hole mounts, so I can raise it as much as I like.

First question I need the lower bolts? Am I over propped? I understand the max RPM is 5600. If I raise the engine, will the two upper through-hull bolts be adequate, or should I plan on drilling the lowers? Which should be done first...the engine or the prop? Finally, what the heck is a blind hole?!?! (I know this sounds silly, but I did my due dilligence with researching old threads) Thanks for the help.

whalersman posted 07-16-2003 03:51 PM ET (US)     Profile for whalersman  Send Email to whalersman     
You definitely need 4 bolts holding your engine on....

Many times the lower bolts are bolted in the engine brackets using the blind holes... How many bolts are showing inside the transom???

The blind holes do not go all the way through the engine bracket... They are threaded in the bracket and you cannot see the butt of the bolt, you should be able to see the head of the bolt inside the transom though...

The blind holes were used on Montauks when the engine was mounted all the way down (and others), because the lower slotted mounting holes were too low to install them in a Montauk because of the splaswell... If you raise the engine one hole up, you should be able to use the very top of the slotted holes thus not utilizing the blind holes at all...

If you already have the holes drilled in your transom for the blind holes, then these blind holes will not line up with the lower slots... At least on most OMC engines.... You would have to fill the holes for the blind setup and drill new holes for the slotted setup if you raise the engine.. The upper bolts are OK obviously, for raising or lowering...

Joe Kriz

ratherwhalering posted 07-16-2003 04:15 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
Whalersman, thanks for the definition! There are only the two upper bolts showing in the inside of the transom well...the upper lag bolts. I do not believe there are any blind hole mounts or lower slotted mounting holes. Yikes. This presents a new question, though. I want to repower in the future, and would hate to drill new lower slotted mounting holes if I am just going to have to fill them later. Now I'm tempted to leave the set-up the way it is, and go with a smaller propeller. I guess the alternative is to raise the engine, drill the lower slotted mounting holes, and then install a jackplate with the new engine (or would the new Johnson e-tech 90 or 4-stroke Evinrude 70 engine brackets will match my old engine bracket?)
whalersman posted 07-16-2003 04:58 PM ET (US)     Profile for whalersman  Send Email to whalersman     
I am under the impression that somewhere in the late 1980's or early 1990's, the bolt patterns for all engines were Standardized...

Someone please correct me if I am wrong....

This was to enable anyone to change engine brands without having to drill new holes and fill in the old ones...

The 1985 Evinrude 70 HP on my Montauk is mounted all the way down and using the Blind holes... Many people find that raising the engine 1 or 2 holes works better for the Montauk... I won't raise my engine because I do not want to drill new lower holes....

I would check into the bolt patterns for all the engines a little more if I were you... If all engines are the same, then I would consider raising your engine. I just measured the hole spacing on my Evinrude 70. The upper holes are 3/4" apart... The blind hole is approximately 2" above the slotted hole... I think to be able to use the bottom slotted hole, you would have to raise the engine 2 holes up....

You say you have "Lag Bolts" for the upper holes and nothing else???
If you can see the head of the bolt inside the transom, and the nut of the bolt on the outside of the engine bracket, then these are bolted all the way through and they are not Lag Bolts....

You might experiement with raising your engine a hole or two before you do anything else.......


ratherwhalering posted 07-16-2003 06:14 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
On 01-08-2001 LGH wrote:
For all mid sized engines, 30HP and up, all bracket mounting holes have been standardized, regardless of brand. If your OMC is REALLY old, it might have been manufactured before this standardization occurred.

I'm not sure when the standardization took place, however. I'd hate to mess this up.

ratherwhalering posted 07-16-2003 06:16 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     
Oh, and they are through bolts, not lags...sorry about the confusion! Duh ;-0

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