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Author Topic:   1977 Evinrude 70-HP Low Full-throttle RPM
Redmanf1 posted 07-20-2015 06:56 PM ET (US)   Profile for Redmanf1   Send Email to Redmanf1  
I just purchased a 1978 Boston Whaler 17 Montauk with a 1977 Evinrude 70-HP engine. The person I purchased it from spent $2,000 on the engine A few of the things: rectifier, power pack, rebuilt power head with timer, one new carburetor, water pump, timer base, many gaskets, seals and rings. [Without a tachometer] I am saying at low RPM it runs nicely, but it just will not get in the upper engine speed range. I just checked cylinder pressure and it is 90-PSI. There is no propeller hub slip. THe propleerl is new and is 13.75 x 15. It is a fresh water boat and engine.

To me it sounds like full throttle is about 3,500-RPM. It seems crazy. The motor starts, idles, changes fwd-rev and does not die. When you start out It throttles nicely and it tries to go but it just cannot get to its upper rpm. Three rebuild kits were used for the carburetors, then, at a latter date when the power head was rebuilt, a new carburetor was purchased.

When I pulled the plugs the middle and lower plugs looked a little darker. Thanks again for any help.

Propeller info: Michigan Wheel P/N 992103, Vortex XHS Aluminum; diameter: 13-3/4; pitch: 15; three-blade, right hand.

I did the compression test, and, yes it was cold; all three cylinders are at 90-PSI with a Mac tester. The engine also has a new starter. So much has been done. I do not want to end up throwing good money after bad. [The boat] is nice enough to repower. I do not want to spend $9,000 on a new outboard. I would look for a nice used four-cycle-power-stroke, a 90-HP to 115-HP.

From my understanding the mechanic was trying to fix this problem. There is a second receipt [for repair to the engine] that has good compression with carburtor rebuild; this was four months before the rebuild. The last receipt has the new carb listed.

The boat weight was good. I checked the compression myself. I just checked the spark with a tester and it will jump 5/8-inch, which is strong spark

The engine starts easily, seems to run okay. but at full throttle it does not have the power to plane off the boat. Any help as what to look for would be appreciated.

Jefecinco posted 07-20-2015 07:22 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
Seventy horsepower seems a little low to me. I recommend you consider the engine mounting height on the transom and a stainless steel four-blade propeller.

There are many here who will be able to recommend some changes which will be helpful.

It would be very useful to post your engine mounting information. Without knowing the RPM your engine is turning it is very difficult for the gurus on the forum to make recommendations to help your performance. You need to find a way to provide the RPM.


contender posted 07-20-2015 10:40 PM ET (US)     Profile for contender  Send Email to contender     
sounds like you have a fuel problem, I would go through the carburetors again and check fuel lines and the bulb (do the easy stuff 1st) If you are getting good compression on all 3 cyl. has to be something simple.
jimh posted 07-20-2015 11:30 PM ET (US)     Profile for jimh  Send Email to jimh     
A simple first step in diagnosis: check for spark on all three cylinders. You might be running just on two of them. Use an in-line spark tester.
Redmanf1 posted 07-21-2015 04:36 AM ET (US)     Profile for Redmanf1  Send Email to Redmanf1     
I did the the compression check as above and the spark test on all cylinders. All ready checked the fuel line and bulb. A couple of videos, the first one I was able to get on plane, the second one it would not.

jimh posted 07-21-2015 07:21 AM ET (US)     Profile for jimh  Send Email to jimh     
Get a tachometer and measure the actual RPM. Your reports of engine speed all seem to be measuring engine speed by ear.
jimh posted 07-21-2015 08:05 AM ET (US)     Profile for jimh  Send Email to jimh     
In the recording seen at

I agree with your aural interpretation of the engine speed. The engine is not running at the normal full-throttle speed range.

Try pumping the primer bulb to help move fuel to the engine at full throttle. If this improves the engine speed, a problem in the fuel system is indicated.

jimh posted 07-21-2015 08:09 AM ET (US)     Profile for jimh  Send Email to jimh     
Is there any indication that the onset of this problem is related to engine temperature? Many electronic components can react to higher temperature.

You report that the Power Pack, which provides the spark ignition primary voltage to the spark coils--was already replaced. Have you checked the spark coils? A spark coil might have developed a fault. The spark voltage may be jumping to ground instead of getting to the spark plug. Try running the engine in the dark with the cowling removed. You may see evidence of the spark voltage jumping to ground somewhere.

dg22 posted 07-21-2015 10:11 AM ET (US)     Profile for dg22  Send Email to dg22     
You could test your fuel pump if you have a fuel pressure gauge or have a marina test it for you. Possibly the fuel pump is not reaching the PSI needed at the higher rpm.

I just ordered a fuel pump for my 1973 50hp Evinrude. They are $114 in Canada but I've seen them online in the US for $79. I ordered mine via the BRP website.

contender posted 07-21-2015 11:28 AM ET (US)     Profile for contender  Send Email to contender     
just because your fuel line is not leaking does not mean its working, use only a factory fuel bulb, and on engines this old you may have an enthanol problem eating the rubber inside the fuel line and other places...Like I said if you have good compression and spark you need to got throught your fuel system/carbs...start with the easy stuff....good luck
Sherman Mohnike posted 07-21-2015 03:35 PM ET (US)     Profile for Sherman Mohnike  Send Email to Sherman Mohnike     
You should do a vacuum check on the fuel system. Many time over the last 40 years with my whaler I had a air leak into the fuel line that caused the symptoms you talk about.
tedious posted 07-21-2015 08:04 PM ET (US)     Profile for tedious  Send Email to tedious     
From your description it is not clear whether the motor was ever working properly since the work was done on it. Can you clarify?

First diagnostic step for these motors is always a "link and sync" - the process of aligning the carb openings and timing advance. If your mechanic does not know what that is, find another mechanic.

Another common oops that can cause that symptom is a piece of lint or something in the high speed circuit of one of the carbs. They may have ruled that out with the second carb rebuild.

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