Compression Ratio

[zerop3]

I have a 1963 40hp Evinrude that I have not been able to find out what the compression should be for a good motor. I checked mine and it is running around 90psi on both cylinders. I belive the motor is tired and needs to be rebuilt and I am considering doing it myself. Is their anyone that has done this and how hard is it. I do have a manual and have been a mechanic for cars for several years. I dont believe it will be that difficult but any known problems would be helpful. Thanks in advance for any help that is given.

 

[rickdb1boat]

Re: Compression Ratio

90 on both cylinders is great for a 63' model. It may be something, but it's not tired.. Is there a problem with it??

 

[zerop3]

Re: Compression Ratio

The motor runs smoothly but does not have very much speed. Without a speedo it seems like it will only do around 10-12 mph. I did notice that with the cowling on and closed up it will not get on plane and is very slow. If I open the inspection cover or take the cowling off it will get on plane, just not very fast. I jsut feel like it should have more power for the size of boat it is on. Any ideas are appreciated. Thanks in advance.

 

[WillyBWright]

Re: Compression Ratio

Sounds like it's only running on one cylinder. Both sparkplugs firing?<br /><br />Not worth rebuilding. Compression sounds okay anyhow.

 

[zerop3]

Re: Compression Ratio

I have checked the firing before and it is firing on both cylinders. If it wasnt the motor would be running very rough and would probably die as soon as it was put in gear. The motor idles smoothly and has smmoth acceleration while in gear, it just doenst have much speed. The boat is only 14ft and I think it should be able to go faster than what it is. And I do believe that it would be worth rebuilding since it is 40 years old and the motor is the original one for the boat which is also 40 years old.

 

[jimmbo]

Re: Compression Ratio

My dad had 63 40hp, it was on a 14 ft boat and usually did about 24 -25mph with an all time top speed of 27. Since it seems to run better with the cowl open, check to see if there is something blocking the air inlet of the cowl. It is located under the carb on the lower engine cowl. There is a round metal thingy about 3/4 in above it. I would pull the flywheel and check the ignition system, clean/replace the points 0.020" gap.

 

[jim dozier]

Re: Compression Ratio

You may also have an exhaust leak which is contaminating the intake when the cover is on. Check the exhaust cover gasket and the powerhead gasket for evidence of leaks. Have you always had this boat/motor combination? If not maybe your prop selection is off.

 

[stairliftoheaven]

Re: Compression Ratio

so, has this set up always been like this or is there a deteriorating condition?

 

[zerop3]

Re: Compression Ratio

I got this boat a year ago from the daughter of the person who purchased it originally. She only had the boat out once and it died on them and they never tried to do anything with it. It sat for a few years before I bought it. I believe she said that it was running fine when she got it from her dad. Other than that I do not know much about its history. I have repalced the spark plugs, rebuilt the carb, replaced all of the fuel lines and the fuel filter. I dont remember if I replace the fuel pump or not. It does not have an adjustable high speed jet, so I dont know how I would attempt to change this. I have not tried to do anything with the prop. I dont think it has an exhaust leak as it is relativly quiet. I do believe that the air intake into the cowling is blocked, I just havent had the time to check it yet. By what I am reading though the compression reading that I got of around 90psi is good so maybe the motor isnt as tired as I thought. Any thoughts are appreciated. Thanks in advance.

 

[G DANE]

Re: Compression Ratio

Hi zerop3<br /><br />It surely doesnt sound like its tired.<br />If putting hood on makes it run lousy, its often because the metal parts on hood hatch, combined with old rotten spark plug leads takes the spark from one of the cylinders. Blocked air intake is VERY rare, would take a birds nest, insulation from hood fallen off and blocking hole, or something like that. There is probably a hole in lower cowling, under carb, too. Try to replace and reroute both sparkplug cords. Your motor has the "lowtension" type points ignition. My experience is they were good, but can be real tricky. Try to test that both sparks can jump a 5/16 gap with strong blue flame. Just put a nail in boot and hold it with an insulated plier. If you short it you will know . If one cant jump the gap, good, replace both points and condencers, set gap at 0.020. My guess is thats your problem, supposingly you cleaned the carbs good enough. And allthough ignition is weak, they can start and run fairly.

 

[zerop3]

Re: Compression Ratio

I was really expecting the motor to be tired because of the 90 psi test. I was expecting it to be more in the range of 150 like most cars. I am very glad to hear that it may be just some other tune up related issue and not major motor issue. I have not changed the points or coils yet. I will have to check the price and availablity from my local marine parts house. It would be great to get the motor running in tip top shape. Thanks for all of the replys that I have recieved.

 

[rickdb1boat]

Re: Compression Ratio

Cars and boats are 2 different animals. Cars are made to run at high compression levels, where outboards should run around 100 PSI, but that is not a set in stone rule. As long as they are even on both cylinders, that is the important thing. And 90 is real good for your motor. Start by eliminating bad parts by testing, not replacing. Points and the coils would be a good place to start. If the coils are showing signs of cracking, replace them. If the points are severly pitted, replace them and gap at .020. If slight pitting, sand them down with fine sandpaper and reset the gap. Just for starters... Many of these parts can be had online if the dealer is no help..

 

[byordy]

Re: Compression Ratio

A couple of thoughts here. As stated above, if the motor runs better with the cowl off that with it on in all likelyhood there is a small exhaust leak from the powerhead base gasket, or one of the exhaust cover gaskets. It doesn't take much and it does not raise the noise level appreciably. A two cylinder 2 stoke will run just fine on one cylinder and often won't stumble etc. It just will not have any power. I had a 40 horse Johnson of that era on a 14 foot Arkansas Traveler and it would do 25 mph with my wife and I and our two (small) kids.

 

[zerop3]

Re: Compression Ratio

If it does have a small exhaust leak how would I go about checking it out? The reason I believe that the air intake for the cowling is blocked is because the boat before I got it was in the woods for a couple of years without anything being done to it. It was not in a garage or anything just out in the open in a heavily wooded area. When I get a chance I will try and make sure whether it is running on two or one cylinder. I just think that it would run rough or at least shake if only one of the two cylinders are working. Also I was wondering if I could be able to go to the 50:1 mixture versuse the 24:1 mixture. If the motor had been rebuilt before (which I would think is a very probable possibility) would the new parts work well with a leaner oil mixture and would that also increase the power some?