40hp merc with 4 cylinders???

[Tripp Halbert]

A buddy of mine bought a runabout w/ a 2 stroke oil injected 40hp merc and i went to check the vessel out yesterday. As i began to pull off the couling, i noticed 2 carbs and 4 plugs..wait 4 plugs?!?!!?..my mind = booooom!!

Anyone know about these things, ive read that they are pretty dependable. I want to help my buddy tune this thing up and all i know are omc motors and normal mercs. Any tips would be appreciated.

Main concerns:

Did not see a thermostat anywhere on the head...does it have one and where is it?

Is this electric shift or throttle?

theres no linkage for the carbs?

read water pumps commonly go bad on these, but this one has a super strong telltail stream.

are these oil injection systems dependable or should we bypass to premix?

are compression readings typical with other outboards? i.e. 120,120,120

 

[Tripp Halbert]

Re: 40hp merc with 4 cylinders???

bump.

please if u have any info guys, were having problems with the motor and id like to start diagnosing it effectively

 

[Chris1956]

Re: 40hp merc with 4 cylinders???

Gee, you might post a serial number. Mercs are never elec shift. Most older inline Mercs do not have thermostat, but some do. There should be linkage to the carbs, but not like OMC linkage which was usually all over the place. Compression readings should be over 100PSI and even on all cylinders.

 

[Faztbullet]

Re: 40hp merc with 4 cylinders???

Did not see a thermostat anywhere on the head...does it have one and where is it?

Does not have one.....

Is this electric shift or throttle?

Mechanical shift and throttle....

read water pumps commonly go bad on these, but this one has a super strong telltail stream.

All impellers fail as wearable item, since no history of engine , replace it.

are these oil injection systems dependable or should we bypass to premix?

If oil tank is in engine cowls its very reliable, if a Autoblend unit in boat,chitcan it and go premix..

theres no linkage for the carbs?

Vertical linkage on inline motors vs horizontal on OMC

are compression readings typical with other outboards? i.e. 120,120,120

If thats actual numbers they are good, less than 100psi engine will have some problems....

 

[Tripp Halbert]

Re: 40hp merc with 4 cylinders???

awesome replys fellas thank you.

yes this is an under the couling reservoir so we will stick with it then, just monitoring closely that oil is being depleted. will do some tests tonight and I will get back to you guys with compression readings and serial numbers sometime tommorow.

 

[Texasmark]

Re: 40hp merc with 4 cylinders???

I just pulled up a '94 sales brochure.

40 hp, 4 in line (old classic design leftover), 43.9 cu in cross flow (not looped like when the 3 cylinders came out), CDI ignition, variable ratio oil injection on electric start models, 2:1 gear box, 177#, no thermostat.

Yepper it exists at least it existed for the '94 model year. Don't know when they went to CDI ignition nor when they started the oil injection. The new design 3 cylinder engines had all the new things plus a thermostat and maybe a popoff valve also for higher rpm operation. That old 4 cyl 2 stroke cross flow was one smooth running machine, but being cross flow and not having exhaust tuning and all it was a gas lover as compared to the new design, loop charged 3 cyl engines.

Mark

 

[Tripp Halbert]

Re: 40hp merc with 4 cylinders???

Kin other words. This is a good trollin motor for fishin and watnot, but gets very hungry if I wanna take a long boat ride haha

 

[Texasmark]

Re: 40hp merc with 4 cylinders???

Kin other words. This is a good trollin motor for fishin and watnot, but gets very hungry if I wanna take a long boat ride haha

That's your cross flow combustion process for you. That's why the Loopers were such a sensation. BTDT

Mark

 

[Tripp Halbert]

Re: 40hp merc with 4 cylinders???

okay guys. not lookin so hot. comp test came back 80 all across. it runs alright though and for about 2 minutes we trolled around and docked back up at the boat ramp leaving it idling as we began talkin about this new boat which my buddy bought, i stuck my foot under the telltale stream and it was boiling temperature. we shut it off. checked oil level, which was lower than before so that was fine. put my hand on exhaust, head and side and all was normal temp.

We then decided to do a test run. Drove it after we let it cool, boat was on plane 3/4 throttle for about 10 minutes down the creek, started losing power, shut off, steam coming out telltale, and would not start back up. Head/block still normal temp. gas in filter was slap full and ball was like a rock. we let it set for an hour and it fired back up. i noticed now that the fuel had let up pressure and the filter was halfway full. we sailed the same distance back to ramp and we got the same results. loss of power, red hot telltale, cool block.

Whats going on here guys?

 

[Texasmark]

Re: 40hp merc with 4 cylinders???

My service manual says 120 is the lower limit before you expect trouble on my '02 90. Would suppose yours would be at least 100. But low compression wouldn't cause a loss of power as you mentioned.

How about some pictures of the pee hose routing. As far as I know those engines had no thermostat. Stats were incorporated in the 3 cyl redesigned engines. So I wouldn't be looking for a stat problem, especially when you said the block was cool.

The test run you made speaks of no cylinder cooling and the pistons expanding to the point where they seized in the cylinders. I had that happen before with an engine with a stat and stuck shut. That should show up as a hot block.

My engine plumbs the pee off the exhaust water jacket which is a different loop from what feeds the block. If that water flow was restricted by something and you are plumbed likewise, then you could have that problem and still have a cool block. Have you checked the temp of the exhaust manifold water jacket? Puzzling thing here is that I don't see a correlation between the exhaust getting hot and the engine loosing power. Something isn't right here. Facts supplied make no sense to me.

Mark

 

[Tripp Halbert]

Re: 40hp merc with 4 cylinders???

hmm i thought the pee route should be entering the waterjacket, filling up, and going thru the block and heads simultaneously because there's no stat? If your correct tho, the temp of the pee stream does not account for the block temp, right?

And yes what really doesnt make sense is the waterjacket is also a normal temp. I can set my hand on it comfortably while the pee stream scolds me......???????????? The pee stream must ultimately be coming directly off something that im not able to touch or see?

im going to run some tests later today on the stand and ill check back in with more results.

 

[ondarvr]

Re: 40hp merc with 4 cylinders???

Most pressure gauges will read differently. I just bought a Merc from a dealer and while I was there they checked the compression with their gauge and it read 80, I used mine and it was 120. I asked him about it and he said he was only looking for it to be even, he knew that gauge always read low.

Unless you know how your gauge reads from experience, the low reading may mean nothing.

When a motor gets hot and shuts down, only to start back up and run normally when it's had a chance to cool down, the electrical system is always a suspect.

There may still be an issue with the cooling system that needs to be dealt with though. Check the spark if you run it again to see if that's why it stops.


I just helped rebuild a 24' pontoon boat that had the exact same motor (same year). It runs very well, but I have never checked the pee temperature to know if it runs hot.

 

[Tripp Halbert]

Re: 40hp merc with 4 cylinders???

thanks for the info. thats what im going to do is check the spark next.

 

[Chris1956]

Re: 40hp merc with 4 cylinders???

Gee, Don't you thimk you should rebuild the waterpump, before you seize that motor?

 

[Tripp Halbert]

Re: 40hp merc with 4 cylinders???

It pees like a racehorse..we are changing impeller tho thx

 

[Texasmark]

Re: 40hp merc with 4 cylinders???

When a motor gets hot and shuts down, only to start back up and run normally when it's had a chance to cool down, the electrical system is always a suspect.

Not always. BTDT as I said!

Mark

 

[Chris1956]

Re: 40hp merc with 4 cylinders???

So it pees like a racehorse, when steam is not coming out the telltale?

Normally a motor like that will have a large exhaust water jacket cover on the port side of the motor. The other waterjacket cover is on the cylinder head. Both should always run cool to medium warm. The starboard side of the motor is not watercooled.

Check the color of the spark plugs. Maybe plug 1 or 2 is running real hot, hence the hot telltale.

 

[Tripp Halbert]

Re: 40hp merc with 4 cylinders???

that would make sense, i will check into this.