Merc 350 problem

[tsutter]

Hello everyone:<br /><br />I very recently purchased a boat with a 1969 Mercury 350 (35hp)on the back. I took it out tonight for my first time and drove it out to the head of the bay. Total running time was about 10 minutes. Max speed during running was about 3/4 throttle, but about 95% of the time was only at 1/3 throttle. About 10 minutes into running I looked back at the motor and noticed smoke. I went to the back to take a look and did not see any pee stream. I ran it for a minute or two more and then decided to kill it and row back to the dock to avoid any further damage. Loaded the boat up and went home. When inspecting it in the driveway, I noticed the pee hole appeared to be clogged with dirt. This dirt or silt was DRY. When I poked through it with the tip of a knife, a couple of drops of water bled out.<br /><br />Questions:<br />(1) What type of damage could have occurred (i.e. water pump, cylinders, pistons, rings, etc...)?<br />(2) What are some of the procedures to diagnose?<br /><br />I appreciate any assistance. Thanks in advance for your help.

 

[andrewkafp]

Re: Merc 350 problem

Hard to tell how much damage, but good choice to turn it off.<br />Should invest in a secondhand small auxiliary.<br />You will have to perform a compression test on it.<br />This will give you an indication of whether either of the 2 cylinders are burned up. If the comp is over about 90psi and are both the same, and it seems to turn over freely you may be in luck. Is the telltale clear and working now ?<br />If one cylinder is much lower than the other (15%) you may have caused damage. Can't see any reason for W/Pump damage, as it was pumping but not exiting.

 

[greasemonkeyozi]

Re: Merc 350 problem

If the motor wasnt running rough even tough you seen smoke, you may be in luck. Do run a compression test on all cylinders and compair them all together. If one is 10 psi different, you got a problem. Fix your clogg and see if it will then pump water again. If not, you may have a meesed up impeller and need to change that out, maybe not though.

 

[tsutter]

Re: Merc 350 problem

Hello again everybody:<br /><br />First of all, thanks for the replies. They are very helpful.<br /><br />Secondly, when I talked to the guy I bought this off of (he is a mechanic), he mentioned to me that if the compression was bad, I wouldn't even be able to start up the motor. Any truth to this? Will the motor start up just fine if the compression is only marginally bad?<br /><br />At this point, I think I have to get the telltale stream going again. If I can't, probably have to replace the impeller. If I eventually get a pee stream, I test the compression and both cylinders should be at least 90psi, with similar compression values (not more than 15% difference). However, as an alternative, couldn't I just throw the thing in the water and try to bring the boat up to speed (assuming I can first get a pee stream)? If it throttles up no problem, would the compression then be OK?<br /><br />Thanks again for your help on this.

 

[The Marine Doctor]

Re: Merc 350 problem

If the engine was smoking there is more wrong than just water not comming out the telltail. You should look into rebuilding the water pump.<br /><br />TMD

 

[tsutter]

Re: Merc 350 problem

Update:<br /><br />I checked the compression of the top cylinder. After 4 turns of the key, I got a reading of 120psi. Also, on the third turn of the key, the motor wanted to start right up.<br /><br />I couldn't check the bottom cylinder compression because the bottom cowl was in the way and I would need a little more time getting that off (only had a few minutes to my disposal). I plan on checking that later.<br /><br />Any thoughts?

 

[tsutter]

Re: Merc 350 problem

Oh yeah, one more thing:<br /><br />before I tried to start, I pulled the starter cord on top of the motor and can't seem to get it to rewind. Any suggestions on getting the cord rewound again?

 

[tsutter]

Re: Merc 350 problem

OK, another update and plan:<br /><br />Put muffs on and started her up. Each time I started the engine (3 times) it started right up as soon as the key was turned. It stayed running in idle and I ran it for no more than 4 or 5 seconds each time. No telltale stream whatsoever. Maybe a small drip.<br /><br />Will replace impeller and give it another try. Will also check compression of bottom cylinder, once bottom cowl is removed. That is the plan for now, if anybody has anything to add (other considerations, etc...) just post a reply.

 

[greasemonkeyozi]

Re: Merc 350 problem

Heres another tip. If your impeller is still kind of good when you replace it, and still doesnt work when you put it back together, there may be a piece of old impellor broken off and is clogging your hose somewhere. Remove all hoses one at a time and blow on them to see if there is a good flow. Look real close at your tail tell in the inside. A piece of impellor could be trapped there. Take it off and see.

 

[andrewkafp]

Re: Merc 350 problem

Tsutter<br />4-5 seconds isn't long enough to check that telltale. It takes at least 20-30 before the motor is filled and it starts streaming.<br />But on the other hand, don't run it for too long if it is not getting water. Run it for 60 - 90 seconds maximum.. If no water by then it's time to start taking things apart.

 

[tsutter]

Re: Merc 350 problem

Thanks for the advice everyone:<br /><br />I will try to run it for a longer duration to see if I get a stream. <br /><br />One other question. My motor has four screw that anchor the lower unit to the drive shaft. I found 3 of them; I can't seem to find the last one. Two bolts near the top on the port and starboard sides, and one near the back in the middle of the anit-cavitation plate. Any idea as to where the fourth bolt might be? I was unable to locate a picture or drawing showing these in my manual.<br /><br />Thanks!

 

[The Marine Doctor]

Re: Merc 350 problem

Remove the trim tab. There should be one underneath it.<br /><br />TMD

 

[tsutter]

Re: Merc 350 problem

Thanks TMD. Y'know, I wondered if that's where it was. Just needed to look!

 

[Chinewalker]

Re: Merc 350 problem

Hi TSutter,<br /> The Merc 350 is a great old motor. I've got two of them - a '66 and '69 and both run fantastic.<br /> It is entirely possible you were pumping water all along, as the pee-hole is not the only exit for cooling water. It comes out the exhaust relief holes in the rear of the motor leg under the lower cowling (as a fine mist) and the lion's share of it comes out with the exhaust through the propeller hub.<br /> If you can find some compressed air, try blowing that back through the pee-hole. Also, when you've got the lower unit off blow up through the copper water tube to see where you get some blow-through. <br /> Regardless of whether you deem it in need of an impeller or not, it probably ought to have a new one installed, unless the previous owner can document that it's been done within the last year.<br /> The 350s do not have a thermostat so when starting up give it about 15-20 seconds for the water to circulate through the motor and exit the pee-stream.<br /> Finally, all traditional 2-stroke outboards will smoke some. If it is in fact smoke, that's quite normal (assuming your fuel mix is correct at 50:1). If it was in fact steam, then you were right in suspecting a cooling issue. When running you should be able to hold your hand on the exhaust cover (port side) of the motor without burning yourself. It should be warm to hot to very warm but not quite ouch-hot.<br />- Scott

 

[tsutter]

Re: Merc 350 problem

Thanks to Chinewalker for the advice and encouragement. Will try her again tomorrow.<br /><br />Have a good weekend y'all.

 

[tsutter]

Re: Merc 350 problem

Well I forgot:<br /><br />Was wondering: Would I need to remove the propellor to take the lower unit off?

 

[The Marine Doctor]

Re: Merc 350 problem

Nope<br /><br />TMD

 

[tsutter]

Re: Merc 350 problem

Hello again:<br /><br />Was busy with the honey do list this weekend so I couldn't get to the lower unit of this thing. I have a new impeller on the way so I'm just brushing up on how to proceed.<br /><br />As far as getting the lower unit of this Merc 350 off, there are supposedly four bolts holding the lower unit housing to the intermediate housing. I found three of them, and TMD mentioned in one of his last replies that the fourth is under the trim tab. Is there any simple way to get the trim tab off? I've tried unscrewing the thing, but to no avail.<br /><br />Thanks in advance for your help.

 

[andrewkafp]

Re: Merc 350 problem

"Honey Do" Like that one..

 

[tsutter]

Re: Merc 350 problem

^^ bump it up dere! ^^<br /><br />Yes that honey do list was long.