'86 Mercury 35HP 2-stroke compression

[markscottsboro]

1986 Mercury 35hp 2-stroke

My service guy is telling me my engine is 'blown up' with it having 150lbs on top cyl and 160lbs on bottom cyl.... says the 10lb differential is enough that Mercury says the engine is bad...

What is the general consensus ?
(my experience is with 4-stroke, and that doesn't seem like enough difference to worry about..)

Also says the engine has a mis-fire, caused by bad trigger... is this a common issue?

 

[Pappy]

Compression wise the engine is okay. Not enough difference to worry about, however, it may be best to tell us what you took the engine in for to begin with.
Also, first and foremost......WELCOME to the forum!

 

[airshot]

In my day, the rule of thumb was the cylinders had to be within 10% of each other. In your case that would be 15 lbs, which you are well within. As far as your mechanic goes, probably just didn't want to tackle the job as his end cost would be more than the motor is worth. He knows that the customer would be PO'd, so just pass on the repair.

 

[markscottsboro]

Thanks for reply - that's what I expected on the compression...

DUH - Some background would have been helpful

Boat had been sitting for several years, outside...
Previous owner stated he had just replaced the water pump impeller.
Couldn't get it into gear (fwd or rev) and parked it... never got time to work on it. So it sat.

We got the boat and cleaned it up.
Drained all fuel. Installed new battery, new fuel filter, new fuel, checked plugs (looked new).

Engine started and ran decently for very short time in driveway - can't honestly remember if it idled.

We dropped the lower unit and got the gear selector shafts aligned and engaged... engine started and ran fine (for ~10min), went into fwd and rev perfectly. All seemed good. Pisser was flowing water great. Engine idled fine as we put into/out of gear at this time.

We then took it to ramp and backed into water, started, and it immediately wouldn't idle... so we ran the throttle up enough to keep it running.
Engine eventually just slowed down and died. Wouldn't start again.
Took back to house, got it to start, but it wouldn't idle and it was missing at higher throttle.
All this time, pisser was flowing water great.

Pulled the carb and it actually looked really good - intake was nearly spotless. We're not 2-stroke familiar so we didn't disassemble the carb, just sprayed it down with carb cleaner.

Re-installed and no change.

This is when we decided to take it to a 'pro'.... wink-wink.
And this is where we're at now.
TIA for help.

 

[airshot]

Get an actual factory rebuild kit for the carb...do a complete rebuild, all the passages need a wire run thru and blown out with compressed air. Sitting for years is hard on everything, gasoline deteriorates and gums everything up, then you add fresh fuel which loosens up all the crud and sends it thru the carb to clog it all up... I would stiil replace the water pump impellor, if it goes out miles from the launch ramp you will have a long row job and when they go out, they often fry the engine beyond repair. No warning when they fail, if the motor is worth fixing, then fix it right !!

 

[MrGiggles]

I've had three different motors that had one hole that was ~10psi lower than the rest. They all ran just fine, as mentioned 10% is a good rule of thumb.

I think what you are seeing here is a thinly veiled case of "I don't want to work on it, take it somewhere else."

You have been given good advice about where to start, when something has been sitting like that the carb is usually the culprit, it also gives you a chance to reset all adjustments and start from scratch if someone has monkeyed with them in the past.

 

[Pappy]

Well said....the only thing I would add is that since the engine is a 1986 it is easily time to replace all fuel lines under the hood when the carb is disassembled and cleaned as per above instructions. Do no short cut on a 2 stroke carb.

 

[mirrocraft16]

If your worried about the 10 lb of compression, I can tell you I ran an OMC V4 for ten years with one hole down about 40 psi. It ran fine, idled fine, never gave me a bit of trouble. I junked the boat due to rot, then hung the motor on a stand after a full season. I decided to retest the compression, and what was 135,136,141,and 90 psi was now 110,117,111, and 105.
It likely had a stuck ring when I got it, and over time, with use it sort of fixed itself.
Now, over those 10 years, I had the carbs off that thing a half dozen times to keep it running, and I replaced a half dozen impellers as well but what was given to me as a junk motor is still a viable motor if I ever need it. ('85 Johnson 115hp).
If my 110 ever give up, I have a spare.

That all said, I have an '85 35hp Mercury here that's headed for the scrap pile. It ran fine one day and when I went to start it two days later, it sounded like tin can full of rocks. I stopped cranking, gave it a rest, and hit the key again and a rod came through the block. When I tilted the motor three bolts fell out from under the power head where the block had been smashed to bits. No surviving parts left inside. That was supposed to be a good motor, and it was right up to the point it wasn't. I hung another motor and moved on. In talking to a few dealer guys they all told me the 35hp twin was a problem motor, but I have an older 40 hp twin that's been aces since I bought it used in 1982. It looks like pretty much the same motor. Not sure what changed but I keep getting told to avoid the later Merc twins. I guess the '85 proved that theory?

 

[LaqueRatt]

I like to do things on a budget, because I get tired of stuff and resell it. Like to get my money back. So I rarely buy a kit for a carb, but what I have had happen before is having to clean it more than once because it's real easy to miss something using the hose it down with carb spray and blowing it out method. It's always a good idea to put a kit in I suppose, pull those weird plugs out to make sure you get it perfectly clean, but if you got the time and don't mind pulling the carb more than once might as well try the cheap method first. Just my opinion and likely a not too popular one. I agree with the rest it sounds like a carb problem. If not I'd take a good look at those coils. I had a coil that worked great until the engine warmed up. Drove me nuts until figured it out. I'll add I'm far from an expert on boat motors, but have been messing with small engines since I was a snot-nosed kid.

 

[Wayne Roberts]

1986 Mercury 35hp 2-stroke

My service guy is telling me my engine is 'blown up' with it having 150lbs on top cyl and 160lbs on bottom cyl.... says the 10lb differential is enough that Mercury says the engine is bad...

What is the general consensus ?
(my experience is with 4-stroke, and that doesn't seem like enough difference to worry about..)

Also says the engine has a mis-fire, caused by bad trigger... is this a common issue?

I have a 1988 year model Mercury 35 hp and my compression is no better and you know what...........engine runs fine. So my take is if she starts and runs............use it and enjoy!

 

[thill]

The trigger having issues is common with that particular model. And worse yet, Mercury stopped making that part. You may be able to find one made by CDI or anothe aftermarket company though.

Mine had a cracked wire leading to the trigger. I ended up bending it where it had good contact and then encasing it in hot glue. Worked great ever since, and I know the guy who is still running it.

Might be worth trying. Run it, and see if it gets better when you move the wires a certain way.

Best to just replace it, though.

That 35 isn't the smoothest idling engine, but they are strong. I hope it works out for you.

 

[Frank Landers]

In my day, the rule of thumb was the cylinders had to be within 10% of each other. In your case that would be 15 lbs, which you are well within. As far as your mechanic goes, probably just didn't want to tackle the job as his end cost would be more than the motor is worth. He knows that the customer would be PO'd, so just pass on the repair.

Sadly, that happens too often these days with costs what they are. Personally, I like old stuff, antiques, hand me downs, well worn and well used lures and boats and motors and tools.

 

[markscottsboro]

Thanks for all the input!
Now just waiting to hear back after the trigger gets replaced.
One thing I forgot to mention is the ignition switch was in bad shape, broken and glued... so it was hard to turn - wondering if the electric choke was sticking due to ignition switch 'sticking in'?

 

[airshot]

Anything that doesn't work correctly should be replaced, why take a chance unless your really fond of rowing ?

 

[Frank Landers]

I’ve just redone an old boat with a 1999 Mercury 25 HP, pull start. Had lots of fuel issues and wound up replacing gas tank/bulb/lines/filters and doing a carburetor kit. During that time checked compression and it’s about 90 on both cylinders. But now it’s running well, even with that low compression. Mercury manual says compression should be 120 or above. Got my fingers crossed.

 

[thill]

Definitely replace the switch. They are inexpensive, and pretty vital to the function of the boat.

Glad you found a trigger. At the time I needed one, they weren't available, not even on Ebay. The friend I sold it to was aware that it needed one, but since it runs great, he has never swapped it out.

 

[MrGiggles]

I’ve just redone an old boat with a 1999 Mercury 25 HP, pull start. Had lots of fuel issues and wound up replacing gas tank/bulb/lines/filters and doing a carburetor kit. During that time checked compression and it’s about 90 on both cylinders. But now it’s running well, even with that low compression. Mercury manual says compression should be 120 or above. Got my fingers crossed.

Compression numbers can vary a lot between gauges, I don't put a lot of faith in the number alone. Last time I sold an outboard, I ran a compression test and came up with 210 psi across the board (four stroke). The buyer did it immediately after with his gauge and came up with 240 psi.

The length of the hose and what adapter you use for the gauge can even make a big difference on motors that have a smaller displacement.

 

[airshot]

I’ve just redone an old boat with a 1999 Mercury 25 HP, pull start. Had lots of fuel issues and wound up replacing gas tank/bulb/lines/filters and doing a carburetor kit. During that time checked compression and it’s about 90 on both cylinders. But now it’s running well, even with that low compression. Mercury manual says compression should be 120 or above. Got my fingers crossed.

After you have put some hours on it, check it again. Sometimes the rings stick and things get stuck from just sitting. You might try a seafoam cleanout and you might find those numbers go up...

 

[markscottsboro]

Update - got the boat back, running great!
Ended up being a couple of the ignition parts (trigger etc).
Installed new ignition switch, fuse panel, and bilge pump - they checked everything out, got the running lights working.
Put in the water on lake, got 30mph (gps) with 1 person and basically no equipment.
Need to replace the (single) antique metal gas tank with a couple of new tanks. Suggestions?
Also - any suggestions on prop? (looks to still have the original prop)

 

[airshot]

Need tach readings to make prop adjustments, otherwise your just guessing !! As far as fas tanks, how much fuel do you go thru on an outing and how often do you want to get gas. Based on my personal usage, I would go with 1 six gallon and one 3 gallon, but if you run longer you may want 2 six gallon tanks. When I was running small tanks ( now have a builtin 17 gallon tank) I would keep a couple 2 1/2 gallon cans mixed with oil on the ready. Easy to add and properly mixed with correct oil ratio. When you keep adding to a premix tank and guessing at oil ratio, things can get off abd mix ends up to rich or to weak...by premixing in known volume containers your oil ratio will be better controled. Use good quality plastic containers with vents, stay away from Chineisum quality with all the epa crap. When I had a small outboard, I would have a six gallon tank and carry a couple 2.5 gallon cans of premix to add if needed. Only you know how much fuel you need for an outing, just make sure you have extra to get you back if conditions get rough.