TOPIC: about to give up... problem 653 with video

Fuel pump swapped for one I had
Carbs cleaned and rebuilt
fuel lines replaced
fuel tank issues fixed


runs perfect on the hose... put it in the water and put her in gear... shuts it right down. We verified that fuel is getting to the pump, verified that it is getting past the pump. Obviously has spark. I checked the sync and linkage adjustment as best I could (I'm no expert for certain). I'm getting to the point that I'm going to swap out the motor for my 85hp but the wife is really objecting.



thanks to space.ace for the videography

I'm not really familiar with the 70HP, but it sounds like it's not getting enough fuel. I notice no smoke whatsoever, so this would confirm that diagnosis? Have you tried opening the idle mixture screws 1/4 turn each? Don't give up, we'll figure it out. Professor, Scott, Jim, anybody??? Maybe one of the Team OMC guys from the G3 forum?

we had the same diagnosis... just cant figure out why. I even held the fuel tank up higher than the motor so the pump wouldn't have to pull the fuel. Maybe there is something I missed in the rebuild?

Feel the sparkplugs after it has run for about 1-1/2 minutes. All should be hot. My guess is that your "magic box" cdi box is shot. It doesn't sound like it is hitting on all 3 cylinders to me. It takes a trained ear to hear it. I could be wrong but that's my best guess. With no load, the engine will rev up, even though it's not firing on all jugs.

I am w/Bruce on this one. She may have spark but not healthy spark on all cyl's.

I agree,listen closely to that video. She sounds to me like she is maybe only hitting on 1 cyl.

That motor does not even come close to sounding properly like a 70 HP should.

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Sounds like it needs teardown and rebuild ( hole in the block ??? )

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No easy fix for it.

the guy included a new stator... think it could be that?

bought myself a new compression tester... all cylinders are above 120 psi. Top and bottom are at about 125 and middle is right at 121 or so. I would think that rules out rebuild...

buy a manual ad volt meter and follow the steps to test the current stator. then test the power pack on it. You have a spark problem. You are loading up on fuel and no way to burn it with improper spark. That is why she quits. Merc's do the same thing when any of the CDI components are failing.

I went ahead and replaced the stator since I had a new one sitting here. Definatly seems to have made a difference. Ill look up the trouble shooting for the cdi box and check it. Prolly have to get a new meter since I think all I have are digital.

Seems odd that this motor has sooooo many problems.

once you clear them up that motor will last forever. that 3 cyl block is proven for sure.

I went ahead and replaced the cdi with a new unit... Its definatly smoking better. I want to replace the fuel line "T" with a bigger one before I go to the lake. I really think it is starving the motor even though it was stock.

My original thought when I listened was that you are running on 2 cylinders. Compression is good.

I would check while idling to see which cylinder isn't running. Start pulling plugs wires one at a time & listen for the cylinder to drop. No change means not running on that cylinder.

Causes (assuming there is no hole or intake leak into the crankcase);

Fuel issue (remember, each cylinder has its own carb). Problem must be after the fuel pump. Otherwise you would have 3 dead cylinders. So, look at the carb, carb mounting flange gasket or reed valve.

Top or bottom cylinder dead, check for a bad crankshaft seal. You can spray a carb cleaner up under the stator to see if it changes while you spray. That would indicate top seal.

Fuel bleed lines from the bearings back to the manifold.

Bad spark. You have changed the stator & CDI, so that leaves the trigger, wires or coils. I would try swapping the coils & see if the problem follows. In my years of working on OMC motors, I can tell you stators & triggers rarely fail. CDI boxes YES.

You also need to verify that the carbs are syncronized to each other and to the timing. The idle is set on these motors with the throttle plates (all 3 carbs) fully closed & the roller begins to open the carbs at the scribe mark. Actual idle speed is adjusted with the timing stop screw.

I had a 55 HP in that same vintage. It would alternate between running on the top cylinder, the bottom or both. Turned out the ignition module went bad in a very odd way. Just saying, your replacing the ignition module was a good bet. They don't always fail completely.

I just noticed your using the silver fuel line .. my dealer said don't use it .... it has a liner inside that comes separated from the outer part and shuts the fuel off from the ethanol in the gas, it eats the liner away also the same thing happens to the primer bulb .. It separates inside Use genuine parts only .. He showed me the inside of the fuel hose ..it looks like a hose inside of a hose !! The fuel hose from west marine and the brand x marine dealer are no good !! The problem could be in the hose

professor wrote:

That motor does not even come close to sounding properly like a 70 HP should.

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Sounds like it needs teardown and rebuild ( hole in the block ??? )

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No easy fix for it.

Sounds exactly how a typical mid sized looper from that era should.

They don't call OMC's rattle traps for nothing...

The motors I rebuild do not rattle like that.

the problem is NOT the fuel line from the tank... can tell you that for certain. Thats the only tank I could get it to run on that day... didnt matter if it was an omc metal tank with omc lines or an after market.

I've got it running pretty good on the hose but, the nearest lake is 30mins and havent had any time. I was hoping to get it there today but, 25mph winds meant the lake is closed (its a raised cooling lake kind of wierd and huge waves on light breeze).

I did try running it in a tub of water but, as soon as I give it a little throttle it chucks all the water out. Which, might actually be good because its not lugging when it does it.

So far following advice from here I found:
Bad fuel pump (all gummed up and the diaphram coming apart)
clogged carbs (metal from the fuel pump)

I also replaced the stator which did seem to help and then the cdi box which seemed to help a little more.

Just an off chance that your issue could be the same as mine: my Merc 1400 would run with no load. When in gear, it would not rev up...

I took to a local dealer and after 2 trips he got it: there was a split in the fuel line after the fuel pump.