1972 Merc. 4HP MN.

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1972 Merc. 4HP MN.

Postby joellerich » Mon Jul 09, 2018 5:34 pm

I have a 4HP Mer. that will not run. 1972 model 40 ser # 3303202. I have tried everything that I can think of with zero success.
It will not start with normal fuel.
It will not start using starting fluid directly in to the carb throat.
It will not start spraying fuel into the spark plug opening.

Here is what I have done so far:
Cleaned the carb
Replaced the spark plug
Replaced top and bottom crankcase seals

Compression test 105 psi
Good spark under pressure and open air
Vacuum outside reed plate with vacuum/pressure gauge about +/- 10psi
Vacuum measured where fuel pump port is attached to crankcase +/- 10 psi
Blown air thru intake port ( all clear)
Exhaust port is clear
Measured where spark occurs by rotating flywheel by hand and observing when spark plug fires. Near TDC

I am looking for any suggestions about what to do next..Getting close to going to junk yard


John 952 890 3885 john.oellerich@q.com.

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Re: 1972 Merc. 4HP MN.

Postby Fisherman6 » Mon Jul 09, 2018 7:48 pm

How did you verify you have spark under compression?
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Re: 1972 Merc. 4HP MN.

Postby crosbyman » Mon Jul 09, 2018 9:06 pm

what does it do with a teaspoon of fuel directly fed in the cylinders

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Re: 1972 Merc. 4HP MN.

Postby Bob D » Mon Jul 09, 2018 9:43 pm

Pull the flywheel and check all of the wiring and the plastic bushing where the wiring connects.
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Re: 1972 Merc. 4HP MN.

Postby 20mercman » Mon Jul 09, 2018 11:44 pm

John, It is Interesting that I just completed a full restoration of a 1974 4hp this week. One thing I would check is the coil. I know you say it is producing spark, but I have found these style coils start to crack, will fire in open air, but will jump to the cindered iron core when under compression. Your insulator in the primary side of the ignition circuit should be OK, or it wouldn't have spark in open air. These systems create a high voltage in the primary circuit that goes to ground when the points close. This allows the capacitor to discharge through the primary windings of the coil to ground. This is why they are called "Maker points" vs. "Breaker Points" These "make" the circuit when they close. This causes high voltage on the points, and they will typically have a significant burned look. This is normal and should not be a reason to replace them. Another issue you may have, is a flooded condition. If you have a failure of your fuel pump diaphragm or the fuel pump is assembled wrong, the fuel will run through the pulse line directly to the crank case, and cause a flooded condition. Is your test tank full of unburned fuel? Pull the pulse line off the fuel pump or the case end, and see if it has fuel in it. This will cause a no start condition as the motor will be severely flooded.

Please keep us posted.

Steven A. Carlson

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Re: 1972 Merc. 4HP MN.

Postby BillW » Tue Jul 10, 2018 6:03 am

Spark on that engine should jump a 3/8" gap in open air.

IMHO, compression is a little on the low side. I'd have been happier with 120. Put a few drops of gear oil in the cylinder, crank it with the plug out just a bit, to disperse it, then put the plug in and try to start. If it fires now, uh-oh....

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