Home › Forum › Ask A Member › 1972 Merc. 4HP MN.
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July 9, 2018 at 9:34 pm #10508
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 sealsCompression 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 TDCI am looking for any suggestions about what to do next..Getting close to going to junk yard
Thanks
John 952 890 3885 john.oellerich@q.com.
July 9, 2018 at 11:48 pm #79294How did you verify you have spark under compression?
-BenOldJohnnyRude on YouTube
July 10, 2018 at 1:06 am #79295what does it do with a teaspoon of fuel directly fed in the cylinders
Joining AOMCI has priviledges 🙂
July 10, 2018 at 1:43 am #79304Pull the flywheel and check all of the wiring and the plastic bushing where the wiring connects.
Bob DJuly 10, 2018 at 3:44 am #79310John, 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.
Thanks!Steven A. Carlson
P.S. (Valerie is doing OK)
July 10, 2018 at 10:03 am #79317Spark 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….
Long live American manufacturing!
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