Defeated by a 4 hp!

So my buddy picked up a 4R73C 4 hp at a yard sale for $20. She looked in decent shape, why not right? After checking it out it was apparent someone had done some head work, as a new gasket and sloppy gasket maker was visible. We tried starting her only to find she'd sputter briefly and die. So, we did the following:

* complete carb teardown, soak and rebuild with OEM kit
* float drop set to 1.375"
* new spark plugs
* new carb/intake gasket

After doing this work we pulled, pulled and pulled some more. She'd fire occasionally and quickly die. It sounded like fuel starvation. We ruled out spark by checking spark quality, removing the flywheel, inspecting points and checking wires. Spark is good.

We found the best luck when we set the high speed needle to 1/2 turn and low speed 1/4. This led me to believe fuel was leaking by the seat, so we tore the carb down again, inspected everything, double, triple checked, then pulled a vacuum on the bowl with it inverted. Everything checked out.

Next I pulled the fuel pump (had already checked screen), inspected the mating surface of the gasket to the crankcase and to the pump housing. All looked well.

Carb back on, problem still there. Pull, pull, pull, she'd sputter and die right out.

So we pulled the carb and then the reeds. Reeds are in shape and functioning properly. Next I pulled the head. No blowby, no water intrusion, no scoring on the cylinder walls and minimal piston play.

We are at a loss with this one. Usually after a carb rebuild on these small engines I set the low speed needle to 3/4 out and the high speed to 1-1/4, then tune from there. We tried that. We tried a whole slew of arrangements.

What are we missing?! It has ot be something simple. I figured it was in the carb - we overlooked, or missed something, but I am telling you we double, triple then double checked again...........

Thoughts, suggestions GREATLY appreciated. Thanks guys!

Compression test??? Stuck rings???? Is there suction when the carb is covered with your hand and the engine is pulled over???? Is the plug wire marked "top" on the correct plug??? Leave the fuel line disconnected and use a squirt oiler containing the two cycle mix to prime it....it should start and run for at least a few seconds...if it does you can rule out electrical issues. I would also try using new plugs...sometimes a conductive deposit is present even though the plug looks to be in good condition. One way to clean a plug is to put it in a 4 cycle lawnmower and run it for a minute or two (assuming the plug dimensions and reach are identical).

Thanks Bobbyc, here are some answers to your questions:

1. my compression gauge bottom out on the top of the piston, so I used some red'n'tacky to seal the threads best I could since I had to back it out quite a bit. 40 psi top, 50 bottom. Today I will buy a nut to fetch up the threads and hold the pressure. Readings seemed a bit low, but could be blow-by around the gauge.

2. Laying the engine horizontal I filled the cylinders with PB and left it overnight. Today after work I will flush with carb cleaner and fill with Seafoam in an attempt to break up any carbon deposits and/or rust. That should free the rings, if stuck, what do you think?

3. Yes, I felt suction, but nothing like what I feel when working on my 75 hp, 90 hp or small block.

4. We swapped plug wires back and forth. The "top" label is long gone.

5. She'll run with a juice of ether, so I don't think its spark.

6. I swapped plugs with new used plugs I had in my shop, but I picked up two brand new ones last night.

I am going to check the head for trueness tonight, polish it down, polish the block and install it. I am also going to pull a carb off a 4 hp Evinrude of similar model to rule out the carb. Fuel, air, spark, right? This little guy is stumping me!!!!

The ignition was designed to put out up to 28000 volts to fire the plugs.-----------So will the ignition on your motor jump a gap of 1/4" or better or did you just lay the plugs on the block and conclude that there is " good spark "

racerone said:

The ignition was designed to put out up to 28000 volts to fire the plugs.-----------So will the ignition on your motor jump a gap of 1/4" or better or did you just lay the plugs on the block and conclude that there is " good spark "

Click to expand...

I made a chepo tester, awhile ago, with a scrap of 1x5 and finish nails. She jumped blue about 1/4"....

Anyway to mark the top plug wire?