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Mercruiser 233 HP Wonbt Start

dmk

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"I have a 1975 Trojan with a t

"I have a 1975 Trojan with a twin Mercruiser 233HP 351 Ford Engines with Prestolite ignition. Before I pulled the boat for the winter, I was experiencing starting problems with my starboard engine. I would have to keep the starter engaged and pump the throttle until the engine eventually started. Once started the engine would begin missing when throttled up beyond 2000RPM. Once I pulled the boat I can't get the engine started to winterize it. I have tried the following lengthy diagnostics:

Replaced: Spark plugs (set@ .30), wires, distributor cap, rotor, coil and gas filter and water separator. Fuel pump was replaced 5 years ago. I rebuilt the carb and had the same problem. I swapped the port carb with the rebuilt and had the same problem. I ran a portable tank with fresh gas and had the same problem. Looking into the barrels of the carb, I can see I am getting gas while engaging the throttle.

I checked the timing by aligning the timing marks at 10 degrees BTC with #1 cylinder in the up position and the rotor aligns with #1 position on the cap. I removed the spark plugs, turned the engine over and all spark plugs were firing. Because the engine would occasionally backfire while trying to start, I loosened the distributor and tried to start it in different positions but no luck.

I performed a compression test and got the following results which I don't think would impact starting:

Cyl Dry Wet
1 125 140
2 120 135
3 140 150
4 130 140
5 135 145
6 135 145
7 135 145
8 140 150

The only thing that I am thinking now is that I may have a vacuum leak that is preventing enough gas to be sucked into the cylinders but I don't know how to test for this without the engine running.

I have reached the end of my rope and would welcome any suggestions."
 
"After a lengthy and expensive

"After a lengthy and expensive diagnostic effort, I have finally solved the problem. The problem was a bad resistor wire from the electric choke to the coil. The coil was only getting 4.5 volts intermittently which mimimicked the engine being out of time. The spark was not strong enough for the clyinders to fire. I corrected the problem by installing a ballast resistor and eliminated the resistor wire. All vintage Mercruiser owners should take note of this because it was not easy to isolate."
 
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