Engine runs smooth for an hour then chokes

[bristolw66]

Hello friends,

I've learned allot over the summer and now it's almost over. Out of the 5 times I went out on the LI sound, I've had to limp or get towed back. This is my last attempt before packing it in....

Consistently, I would launch smoothly and run at around 10-15 knots for an hour. Then after the hour the engine would start to backfire, loose power, and even stall out (with a strong fuel smell). It definitely has a "flat spot" in the throttle that I have to play with to get/catch enough power to limp back to the dock. Then the next day.... it runs like there is no problem again.

So far I've:
Changed the Alternator
Added another Battery w/cut off
Re-terminated all ignition wires looking suspect
Found the coil not working when it refused to start at the mechanic
Changed the coil, cap, fuel filter, and plugs (but not the wires)... when the problem presented itself
Fixed the outdrive (I burned out ignoring the alarm... see previous post)

The mechanic I've been taking it to took it out (of course the ride was perfect) and he can't definitively find the problem (root cause).
However, he indicated finding a huge deposit of fuel when pulling the carburetor.

I've read all related posts to timing and the accelerator pump. My buddy recommended removing the carburetor and checking it again. I was wondering if the gas mixture is too rich and is there an adjustment that can be made.

I want to try one more time this last weekend of the summer.
Any thoughts?

[mmwjr]

When it dies have you checked for spark? If not I would do so to confirm it’s either an ignition issue or not to help break down the problem. If you are loosing spark I would replace the ignition sensor in the distributor as there are know for this type of failing. I would also say the ignition coil but you said that was already replaced.

[bristolw66]

Yes but would the ignition sensor be intermittent? The boat starts right up again, but sputters and backfires before catching power. I come back the next day, and the boat runs right a rain... Please send a photo/part number of the sensor, If I can replace it myself then I will.

[mmwjr]

Originally Posted by bristolw66

Yes but would the ignition sensor be intermittent? The boat starts right up again, but sputters and backfires before catching power. I come back the next day, and the boat runs right a rain... Please send a photo/part number of the sensor, If I can replace it myself then I will.

The ignition sensor fails after heating up, typically about an hour. Failure can be complete loss of ignition till it cools down, ignition breaking up resulting in loss of power again works after cooling down, ,…

Go to Mercruiser parts .com and use you engine serial number to look up the part. The new sensor has 3 wires where the old one has 2 wires. The new wire goes to ground. It will come with instructions.

[mmwjr]

It will look like what’s in the following link.

https://www.boats.net/product/mercur...iABEgLPV_D_BwE

[bristolw66]

Incredible!!! What is the purpose of this device? Is it safety? Why didn't my mechanic think of it? I am floored, and will order one today. Where is it located in the boat?

[mmwjr]

Originally Posted by bristolw66

Incredible!!! What is the purpose of this device? Is it safety? Why didn't my mechanic think of it? I am floored, and will order one today. Where is it located in the boat?

It’s inside the distributor and is the trigger mechanism for the spark to each cylinder. I would first verify that you are loosing spark when the engine dies

[bristolw66]

Wouldn’t that have been part of the distributor cap kit that was changed out during the “tune up”?

[mmwjr]

Its #11

[ATTACH][/ATTACH]

[mmwjr]

Originally Posted by bristolw66

Wouldn’t that have been part of the distributor cap kit that was changed out during the “tune up”?

No. It’s the electric trigger and would only get replaced when it fails. You’re thinking like an old points distributor.

[bristolw66]

I hope you don’t think this a dumb question but where do I find the engine serial number? And if I can’t find it, what are my options? Taking the unit out and trying to see/match the part number?

[bristolw66]

I am quite sure I’m loosing spark by the amount of gas I smell and the pool of gas in the carburetor when I opened it up.

[mmwjr]

Originally Posted by bristolw66

I hope you don’t think this a dumb question but where do I find the engine serial number? And if I can’t find it, what are my options? Taking the unit out and trying to see/match the part number?

Post a picture of your engine. There should be a plastic cowl that overs the carburetor which has the engine and drive serial numbers. You can also search the Website using engine size and year.

[mmwjr]

Originally Posted by bristolw66

I am quite sure I’m loosing spark by the amount of gas I smell and the pool of gas in the carburetor when I opened it up.

Unburned gas accumulating can be a sign of no spark. Without spark the fuel isn’t getting burned and when the engine shuts down the carburetor is still spraying fuel which will just lay there. Another way this can happen is if the choke plate were to slam closed choking of the engine while fuel get dumped into the intake. This is why I say you need to verify if you are looking spark when it dies otherwise you’re chasing your tail.

[bristolw66]

Originally Posted by mmwjr

Unburned gas accumulating can be a sign of no spark. Without spark the fuel isn’t getting burned and when the engine shuts down the carburetor is still spraying fuel which will just lay there. Another way this can happen is if the choke plate were to slam closed choking of the engine while fuel get dumped into the intake. This is why I say you need to verify if you are looking spark when it dies otherwise you’re chasing your tail.

How do I upload a photo here, and how do I verify the spark in the stalled condition (without going on the sound and waiting for it to die).

[mmwjr]

Originally Posted by bristolw66

How do I upload a photo here, and how do I verify the spark in the stalled condition (without going on the sound and waiting for it to die).

Don’t use the quick reply but use the post reply then at the top of the window there is a paperclip symbol that allows you to attach pictures.

Get a spark gap tester. It gets installed on any spark plug wire and then connected to ground. You adjust the gap to the same gap specified for your plugs. Crank the engine and see if there is spark.

https://www.amazon.com/Deal%E3%80%91...5692845400def3

[bristolw66]

Will do, here are the numbers found

[mmwjr]

The engine serial number is in the first picture on the top of the flame arrestor.

[bristolw66]

Originally Posted by mmwjr

The engine serial number is in the first picture on the top of the flame arrestor.

Ordered on Amazon and hopefully will be on the water with no issues this weekend.

THANKS!!!

[bristolw66]

Originally Posted by mmwjr

Don’t use the quick reply but use the post reply then at the top of the window there is a paperclip symbol that allows you to attach pictures.

Get a spark gap tester. It gets installed on any spark plug wire and then connected to ground. You adjust the gap to the same gap specified for your plugs. Crank the engine and see if there is spark.

https://www.amazon.com/Deal%E3%80%91...5692845400def3

Got this instead, remember the good old days ;-)

https://a.co/d/3PugQQZ