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Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3308
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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Merc 90 hp seems as though one cylinder is dropping out for

Customer Question

Merc 90 hp seems as though one cylinder is dropping out for a few seconds then comes back online for short pediod of time then drops out again. It does this until it get really good and hot. Have installed new plugs and fuel filter.
Submitted: 1 month ago.
Category: Boat
Expert:  Charles replied 1 month ago.

Hello, my name is***** will assist you.

Expert:  Charles replied 1 month ago.

Has the engine been sitting for some time?

Customer: replied 1 month ago.
It sat over the winter and was sluggish when I first put it in the water. But then it ran OK for a while. Now it is acting up all the time. I installed new plugs and fuel filter and put conditioner in the gas.
Customer: replied 1 month ago.
are you online?
Expert:  Charles replied 1 month ago.

Yes, I m here. I was looking up your engine.

Expert:  Charles replied 1 month ago.

Were the old plugs fouled at all?

Customer: replied 1 month ago.
The old ones were installed by a boat mechanic but they were the wrong number for the engine according to the manual and the gap was very wide. They did not seem to be fouled.
Expert:  Charles replied 1 month ago.

Ok, can you do a compression test on the engine. Lets make sure that the block is in good shape.

Customer: replied 1 month ago.
I will not be at the location of the boat until next weekend. I can run it then.
Customer: replied 1 month ago.
What is the range of numbers I should be looking for when doing the test
Expert:  Charles replied 1 month ago.

Thats fine, there are no time limits here. While there, look at the plug tips and see if they are all the same color. The readings should be over 90 PSI and all with n 15% of each other. If that is good, Remove the drain screws from the side of the carb bowls and try to catch the fuel that comes out. Look for any debris in it.

Customer: replied 1 month ago.
I removed the 3 plugs and wiped them off. The bottom plug looked like new. The others were slightly brown.
Expert:  Charles replied 1 month ago.

Ok, so its not running on the bottom cylinder. That would be why the engine is down on power. Compression, and spark need to be checked first. If those are both good, it points to a blockage in the carb.

Customer: replied 1 month ago.
Will check out this weekend and then get back to you.
Expert:  Charles replied 1 month ago.

Sounds good. I am going to send you a phone offer as well. There is an additional fee for it, but I can be on the phone with you as you are at the engine.

Customer: replied 1 month ago.
On Saturday I ran compression check - all three cylinders OK. I then checked each plug wire to see if all are firing and they are. I then removed the carb air noise cover and found a piece of the cover's gasket - about 3" long and 3/16' wide - laying in the throat of the lowest carb. I removed that, reinstalled the cover, and took the boat out for a check run. It started out OK but after about 3 o4 4 minutes it bogged down. I pulled up to my neighbors dock and he got on the boat. I took if for another run intending for my neighbor to squeeze the primer bulb if it bogged down. It ran great for several acceleration runs and flat out runs. I let it cool down for about 2 hours, took it for a test run and it ran fine. I let it set overnight and took it out this AM. For the first 3 or 4 minutes it ran great. Then it bogged down. Pushing the choke button and pushing the priming bulb did nothing. I almost did not get back to the pier. I let it set and run for quite some time to see if total warm up would help, took it out and again, it ran great for the first 3 or 4 minutes and then bogged down.What now? (Probably will not be back on email until Monday PM.)
Expert:  Charles replied 1 month ago.

Take a spark tester with you on the next trip. When the engine cuts out, quickly do a spark test, see if one of the coils is failing when it gets up to temp. If it is, replace that coil?

Customer: replied 18 days ago.
We tried a spark tester and all were good. I dumped the (new) water separating filter and found pieces of varnish in the gas. Also, my neighbor who was helping me, noticed the in-line filter (new) was only half full. We pumped out all the old fuel, put half a can of Sea Foam in the water separator, the other half can plus a whole can and five gallons of fresh gas in the tank. The motor started running better and continued to get better as we ran it. That was last weekend. It sat for a week with the Sea foam in the system and I ran it today - ran great. It did lag a couple times when, at idle, the throttle was fire walled. The idle is a bit rough but I didn't try to adjust it. Hopefully the Sea Foam will continue to work and the idle will improve. I won't be running it for about two or three weeks so the Sea Foam will have a lot of time to soak the fuel system parts. Any suggestions on how to improve the two minor problems mentioned? Also, what is the best thing to use to keep the gas from going bad over the winter. Thanks.
Expert:  Charles replied 18 days ago.

Check the fuel line from the boat to the engine. Make sure that it is not coming apart on the inside. That could be what was seen inside the filter.

It does sound like the carbs are stopped up. If it does not get any better the next time that you run it, you will need to remove the carbs and clean them.

Any good marine fuel stabilizer will work for the storage of the fuel. Just make sure that you run the engine for at least 15 minutes at idle on a hose to ensure that the product gets into the carbs.

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