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Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3473
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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I have just purchased an 18ft Xpress with a 97 Mercury 150.

Customer Question

I have just purchased an 18ft Xpress with a 97 Mercury 150. Compression tests show: 121/115/119/115/111/116 top left to bottom right. At 1700 rpm it starts to lose power. I can push the "choke" on the key several times to get it up above 2000 rpm and then it will run fine up to 4800 rpm WOT. Prop is QA2014X 18p. I realize WOT should be higher rpm and that may be the prop size, however, until I get it running smoothly past the 1700 rpm there is no reason to look at prop size.
I had a local shop (part time boat motor mechanic) check it out because it would not idle. He "removed & cleaned the top carb" because there was spark on all 6 cyls but pulling the wires off the top 2 didn't effect how poorly the engine idled. After the carb cleaning, it idles smoother although still dies sometimes.
Do I need all 3 carbs rebuilt? It doesn't have the oil injector (removed by previous owner) and I have fresh gas oil mixed 50:1 with a can of Seafoam in it. Before going back to any mechanic I'd
Submitted: 1 year ago.
Category: Boat
Expert:  Charles replied 1 year ago.

Hello, this is Charles. Thank you for requesting me today.

Your compression numbers are fine. The carbs have nothing to do with the actual spark on the engine, just allowing the fuel to enter the chambers.

Has your engine been sitting for some time?

would you please post the serial number of the engine?

Customer: replied 1 year ago.
I have no idea of the serial number. The tag on the mounting plate is missing because the motor was painted to match the boat...UGLY! Anyway, I've looked for the number on the freeze plate, but I'm leary about taking things off the motor to find it.
Customer: replied 1 year ago.
After looking at a lot of motor diagrams, I've got it down to MERCURY > V-150 > 0G303046 THRU 0G760299 based on cowling and other parts listed. I've tried calling Mercury to ask some questions and that is what they agree with as well.
Customer: replied 1 year ago.
Customer: replied 1 year ago.
WS carbs #1374-5427-C
Expert:  Charles replied 1 year ago.


the thing that I was wanting to look at is the coils. Are there 2 wires with nuts going to each or a multi wire pin going to them?

Are you sure that there is spark on all 6 cylinders?

Customer: replied 1 year ago.
Single wire (green w/ red stripe) plus grounding wire.
Customer: replied 1 year ago.
there was spark to all a couple of days ago.
Expert:  Charles replied 1 year ago.

Ok great.

Do you know if the motor has been sitting for some time?

Customer: replied 1 year ago.
it was, but I've run out for about 2 hours in the last 2 days.
Expert:  Charles replied 1 year ago.


what you need to do is to clean the other two carbs. Take them off and tear them apart. Pat attention to the jet sizes and where they come from. After getting them clean, using a good quality carb cleaner, blow every passage out with compressed air. Replace all gaskets and the needle valve. reassemble making sure that the float bowl sits level with the top of the bowl when turned upside down. Reinstall, set mixture screws to 1 1/2 turns from a light seat and give it a test.

Customer: replied 1 year ago.
Sounds like I should rebuild all 3 carbs if the 1 was just cleaned. I see several different jet sizes in rebuild kits. How do I know what to get?
Expert:  Charles replied 1 year ago.

Just get the carb gaskets kit. You do not need to replace the jets, just clean them.

Customer: replied 1 year ago.
Okay. Will I need to adjust past the 1.5 turns to balance the carbs? How will I know when they are right?
Expert:  Charles replied 1 year ago.

you may need to go in or out with the needles. As you throttle the engine up, if it sneezes, it is lean, open the screws, if it bogs, it is fat, turn them in slightly.

Customer: replied 1 year ago.
I'll give it a try. Thank you. Anything else I should look at?
Where do I find the serial number on the block?
Expert:  Charles replied 1 year ago.

If the powerhead is original, there is a freeze plug on the top of it with the serial number stamped unto it.

Customer: replied 1 year ago.
Okay. I'll look there. Do I need to remove anything to see it? Mercury said it was on the side, but I'd need to remove some things.
Expert:  Charles replied 1 year ago.

just the air box.

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