Spark plugs for older Merc (80s)?

[Robbyracks]

Hello folks,

First time poster! I recently bought a 16' fishing boat paired with a mid-80s 18HP Mercury XD. The seller believes it is a 1985. I still need to confirm this. But when I googled images of 85 Mercs, they look the same. Motor is idling rough and stalling while in neutral. I am going through the process of diagnosing the problem. I figured I change out the plugs while I am at it (the old ones were fouled) and lower unit oil. I am thinking a carb clean could help. While in forward or reverse, it seems to be okay, although I've only done this in a bucket, not on the lake.

What are the proper plugs for this motor? The old ones in the motor are NGK BUHW however I cannot find these plugs at my local stores. I looked up plugs that are compatible and one site listed the Champion L77JC4. Will these plugs work?

Also, what fuel filter (part/number) does this engine take?

Thanks so much in advance!

 

[Faztbullet]

The plugs listed for your motor are: L76V, V40FFM or BUHW. The L77JC4 will not work

 

[ThomW]

Do you have the serial number from the motor? That's the easiest way to verify the fuel filter you need, as well as what plugs it takes.

Most likely your fuel filter would be 35-87946Q04...
but a serial number would verify...

 

[Robbyracks]

Do you have the serial number from the motor? That's the easiest way to verify the fuel filter you need, as well as what plugs it takes.

Most likely your fuel filter would be 35-87946Q04...
but a serial number would verify...

Thank you. to both.. will look up serial...

 

[jimmbo]

Welcome to iboats
Before you start spending money replacing parts that are likely fine...
Can the Ignition system produce a Spark that can jump a 3/8" Gap, on both Cylinders?
Have you tested the Compression?
Does the Prime Bulb get firm and stay firm?
If it still doesn't fire, try spraying a bit of Premix into the carb

 

[Robbyracks]

The plugs listed for your motor are: L76V, V40FFM or BUHW. The L77JC4 will not work

I found this at one of our local box stores. Is this the one you are referring to?

Champion 827 L76V​

 

[Robbyracks]

Hey folks, where are the serial numbers on these motors?

 

[Robbyracks]

Welcome to iboats
Before you start spending money replacing parts that are likely fine...
Can the Ignition system produce a Spark that can jump a 3/8" Gap, on both Cylinders?
Have you tested the Compression?
Does the Prime Bulb get firm and stay firm?
If it still doesn't fire, try spraying a bit of Premix into the carb

Well, it starts fine, runs pretty good in forward and reverse... it's the neutral position where it hiccups... stalls and sputters... weird. A mechanic at Basspro said it's likely carb issue...
I am going to attempt a compression test.

Bulb wasn't that firm.. what does that indicate?

 

[jimmbo]

Bulb wasn't that firm.. what does that indicate?

Wasn't that firm when? When you are squeezing it? When the engine is running?

 

[Robbyracks]

Welcome to iboats
Before you start spending money replacing parts that are likely fine...
Can the Ignition system produce a Spark that can jump a 3/8" Gap, on both Cylinders?
Have you tested the Compression?
Does the Prime Bulb get firm and stay firm?
If it still doesn't fire, try spraying a bit of Premix into the carb

For a compression test, should I do this with a cold engine or warm? Are there any safety tips I should be aware of? I watched a Youtube video where the Youtube basically removed the boots and plugs screwed/attached the tool that comes with the compression kit and cranked the motor five times then came up with a reading... should be close reading for both and preferably over 70 PSI or so, correct?

 

[jimmbo]

Some people prefer warm engines, but if the engine won't start and run, it is not going to get warm. It rarely makes much difference,
You keep cranking the engine, until the Gauge is at its highest reading.
On multi cylinder Engines, there shouldn't be more than a 15% difference between the Highest and the Lowest numbers.

 

[Robbyracks]

So any risk of shock? If so, how do I prevent?

TY

 

[jimmbo]

Ground the Plug Wires

 

[Robbyracks]

Sorry, I'm really inexperienced at this.. trying to learn as I go along... how do I ground the plug wires? thanks

 

[Robbyracks]

This is the serial number I found. I looked it up and couldn't find anything on it. It's A701232

 

[jimmbo]

The Part# given ThomW in Post #3 is correct

As for the Plug Wires, I never grounded them, and I never got a shock while checking compression. I have gotten plenty of shocks while touching the Wires, attached to the plugs, when the engine was running

 

[Robbyracks]

The Part# given ThomW in Post #3 is correct

As for the Plug Wires, I never grounded them, and I never got a shock while checking compression. I have gotten plenty of shocks while touching the Wires, attached to the plugs, when the engine was running

Thanks... so the above serial number I posted - is this correct? Why is it not showing up?

 

[Robbyracks]

those fuel filters are a real treat to remove. I cannot get that filter off... it must have never been changed!

 

[dwco5051]

Thanks... so the above serial number I posted - is this correct? Why is it not showing up?

Try here. It appears to be a 1985

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[jimmbo]

Interesting, the Spark Plugs are shown to be L76V, but in Canada QL78V.
All are Surface Gap Plugs.