Could Poor Perfomance=Wrong Plug?

[jmills224]

Hello,

First I want to say great site. I've learned a ton from iboats.com and the members. On to the problem.

I recently bought a boat with a '97 Merc. 4.3Gen+. The engine has a problem with run-on(dieseling) some of the time. It is also somewhat hard to start when cold, doesn't reach peak RPM and sometimes dies when under a load (twice).

I read the online repair manual #18 from Boatfix.com and went to get the proper plugs. I pulled the originals and found they had Champion RV15YC4, when the book calls for Champion RS12YS Plugs. From what I can tell the RV15YC4 plug is for marine big blocks

The old plugs are approximately 1/8" shorter which is what has me worried. I want to double check that I do indeed have the correct plugs now.

So the questions are:

a)Do I now have the proper plug?
b)Could the wrong plug cause some of the problems I've been experiencing?

Thank You,

~J

 

[Bondo]

Re: Could Poor Perfomance=Wrong Plug?

a),..... I run ACs,..... So in My book you still have the Wrong Plugs,.....

Being a 97, you might have the Vortec heads on that motor,.... They take a Longer plug.....
BBCs,+ SBCs take the Same plugs,... Sometimes,.... Depending on the Vintages......

b),..... Nope,........

Other than the Dieseling,.... It sounds like Fuel Starvation, or uneven Fuel Delivery..........
Not the Sparkplugs though......

 

[Dakota47]

Re: Could Poor Perfomance=Wrong Plug?

Be careful, if those NEW Plugs are to long they can damage the piston..

 

[Don S]

Re: Could Poor Perfomance=Wrong Plug?

I'm with Bondo on using Champion, even with the right PN, they are the wrong plugs for marine engines. The book gives you 3 choices for your 4.3LX Gen+
AC - MR43LTS or the NGK BPR6EFS would be the only ones I would consider using. You also need to check the timing, and adjust the carb and idle speed.
One note, you should have the Thunderbolt V ignition system, so you do need to put the distributor into base timing mode in order to check/adjust the timing or to adjust the carb.
The dieseling is commonly from idle speed to high, or shutting down after a run without allowing the engine to idle for a minute or so.

 

[f_inscreenname]

Re: Could Poor Perfomance=Wrong Plug?

I also use AC's (like rounds in a chamber ), never had a issue with them and I stick with what works. I have tried all the top end plugs and found the only thing you get out of them is you end up keeping them longer because they cost more. So you get almost no benefit up front and then keep them to long because of the cost. With AC's its set'em and forget'em until the next time.
I also think it is timing and the carb but I think its more timing.

 

[jmills224]

Re: Could Poor Perfomance=Wrong Plug?

Okay, thanks for all the replies. I checked the timing and its spot on, I put it in base timing mode. Idle RPM is steady at 500rpm. I went ahead and changed the plugs and the engine does run considerably smoother w/no hesitation or dieseling.

I will be putting AC plugs in. The local part store didn't have the proper AC plug so I went with the champion(which are only good for lawnmowers IMO) just to get me going.

I plan on running the boat today on the lake. Will report back after a tank of fuel. Thanks for the help.

~J

Edit: After reading the maintenance section of the manual I found that the 4.3L and LX models call for the RV15YC4 plug while the LX Gen+ calls for the RS12YC. I wonder if this indicates the Gen+ has the Vortec heads like Bond-O suggested?

 

[180shabah]

Re: Could Poor Perfomance=Wrong Plug?

YES, the Gen+ has vortec heads.