Gunky Spark Plugs and Occasional Backfires

[BrodyO]

Hello All,

I've got a 1996 Larson 196 with a 5.7L Volvo Penta GL and an SX drive with a 17 pitch prop. I mainly use the boat to pull wakeboarders.

Since I bought the boat, the corner spark plugs have gotten gunked up (almost fouled) but my middle cylinder spark plugs always look very good. When I bought the boat it had those Dorman anti-foulers in those cylinders. The plugs I'm running are a Champion RV15YC4 S. Since I bought the boat I've done head gaskets twice(second time was because the engine needed to come out anyways to fix a broken starter bolt) and both times the heads have been gone through and had new seals put in. This last time I put all new valves in as well. I have debated putting a hotter spark plug in, even just those four corner cylinders but I've seen some posts warning against that because of the constant high RPM in a boat.

Another issue I've had is an inconsistent backfire when pulling the wakeboarder out of the water. This is more consistent if the person wakeboarding is heavy or we have more people in the boat (more load). I'll hear 1 or 2 large pops when I go to sudden WOT pulling the person out of the water. I'm unsure if this always happens on the same cylinder or if it's related to the gunky spark plugs.

So, any advice on how to proceed? I've verified timing, I'm currently running slightly under the specified timing, I always run 91 octane non-ethanol fuel and the last time I took the carb apart it was spotless.

Hotter plugs? Throw the anti-foulers back in? New boat?

Thanks

 

[Lou C]

Valve guides and seals?
what about carberation? Overly rich?
Its odd that it is just the corner cylinders

 

[BrodyO]

Valves and valve seals replaced but not the guides.

Carb is clean, boat does run well and steady at WOT but my issue is usually at high load, low speed. I didn't check jetting the last time the carb was open but the center cylinder plugs look good so I would be hesitant to go much leaner.

 

[Lou C]

How many total hrs on the engine? Or cyl heads?

 

[Lou C]

It the seals were replaced and what you see is still oily plugs the only other thing I can think of besides valve guides is piston rings not sealing properly...blow-by excessive?

 

[72fj40]

How much oil does it use? Have you done a dry and wet compression test? The po installed anti foulers for a reason. I once had a 350 with 1 weak cylder and instead of buying new plugs, I would just rotate them.

 

[BrodyO]

Good questions. Engine has about 650 hours on it, assuming the cylinder heads are original. This is a non-vortec head.

When I had the heads off I looked that the cylinders, they didn't look the best, still had crosshatching but also some scuffing, nothing deep enough for my nail to catch. Does have some blowby, I wouldn't say excessive but not great.

Funny enough that when the starter bolt snapped off in the block last year I was compression testing it. I tested it when setting valve lash and all were within 15psi of eachother.

Boat has ran pretty good so far this year after the fresh gaskets and valves, haven't checked plugs yet but with 7 people in the boat and pulling up a wakeboarder I started to hear the backfire again about every 4th time I'd go WOT

 

[Bondo]

Ayuh,..... The backfire is from a lean condition,...... Maybe the acc. pump is failin',.....

 

[BrodyO]

Easy enough to replace. Can someone confirm that the 703-36 kit is the correct one to use?
I don't have the carb P/N on me but I know it's a 2bbl Holley

 

[kenny nunez]

It almost sounds like the power valve is blown out. And any time a Holley suffers a back fire through the manifold that is what happens.
There is a kit that places a small check ball in the carburetor that will prevent the blow out.
Can you post a picture of the carburetor?

 

[BrodyO]

Here's a couple pictures I had on my phone, sorry they aren't better, I'm not near the boat during the weekdays

 

[BrodyO]

Gasket kit look correct based on the pictures?

 

[kenny nunez]

Thanks for posting the picture, I was concerned that your carburetor had been swapped for an automotive type.
The Holley web site will show how to install the check ball that protects the power valve.

 

[kenny nunez]

Your carburetor may have the check ball but if it does not this is the Holley #Hly-125-500. Summit @ $18.02