4.3L carb tuning question and plug w/shattered electrode

NHGuy

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I wouldn't be surprised if you find water is getting in the starboard side combustion chambers, perhaps through the exhaust. Could be via an exhaust manifold, riser or gasket leak between them. Remove the riser and have a look in the manifold. Any water? Water in #4 could have hydrolocked & mashed the plug.
Check that stuff before changing the carb setup. If there's no water and it is carb related you will need to get everything cleaned up and see if there's low fuel supply getting to the starboard side, but If I recall the intake manifold sends one side of the carb to 2 cyls on the starboard side and on on the port. And the other side of the carb goes to the remaining cylinders, so 1 side of the carb going lean isn't as likely a suspect.
 
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Caddy_Kid

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In this case, it actually has 1-piece OMC exhaust manifolds/risers. The story is that the guy I bought it from got it about a year ago without an engine and put this 4.3L engine in and put the OMC manifolds on it to re-sell it this summer (that's. what he does as a side business, he is a marine mechanic at a local [to where I bought it] boat shop). So I could pull the complete manifold and check.

I can also remove the carb and swing it by my carb guy and see what his take on it is. If nothing else, he should be able to tell me whats going on inside it.
 

Caddy_Kid

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After some research, it seems that the plug that is in the boat now (Autolite 25) is pretty much 100% wrong. Its far too short and is a copper resister, when the research I have done points to a platinum or irridium long plug. Not that it will explain all of the issues, but it certainly isnt helping any.
 

funk6294

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Well the wrong plugs sure wont help the situation and could very well be the reason the one melted if they are not reaching far enough into the cylinder. Have you run a cylinder leak down test or comp test to make sure that cylinder still looks ok? If that looks ok or you want to skip ahead then would put in new plugs, and check the rest of the ignition parts to make sure they were not worn. Once all of that looks good hook up a vacuum gauge and use that to set up the carb idle mix and check for a leaky intake manifold gasket that might lean that cylinder out. If your not familiar with the process I have attached an article that gives a pretty good description of how to do this.

http://www.tuneruniversity.com/blog/2011/05/check-your-engines-health-with-a-vacuum-gauge/
 

Caddy_Kid

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I actually just discovered this issue this afternoon and have not had time to look beyond the bad plug, will hopefully have some time this weekend. I am familiar with compression tests and basic vacuum testing (although your article certainly did go into more depth than I was familiar with, thanks much for that!) but have never done a cylinder leak down test. I used to just do a dry compression test, then put in a bit of oil and re-try and see if the reading went up.

In any case, I will get a good set of correct plugs, do at the very least a compression test (I do not yet have a leak down tester) and see what the vacuum readings are like when its running. Up to this point I have only heard it run on a trailer as I just bought it and have not had a chance to get it to the lake. Perhaps that's a good thing.
 

Caddy_Kid

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I called my local Chevy dealership and they say that an engine of my vintage and size calls for an Iridium AC Delco plug number 41-993. I have read on here that marine engines seem to often use a standard copper resistor AC Delco MR-43LTS rather than the 41-993.

Truth be told, I don't really much know what the difference would be. I don't have a fancy HEI or MSD ignition system, just the old school separate coil, so the copper resistors may work just fine, not sure. The cost difference doesn't really bug me, I just want to put the right plug in the engine for how its set up.
 

alldodge

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I called my local Chevy dealership and they say that an engine of my vintage and size calls for an Iridium AC Delco plug number 41-993. I have read on here that marine engines seem to often use a standard copper resistor AC Delco MR-43LTS rather than the 41-993.

Truth be told, I don't really much know what the difference would be. I don't have a fancy HEI or MSD ignition system, just the old school separate coil, so the copper resistors may work just fine, not sure. The cost difference doesn't really bug me, I just want to put the right plug in the engine for how its set up.

Use one of the following plugs listed below. Delco 41-993 is acceptable for auto's but not marine. You want plugs which will hold up in a moisture filled environment, auto plugs will rust out.
AC - MR43LTS
NGK - BPR6EFS
 

Caddy_Kid

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That was my thought as well, thanks. I personally am partial to NGK's for one reason or another but will see what my local boat shop has and go from there. Thanks!
 

funk6294

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Also there will be a difference in heat range between the plug specs for an auto application and the marine. I wouldn't worry so much about iridium or platinum as those were more for longevity than anything else. I am with you as a fan of NGK, and I was able to snag my last set from autozone. If you get them from your boat shop just don't let them sell you Champion.
 

Caddy_Kid

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Yea, I figured I would call them first and check. I owe them buying something there as they have helped me out a bit recently. As I recall, marine usually runs a lower temp range on plugs than auto?
 

funk6294

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Sorry to hear about the compression results. All may not be lost though. With that many cylinder low it could be simply the valves are out of adjustment. Did you put any oil in the cylinders to see if comp improved? If it ran lean and burned all the exhast valves you might get by pulling the heads and having them gone through.
 

Caddy_Kid

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That is kind of my hope. There is no blow-by and no blue exhaust. And it sure starts and idles well, so to me that would also point to the heads rather than rings. I will do the oil trick this weekend and see what happens. Right now I just got done running it, just so I could hear it, and things are pretty warm at the moment. The heads are factory roller rocker heads and hydraulic lifters, I thought you pretty much torqued them down to speck and that was it? I didn't do any of the engine work, but thought that they were pretty much in adjustment as long as they were torqued correctly? So I would also think that its more like burned valves, but no easy way to figure that one out.
 

keith2k455

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So how does it run? Before you get carried away, put the new plugs in, fire it up for a few minutes on muffs and then pull the plugs again and see how they look. If they look halfway good, dunk it for a short trip and check the plugs again when you get it out of the water. If you see anything suspicious in either test, you have a reason to tear in to things further. If nothing looks too suspicious, enjoy!
 

Caddy_Kid

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I did the oil in the cylinders trick and it didn't change the readings one bit. One did read 10psi higher (70psi), but since the others were the same my guess is that I just didn't read the gauge quite right the first time.

Put in new, correct plugs and ran it a couple of times in a garbage can for probably 15 minutes each time with no issues. Fires right up, idles down like it should, and no blow-by or blue smoke at all, hardly even a warm oil smell out of the blow-by tubes. I need to check the timing just to be sure, but it starts and idles down smoothly, so its probably within spec.

The plugs come out a brilliant white, so I pulled the carb and will take it to my carb guy next week along with the plugs and see what he thinks. Looks like re-jetting wont be as easy as on my Holley, but I'm sure he can get me going in the right direction.

At the very least, I would like to bump up the jets and get the plugs to read a bit more like they are supposed to. Also fresh ethanol free fuel. That way at least I will hopefully stop the damage from continuing and leave any big projects to be re-examined this winter.
 

Caddy_Kid

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I took the carb apart and took it over to my favorite marine mechanic. He looked up the specs for that engine and it does have the factory size jets (155) in what is a factory Mercury carb. Mercury did list an optional 160 jet, so that might be one to try.

When taking the carb apart and cleaning it, I did remove the factory block out plug from the idle mixture screw and found that it was only 1 turn out, which seemed pretty lean. Put it back on the boat and proceeded to richen it up a bit, which made it run better at idle and the plugs are starting to get some color on them for a change. A half turn in from its initial setting made the engine die, so I have a feeling it was a bit lean from the get go. I wont be able to do a real tune in that area until I'm on the water and can jump on it and see how it reacts from idle to speed, but at least its taking an adjustment.
 

funk6294

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I did the oil in the cylinders trick and it didn't change the readings one bit. One did read 10psi higher (70psi), but since the others were the same my guess is that I just didn't read the gauge quite right the first time. Put in new, correct plugs and ran it a couple of times in a garbage can for probably 15 minutes each time with no issues. Fires right up, idles down like it should, and no blow-by or blue smoke at all, hardly even a warm oil smell out of the blow-by tubes. I need to check the timing just to be sure, but it starts and idles down smoothly, so its probably within spec. The plugs come out a brilliant white, so I pulled the carb and will take it to my carb guy next week along with the plugs and see what he thinks. Looks like re-jetting wont be as easy as on my Holley, but I'm sure he can get me going in the right direction. At the very least, I would like to bump up the jets and get the plugs to read a bit more like they are supposed to. Also fresh ethanol free fuel. That way at least I will hopefully stop the damage from continuing and leave any big projects to be re-examined this winter.
Just checked back in and it sounds like your making some progress. The upside on your results is all signs point to the top end so at least the rings and bottom end are in good shape. If it idles out ok and seems smooth take it out and run it. If it runs well enough enjoy it for a bit and pull the heads over the winter
 
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