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Old 11-24-2002, 04:43 PM
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Post Melting Spark Plugs

Hi Guys,

I am not a hot rodder as such but I am installing a 350 Chevy in a Ski boat and I thought this forum would have the best of all worlds for engine questions.

The problem is I am melting spark plugs. The plugs are melting in minutes after a quick run. It is always the same 3 plugs too. The engine has been rebuilt and am running electronic ignition. I have installed a 650 Holley carb. Does anyone have any suggestions on what to look for that might be causing this problem. I have hit a dead wall and do not know where to look from here. So if anyone can help it would be greatly appreciated.

Thanks in advance,
Waterhead

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Old 11-24-2002, 05:04 PM
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Sounds like you are way too lean. as in not enough fuel. This is not good!!! Some of the high tech guys will have more for ya!!!
Let us all know what you find out.
Slider in Wa.
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Old 11-24-2002, 05:38 PM
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Hey waterhead,
I've been around some boats in my day. What have y'all got?
Slider...is right you are way too lean!!! at first take. maybe wrong plugs.
Need more data as ET says, to help you.
kind of boat...
Motor & size...
compression....
cam............
carb...........
ignition.......
ign. advance...
type of plugs std or extended and heatrange
rpm range.

bentwings
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Old 11-24-2002, 05:41 PM
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Sounds like you might have a vacuum leak in your intake manifold. How do the plugs look? Is the color good on the other plugs?

We need more info.
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Old 11-24-2002, 05:46 PM
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Another long shot, do need more info. But will say I`m not sure of the cooling system your running, but make sure there is no air pockets, we ran a inboard V6 on a boat years ago, and had a problem similiar to yours, a air pocket had prevented any coolant flow and thus caused all kinds of problems, if this doesn`t help, I agree with the other posts.
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Old 11-24-2002, 06:24 PM
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Ok, a bit of history. The boat is a Hallet Skimaster (I believe you can only get them in Australia)with a centre mount 350 Chev small block. I brought the boat without the engine installed but the engine had already been rebuilt. I have written down everything that the rebuilder could remember putting in but don't have that with me at the moment (will post later). The engine is raw water cooled with water cooled manifolds (water enter thru pickups and out exhaust and we can see water coming out of both exhasts). The ignition is electronic (we believe it is an msd Electronic distributor but are unsure of the control unit). We are running a 650 holley carb which has just been reconditioned (jets cleaned, not replaced). It has been bored out 30 thou. Just replaced the fuel pump and the alternator.

We have run the boat twice and it only lasted 5 minutes the first time before we lost 3 cylinders. The plug looked like it had been hit and the gap closed up so we originally thought we had something in one of the cylinders but the other two plugs look like the top of them have melted. After replacing the plugs and running for a while all sounds good then we put back in the water and the same thing happens. The other plugs all look ok. Not sure of plug types but will check. I have also been told that it is possible we have the wrong coil?? How can I check for air leakes or how do i know it is running too lean??

cheers,
Waterhead
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Old 11-24-2002, 06:59 PM
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Just wondering can u give the #'s of the 3 cylinders with the problem??
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Old 11-24-2002, 07:44 PM
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A smashed side electrode and losing gap can mean only one thing...detonation. What are you running for initial timing?
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Old 11-24-2002, 08:36 PM
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i've built many small block boat engines and most are hard pressed to make 160% temp while running. what gas? what plugs? what is timing set at?,check for leaks at the intake gasket. does the carb seem to respond ok?
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Old 11-25-2002, 02:04 PM
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Some more info on my motor:

- 350 chev small block
- 650 cfm Holley Carb
- bored 30 thou over size
- new pistons: hi protech piston: solona
- cam bearings ground
- new cam: dynamic kinetics
- con rods closed and honed
- single molly rings
- New cam followers
- New push rods
- Head serviced
- new melling oil pump & new drive
- Double row timing chain set
- New ARP head bolts
- Spark Plugs: bosch w8dc washer seated
- Electronic ignition
- Timing is set as per a standard 350 chev manual

I have been talking to a few of my local contacts and they all seem to think it has to do with the type of spark plugs I am using. We a using 20mm spark plugs, maybe I should be using 10mm ones and a colder heat range plug. The 20mm plugs would be putting the spark plug right in the middle of the explosion. Another possibilty is that we are using the wrong coil. Is it possible this could be my problem.

I have not ruled out an air leak in the intake manifold, would you expect that you could hear the air leak? How can I check for one?

The cylinders that keep melting the spark plugs are 4, 5 and 6.
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Old 11-25-2002, 03:52 PM
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I'm certianly no boat expert, However, I'm not entirely sure those are the right plugs. I had thought the chevy plugs were tapered seat, no gasket.

Not because of the electrode being to far into the explosion but (someone correct me if I'm wrong) because the tops of the pistons are so close that they are being hit and bending them. Can you post a pic of them. Maybe the cylinder heads weren't milled right. To much angle one way or another. You sure their melted, not just bent?
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Old 11-25-2002, 04:42 PM
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One looks like it is just bent, two appear to be melted. But its always the same three cylinders.
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Old 11-25-2002, 05:52 PM
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all of your plugs are in cylinders that have siamesed exhausts = heat, however i think your plugs are too long, but could NOT, if that is the case, explain why you're only blastin 3 of them! also, there are some intakes/gaskets that dont allow water to get to the intake manifold.

in any case if i were you i'd sure be jackin up the guy who built the engine

[ November 25, 2002: Message edited by: bullheimer ]</p>
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Old 11-25-2002, 06:43 PM
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Im gonna hazard a guess thats a little funky... i think weve been missing the boat so to speak... this is definatly detonation... but if hes cooling this think with raw water, this is gonna run cold... as in 135-160... isnt this gonna substansially change timing requirements? as in alot less? I know my cars always rattle a bit cold, but a car usually clears 160 in a min or so after startup... obviosly a vacumn leak could lean it out... detonation like that, im suprised you havent joined the gasket club... ben there.. done that... remember to change the oil often... that cold will never bpoil water, etc outa the oil...
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Old 11-26-2002, 09:58 AM
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Question

Not sure I understand the reference to the 20mm and 10mm plugs. If this is the reach (length), it's possible that the long plugs are hanging out in the breeeze in the chamber. As for only 3 cyl. getting killed.. it may be that these are the most critical and closest to the 'edge'...these go first the the motor runs like crap so the others cool off.
what do the other plugs look like??
also look more carefully at the ignition. I just worked on a mech inj SBC and ripped out my hair trying to get it to run. Come to find out that the MSD has programmable cyl count, advance, and rate. Once re-programmed the thing ran like he...ll. Never knew that a v-8 would even run on 3 cyl 2 cycle ign!!! Also be aware that unhooking the battery can lose the program.
Boats run hard all the time just like a dyno pull so go easy on the total advance and don't get it in too soon. This motor should run pretty good.
I used a restriction on the water outlet on my boat. It took a little adjusting but I finally was able to get it run at 180 all the time. You can't easily run a thermostat as the inlet water pressure will force it open at speed. I did use a thermostat, water pump, and stock water neck. This kept the motor warm at speed as well idle. the intake water was run thru the exhaust manifolds to preheat it then back into them after passing thru the motor. Very complex. It's much easier to use a heat exchanger system. Just the way my boat was setup.
The vacuum leak possibility is very real too. Just pulling a small amount of oil in can cause massive detonation and not even smoke. The leak may be from the bottom side of the ports and will be difficult to find. A very low vacuum reading might show it up. You may even have broken rings or improperly installed rings allowing oil to get into the chambers. An upside down ring can cause this...saw it in a circle burner.
Possible that the power valve is not opening up enough or at all causing a very lean condition. I would think the motor would act up...missing, running rough or not taking throttle however.
just some more thoughts. Tough problem. keep feeding us informationa and we'll try to help you.
bentwings
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