Mercruiser 350 and 305 burns hole in piston #6

delmarjr

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I have a problem with #6 cylinder on three different engines possibly four that have been installed in my boat. The first engine was a Mercruiser 350 V8 and #6 hyperutetec piston top broke off and then I installed a second 350 engine and #6 sparkplug center electrode melted out. The third engine was a 305 and it only lasted less than 10 hours before it melted a hole in #6 piston. I set the timing at 8 BTDC used the correct marine sparkplugs ac mr43t - 350 and ac mr44t - 305 in the engines. I think it has to do with my Thunderbolt IV ignition - distributor and or the module that mounts on the port side manifold riser. It says 22a on the module and all I know is that I could see it advancing with a timing light. The second engine had a different carb on it but the first and third had the same Rodchester Qjet carb. All of the other pistons look fine but #6 is the only one with overheating signs. I feel that there must be something wrong with the ignition or the carb. Also the orginal engine blew up and the owner said he was told he had the wrong spark plugs in it but I believe it was probably #6 problem then again. When I installed the engines I reused the carb, ignition starter alternator, etc. so it's got to be something that is being reused on a different engine. Any ideas? Why predetonation on #6 only? Please respond.
 

pine island fred

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Re: Mercruiser 350 and 305 burns hole in piston #6

Are you using the same intake manifold on the different blocks? Crack or warpage allowing air in resulting in a lean mixture at that port. FRED
 

Robj

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Re: Mercruiser 350 and 305 burns hole in piston #6

That really sux!!!

I agree with Fred, sounds like a lean condition at number 6. Maybe your manifold is cracked, warped or damaged and letting air in.

Good luck and have a great day,

Rob.
 

flashback

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Re: Mercruiser 350 and 305 burns hole in piston #6

I would also check the exhaust manifolds if you haven't already..water getting in there can really tear things up...IMHO
 

dcg9381

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Re: Mercruiser 350 and 305 burns hole in piston #6

I don't know how to prevent it.. Odd problem for sure, but if you've got it timed right - generally carbs distribute fuel evenly.

I'd run them for a short amount of time, read the plugs upon install of the next motor... As the spark is distributed by the cap/rotor, it'd be odd to have the timing of a single cylinder off...

Tough one.
 

Buttanic

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Re: Mercruiser 350 and 305 burns hole in piston #6

Does the intake manifold have a blocked off EGR port. It is usually closest to # 6 and if the gasket is bad on the block off plate it could be sucking air and diluting the mixture to #6 the more than the other 7
 

DHPMARINE

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Re: Mercruiser 350 and 305 burns hole in piston #6

A few years back we had a 4.3 V6 do that.Three pistons,three seasons.I think it was #3 piston.No one had an answer,but then we found it had a 1.5 ratio (V8) outdrive.Changed to proper ratio (1.84 as I recall) still out there running.

DHP
 

delmarjr

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Re: Mercruiser 350 and 305 burns hole in piston #6

Each engine had a different intake manifold but I did use the same water exaust manifolds.
 

delmarjr

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Re: Mercruiser 350 and 305 burns hole in piston #6

I think it has something to do with the thunderbolt IV ignition module or the distributor. Is it possible it is firing to soon only on #6 causing predetonation on that cylinder only? I never heard any pinging. The boat has less than 200 hours and it has wasted four different engines by two different owners. Please respond with any answers to this weird problem.
 

John_S

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Re: Mercruiser 350 and 305 burns hole in piston #6

The timing per cylinder is via the window of the rotor and distributer cap, as mentioned above. The ICM has no idea which plug is firing. #6 fires 360 degrees out from #1. You should be able to swap plug wires with timing light pick-up to compare between #1 and #6. For the timing to be off on one cyl, seems highly unlikely.

The intake manifold leak seemed the most plausible, until post indicating different intakes. Any vacuum tests done?

If water was leaking into chamber, it should be very clean and obvious since it has been disassembled. How about some pictures of #6 head chamber and piston?
 

JustJason

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Re: Mercruiser 350 and 305 burns hole in piston #6

verifying the ign timing is time consuming but simple enough. pull all 6 plugs. rotate #1 up to tdc compression stroke and the timing mark on the harmonic balancer should line up with the timing marks on the t-chain cover. next, rotate till #2 is up to tdc compression, with a little white out, write the #2 on the harmonic balancer and also put a mark where "0" is indicated by the t-chain cover timing marks. Do this for all 8 cylinders. Basically what your doing is indexing. It will help if you stick a screwdriver down the plug holes to feel the piston come up as you rotate it by hand, and may have to pull the manifolds. Where you put the mark is sorta critical, so make sure your at exactly tdc on every cylinder or else the test is useless. after you button it all back up, start it up and put your timing light on 1 through 8 and 1 through 8 should come up on the balancer... if it doesn't or if 1 of the numbers is bouncing and not holding time then you probably have a bad tbolt module.
 

John_S

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Re: Mercruiser 350 and 305 burns hole in piston #6

If you want to go through looking at all cylinders as suggested, I'd recommend just getting a degree tape for your balancer.

If you do find it erratic on only one or two cylinders, than look at those specific cyl first, dist cap terminal, cap connection, plug wire, plug wire routing, and spark plug. If happening on most, than check distributer pick-up, rotor, and distributer end play. The ICM would be about the last thing to just swap out. Thats an expensive guess.
 

delmarjr

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Re: Mercruiser 350 and 305 burns hole in piston #6

Is it possible that the ICM could cause predetonation only on cylinder #6? Or would it only be possible that the distributor is causing predetonation on cylinder #6 only. The spark plugs, wires, distributer cap and rotor were all replaced after the first engine blew up.
 

John_S

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Re: Mercruiser 350 and 305 burns hole in piston #6

A T4 ICM does not know which cylinder is firing, so it can not treat number 6 different than the rest.
 

whywhyzed

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Re: Mercruiser 350 and 305 burns hole in piston #6

Did the other 7 pistons look 100% ok, or is #6 just the first to die each time?

I hate to suggest this - but sounds like someone repeatedly messing up the firing order with the plug wires...

Like the others, I cant see the ignition module causing a single cylinder issue.

At 4000 rpm, the module is making 16,000 sparks every minute. That's 267 per second. Of those 267, thirty four are for plug #6. Can't see how it could mess up on those 34 and get the others all spot on.
 

tommays

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Re: Mercruiser 350 and 305 burns hole in piston #6

"A few years back we had a 4.3 V6 do that.Three pistons,three seasons.I think it was #3 piston.No one had an answer,but then we found it had a 1.5 ratio (V8) outdrive.Changed to proper ratio (1.84 as I recall) still out there running."


I still think the above needs to be looked at

What kind of WOT and speed was it getting when it was running as over proped would cause the failure



Tommays
 

Silvertip

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Re: Mercruiser 350 and 305 burns hole in piston #6

Some years ago Ford had a similar problem with engines burning up (I think) #7 cylinder. The problem was (if my memory serves me correctly) spark plug wire routing. Cross feeding from one wire to another affected that cylinder. Proper spacing of the plug wires apparently solved the problem but a lot of engines were junked because of it. This may be the problem here as well but I, like the others, feel this is a lean mixture condition on that cylinder.
 

delmarjr

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Re: Mercruiser 350 and 305 burns hole in piston #6

I installed new 8mm silicone wires before and hooked them all on the wire seperator tabs on the back of the engine and checked the firing order several times, the prop is a 21 pitch Mirage and WOT was about 4400-4600 rpm at about 55mph in a 1990 Stingray SVC 200 19 ft boat. Mercruiser 350 mag with an Alpha 1 outdrive. It has a Mercruiser Thunderbolt IV electrinic ignition and the module says 9D13A V8-22A and is mounted on the port water manifold riser. I was told my a marine mechanic that he believes it is the ignition module because he thinks it is probably advancing too much and that #6 and #7 are the first to go. All of the other pistons look great with no signs of predetonation at all, only #6 every time. The only parts used on all engines are ignition, distributor, exaust water manifolds, altenator, starter, thermostat housing, power steering pump. Anymore ideas?
 

John_S

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Re: Mercruiser 350 and 305 burns hole in piston #6

The V8-22A timing curve is not agressive at all. The 350 mag usually has a more agressive one like V8-24S. The 22A is very gradual compared to others. If its advancing too much, you will see it on the timing light. As I said, you can easily check #6 to compare to #1. I would think you would do everything at this point to verify what the issue really is, vs shot-gunning an expensive part and possibly another engine. If your timing is jumping around, the distrubuter is much more likely to be the source.

See timing curves in: http://www-alt.mercurymarine.com/mnetdata/Service/Cruiser/Techndbk/94/94hg4b.pdf
 

CharlieB

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Re: Mercruiser 350 and 305 burns hole in piston #6

GM Q-jet intake manifold are NOT as even flowing as many people think.

#7 typically runs slightly lean , consistantly, wet flow air streams have fuel 'fall-out' differing cyl's differing rpm.

No one asked, is this a single engine application, normal rotation?

What carb is used, was it the same on all engines?

Square-bore carbs have been 'stagger-jetted' to compensate for the fuel distribution problems of an intake.

Double check, pressure test that exhaust also, just to be sure.

Another thought, cam shaft reused between engines, low exh lobe on #6?
 
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