Mercruiser 350 and 305 burns hole in piston #6

[CharlieB]

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

?????????????? 8 degrees initial, @ idle 600- 700 ????????????

I gottaaaaaaa go find a book but I'd swear that's too much!

I rather set total timing at 2800 and don't worry about initial.

Did you ever check total?

 

[alaska_av8r]

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

Delmar pressure test those exhaust manifolds just to be on the safe side, that seems to be one of the common things between all the engines per your post above. I had a manifold that was slow leaking on me and of course only showed up after the engine was shut down for a period of time, ironically it was leaking just a little into # 7 and a lot into number # 6 ....

Just a thought, and at least you could eliminate those as culprits after a pressure test.

 

[CharlieB]

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

Checked an OLD factory manual, small chevy's used up to 10 degrees at idle.

Also called for 93 octane. (with lead)

Which brings up another ? What's your 'usual' fuel?

Generic gas stations in my area, the fuel burns SO HOT, I can't/won't run it any longer in anything I own, AND, I tell my customers to run mid-grade NAME BRAND fuel.
The generic has caused the fuel to BOIL in my chainsaw, squirting out the vent.

 

[delmarjr]

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

Anymore ideas? I was using 87 octane WAWA gas. I was setting the timing at 8 BTDC with a timing light at a reduced idle-around 500rpms. I hope that was the correct timing procedure. I did notice the timing mark advancing way up off the scale when I would rev the engine up with the timing light on it. I thought that was normal but without using an advanced timing light I'm not sure it was advancing way too much or if that was normal. The engine never pinged or at least I never heard it, but it does have thru hull exaust and it is fairly loud. I'm leaning towards either the distributor or the module has a problem. I think it's got to be an ignition problem. I pressure tested the exaust manifolds with a garden hose while blocking the exaust end and I didn't see any leakage but I'm not sure if maybe when it gets hot that a small crack could open up, but not leaking when cold. Anymore thoughts?

 

[Flukinicehole]

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

Is your through hull exhaust factory? If not, engines need some back pressure and if not done properly this could cause a lean condition. What Charlie says is true, usually it is #7 thats lean but thats because of the 5,7 firing order.

 

[DHPMARINE]

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

Lots of guesses and speculation here because that's about all we can do on line.But have you actually checked the ratio of the outdrive? It's a bit involved because you have to pull the cover and count teeth,but it reallly could be your problem.

DHP

 

[delmarjr]

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

The thru hull exhaust tips are factory and made by Gil but they are open inside.

 

[flabum]

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

When you changed the rotor, did you change the plate as well or re-use the old one? If the old one had a deformity on #6, it could cause a repetitive problem.

 

[delmarjr]

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

I changed the rotor plate as well because it was attached with three small screws and came attached to the new rotor. Anymore ideas? I had an old salt marine mechanic tell me it's the ignition module and he says it's probably advancing the timing too much. He says #6 and #7 cylinders are the weak link in a chevy 350. But I don't see any overheating on any of the other pistons, only #6. Has anyone ever seen this problem before?

 

[flabum]

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

The timing on these engines are quite conservative. If the timing was set so close to the threshhold that it burned a cylinder, then the timing was improperly set.

That being said, I still think it was that loose spark plug.

Other possibilties include a cracked intake at that cylinder, poor gasket seal there, poor fuel quality or anything that can introduce more air to that cylinder or less fuel.

 

[MikDee]

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

The timing on these engines are quite conservative. If the timing was set so close to the threshhold that it burned a cylinder, then the timing was improperly set.

That being said, I still think it was that loose spark plug.

Other possibilties include a cracked intake at that cylinder, poor gasket seal there, poor fuel quality or anything that can introduce more air to that cylinder or less fuel.

Loose spark plug? I don't remember reading that here anywhere , but if it were loose, it would create a vacuum leak in that cylinder, causing it to run too lean, that would be a good probability then.

 

[flabum]

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

Loose spark plug? I don't remember reading that here anywhere , but if it were loose, it would create a vacuum leak in that cylinder, causing it to run too lean, that would be a good probability then.

Sorry, wrong thread....there was another who had a lean piston and he found a plug loose in that cylinder. http://forums.iboats.com/showthread.php?t=230019

 

[CharlieB]

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

One, Any pictures of the toasted pistons, from the current and prior engines?

Two, can you get your hands on an advanced timing light. I really would like to know what the total advance is on that module. It is possible that the module has too much advance for your base timing, knowing how much you should be able to reset your base # and be safe.

Three, any of these engines re-use the water pump? Could be be a slight restriction in cooling flow to that bank?

 

[oops!]

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

hey del. welcome to i boats. and merry christmas.

thats one stupid problem there. i hate thoes kind!

people here that have replied, know a lot more than i do.

however....when i was 16 years old ......omg .....30 years ago.....
i worked at a tune up shop....we had a simmelar problem, if memory serves....not !...
we took out the guys dist. and put it in a machine that showed where it was firing and the mechs....good ones... determined that the dist shaft was bent. or somthin like that. the actual shaft was not sending spark evenly as the shaft turned. it was out just a hair but it was enough.
id have yours checked.

other than that, the only thing i know of that will cause super heating in a hole is mixture.
my suggestion is check or replace the dist. and check or replace the mani. it will be cheeper than another block and a re & re. anything that youve used on the previous motors is suspect.....!

good luck
and again, im not a pro....i just remember somthing like that in my jadded past...
cheers
oops