1976 1500 cylinder going?

[rriddle99]

HI all. Got another issue on my 150hp monster. This has happened twice now at high speeds I lose a cylinder. Engine RPM above 5K. Yesterday it was about 5400rpm when it happened. It seems to be "fouling" a plug with aluminum which I know is not good at all. Quest I have "is there a way to save it?". Can something be adjusted to stop this issue even tho its only on 1 cylinder? Here is the pic of the cylinder after the 2nd time it failed:



:facepalm:

This next pic is the next 2 plugs. The right one is number 2 and the left one is cylinder 3:



Anything I can do? I'm going to clean the plugs and go fishing with the boat here in a few minutes. I wont be running it hard today and hopefully it will be good to me. I would assume it it was a lean condition or timing I'd see it on at least 12 more cylinder.

 

[rriddle99]

Re: 1976 1500 cylinder going?

The first time this happened the plug was wet with just a couple specs of aluminum. This time it's covered with aluminum and dry???

 

[Chris1956]

Re: 1976 1500 cylinder going?

Clean your top carb, install a new waterpump impeller and make sure the max timing is no more than 21* BTDC. Oh yes, say a prayer to your God.

 

[rriddle99]

Re: 1976 1500 cylinder going?

Thank you Chris for the fast response. I'm working on impeller now. The telll tale was flowing pretty good. If that is the case is there a water passage that needs cleaned or checked for the top cylinder?

Not sure if you remember but this is the one that cant run at 21* for whatever reason. It's at 28 and was running nicely until this. IF I put it at 21 I cant even get it to plane. Just jogging your memory. I'll try to check it out and verify the timing. I did verify the compression and cold it's about 118psi just a cylinder 2 and 3 are.

 

[wired247]

Re: 1976 1500 cylinder going?

The 28 degrees is why its detonating. You WILL lose that piston.

Is it a '73 "Circle C" casting block?

 

[rriddle99]

Re: 1976 1500 cylinder going?

Is it a '73 "Circle C" casting block?

I dont know. The block does not have a SN on it. I was told in another post it was most likely replaced under warranty and the dealer did not stamp the SN in it. My guess is because it's a 1976 SN the block would have to be close to that or newer.

I backed the timing off a little and am going to take it to the lake. I just replaced the impeller and checked the carbs for dirt which they were clean.

I'm going to run it on the muffs then check the plug again before I hit the lake.

 

[wired247]

Re: 1976 1500 cylinder going?

Most 1500 dealer service blocks were early production blocks that were set aside for service replacement use. During the first few months of production Mercury got a bunch of complaints about the 1500 being a total dog on the bottom end. The ports were just too high. It prevented decent compression from building and the timing of the ports didnt work well for low rpm with bigger boats. Part of my total dislike for inlines comes from a few run ins with the early 1500 blocks. They run like crazy on the top end but are dogs down low.

You can recognize the block by a "c" cast into a round raised area on the side of the block. Being a service block I'm betting you'll find one. They are similar to the Circle J blocks in their port timing but are even more agressive in their lack of bottom end.

Whats the cure? Use a later block when you rebuild it ( it will be a short time off with 28 degrees timing ) ...or... Use a lower pitch prop in the 15-17 range. You can try venting the prop to get it up into a usable RPM range before it bites.

 

[Chris1956]

Re: 1976 1500 cylinder going?

So you are melting the top of your piston. You might pop off the top transfer cover and see how bad it is.

 

[rriddle99]

Re: 1976 1500 cylinder going?

I changed the impeller, flushed carbs, cleaned the plugs and backed the timing off a little. It ran ok on the lake but I was careful not to give it WOT very often and not for long when I did. I'll check the plugs when I pull it from the lake. why would this be only 1 cylinder? When it happened the first time the other 5 plugs looked better than any plug i've ever pulled from an engine. Very clean with just a hint of carbon between the electrode an the outer ground.

Chris... I know this might be a stupid question but will I have to replace the gasket when I do that? I know it is normally suggested however I'm not sure how long it will take me to get one.

 

[wired247]

Re: 1976 1500 cylinder going?

Its an inline. #3 runs lean. Not normally a problem until you advance the timing to 28 degrees.

 

[rriddle99]

Re: 1976 1500 cylinder going?

Here's an updated pic from Cylinder #1 after running it today:



I did not see the "C" cast into the block anywhere. Might it be in an area that is not visible with all the framing in place?

Anyway... it ran great on the lake today. As I said I backed off the timing a little. I'll take it down more but I think this whole thing makes sense now. I was forced to go to a higher timing setting possibly due to an oversized prop. I know Chris and I discussed that in one of my previous posts however we somewhat dismissed it until now. I'm going to look into getting a lower pitch prop to see if I can get the timing down below 21* as its been suggested to land around 19* with today's fuel.

If this is a "C" block what steps do I need to take to improve performance without rebuilding engine and changing blocks. Is there a different trigger plate for the distro that can help this?

 

[Faztbullet]

Re: 1976 1500 cylinder going?

Thats preignition damage and looking into the spark plug hole you will see the top of piston eroded.This usually happens from a cooling or ignition problem(incorrect timing) as carbs wont affect a single cylinder. The distributor models are bad about this......

 

[rriddle99]

Re: 1976 1500 cylinder going?

Based on all this good information I'm going to look at venting my Prop. Anyone have any links on how to do this correctly? I've read it's not that difficult however I'd like to have a blow by blow description of steps to make sure I dont destroy this prop. The one I have is a 48 72762A4 19P and it's stainless. I'd like to vent that before I buy another prop because I know venting is reversible just by plugging then vent holes.

 

[wired247]

Re: 1976 1500 cylinder going?

Reduce that timing and you may end up going to a 17 pitch which you can still get 45 mph out of.

 

[Chris1956]

Re: 1976 1500 cylinder going?

Good luck drillling that SS prop. Start with a real small drill, and graduate a few times. Go real slow and use lots of oil to keep temp down, or SS will case-harden. 5/16" hole was the best, if I remember correctly.

Crap, After you are unable to drill that SS prop, buy a Turning Point Hustler and use it. It comes pre-drilled for about $100.

 

[rriddle99]

Re: 1976 1500 cylinder going?

Thanks Wired and Chris! I should be ok venting my prop. I have carbide drill bits :joyous:

Do you have a link or PN for the Turning Point Hustler prop?

I've searched for them but I can't tell which ones are pre-drilled.

 

[rriddle99]

Re: 1976 1500 cylinder going?

ok... sorry for that Chris. I see they are all drilled.

 

[Chris1956]

Re: 1976 1500 cylinder going?

RR, Those Hustler props give nearly the same performance as my SS Rapture prop, which was real good. Hurricane Sandy got all my props, including the Rapture.....Rats.