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Caddy 500 melted spark plugs

Posted: Sun Sep 17, 2017 12:45 pm
by AirPat
Anyone know what could cause this?
I was loaded down with feed during Harvey floods pushing cattle to high ground. Three burned up plugs all next to each other. Pulled head and found no scratches in cylinder walls. A few nicks in one piston. Re assembled and did compression check and all checked out good.
Motor is a 500 caddy DD with 10 to 1 heads
Edlebrock intake and carb.

Note: before this outing my ignition coil was failing ( bolts stripped out and could not tighten wire down) and I replaced it with an Accel brand one off the shelf from orielly. The ingnition coil over heated and shut the motor down. I limped back to the ramp on old coil. Got different coil with correct voltage and motor ran good. Could the damage have already been done?

Re: Caddy 500 melted spark plugs

Posted: Sun Sep 17, 2017 8:09 pm
by Deano
If you are already running good gas, almost certainly to much timing. Possibly running lean as well.
Either can and will create/contribute to detonation that you don't want and often can not hear.
At that compression, you have to run 93 minimum. If you aren't . . . that will do it to.

Detonation is likely the most prevalent Caddy killer (especially with higher compression).
To much timing and/or sub-standard gas and/or running lean could, and will, all contribute to that scenario.

Heads and pistons determine compression ratio. Are you running early factory heads with OEM pistons
or if not, what heads and what pistons do you have? What is your total timing?

If you're running the original 45 yr old points distributor, I recommend cutting through that BS and just replace it
with an HEI. More reliability, fewer moving parts, less maintenance . . . considerably less grief all the way around.

Others may have made out differently, but I've had no problems with the likes of these generic ones: ... 1312901139
Given a 10 dollar curve kit, that is about the most reliable and cost effective alternative for a Caddy DD, imo.

Not to imply that it is the cause of your problem, but have you checked the condition of your timing chain? Excessive
gear keyway wear will allow the timing to float and jump around and is guaranteed to complicate any timing efforts.
After removing the cap, turn the prop CW until all slop/slack is removed and note and/or mark the rotor position.
Then turn the prop CCW and see how far it will go before you see the rotor move from its position. A good timing
chain may allow some, but not much slack. If you can move the prop a measurable amount, it likely needs replaced.

Re: Caddy 500 melted spark plugs

Posted: Mon Sep 18, 2017 7:54 am
What degree of advance were you running with the timing and how was the distributor configured?
b)vac advance
c)mech curve

Were these roasted plugs on the port or starboard side of the engine?

What fuel were you running and what engine temperature were you experiencing under your heavy load period?

The engine, has it been rebuilt or is it a stock take out ?

Direct drive (rpm you run wide open) ?

The use of the Edelbrock intake concerns me, the "Cadillac Master" has taught me that they do not fit the engine properly. They are poorly cast and of the 100s that he has handled in the past 20 years, only one was out of the box acceptable. He has taught me to place the manifold on top of the assembled engine with no gasket and inspect the fitment. What we find every time is the casting is slightly off which does not allow for a proper fit. If you just push thru not knowing, you will have one side seal just fine and the other will have a gap that likely will create a vacuum leak. Our intakes go to the machine shop and get "cut" to make a perfect fit. Mr. Branch has notified Edelbrock of this problem some years back but they choose not to make any changes to the product.

This engine failure likely is a product of several factors. The response should be geared at making sure none are an issue in the future.

1) Replace intake with a stock cast iron (more torque) or verify fitment and weld center plenum on Edelbrock
2) Verify proper timing and follow Deano's direction on checking your chain for key way play. Suggest u run a delayed mechanical curve-verify
3) Obtain fresh 93 octane fuel and blend in 10% AV gas in weather where you experience high heat use and also use a quality electric pump
4) Inspect the cooling system. Verify that a functional (CADILLAC SPECIFIC) stat or if removed the freeze plug was properly installed under it.
5) Take steps to ensure engine is running cool (under180')
6) Use high quality aftermarket intake gasket such a the MTS or SCE with printed sealer bonded to its surface
7) Inspect vacuum caps for damage / replace

CHECK YOUR VALVES and seats ! They had to chew up some debris

Re: Caddy 500 melted spark plugs

Posted: Mon Sep 18, 2017 9:46 am
by AirPat
Thanks for the responses.
let me see if I can answer some questions.
Motor was rebuilt by myself about 3 years ago. Had block, heads and crank at machine shop to bring into spec. Block is .30 over. I believe the pistons I used were sylvalite? Idk I will have to check when I get home. I wrote everything down I did and parts I used. They are flat top.
HEI distributor with 10 deg. Adv. no curve kit.
Cloyes timing chain. ( I will check for slop in it)
Had a MTS gasket on intake
Roasted plugs on starboard side.
Motor runs around 180 to 190
2900 Rpm on trailer max
There is a crack in the vac plugs on carb.
Had one bent valve that I replaced in broken plug cylinders ( got nervous when you guys said parts are hard to find so I have an extra set of heads and an entire other engine in the garage for parts)
Boat has been sitting since January with fuel in it. Topped off tank with 93 octane. Maybe 8 gallons out of 30.

Maybe worth noting:
I rarely ever run the boat as hard as I did that day. Maybe never have.
I can not leave the boat in the rain. Water leaks in some how. Could this be thru the intake gasket?
Compression tester reads 200 psi in all but one cylinder at 190. (Not one with damaged plugs)
I wouldn't say my mechanical knowledge is great. I built this motor from reading yalls post and a book I bought off So if my answers seem vague please forgive me.
The main thing I don't really understand is the difference in mechanical and vacuum advance or curve kits. I basically am running the same set up that came on the donor motor I used to build it. There is no vacuum lines so I am assuming its mechanical.

Re: Caddy 500 melted spark plugs

Posted: Mon Sep 18, 2017 2:40 pm
by Deano
When you get back to your notes and can confirm what pistons you have, please identify the head casting number(s) as well.

While I most certainly hope it does not end up being the case . . .
if by chance you have early 76cc heads and the pistons are KB353s (which are flat tops), there will be little mystery left to what happened. I don't think Silv-O-Lites were cast in that configuration for this engine family.

That head/piston combination will yield compression upwards of 13:1, by which point, exotic fuel becomes absolutely mandatory.
Those particular pistons are designed to bring the compression up when using the later 120 cc heads and are good for that purpose.

When you've identified the head casting numbers and which pistons you have, we will better able to draw some definitive
conclusions. No need in theorizing about what might have happened without knowing exactly what you're working with.

The one other thing you didn't mention was the cam. Is factory, or was it replaced, and if so, do you have its specs ?

Re: Caddy 500 melted spark plugs

Posted: Mon Sep 18, 2017 5:38 pm
by airduds
"There is a crack in the vac plugs on carb. "

She's running lean.

Re: Caddy 500 melted spark plugs

Posted: Tue Sep 19, 2017 7:39 am
If you have it off the boat you may want to check the bearings and rings. That level of detonation can waste your rod bearings and crack piston rings.

As Deano advised get your casting numbers because your compression calculations are more than likely off. Usually a true flat top will put you at either 9:1 with 120cc or nearly 13:1 with 76cc heads. It would be hard to believe a plain cast piston could survive that degree of abuse. That said if any piston had a chance to survive a true flat top would offer that and Silv-O-Lite makes the KB so they have a good alloy.

Clearly you have several issues that came together at the same time to yield this result. A large cracked vacuum cap can really cause problems if that was the case it is a piece of the puzzle. Any air flow that does not go into the carburetor will not pass by the venturies and as such it decreases the volume of fuel. This compounded with the intake selection, hot environment and older fuel starts to stack up against you.

On a side note:

Pump fuel with ethanol loses about 3 octane points after storage in a vented system beyond a few weeks in a tropical environment.
We do everything we can to preserve our octane rating but the key is getting proper fresh fuel and blending in Av or a race fuel is good insurance. As Deano stated if in fact you accidently built a 13:1 engine you are talking a need far above 93 octane and a huge cost to feed that requirement. If you do nothing else add a bottle of Marvel Mystery oil to your tank if your boat is going to sit a few weeks that added can slow the degradation.

I'm told by old Cadillac racers that Champion plugs are a better choice fyi, there experience had been they stay together a little better if you get into detonation.

Re: Caddy 500 melted spark plugs

Posted: Tue Sep 19, 2017 9:28 am
by One Eyed Gator
Get the Silicon Caps they last a lot longer than the black rubber caps.

Re: Caddy 500 melted spark plugs

Posted: Wed Sep 20, 2017 10:15 am
Wow that was some good reading ,i moved past. Caddys 20 years ago because of a bunch of things happening like this we could never figure the issuse out .this artkcle is very infomative a.nd surely will help the new caddy guys out ,

Sent from my SM-G935P using Southern Airboat mobile app powered by Tapatalk

Re: Caddy 500 melted spark plugs

Posted: Wed Sep 20, 2017 10:56 am
by AirPat
Sorry guys. Been bowed up at work trying to get concrete placed. Harvey jacked our schedule up and now it's make up time. Working late and going in early. Should have some time this evening to get the info.
Thanks for the replies.

Re: Caddy 500 melted spark plugs

Posted: Wed Sep 20, 2017 6:56 pm
by OneBFC
Pump fuel with ethanol loses about 3 octane points after storage in a vented system beyond a few weeks in a tropical environment.
Swamphunter, do you have a source we can read for this claim?


Re: Caddy 500 melted spark plugs

Posted: Wed Sep 20, 2017 8:41 pm
by AirPat
Ok. The pistons are not flat top. They are Sylvia-o- lite pistons like the one in the picture. The boat is in storage so I'm going to swing by there tomorrow and get casting numbers. I checked into them before purchase and referenced an old thread on here and am 99.9 percent sure they are the 10.1 heads.
Here is my plan for re assembly:
Drain all the gas
Use highest octane fuel I can find. I'm assuming you can buy AV gas at the airport. There is a small one down the road. Plan to swing by there and get some and mix in.
Fix vac plug leaks
Use stock intake. ( I am still debating on this. I might just take the edlebrock to a machine shop and have it machined to fit and weld up plenum and exhaust cross over.)
Check for slop in timing chain
Check timing
I already re installed head with new gasket.

What exactly is detonation. Can you hear it?
Is there any way to check for leaks in the intake?

Re: Caddy 500 melted spark plugs

Posted: Wed Sep 20, 2017 8:56 pm
by AirPat
Swamp hunter.
Motor is still in the boat. I pulled the oil pan and looked around the best I could and didn't see anything alarming. Motor was still holding good oil pressure so I'm just going to hope for the best. Thanks for the advice though.

Hate to say it. But if the motor is coming out of the boat a Chevrolet and reduction are going back in. It's been fun with the caddy over the years and I learned a lot of what not to do but it would be just the extra push I would need to Un a#* the cash and go with the reduction.

Re: Caddy 500 melted spark plugs

Posted: Wed Sep 20, 2017 9:53 pm
by Deano
Those are factory replacement pistons for the later 120 cc heads. They are very similar to and are the standard
replacement for the OEM pistons used from '74 -76'. They will ultimately yield about 8.25 - 8.5 with the later 120 cc heads.

Understand that there are no " 10:1 " heads. This is likely slang for the early 76 cc heads used by persons who saw those heads on an engine that produced that ratio, but were not well versed in Cadillac anatomy. The early heads were used by the factory with different pistons to arrive at different compression ratios. Depending on the piston used during which year, those same heads were used to produce 10.5:1, 10:1 and 8.5:1. This is why I said earlier the compression ratio was contingent on the head and pistons.

I will guess, based on your cranking pressure, and what has transpired, that you do have the early 76 cc heads and your compression is consequentially much higher than was your intent. Again, this is a guess, but should be a reasonably good one.

The early 76 cc heads with the factory 10:1 pistons that they used will produce 165 - 185 lbs cranking pressure.
(Be aware that those pistons have a much larger dish than do the ones pictured.)
Having 200 lbs cranking pressure implies a higher comp ratio than that. Conversely, the later 120 cc heads with
those pistons yielding a skinny 8.5:1 will generate more in the neighborhood of 145 - 165 lbs.

Naturally, those are ballpark numbers dependent on other variables, but should suffice to illustrate the point.
With early 76cc heads and those pistons, your compression would not be 13:1, but would be well beyond the 10:1 that would
happily run on 93 octane.

If you were mistaken and have the later 120 cc heads, then that would be a good thing as your fuel requirement
would be an absolute non-issue, and you don't have a head/piston mismatch. If this is the case, your problem may
well be far less serious than what has been outlined as a possibility based on the wrong heads.

With that being what it is, we arrive back where we were, needing to confirm which heads you currently have.

Re: Caddy 500 melted spark plugs

Posted: Thu Sep 21, 2017 7:26 am
by One Eyed Gator
When I welded the cross-over in my aluminum intake, I ended up have to add small piece of thin metal over the exhaust passage or it would burn through the fel-pro type gaskets in no time. This was even after i had head and intake surfaced.

airpat I was a diehard caddy fan ran a DD caddy for 10 yrs plus. Swapped over to a carbed 5.3 with a 2.68 geared box on same hull and rigging. Boat performs much better.

Re: Caddy 500 melted spark plugs

Posted: Thu Sep 21, 2017 9:04 am
Not to hijack the thread but to answer Russ.

There have been several sources that I had read about ethanol fuel degradation, don't have a link for you but I first found issues with ethanol enhanced (oxygenated) fuel in a vented system about 7 years ago. The scientific name is "phase separation" in which the ethanol falls from the fuel blend. Ethanol is used to obtain the octane rating in the blended fuel. The ethanol typically has a RON octane rating around 109 -112 as such it's addition is used to boost the base fuel to it's rated octane number. The open system of a vented marine application suffers when the alcohol absorbs up humidity and bonds to water molecules. The ethanol is lost from the fuel in it's usable form and becomes a watery oily by product that when agitated can become milky in color. A dryer fuel will remain clearer but a more oily fuel will cloud up. The three point reference comes from the belief and understanding that todays Recreational fuel 89 octane is the base for Premium Oxygenated (10%) enhanced 92-93 octane which is your typical premium fuel. The addition of the alcohol and a few additional additives raise the base fuel 2 plus octane points. I use the 3 point reference to error on the side of caution and feel I am very close on my calculations. I know this is a topic of concern in the recreational aircraft arena for pump gas conversion waivers. I don't think I would put it into a plane or ultra-lite personally but some folks do.

There is no across the board hard formula to adapt to fuel. In fact non-ethanol fuel in its very early stages of decline actually see's a sight octane rise as the more volatile elements evaporate. That was revealed in a study done by BP. Without a lab report on every batch and tank of fuel we never really know what we are buying. I have tested pump fuel and found batches anywhere from 0-15% ethanol at the pump. You can get a tester for about $10 to verify ethanol percentages but it will not help us know the octane.

Bottom line is your best with fresh fuel in a vented system if ethanol is blended. Sealed systems like a modern FI automobile not much of a problem as the humidity is kept out.

Mercruiser Marine offers a very nice ethanol fuel ratio device in a glass test tube format