Spark plug detonation
 

HP500 Carb motors, otherwise stock, and this has happened three times in the last two years. Porcelean blows out of the plug base, and I'm trying to understand if the damage to the electrode is a result of cylinder detonation or impact with side of the boat.

Changed the plug and seems solid again.

Anyone seen this?

Thanks,
Brian

I remeber hearing about this happening more often with a certain brand of spark plug. I don't remmeber which brand though. What brand are they???

Cant answer any questions but I can tell ya a buddy of mine had an Autolite do the same thing in his strocker motor small block. We never figured out why? Rag's

looks like you are way lean

The plug pictured is an AC Delco. I'm currently using Champions.
Doesn't seen to matter what brand. Random cylinders, too.

Earlier this spring I had a piston detonate in the #6 hole, but the plugs have only failed in #1, #2 and #4.
While the motor was apart, I saw no other evidence of detonation in these cyls.

Timing is 34 @3000.

Only happens on one motor, too. No problems with the other one.

This is kicking my ass, and I lost a race this weekend because of it.

Should I be looking at jetting? Flaky fuel pump? Ignition upgrade? what are the "hottest" plugs out there?

Thanks again,
Brian

Your not running an MSD box are you?

No, but considering it. Why?

Right now, the carb, dist, coil are all stock HP500 stuff from Merc.

Oh, the fuel pumps were switched to Holly Blue's by the previous owner.

Sometimes the electronic boxes will intermitantly advance the timing enough to cause problems. Ive never heard of a Merc box doing it but you might want to change it to be safe. Never had a plug do that.

Detonation is what is tearing the plugs up. The good part is the plug is taking the hit. Detonation can/willl destroy an engine quickly. Need to find the problem. Hotter plug is not the answer. When you had motor apart could you get a guess on the compression? I would check the jetting.

The replacement pistons have a slight (2mm) dome, and were punched out .30 over.
I believe this equates to about 8.75 to 1, and is as close to stock HP500 as I could get.
Heads and combustion chambers are stock.

I've never played with jets, and these motors have about 110 hours each. The
problems seems to be something intermittent, because they run fine most of
the time. That's why I'm thinking my fuel pump
may be flaking out. Floats have been set with the boat sitting in the water.

How can I figure out jet size? Are sizes stamped in the jets?

Thanks again.

I been through this same problem before with our 1990 Baja, stock 454/330`s. We put 800+ hrs on her over 12 seasons. If I was running AC MR43`s it would blow out the center of the plug also. It happened 3 or 4 times over 12 yrs. Never did it running another brand.

might have a broken valve spring ,bent push rod,cloged jet

Could be water getting into the cylinders due to leaking exhaust. Plug looks very clean. If that is it and you keep it up, and a valve head with shatter (embrittlement). Water in the cylinder will cause detonation due to decreased cylinder volume (high compression). Look for water trails in the exhaust runners.

Just a guess,

BT :cool:

Thanks Blue,

What do these "water trails" in the exhaust runners look like? I'm running Imco Powerflow Plus exhaust.

The incident earlier this spring detonated a piston in this motor and filled the block with water, cracked
the head, cracked 3 valves. I'd rather not do this again. I've never determined the root cause of this.
Water in the exhaust is a possibility.

I'm pricing out an MSD 6M-2, Soft Touch, a billet distrib and Blaster coil. MUCH cheaper than pistons!

Other opinions on plugs?

Thanks,
Brian

Will be looking at the jets tonight. What are stock sizes for HP500 carb motors? How do I determine
the size/type of the power valve(s)?

Thanks again,
Brian

Double check your timing...use different timing lights, check the advance springs in your dist. Make sure there not corroded or broke. If you can put them on a dist machine and check the curve. Set up your carb like the good one or switch carbs around. Your either leaning out somehow or the timing is wrong. Also..does all your fuel come from 1 tank?

Dan

If you are doing all of that damage you have a major problem. Water in the engine cools the intake charge & it helps fight detonation. My vote is that is not the problem. Detonation is caused by poor fuel (low octane), lean mixture, 2 hot of sparkplug. too much timing, high engine tempetures (a motor runing hot while under heavy load (wot)), &/or High intake air temp. I have never messed up a N/A motor but have distroyed my share of blower motors. Is the motor messing up under wot or at cruise speeds. An engine runs the best on the verge of detonation. You cross the line & it is going to cost you.

A msd is not going to make your problem go away unless the timing advance is going wild.

Reversion will cause a cooling affect evenly across all cylinders. Leaking exhaust is different. More water localized. I have blown up engines due to this. Both times the symptom was a spark plug that look like the one at the top of this thread. Fixed the leak and no more blown plugs or engines. How much cooling do you think water will provide under the pressure produced at say 20:1 compression with pump gas? Maybe pre-ignition would be a more appropriate term. Not sure if it happens before or after TDC, but leaking exhaust WILL produce this result.

If you haven't done so, do a plug reading of all plugs at once and see how they vary.

BT :cool:

P4 - You could see discoloration in the normal "soot" inside the exhaust runner leading from the leaking area and heading towards the exhaust valves. Removing the plugs and looking for moisture after sitting a short while should show water if this is the problem. Are you getting any milkshake oil in the crankcase?

Again, how do all the other plugs look? If this is a tuning problem it would be more widespread, no?

BT :cool:

If my memory serves me correct detonation only takes place 15?? - 35 degrees after the plug fires. Preigination takes place before the plugs fires & is instant death.

If problem is leaking exhaust it will be easy to do a pressure test & see.

P4-33,

You have a serious problem here. It may be caused by any of the above. My advise to you would be to take a really good look at your ENTIRE fuel and ignition system. I don't mean to insult you, but if you are asking how to determine what size jets you have, then maybe you should be looking for a person in your area that has more experience with diagnosing this type of problem. Simply putting in a new piston and spark plug then letting it rip again certainly will yield the same results. You have to concentrate on fuel flow and pressure problems, jetting, power valves, timing, water ingestion. The list goes on . By the way, Holley jets have a number stamped on the side, that is the jet size. The same is true for their power valves.

Check the PVCR orfices to make sure one of them is not partially pluged with that white corosion crap. These orfices are located under the power valve. Two .050 dia. holes per power valve.

Hello again. Kinda busy, but here's what I've found:

First, the carb gasket had a very poor seal where the throttle body meets the linkage plate, possible air leakage.

Jets haven't been messed with by previous owners; primary 81 port, 75 stbd, secondary 89
port, 93 stbd. and 6.5 power valve. These are HP500 carb stock sizes.

No corrosion or other deposits in bowl or clogged orfices.

Haven't had a chance to read the other plugs, that'll have to happen tomorrow night.
Exhaust leaks are unlikely as the gaskets are only 3 months old. This problem has
occurred both before and after rebuild (new exhaust gaskets)

Answers to a couple of questions: I have individual tanks, so it's possible there's crap in one.
And speaking of gas, the first two plugs I detonated last year happened after getting
some questionable 89 gas on the water. They happened 3 days apart. What can I
say - had to get home.
This weekend, I also filled up with "unknown" gas, Amoco 93 from a station near
the race site. I typically run Sunoco 94 that I always get from the same station.
Time to think about buying it by the barrel.

WOT fuel pressure stays at 7.5 lbs on the guage leading into carbs.

Here's my thoughts, and I welcome input from the critics out there. ;^)

My engines run great at WOT, and the only time I've had problems is with unknown fuel.
Certainly possible air leak at carb gasket is causing slight lean condition. This has only
happened running WOT.

Merc ignition isn't top notch, so I'm probably going to upgrade the entire thing to MSD -
box, rev limiter, billet dist (which I'll have set up by my shop with curve measurement tools),
wires, and Master Blaster coil. I could also use a couple more mph for another boat on the circuit.

Also, thinking about going one step colder on plugs. Any thoughts on gap with 8.75-1 compression? I currently set them to .050

I'd rather not touch jets/PV as they are stock and have performed well for over 100 hrs.

I'll follow up tomorrow after looking at plugs.

Thanks again,
Brian

I have seen that numerous times,the only problem that I have ever had cause that was loose spark plugs!

my take on things.is the same side of the engine with random cylinders doing this or is it both sides? if it is both sides did the same manifold and pipes go back on the sides they came off of. one drop of water in a closed cylinder could do this so a good check of the exhaust is in order.

now,one would have to say if it was questional gas quality it would happen to both engines. a vacuum leak even a small one at the base gasket would cause undesirable idle characturistics,you did not complain about your idle.

if the engines are the same,anything that is comon between them would cause the condition in both. ie ignition systems , carbs , jetting , ect. most times when componants fail they fail. by process of elimination it sounds like water entering the cylinder from the exhaust (and it wouldn't be alot) or a loose spark plug.
if the plug is loose the heat from the combustion chamber travels up the outside of the shell,heating it up,making it soft alowing it to eject the porcilin and susequent damage to the plug.

The problem has happened 3 times, twice on one side, once on the other, on the same motor. The
only problem I've had with idle is the motors loading up while putting around at no-wake speed. I've
just assumed this is a charactistic of carb motors. This happens to both, and is a primary reason I'm
considering an MSD setup. That, and a touch more juice on the top end.

Loose plug is a possibility. Last year's plug incidents occurred with plugs that came with the
boat, and when changing out the set, a couple weren't very tight. This time, the plug base that
was left in the hole was just "snug" and not what
I would consider tight.

So that brings up another question: how tight should these be? I'm using a 3/8' drive ratchet,
and would guess that I can get about 35 ft/lbs on them while performing contortionist acts. Do they need more than that?

Last question: I've seen one recommendation to go with AC 400 plugs, one notch colder. What
type/brand of plugs are you folks running out there?

Going through my exhaust system is a significant task, and the motors would need to come out.
I'm not yet convinced that I need to go this route.

One more race and then they'll come out for the season and I'll go through everything. This is a
rough water race on Lake Michigan and there will be a whole bunch of throttling and not much
on long WOT stretches.

Thanks again, guys. Good stuff here.

Brian

Sounds like you may have answered your own question, plugs loose. I probably get mine too tight as I break one every once in a while but I use the same wrench as you. Ive heard that detonation can loosen a plug but its never happened to me. Were you personally positive that the last plug that blewup was "tight" when installed? If not, maybe it just wasnt tightened properly. If yes, then detonation loosened it maybe and you need to look deeper. Good Luck and keep us informed, never too old to learn.

Had this happen on 98 hp carb
 

Had major loss of power, upon checking, the
back plug port side looked like yours.
It had fired between cylinders and burned a
hole in the block and head. Never figured out
exactly what had occured, but it was ugly.
Hopefully you have better outcome. John

Loose spark plugs are typically a sign of detonation. I get worried when I find loose plugs. I don't use a torque wrench but just guessing I apply about 25 to 30 ft-lbs of torque. I run aluminum heads so my plugs are probably different than yours, but with the cast heads I ran R43TS AC.

You really need to do a plug reading promptly after a WOT blast. Sounds like you are plenty fat at idle. A cylinder leak test may tell a lot too, but may tell too much for your liking at this point in the season.

BT :cool:

I have a cylinder leak test guage on the way, and will check both motors - for comparison as well as a sanity check.

I'll take a look at plugs tonite, install fresh ones and take another look this weekend after a WOT pass.

Anyone have luck with AC Gold plugs?

BR6FS NGK.......and make sure there tight and checked or replaced after every race!

As I was working on my motor last night changing the oil I decided that the problem is in the fuel system somewhere. If pressure is good, then possibly trash in one of the jets or passages. I am assumming the motor is not getting any hotter then the other one, has correct heads, pistons, & timing is correct..

Thanks again for the tips.

So here's the game plan: Cylinder leak test tomorrow night, and then new NGK BR6FS's to both motors. Plugs tight! New base gaskets for both carbs. While doing gaskets, I'll swap carbs between motors. At least if is a carb problem and I detonate something serious, it'll be the motor I haven't rebuild yet. This will also tell me if the plug problem "moves" to the other side.

WOT runs this weekend and get some plug readings. Assuming things look fine, I'll install the MSD box, billet dist, rev limiter, 5200 chip modules, and wires that are on the way from CP Performance early next week.

My engine shop suggests that I lock down my timing at 34 with no advance, seeing this isn't a pleasure boat anymore. They say this is common in race vehicles, and MSD distributors have provisions to do this. Any thoughts on how this could affect drivability?

If all goes well, that Boatworx Formula has some surprises to deal with in Grand Haven.

Thanks again,
Brian

do not lock your advance at 34 degrees,you could detonate your motors during accellaration,your rev limiters are built into your modules so using msd rev limiters are kinda a waste of money,and they should be limiting at 5350 now,you may also want to put a set of carb spacers under your carbs since you already have them off,usually good for about 10 plus horse on each motor,dumb question,but hopefuuly you used the right head gaskets in you motor right?

I had my HP500++ locked at 34 but I was using a Crane ignition. Never had any problems. The multiple fire feature of the ignition is such that it fires +/- 20 degrees under 2000 rpm if I remember right. In other words the first spark happens at 14 degrees when starting and idling. You will notice that when you try to use a timing light. It will only work for setting the max timing and not idle. Never tried it with an MSD box.

Offshore - these "modules" you speak of... mine are HP500 carb motors, and I'm not aware of any modules where rev-limiting currently takes place. Are you thinking of EFI motors?

Head gaskets were installed by a reputable performance marine engine builder.

The MSD boxes have a similar feature to the Crane box formula21 describes where plugs fire 3 times under 2000 rpm. This should take care of any pre-ignition problems by having more advance on the lower end. How much heat could be created during the short period I spend getting up on plane?

Anything over 3000 is status quo, I'll still be at 34 degrees.

But hey, I'm willing to listen...

Carb spacers are a good thought. Thanks.

on the rear of your starboard cyl head there should be a bracket,on the rear of that bracket should be your ignition module,and programmed in that module is your rev limiting capabilities!as far as locking your ignition at 34 degrees ,all that is is setting your total advance at 34 at 3000,thats the way its sopposed to be done,I may have just misunderstood it to where your ignition would have been at 34 degrees all the time,thats crazy on marine motors in offshore conditions,to much timing with not enough fuel under severe planing loads!