blue thunder

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

Turbojack

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.

Vinny P

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.

REDCHECKMATE

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.

p4-33

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

Offshore Addiction

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

mrv8outboard

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.

p4-33

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

formula31

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.

fountain27ho496

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