p4-33

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.

p4-33

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.

Biggus

iTrader: (1)

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.

Dixie Doug

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

blue thunder

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

p4-33

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

p4-33

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

DanB

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

Turbojack

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.

blue thunder

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