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Faithful, I need your help. I repowered my 31 this past winter. We spared no expense. 541 CI Merlin Blocks Oval Head Ports, Edelbrock Carbs, Davis Unified Ignition, Roller valves. You name it, we overbuilt these engines. They were installed by Southwick’s in Beach Haven and build and assembled by IDM in Mayetta. I have worked with these guys for years, as my dad di and his dad; trust is not an issue. May 2017, launch the boat, runs great. Port motor starts to lose RPM. Running at 3000 then drops 200-300 RPM and goes back, then would backfire. It was intermittent. I would tell marina, they would test for 2-3 hours no issue, then I would get on boat and it happened again. Finally we noticed oil in the DUI electronic ignition. So we sent to Davis and the gear was super tight, the bearing hade ceased on the port motor (reverse rotation) so we installed an old gm electronic distributor and she ran beautiful. No issues with Starboard notor. The new distributor comes back we put in, runs beautiful for a few weeks. The labor day out for a run it backfires and has that brrrrrr sound like its not firing. Turns out we blew the pistons 4 & 6 and cracked the block. Pull the motor redo use a diff. distributor and have about 20 hours on motor. Turkey day comes, 5:30 am departure from Beach Haven for Turkey day troll. Round the inlet and I hear from below we got oil! Open the hatch and oil everywhere. Checked all lines no leaks, no backfiring no weird sounds. Add 2 quarts, and watch as we start to increase speed, smoke out the blowby. Shut her down, diagnose at marina. Blown #4 & 6 on starboard engine. No warning signs, nothing. I am at a loss. These motors are over built and yet I have blown two, The first one we thought the distributor was misfiring (we can’t find a way to prove it—HELP), now I have no idea what to think? Could the Davis (DUI) be bad on both? I only use 93 Valvtec treated fuel from reliable source, the timing is not advanced, no pinging, the plugs are all identical in that they are dark not white. Any thoughts, feedback anything.

Mike

Blown Pistons. Could be a number of issues, but strange it is the 4 & 6 Cyl. on both engines. I tend to lean towards re-builder issues. Did the same guy work on both engines? What would be different on the 4 & 6 cyl. that would make the re-builder make the same mistake on both engines?

Take the re-builder out of the equation and then you are down to a couple of things:

1) Defective Pistons.
2) Detonation issues
3) Something hitting pistons (valve slap?)
4) Over heating

I think we need Bruce on this one.

Original fuel tank?

Many years ago I had 2 similar experiences with stock plain jane engines.

The first was a V-6 Buick engine. I improperly installed the distributor. Then simply rolled the engine over until #1 came up on the power stroke, looked at the rotor and called that #1.....installed the plug wires totally disregarding the fact that the distributor cap was numbered to correspond with the related cylinder. I thought they were numbered for convenience....and not that it was detrimental to the engine function.

It cranked and seemed to run just fine.......except that under Wide Open Throttle it would start backfiring thru the Carb and dropping RPM. I had no idea what the problem was. After running it like this several times it finally burned a hole in one of the pistons.

It was only then that I learned that this engine was an "Odd Fire" engine and what was really going on was that the cylinder that burned up (and possibly 2 others) were running in a severely advanced timing situation.

Installed a new piston and correctly installed distributor and we were back in business.

The next time I had an experience was when I installed the wrong spark plugs in a Chrysler 318 engine in a house boat. The spark plug heat range was way too hot for a marine engine. At cruise the engine ran fine for a mile or so then started backfiring thru the carb.....this time I knew the only thing it could be was wrong spark plugs....since that was the only thing I changed. Soooo spark plugs with the proper heat range cured the problem before I destroyed the engine.

I'm not sure how or if this relates to your engines....but backfiring and destroying pistons sure seems to correlate with my experiences...

It's sure disappointing I'm sure.....hope you fine the problem and share it with us.....sure got my curiosity up!! ! !

Raybo Marine NY wrote:Original fuel tank?

That is what I was thinking...original fiberglass tank and ethanol fuel do not mix. Hmmm actually they do mix...and engines do not like eating resin laden fuel.

New aluminum tank out in on 2008 when I purchased. Ran from 2008-2016 on 465’s
The engines were from 1992 and I just felt it was time, Boy was I wrong!

are you getting enough fuel? pretty long fuel lines to the engines

The run may be 6 feet. Lines are all new and we never had any hesitation when accelerating. The plugs were not white they had a nice uniform color to them. We could start from idle and accelerate to full RPM 4400 @46 knots without any hesitation.

Mike,
What is the rpm/hp curve these engines were built for?

Torque starts building from approximately 1800 and stays flat through 4000rpm. Horsepower peak came in at 4400 RPM and stayed flat through 4800. A cruise RPM of 2800-3200 is most efficient

mikepelham wrote:4400 @46 knots without any hesitation.

I have geezer 454 gassers in mine and when I spin up to 4400 I'm in the low 30 kts range. Might you be over propped for the normal operating RPM?

Good luck,
Stephan

most important question what is the designed compression ratio ? when you say blown engine DID a valve strike top of piston or is it a spun rod bearing (on the subject are you using Mellings 55C HV oil pump ? if so what oil pan pick up?

i am pretty shore i know the problem just want to make sure . i need "cam card" from manufacturer this is the specification sheet you receive with the cam i assume a roller cam .

Too lean? Marine engines should be set up to run MUCH richer than automotive engines. Sustained high rpm's with a lean fuel mixture will burn holes in pistons.

Mike what kind of speed were you getting out of those babies?

i had a good friend with a 38 Awesome Cat. He had the engines built by the same guy who does some name brand monster truck engines. We kept blowing engines and no one could tell why. Turns out the expensive roller lifters were seizing in the bores and crashing the valve train. I cannot remember if they were Gen II or IV blocks. They figured that out right after he sold the boat. It was fun when it ran! 116 on the Garmin GPS.

for what it is worth I had custom engines built by white brothers in Tennessee and couldn't get the performance ignition to run the motors on the dyno either. Switched them back to GM HEI's and ran perfect. I even called MSD and tried all 3 of their wiring methods for different GM set ups. The would fire and kill.

as far as why they keep dying..... see what bob thinks.

To me sounds like a vacuum issue caused by the cam sucking in water via exhaust or a cross fire on a chevy can be common as well if wires are not ran properly. I wouldn't suspect under or over fueling as you would just run out of power or RPM curve. Mine are purposely under fueled. I wanted a 650 with vacuum secondary set at 3000 rpms. Any lower and I'm running on the perverbable 2 barrels. My engine's HP and torque curve look like a cliff at 4800 RPMS.

Edit. I would also hope custom built gassers have inconel valves.

the cam sucking in water via exhaust-------un---huh

Same Cylinders on both engines. That means something in common on both engines.

i need camshaft specification card from the manufacturer, most likely Competition Cams roller cam.

Erson Cam

Grind RH264/300
RH272/300
Lope Separation angle 117
Overlap 32.5
Intake Opens 14.5
Exhaust Closes 18.00
Intake Closes 68.5
Exhaust Opens 72.00
Running Clearance 0.00
Valve Lift (Int/EXH 0.514 (0.513 Hot)
Duration Int/Exh 263 (270 Hot)
Checking Figures @ .050" Tappet Rise
Overlap -23.5
Intake Open -13.50
Exhaust Closes -10.00
Intake Closes 40.50
Exhaust Opens 44.00
Duration intake 207.0
Duration Exhaust 214.0
Intake Center Line 117
lobe Lift Intake 0.514
Lobe Lift Exhaust 0.513
Rocker Arm Ratio 1.72

It is a high volume oil pump. Valves have 160 thousandth clearance

Spark Plugs were not white at all, nice dark color indicating it was rich. Only had 50 hours on motor. Never ran WOT for more than 10 seconds. Most of my running was 3200 RPM

bob lico wrote:i need camshaft specification card from the manufacturer, most likely Competition Cams roller cam.

I hope competition roller cams aren't the problem! They are in mine..... custom ground however....not off the shelf..... that scares me a bit more now....... DAM IT!!!

no, no, no on the contrary I mention competition cams as a standard in the industry and competition cams. race roller lifters are one of the best .i used them on my 408" Corvette engine, actually, the two best are competition cams and Morel.trust me at $ 700.00 a set you get your monies worth. needless to say in the world of roller cams those lifters become a major topic for durability when they let loose the engine is totally destroyed. little needle bearings everywhere --------ouch!!!!

in my humble opinion, there is something radically wrong with the lobe separation angle at 117 degrees really out of wack causing water to enter combustion chamber from the overlap. 110 degrees would be all you could do in the marine engine running 4000 rpm . my brain tells me intake centerline at 106 degrees duration @ .050 211 intakes, exhaust at 221. valve lift intake .476,exhaust at. 507 exhaust close at 22 ATDC and open 70 BBDC intake opens at 21 BTDC and closes at 58 ABDC. this will make a happy engine on the water with maximum torque at mid range in a 31 Bertram.

Looking at the brain of a carbureted engine, lobe separation angle has an inverse relationship with reversion. The higher the lobe separation angle, the less overlap an engine will experience. Hence less of an opportunity to ingest water. All stock mercrusier cams are ground between 114 and 116 separation angles.

I am at a loss

Tony Meola wrote:Same Cylinders on both engines. That means something in common on both engines.

Or the same mistake is being made twice by whoever is doing the work. I would start with an audit of the cam to make sure there is not a repeated manufacturing defect on the lobes for that cylinder. Same with the heads, intake manifold, ignition and exhaust manifolds. It would also be logical that an installation mistake would be repeated for both engines (i.e. intake or exhaust manifold gasket upside down or flipped, directional piston, etc) so it may not jump out at you. It is there and when you find the problem, it probably will not seem that significant-- but it will be the problem. You just got to keep looking.

cannot get into a long explanation because 100% of my mind and body is needed for love one will be off the board a while. take it for what it is worth the cam is no good for your purpose, not just lobe separation it is in conjunction with the valve opening, look at my specification you can take that to the bank.please consider before going a third time.----------your friend Bob.

Bob, God Bless you and your loved one!

Bob

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