Mercruiser spark knock issue

[Sascwatch]

My Dad purchased a boat in 2016 from a marina we have used and trusted for over 20 years. But this last experience has been nothing but problems.

The boat is a 1988 Searay Sundancer 268 and has twin Mercruiser 3.0 motors. We have only ever had issues with the port side motor, the motor from day one has never lasted more than a few hours before it loses power and blows the head gasket. Finally after the 5th motor swap the marinas owner decided he wasn’t going to fix it anymore and kicked my dad out of the marina. He accused him of advancing the timing so we could slow the boat down and troll with the main motor even tho we have a kicker installed.

So with the current motor which as far as I can tell is a 1991 3.0 in-line 4. I’m assuming this because the serial number is nowhere to be found (sticker on the valve cover is worn out, and the tag above the starter is missing. The block is code comes back as a 1991, but I am unsure of the year of the head (outdrive serial number on the cover comes back as 1988)

So going by the 1991 motor or 1988 head it should have MR43t plugs in it, but it has MR43LTS.

So onto the problem, the motor sound like a can of marbles from what I believe is spark knock. We took the boat to a licensed merc mechanic and he upgraded the motor to a pertronix electronic ignition and adjusted the timing. The motor runs fine on muffs but once under load it sounds like crap. Before it would start making the marbles in a can noise and lose power then boom the head gasket would blow.

The spark plugs are carboned up badly, so maybe pre-ignition caused by the buildup. Or incorrect plugs (should have MR43T by its year range 89-91)

The fuel is fresh, and filters have been changed.

I dont know what the mechanic set the timing at but he is known for good work and is experienced with these motors.

We also checked the exhaust flappers and they are intact but we are replacing those anyways. No restrictions are visible.

So the quick version is spark knock under load, loses power and blows head gasket. Heads and blocks have been swapped 5 times with fresh rebuilt motors. Bad carbon on the plugs (rich?) but spark knock (lean/timing/fuel supply issue?)

 

[Sascwatch]

I forgot to mention that we are running fresh 87 octane pump gas from a shell station in canada.

I am am going to get my hands on a timing light a double check the timing tomorrow. From what I’m reading it should be 6-8 degrees btdc. I will also swap in some MR43T plugs gapped at .035-.040.

 

[achris]

If the initial timing is set correctly and the engine continues to pop head gaskets, I would be checking the total advance of the distributor... Here's the timing curve for that engine...

 

[alldodge]

Yes, you should have MR43T plugs, the LTS are longer and puts the spark to deep into the chamber. Plugs carboning up is from to much fuel or oil

 

[Sascwatch]

Thanks for the info guys. I will swap the plugs tonight and double check the timing. I also forgot to add that the plugs were carboned up after running the boat on muffs for about a half hour. The motor in question has the TKS style carb and runs really rich for the first several minutes until the enrichment gets shut off. I removed and tested the TKS system and it heats up fine and extends the rod after a few minutes. Inside the carb looks good.

 

[alldodge]

going by the 1991 motor or 1988 head

motor in question has the TKS style carb

The TKS does provide more fuel and appears to be working, but reading plugs with out running on muffs is not a good read

Not sure if 8 BTDC would be correct, or 1 BTDC. The points and DDIS used 8 and EST used 1 BTDC. You have petronics so compare the pet curve with the one posted by achris

Don't see your issue as preigintion, more to much fuel from the carb, and what did the TKS carb come off of?

 

[Sascwatch]

I’m not sure to be honest, with all 5 engine swaps they have carried over the carb, distributor, alternator, starter and power steering pump. I will look for a part number on the carb so I can figure out which year it is.

I swapped the plugs over over to the new MR43T and sprayed a can of mercury carbon cleaner through the motor. The whole driveway is black and it’s seems to run and idle smoother now. The real test will be tomorrow once we put it under a load.

 

[Sascwatch]

So we made it about a mile today without issue then the motor started knocking again, but nowhere near as bad as before. We throttled back and all was well, on the ride back in it was about the same. A mile or so at 3000 rpm and the knocking returned.

Its looking like a fuel starvation issue to me now so we’ve ordered a new fuel pump and a rebuild kit for the carb.

 

[Rick Stephens]

I’m not sure to be honest, with all 5 engine swaps they have carried over the carb, distributor, alternator, starter and power steering pump. I will look for a part number on the carb so I can figure out which year it is..

When you say carried over, do you mean using the same carburetor and ignition system? Only change being swapping to Pertronix? I'll tell you what, if that is so, it was asking for repeat. The marina is insane to do the same thing over and over. Get rid of the TKS carb and put a Mercarb matching the other engine.

Just my very humble opinion. But FIVE motors? Really?

Rick

 

[achris]

...We took the boat to a licensed Merc mechanic and he upgraded the motor to a Pertronix electronic ignition ...

Was this upgrade a simple module to replace the points, or a whole new distributor?

Chris..........

 

[Sascwatch]

The marina that we purchased the boat from replaced the block/head 5 times and used the same carb, distributor starter, fuel pump and alternator on each swap. After the 5th blown gasket they accused my dad of advancing the timing on each rebuilt motor so he could troll with one of the main motors even tho he has a brand new 9.9 kicker on the swim platform.

After the last swap we found out that NONE of their mechanics are licensed and it seems they have no idea what they are doing.

We had the pertronix igniter II installed along with a new distributor body, cap, wires and coil. Timing was set by a licensed Merc mechanic and he claims it is now dead on, and the weighted timing advance is functioning correctly.

It seems our issue might be fuel related which is what we’ll chase next. Fuel pump not keeping up? Or carb dirty/incorrect jetting?

On todays test drive the motor that has been troublesome did the same thing that the 4 previous motors have done. After a short period of running the motor at 3000-3500 rpm it will start ticking (sounds almost like a choke flapping but this motor has no choke). If it is allowed to continue at those rpms it will lose power/rpms, if the throttle isn’t backed off at this point it will blow the head gasket.

Ill add that it’s not temp related, it never goes over 145 degrees. Gauges and thermostat are new. 40 psi oil pressure aswell.

 

[Sascwatch]

I took a video today to record the sound, we backed the throttle off and began fishing there. Once fired back up the noise is gone for 1-2 miles at 3000-3500rpm.

https://youtu.be/aVfswnf8kQQ

 

[achris]

There is no 'licensing' for mechanics. They do their 'time' (as an apprentice to a master mechanic, and trade school) and then they are 'qualified'... Most will then work for a shop. Some will open their own shop. Some manufacturers, like Mercury, and most of the car makers will run 'schools' for mechanics to train them on their specific machines, and then they are 'factory qualified'. That's all it takes to be a mechanic.

Now, your blown head gasket problem... Were these guys replacing a complete long motor? (block, head, rotating gear and 'tin') And just bolting on the accessories. In that case, the real question becomes, what long block was being used, and was the timing being set for THAT block? It may be that the timing was being set for an earlier engine, and THAT was what was killing the new ones.

Early 3 litre engines ran at 8°BTDC, where as later engines required the timing to be set at TDC or even as late as 2°ATDC. You need to check yourself, or ask the new Mechanic where he's set the timing.... Then find out what engine block you have, and check it's the right timing for that block.

HTH,

Chris..........

 

[Bondo]

It seems our issue might be fuel related which is what we’ll chase next. Fuel pump not keeping up? Or carb dirty/incorrect jetting?

Ayuh,...... After 5 motors, it's highly likely the boat's side of the fuel system is the Problem,.......

Try a test run with a can of fresh clean gasoline hooked directly to the fuel pump,......

If yer not runnin' canister type fuel filters, buy a couple, 1 for each motor,.....

 

[Sascwatch]

Yes, the marina we purchased the boat from has replaced the long block each time the head gasket has blown. There is no serial number tag on the block but the block I’d code is RM which is a model 3.0L 1990–1991, 2.5L cyl. head, start full circle rear seal.

The fuel is fresh 87 octane and the water seperators/ filters have been changed. The port side motor runs perfect and has never had any issues, just the #1 starboard motor.

 

[alldodge]

Look for the casting letters on the block

https://www.perfprotech.com/blog/bo...ines-block-id-codes-4-cylinder-marine-engines

 

[Sascwatch]

Look for the casting letters on the block

https://www.perfprotech.com/blog/boa...marine-engines

With the block id code I get a 1991, 1 for the year and RM for the last two digits.

 

[Bondo]

The port side motor runs perfect and has never had any issues, just the #1 starboard motor.

Ayuh,..... So run the starboard motor on a remote tank, 'n test the fuel pump pressure,.....

 

[alldodge]

Remote tank is a great idea, suggest checking fuel pressure at the carb if it does not go away

 

[Sascwatch]

These motors have mechanical fuel pumps mounted on the block, with a solid line running to the carb. Is there a special gauge set that will hook up directly to the carb inlet?

Also we purchased a brand new merc fuel pump and will be swapping it out today and testing tomorrow.