Mercruiser spark knock issue

[Bt Doctur]

Just for comparison, check the port timing and full advance against the other motor

 

[Sascwatch]

Will do, I’m going to pickup a timing light and compare the two tomorrow when we put the new fuel pump on. The other motor is a 1986, the only difference is the port motor has a merccarb on it and the points/rotor ignition system.

Timing should be be the same on both even with the year difference 1986/1991 since they one is the pertronix and the other is the points/rotor.

 

[alldodge]

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?
.

At an auto store get some fuel line, fittings and a Tee. Remove the hard fuel line and make it up in rubber to measure pressure.

Either that or get a fitting linke Mercs 91-18078

 

[Sascwatch]

I have been unable to get my hands on a pressure gauge or timing light but we went ahead and put on a brand new merc fuel pump and tested it out. First trip was around 6miles each way at 3000-3200 rpm and the noise was gone, this was on a relatively calm day with under a half a tank of fuel. No issues at all and the motor ran great.

We went out again yesterday and made it about 2-3 miles at the same rpm range and the noise returned but no nearly as bad as before. This time the fuel tank (100 gallons) was nearly 3/4 full and the seas were a lot rougher. If we lowered the rpms the noise would leave completely.

We also swapped fuel the pickup lines from the tank around and it made zero difference.

I’m still not sure if fuel or timing is the issue, but I described the issues we are having to a few mechanics and they all suggested spark knock. For anyone that has had spark knock issues on their mercruiser does the video sound similar?

thanks for all the help so far, hopefully we can get this figured out before winter. I think the next step is either replacing the carb or rebuilding the one that is on there. Getting rid of the TKS carb would be ideal.

 

[alldodge]

Rough seas and more weight (fuel 3/4 full) so the motor is working harder. Need to check timing

 

[Sascwatch]

Carb is torn apart and I’m seeing the 3 main jets, two are 170 and one is 70. Going by the parts sheet it should have 2x 160 and I’m not sure about the third.

Carb number 866140.

I guess I will have to get a timing light to make sure that isn’t a problem, the shop we took it to says it’s set perfect but something is still off.

New spark plugs were carboned up again, to much fuel?

 

[alldodge]

Carb number 866140.

This is one of the rare cases that knowing the carb number may not help. The carb number tells us what the carb should have for the given motor. The issue is in 1991, there were no TKS carbs available, not yet invented.

In 91 it used the standard Merc 2 bbl carb with a choke

Plugs carboning up is to much fuel, so maybe 160 would be better, don't really know. This can also be caused by timing being off

 

[Sascwatch]

What a mess mixing parts up will get you in, chasing problems that shouldn’t exist. I’ll pick up the timing light and I think we’ve found to correct mercarb off a running 1988.3.0.

 

[nola mike]

Any reason you haven't swapped carbs between the engines? Or the ignition system for that matter?

 

[tank1949]

I am familiar with TB5 Inition sustems with knock sensors. But I am not familiar with your specific ignition upgrade. If my memory is correct, initial timing is set to 0 on TB5 before u attach knock module. If your ignition system matches TB5, u are way too high. Just my 2 cents... Good luck!

 

[Scott Danforth]

In addition to the fuel system.....

Look at the pertronix and the dizzy. A bad dizzy bushing and a bit of corrosion on the advance weights will have timing all over the map.

Maybe toss the dizzy and get a delco est

 

[Sascwatch]

Weights look new and move freely, I removed the pertronix unit and inspected the advance. No play in the shaft either. I’m pretty sure it should have the same timing as points/rotor that the pertronix replaced 6-8 btdc.

I suggested we swap the carb and distributor around and see if the problems switch motors but my dad doesn’t want to touch the port side motor since it works perfectly.

Going to borrow the timing light light and check timing under load and hopefully we can get our hands on the proper style of carburetor.

 

[achris]

I am familiar with TB5 Inition sustems with knock sensors. But I am not familiar with your specific ignition upgrade. If my memory is correct, initial timing is set to 0 on TB5 before u attach knock module. If your ignition system matches TB5, u are way too high. Just my 2 cents... Good luck!

This is a 3 litre engine, never fitted with Thunderbolt ignition systems, ever. No knock sensor either. It's a points ignition converted to a Petronix module. So the original weights and springs are doing the advancing.

OP, what is the engine pulling at WOT? I'm wondering if you're just in the bottom of the range and the extra load of a fuller fuel tank and rougher seas is pushing the engine into overload, hence the knock...

Chris.......

 

[Bt Doctur]

Could also just rotate the dist 3/8ths of an inch clockwise to retard the timing

 

[Sascwatch]

We haven’t pushed this newly rebuilt motor past 3500 rpm yet, would be nice to get a few trips on it problem free before we open it up. The boat will stay up on plane easily and moves about 20mph at 3500rpm.

from day one I’ve thought it was to much boat for the tiny 3.0 motors, they are in a Searay Sundancer 268. 100 gallon gas tank and probably 5-600 lbs of gear onboard. Usually two people and water tank/heater is empty. Factory brochure lists twin 4.3 v6’s and the smallest option.

I will check the timing tomorrow out of gear and under load around 2000+ rpm. Haven’t had a calm enough day to be around the motors with the hatch open. I have no problem retarding the timing a few degrees so the motor doesn’t blow up, the small amount of power lost won’t be missed.

 

[Bt Doctur]

Get it to 3500, get on plane, reduce throttle to stay on plane and be happy with the results. You are seriously underpowerd for that size boat and pushing the motors to see how fast it goes will blow the head gaskets.That boat should have never left the factory and most likely was made just for a boat show sale.

 

[tank1949]

This is a 3 litre engine, never fitted with Thunderbolt ignition systems, ever. No knock sensor either. It's a points ignition converted to a Petronix module. So the original weights and springs are doing the advancing.

OP, what is the engine pulling at WOT? I'm wondering if you're just in the bottom of the range and the extra load of a fuller fuel tank and rougher seas is pushing the engine into overload, hence the knock...

Chris.......

Sorry, I thought that I read in his beginning that he did indeed have knock sensors.

 

[Sascwatch]

Update, it’s been a few days and we’ve had the boat out several times with zero issues after the carb rebuild. When I rebuilt the carb I may have discovered a few things.

first off there was a bit of debris and even some small metal shavings in the float bowl. The float itself was set wrong on both the high and low sides. And I’m pretty sure the jets were in the wrong order. I believe the two in the middle should be the same (170 in my case) and the third which is offset to the front of the carb should be the 70 jet.

Well they were installed 170/70/170 where i think it should be 70/170/170? Correct me if I’m wrong because I’m not to familiar with carbs.