350 mag won't rev over 2k in the water

[dirty diesel]

I cant get my boat over 2k in the water. Boat is a 1995 but it has a newer 1998 350mag 0w015536 with vortec heads and carb'd. New plugs, wires, cap is good, fuel pump thumps away, tank was removed and welded, new fuel separator, fresh gas, and carb was rebuilt. It's no prop issue as it worked before. One thing I did notice is its a Thunderbolt V and when I ground the purple/white wire to do base timing it idles and revs steady, but when I unground the wire the rpms pulse up and down 100rpm either way. Also I dont have a timing tab, only the one big notch cast into the timing cover and a single line on the balancer. Is this line TDC or 8 BTDC? Also the inline fuel filter only has about 1/4 of the fuel in it when running, is that normal?

 

[Fun Times]

I cant get my boat over 2k in the water. Boat is a 1995 but it has a newer 1998 350mag 0w015536 with vortec heads and carb'd.

Hi there, just so you're aware, your engine serial number you posted has your engine model to be a 5.0 fuel injected engine (MERCRUISER 5.0L MPI ALPHA) which is not considered a Carbureted 350 Mag from new. Do you happen to have an overall top photo of the engine that you could post? It may help clear up what you truly have there as it's important to know. Also with a serial number that starts out with 0W01 would put the engine year closer to 2004ish year not 1998. Have a photo of the engine serial number too?

New plugs, wires, cap is good, fuel pump thumps away, tank was removed and welded, new fuel separator, fresh gas, and carb was rebuilt. It's no prop issue as it worked before. One thing I did notice is its a Thunderbolt V and when I ground the purple/white wire to do base timing it idles and revs steady, but when I unground the wire the rpms pulse up and down 100rpm either way. Also I dont have a timing tab, only the one big notch cast into the timing cover and a single line on the balancer. Is this line TDC or 8 BTDC? Also the inline fuel filter only has about 1/4 of the fuel in it when running, is that normal?

Sure sounds like you may have a carbureted engine with this description though.

The fuel filter should be full. Be sure the filter is not accidently doubled O-ringed as the O-ring could stick to the top of the filter housing base assembly sucking air.

You'll want to check fuel pressure as you'll need to know where it's at 'at all engine RPM. But you'll need a special fitting which is Fuel Pressure Connector (Carburetor) Mercruiser part number 91-18078. http://www.ebay.com/sch/i.html?_fro...l1313.TR0.TRC0.H0.TRS0&_nkw=91-18078&_sacat=0

The line for the timing is 0 top dead center. To check and adjust the timing, you'll need a timing light with an advance dial such as one of these for example, http://www.bing.com/images/search?q=timing+light+with+advance+dial&FORM=HDRSC2

You put the dial of the light to 8 or 10 degrees and line up the two marks turning the distributor assembly.

If your engine really is carbureted, Your engine idle RPM should be right between 643 - 648 RPM when running/adjusted correctly while watching a digital tachometer gauge. MPI is a different RPM depending on actual serial number/year it is.

 

[alldodge]

When the motor is in base timing mode the rpm's are increased which appears to help stabilize it. When not in base timing mode the rpm's drop and it starts hunting. Leans me toward the carb may need a bit more cleaning, adjusting, or you have a vacuum leak. Agree your fuel filter should be full

BTW: If you measure your harmonic balancer you can get some timing tape and put it on the damper. Once there you can use the a regular timing light

 

[dirty diesel]

The only vacuum port is up on the top of the carb which is a weber 4bbl. Should that be plugged?

 

[alldodge]

The only vacuum port is up on the top of the carb which is a weber 4bbl. Should that be plugged?

In most cases the port on top the carb is a fuel pump over flow fitting. There is no vacuum being found there. To check put you finger on it with the motor running, you would fill a suction. There should be a clear line connected to it coming from the fuel pump.

The vacuum leak I'm talking about would come from a gasket at the base of the carb or intake manifold. Don't know if it has one the motor just acts like there could be a problem there.

 

[dirty diesel]

I put a new base gasket on when I rebuilt the carb. You were right about the port on top, no vacuum there. When I put the timing tape on the btdc marks would start to the right of the 0 line as viewed from the front of the engine, correct?

 

[alldodge]

When I put the timing tape on the btdc marks would start to the right of the 0 line as viewed from the front of the engine, correct?

The motor is a Left Hand rotation motor which means it rotates clockwise as you're facing the front of the motor. So to get the firing to happen before TDC the tape should be to the left facing the motor.

 

[dirty diesel]

Wouldn't the tape be on the right facing the motor? As the crank rotates clockwise (viewed from the front) btdc would be to the right of the tdc line, right? However if I set it up that way the ignition advance actually retards toward tdc then into atdc.

 

[Fun Times]

Wouldn't the tape be on the right facing the motor? As the crank rotates clockwise (viewed from the front) btdc would be to the right of the tdc line, right? However if I set it up that way the ignition advance actually retards toward tdc then into atdc.

As you can see from the attached image below, Before Top Dead Center on GM engines are to the left when facing the front of the engine. I "believe" you can rent timing lights at auto stores if needed. With the dial light like mentioned above, you won't need a timing tape.
https://www.perfprotech.com/blog/articles/mercruiser-engine-timing

 

[dirty diesel]

yes but i have this cover with only one line on the balancer

 

[Fun Times]

^ Yes I know.
If you can get ahold of an advance timing light, all you have to do is put the degree setting on 8 on the advance timing light, then line up the center of thick line on the balancer to the centerline "V" on your cover and be done.

Then next if only desired by you, knowing how the image above looks number and distance wise for 8 degrees, you could put the advance timing light to 0 and watch the line on the balancer move up/to the left a few degrees.

http://www.bing.com/images/search?q=...ht&FORM=HDRSC2

 

[dirty diesel]

Ok did some more testing. Got an advancing timing light and checked the timing in base mode. Set perfectly to 8* btdc. Shut her down and ungrounded purple/white and restarted to check the advance. When revved up to 1500 the timing retarded and danced around a little. So in doing some tests I unplugged the knock module and fired it up and it ran perfect. The timing advanced correctly and was very steady. The blue wire from the module to the knock sensor has no resistance, and with the key on not running there is 12v to the purple wire on the module plug. What other tests can I do? Was going to get a new knock sensor tomorrow. Sounds like it's either the sensor or module and the sensor is only $30 so I'll shotgun that on first.

 

[alldodge]

Knock sensor testing

 

[dirty diesel]

Ok went through the procedure and everything checks out. 12v power on purple to knock module, good ground, signal wire to sensor is good and not near plug wires, and purple/white wire is good. Still retards timing when revved. When running the purple white lead has anywhere between 6 and 9v. If I unplug the sensor no change. Now the weird thing is while it's running if I unplug the whole knock module everything runs perfect. Oddly there's a constant 7.5 volts on the purple white wire still which I thought only got voltage from the knock module. This is making me think the icm is bad and injecting voltage onto this lead screwing everything up. The purple white wire is not frayed or shorting to anything.

 

[alldodge]

Don't know what to say other then, the write up says voltage should be between 8 and 10 volts. You had 7.5V which is lower and yet this could also be a meter issue. yet when the wire is unplugged it runs good.

So if you did the test exactly as posted it should provide the answer, but appears it may not have. Don't know, I'm to many electrons away

 

[NHGuy]

Have you tried it in the water since you set things up correctly?
I have seen it written in here that the TB V will look like it's doing weird things without a load on the driveline. That retard may be the knock module adjusting for whatever the sensor is hearing. So give it a go in the water if you have not done so yet.

 

[dirty diesel]

Yeah I have been with no success. I'm waiting on a new icm right now.

 

[Fun Times]

Now the weird thing is while it's running if I unplug the whole knock module everything runs perfect. Oddly there's a constant 7.5 volts on the purple white wire still which I thought only got voltage from the knock module. This is making me think the icm is bad and injecting voltage onto this lead screwing everything up. The purple white wire is not frayed or shorting to anything.

On the 5.7l engine the purple/white wire is connected into both the Ignition control module and the knock module along with the purple "power" wire coming from the ignition key is also connected into both modules so only unplugging one harness (knock) could be the reason you still seen 7.5 volts.

Since it seems to work with the knock module disconnected, that may still indicate a bad knock module. Plus since you tried your ICM on another boat, that seems to indicate the ICM is okay..... Does your friends boat have a knock module he'd let you try?
If not, If you shop it right online, you can find lower costs on either GM or Mercruiser knock modules or you may even be able to match one up from an auto parts store for less money using a GM truck part form the 90's. Here's some examples with prices from high to low, http://www.ebay.com/sch/sis.html?_nkw=Mercruiser+Knock+Module+806611T

Get the numbers off of the knock sensor on your engine and input them into both EBay and online for possible other lower costs just so you can rule out the knock module.

We/you really didn't clear up the miss matched serial number info mentioned in post #2 so not to sure of the history of the engine and swapped out components..Was it a complete running engine you installed? If so do you know if it was a bravo or Alpha stern drive that was attached to it originally?

If the sticker on the ignition control module is still legible, it should have the wording Bravo or Alpha on it.

But before suspecting the modules, I would also be suspecting a possible bad module wire harness plug connector for things like a wire backing out of the connectors or corrosion buildup between wire pin connectors allowing wires to contact each other. So be sure to thoroughly/internally inspect the module plug connectors.

While this guy in the following link doesn't have a knock module for his 5.0L Alpha carbureted engine due to the 5.0L Alpha drives don't come with a knock system...Only 5.0's with bravos do, he seemed to sort of have similar enough issues as you seem to be having and it turned out he said he found a bad fuse....So you may want to thoroughly inspect all your fuses too. Also it wouldn't be a bad idea to trying jumping the engine cannon plug connector like seen in the video linked below to see if the engine will run better too.

http://www.justanswer.com/boat/70012-thunderbolt-timing-advance-problem-i-98-mercruiser.html

https://www.youtube.com/watch?v=NUoXFIo2Ab8

 

[dirty diesel]

So incase anyone has the same problem, I replaced both the knock module and ignition module and that fixed my problem! Thanks for all the help!

 

[Pete104]

Thank you for sharing results! :laugh: