Mercruiser 5.7 Vortec Carb - Ran OK tuned up and now runs bad

[leeirvin]

Hi,

I saw the thread on how to ask, and apologies but I could not find in searches.

I have a Mercruiser 5.7l 2bbl Thunderbolt V.

Serial 0L692104

As I have found out, she is the one with vortec heads. 8 bolts not 12.

She ran ok, but hard to start cold or warm. So I bought the Sierra tune up kit. I realized when I was replacing the sparks plugs that the plugs I pulled out were longer than the new plugs going in. (Clue #1) I came back in and went to the Mercruier site put in my serial number and even the picture showed the short plugs. I put everything on, new distributer and rotor, new wires, etc.... I crank it up here at the house with the muffs on. Still hard to start, but OK.

I took it out on the water, cranked it up and when I went to go up on plane, the engine wouldn't go above about 1500 or 2000 RPM. She made a notice like she was pinging. I pulled over in the cove and double-checked the wires, all was good. I could idle back to the dock and get on the trailer.

I get back and this is when I realize I have the Vortec heads. The Mercruiser site has a twistie on the Spark Plug and it shows both sets of plugs for my serial number.. I order the right plugs, that match what came out of my boat.

Put the plugs in and start it up. Now I know what to listen for. It has a shutter to it when it idles. The coil looked roster, so I ran down to AutoZone (no marina near) and get the Pertronix Flamethrower. Engine started up, and it still shuttered. I checked the plug wires. I removed the DIst Cap and looked under it. It's notched, so I figure only one configuration.

Try to start it. no start. Looked back under cap, nothing out of place. Tried starting, and she fired up. Still not running right, so I put her in the water and still the same issue. Idels rough ( but always did) but won't get up over 1500RPM. Still sounding like pinging when trying.

Now I noticed some corrosion on the Timing Sensor. Could that be bad?

I haven't had a motor that required a timing light in thirty-five years, so I do not have one. I'll pick one up. A voltmeter is fine on the coil.

How did I screw up the engine doing such a simple part swap?

 

[Rick Stephens]

Sounds more like crud in the carburetor than electrical to me. Dump your fuel water separator into a clear jar and see if anything but clean gas in there. I'd be thinking carb rebuild.

Note, I ran the short plugs for a while in my 4.3 when it got updated to Vortec. After a couple years I figured I should try the longer ones that are appropriate. Made no difference at all. Left them in, but motor runs the same.

 

[Bt Doctur]

firing order incorrect

 

[xltier]

i had a misfire on a 5.o mercruiser and it was a rusty sensor in the distributor.just a thought.

 

[leeirvin]

I wound up adjusting the timing a bit by ear last night, runs a whole lot better. I'll put her in the water today.

Before I go, I'll check the water separator. I've seen a lot of water in these things, given the humidity. Just got this boat, so definitely worth a check. Given that the timing sensor was considerably rusty, I'll go ahead and order a new one now.

Firing order was one of my first suspects. The engine cowling has the firing order on it. 1-8-4-3-6-5-7-2. I left the Dist off and kicked the started to confirm the rotation and rechecked the wires. I was fairly diligent changing the wires and have numbers on the wires from the sparkplugs.

 

[Rick Stephens]

Adjusting the timing by ear is a good way to burn a hole in a piston.

 

[leeirvin]

New Ignition Sensor and new timing light. Timing is OK and the new sensor seems to have fixed the issue. Plus, the sensor has been improved and sealed in plastic to help prevent future corrosion.

I'm glad, spoke to two of the local Mercruiser mechanics and it's about 4 weeks to get something looked at.

Thanks for all the help.