5.7L 1984, Rough idle at start up, then ok after a minute or so

[nola mike]

I'd rule out an intermittent fault to the coil pos as well. Maybe run a jumper

 

[isaacs]

Sounds like you may have put it to bed for the winter but when you can, take a look inside your distributor cap. Mine takes on a mixture of oil and water which collect all over the inside of the cap when the boat is not used. Cool moist weather makes it worse. After I run it for a while it clears up. Once it gets bad enough I have to either clean the cap with WD40 and a paper towel or replace it. You may find a some blue boogers hanging from the little contacts that get the spark from the rotor; these can usually be knocked off with a pocket knife.

Not sure about the tach, perhaps it's just reacting to a poorly running motor. Mine did some strange things when my ignition sensor died.

 

[Drcoffee]

Way back, when I had ‘71 corvette I had a misfire issue under load. Turned out it was a bad plug wire that intermittently arched to the engine block on the underside, hard to see in daylight. Run the engine in the dark and see if the plug wires are faulty.

 

[tfret]

Ok here's the latest update and summary:
misfiring / stuttering bad under any load at all. By the end of the season last year we couldn't even use it. It got progressively worse all last year. What I've done so far:
Sprayed fuel directly into carb while sputtering to see if it was starving for fuel. This made no difference.
Listened with stethoscope to see if a valve was sticking and everything sounds normal.
Performed a compression test and everything looks good.
I use ethanol free premium fuel.
I dumped the contents of the fuel water separator and found no water.
And yesterday I replaced all plugs, wires, rotor, and distributor cap.
Now it idles just fine, but when I run the throttle past 1500 it starts missing really bad. I run it up to around 2000, same thing. Again, the tach acts bazar. At around 2000 rpm at times the tach will lay on zero for several seconds. This baffles me and still leads me to believe it's an electrical issue like a bad ground wire that is making poor connection. Could the shift interrupt switch be flakey? Ignition coil, ignition sensor? Can I isolate the ignition system from the rest of the electrical? I'd like to remove all connections and install jumpers where needed to just keep the ignition system alive, and disconnect everything else. It may be more than coincidence, but I did have a mouse in the boat last year. He only chewed up one jacket, and put a hole in my Bimini. I've been all over that boat examining the wiring, and moving the wires around while the engine is running and haven't found anything. Sorry for the long run on here, just want to be thorough and hopefully this rings a bell with someone. Thanks again for all your feedback, good ,bad, or otherwise.

 

[isaacs]

Try a jumper from the battery + to the coil +.

 

[poconojoe]

Did you change the spark plug wires?
Seems to be an inexpensive way to take that out of the equation. They probably need changing anyway.

That ignition module @achris was talking about. Mine died and I couldn't believe the original was an unsealed exposed circuit board! It was totally corroded. The new one was completely sealed. Duh!

 

[dubs283]

You mentioned early on in the thread the engine ran great with the tach disconnected but you fail to mention if you further troubleshot the tach/wiring.

Try running it with the tach disconnected. First disconnect from the gauge and run it. If the symptoms are still there disconnect it from the coil and run it.

If the issue still persists it could be the ignition sensor in the distributor. I've heard claim they either work or they don't but I have seen first hand intermittent issues with the older style mercury ones and aftermarket ones. Best to use a latest style oem three wire with the ground lead next to the main battery ground on the flywheel cover stud.

 

[isaacs]

My old style ignition sensor was intermittent when it bit the dust--tortured me for an entire summer 'till I tracked it down. The engine would cut out, mostly when I was on plane. It was as if someone was briefly switching the ignition switch off and back on again--all my gauges flipped to zero when it happened. But didn't the OP say he has replaced the sensor twice?

 

[dubs283]

But didn't the OP say he has replaced the sensor twice?

Yes, but we fear they have been aftermarket. For the thunderbolt ignition sensor in the distributor the OEM (mercury/quicksilver) latest style fully potted three wire sensor is the only acceptable replacement.

I will say if one is running an older style ignition module (large 5"×5"×2") box mounted on the exhaust riser there will be a 4-5 degree timing jump when trying to set initial idle timing. It is very frustrating as the timing will jump up and down when trying to get everything at 8 degrees and idle speed set at 750ish in neutral.

My only advice is to be patient with the system, it will annoy you very much but you can get the timing/idle to stabilize. It may require a carb rebuild and adjustment to get right.

 

[tfret]

Did you change the spark plug wires?
Seems to be an inexpensive way to take that out of the equation. They probably need changing anyway.

That ignition module @achris was talking about. Mine died and I couldn't believe the original was an unsealed exposed circuit board! It was totally corroded. The new one was completely sealed. Duh!

Yes I replaced the wires. The Thunder Bolt IV I have is a sealed box.

 

[tfret]

You mentioned early on in the thread the engine ran great with the tach disconnected but you fail to mention if you further troubleshot the tach/wiring.

Try running it with the tach disconnected. First disconnect from the gauge and run it. If the symptoms are still there disconnect it from the coil and run it.

If the issue still persists it could be the ignition sensor in the distributor. I've heard claim they either work or they don't but I have seen first hand intermittent issues with the older style mercury ones and aftermarket ones. Best to use a latest style oem three wire with the ground lead next to the main battery ground on the flywheel cover stud.

Sorry, I later determined the problem still persists even with the tach disconnected. I’ve replaced the ignition sensor twice prior to this issue. Both times it just completely died when the sensor failed. I don’t recall if it is two wire or three wire. But I’m thinking it’s two. I may swap it just to see.

 

[tfret]

My old style ignition sensor was intermittent when it bit the dust--tortured me for an entire summer 'till I tracked it down. The engine would cut out, mostly when I was on plane. It was as if someone was briefly switching the ignition switch off and back on again--all my gauges flipped to zero when it happened. But didn't the OP say he has replaced the sensor twice?

Yes I replaced it twice prior to this problem. Both times it just completely died. I have not replaced it yet while dealing with this problem.

 

[tfret]

Yes, but we fear they have been aftermarket. For the thunderbolt ignition sensor in the distributor the OEM (mercury/quicksilver) latest style fully potted three wire sensor is the only acceptable replacement.

I will say if one is running an older style ignition module (large 5"×5"×2") box mounted on the exhaust riser there will be a 4-5 degree timing jump when trying to set initial idle timing. It is very frustrating as the timing will jump up and down when trying to get everything at 8 degrees and idle speed set at 750ish in neutral.

My only advice is to be patient with the system, it will annoy you very much but you can get the timing/idle to stabilize. It may require a carb rebuild and adjustment to get right.

It idled good. Just misbehaves under any amount of load. Both times I replaced the ignition sensor I think I bought the Sierra potted unit. But that’s been a few years back.

 

[dubs283]

Sorry, I later determined the problem still persists even with the tach disconnected.

Tach disconnected at the gauge or the coil? Could be an issue with the wire/harness that can be eliminated by disconnecting the gray lead for the tach at the coil

And as stated, only use the latest style oem sensor in the distributor. Can't stress this enough, I've seen way too many issues similar to yours from aftermarket parts

 

[tfret]

Try a jumper from the battery + to the coil +.

I thought about that. I’ll add a ground jumper for the ignition module also to be sure the system has a solid power connection.

 

[tfret]

Tach disconnected at the gauge or the coil? Could be an issue with the wire/harness that can be eliminated by disconnecting the gray lead for the tach at the coil

And as stated, only use the latest style oem sensor in the distributor. Can't stress this enough, I've seen way too many issues similar to yours from aftermarket parts

I did disconnect the tach at the coil as you recommended. I’ll be sure to order the OE ignition sensor. I guess that would be Mercruiser?

 

[tfret]

I did disconnect the tach at the coil as you recommended. I’ll be sure to order the OE ignition sensor. I guess that would be Mercruiser?

Iboats sells CDI and Sierra brand sensors.

 

[isaacs]

I did disconnect the tach at the coil as you recommended. I’ll be sure to order the OE ignition sensor. I guess that would be Mercruiser?

I got mine a couple of years ago at a local marina. It was either Mercruiser or Quicksilver (same thing).

Here's Amazon: https://www.amazon.com/Quicksilver-...cruiser+ignition+sensor&qid=1653312049&sr=8-3 This looks a bit pricey but who knows these days!

 

[todhunter]

I got mine a couple of years ago at a local marina. It was either Mercruiser or Quicksilver (same thing).

Here's Amazon: https://www.amazon.com/Quicksilver-...cruiser+ignition+sensor&qid=1653312049&sr=8-3 This looks a bit pricey but who knows these days!

It's gone up about $8 from last year when I bought mine.

 

[dubs283]

Iboats sells CDI and Sierra brand sensors.

Go to your local mercury/mercruiser dealer, they will have the correct one. Its a quicksilver brand product, comes in a black, blue and gray box with instructions and additional wiring to connect to older style igniton modules