Perfectly for 15 minutes then, surging, knocking, and jumping tac

[mthwrt]

Sorry if this has been answered but I can't find it.

I have a 2001 Mercuriser 3.0 with a carburetor. I bought it a couple of months ago. Every time I take it out it does the same thing.

I have to give it some throttle to start it. It runs great for 15 minutes of cruising. Then after 15 minutes it starts to surge & hesitate, with an occasional knocking sounds, and the tachometer jumps around (often showing the revs maxed out even though the engine hasn't sped up). The tachometer behaves perfectly before the issues start.

So far I've tried replacing the fuel filter with no success. Can I get some suggestions of what I should try next?

 

[Davetowz]

When it cuts out, remove the flame arrester, pump the throttle and see if it ran out of fuel in the bowl. Isolte, Fuel or Ignition.

 

[Rick Stephens]

Next time it does it open the fuel filler cap and see if it sucks air. Would indicate a plugged tank vent.

Other thing to try is put a pressure gauge on the fuel line and see if you are maintaining fuel pressure.

 

[mthwrt]

Next time it does it open the fuel filler cap and see if it sucks air. Would indicate a plugged tank vent.

Other thing to try is put a pressure gauge on the fuel line and see if you are maintaining fuel pressure.

I like it. It fits the issues and it's very simple to try the fuel cap.

 

[mthwrt]

When it cuts out, remove the flame arrester, pump the throttle and see if it ran out of fuel in the bowl. Isolte, Fuel or Ignition.

Just to check what you are suggesting. I take off the big round filter and push the throttle level back and forth to see if fuel goes into the bowl?

 

[alldodge]

Being a 2001 it probably has a TKS carb, and if so might just need a carb rebuild. Takes about that long for the module to shut off the extra fuel

Serial number would verify

And yes that's what davetowz was talking about

 

[nola mike]

Disconnect the tach wire first, but Guessing you have a problem in your TKS system, if that's what you have. Temp switch closes, goofy things begin.

 

[mthwrt]

Disconnect the tach wire first, but Guessing you have a problem in your TKS system, if that's what you have. Temp switch closes, goofy things begin.

Why do I need to disconnect the tach wire? It's a good warning things are going downhill.

Thanks after I try the vent plug idea. I'll see what's involved in a TKS carb rebuild.

 

[nola mike]

Why do I need to disconnect the tach wire? It's a good warning things are going downhill.

Thanks after I try the vent plug idea. I'll see what's involved in a TKS carb rebuild.

It might be grounding out your ignition causing the symptoms. Sounds electric to me, and I think you need to chase TKS wiring rather than the carb itself. As I frequently recommend, and nobody ever tries, is to put a timing light on there while you're having issues. It will show an erratic spark easily.

 

[mthwrt]

It might be grounding out your ignition causing the symptoms. Sounds electric to me, and I think you need to chase TKS wiring rather than the carb itself. As I frequently recommend, and nobody ever tries, is to put a timing light on there while you're having issues. It will show an erratic spark easily.

Well there is no vacuum in the fuel tank so no easy fix.

I'll disconnect the tach wire to rule that out but I don't understand how that fits the symptom of the issue starting when the engine is warm.

Similarly with ignition problems, what would cause them to only manifest when the engine is warm?

 

[alldodge]

Similarly with ignition problems, what would cause them to only manifest when the engine is warm?

post 6

 

[nola mike]

post 6

And 7

 

[Davetowz]

All diagnosis starts with splitting the systems. Look into the 12 step diagnostic procedure. The Gov spent a lot of tax money developing it. Figure out where the problem is then narrow down.

 

[sowilliams]

What is the 12 step diagnostic procedure?

 

[mthwrt]

Ok, I ran the motor with an ignition timing light attached. It was very interesting thank you for the suggestion (if I number the cylinder with 1 nearest the belts drives and nearest the bow)

Cylinder 1: intermittent flashing
Cylinder 3: continuous stream of flashes with occasional missed flash
Cylinder 3: continuous stream of flashes
Cylinder4: continuous stream of flashes

The results were repeatable over good running and poor running states. Another words it happened both when the engine was cold running well and the tach working, and hot when the engine was surging and dying and the tach was jumping and maxing out.

Any suggestions on how I use this information to figure out what is causing the issues with the first 2 cylinders?

 

[alldodge]

It shows your issue is most likely not electrical, its fuel

Best method is to put a fuel pressure gauge Tee'd in the fuel line before the carb. This is Merc 91-18078 but can make your own with rubber fuel line and fittings for temp use. Need a vacuum fuel pump tester. Should see at least 3 psi running under load.

If fuel pressure stays up then were back to the carb

 

[nola mike]

It shows your issue is most likely not electrical, its fuel

?
Intermittent flashes mean intermittent spark. Definitely a problem. See if you can tell if you're erratic with the light on the coil wire. If not, your problem is with the cap/rotor/wires. If so you have another problem. Agree though that if the flashing is the same regardless of symptoms then that isn't your immediate problem. But if it's fuel I don't have an explanation for the tach...

 

[alldodge]

Different opinions
OP said it was correctly and no issues, and later with issues and same flashing

Timing light picks up off how well the the pickup works. Was the issue the differences in resistance of the wires, the wire or there was actual missing? Don't know, it would take 4 spark gap testers to verify

 

[nola mike]

Different opinions
OP said it was correctly and no issues, and later with issues and same flashing

Timing light picks up off how well the the pickup works. Was the issue the differences in resistance of the wires, the wire or there was actual missing? Don't know, it would take 4 spark gap testers to verify

Like I said, agree that the immediate issue isn't the flashes (though it may mean the engine isn't performing as well as it could).

But how do you explain the tach?

 

[alldodge]

Don't care, tach moves same issue. Tach disconnected same issue