2000 4.3 Bravo run on (dieseling) after shutdown

[newfiez]

Happens after a run, or when engine is hot. I did a full tune-up (plugs, wires, cap/rotor, carb rebuild by carb shop), set the timing to 10° (locked in base timing mode), idle mixture to 1 1/4 turns, idle speed 650 rpm. I assume there’s carbon build-up causing hot spots in the cylinders?

Note: this engine lost the impeller and overheated to the point where it melted the riser coupler tubes (or bellows). There’s new carb mounting gaskets and I retightened the intake manifold bolts. Engine performance is great, starts and runs like a champ. Same performance as the twin 4.3 in the boat, which didn’t overheat.

 

[Bondo]

set the timing to 10° (locked in base timing mode),

Ayuh,.... Is that 10* spec,..??

Sounds hot, I'd have guessed 8* or maybe 6*,.....

I'd try 8*, just to tame the dieselin',...

 

[Lightwin 3]

"Dieseling", "run on" or whatever you want to call it typically means it's running lean.

I suspect you have an uncontrolled air leak into the engine: vacuum hose, intake gasket, carb gasket, etc.

The timing seems a bit high also.

 

[Bt Doctur]

idle mixture to 1 1/4 turns
Not really, idle mixture is what runs the best. keep opening until a stumble, then close for best idle speed

 

[Scott Danforth]

after you adjust the carb like Bt Doc mentioned, and get it running right, if it keeps dieseling, then let it idle for a minute after a long run.

 

[alldodge]

Need a serial number to verify but you should have a flat distributor cap, and if so the timing is not set with a timing light.

Bring cylinder number 1 to TDC on compression stroke, then align the distributor as shown below




Edit, disregard you said you have a carb

 

[newfiez]

Letting it idle after a long run should work but doesn’t. Mercruiser manual says 10° timing at idle, and the other engine is at 10°, both start and run “click buzz”, accelerates very fast and responsive with no lean backfire. This only happens when key off after running at a high rpm (or in neutral at say 2500 rpm) when the engine is hot. There’s no vacuum lines or pcv on this engine. I need to find a way to do a vacuum check I guess

 

[alldodge]

Still need serial number to verify

Might have TB-V and are you setting timing using base timing mode?

Agree 10 BTDC is the correct timing

What are you idling at?

 

[newfiez]

0L693979.
650 rpm at idle.
Set the timing as per the manual:
Ignition Timing
Thunderbolt V Models
1. Connect timing light to number 1 spark plug wire.
2. Connect a shop tachometer to the engine.
b
IMPORTANT: Before starting the engine, connect a jumper wire from the ignition tim- ing lead to a good ground. This has to be done before the ignition key is turned ON to lock the ignition module into Base Timing Mode.
3. Before starting the engine, connect jumper wire from timing lead to a good ground.
NOTE: The PUR/WHT timing lead is located towards the front of the engine near the fuel line or near the distributor, as equipped for your model.
4. Start engine and run at 1300 rpm until it reaches normal operating temperature.
5. Disconnect throttle cable from the carburetor.
6. With engine at idle rpm, adjust the carburetor idle rpm screw to the specified engine idle rpm.
7. With the engine still at idle rpm, check the ignition timing. If incorrect, rotate the distributor until timing is correct. Torque clamping screw to 18 lb-ft (25 Nm).
8. Adjust the idle mixture screw. Inward is LEAN, outward is RICH.
9. Recheck ignition timing.
10. Stop engine. Remove timing light, jumper wire and shop tachometer.
IMPORTANT: Timing jumper wire has to be removed or the ignition module will stay locked in the Base Timing Mode and it will not be able to advance the ignition timing correctly when the engine rpm is increased.
11. Adjust and reinstall throttle cable. Open and close remote control throttle lever. Ensure that the carburetor throttle lever is contacting the idle rpm adjusting screw every time.
12. Restart the engine, increase rpm to 1300 then return to idle position slowly and shut the engine off. Ensure that the carburetor throttle lever is contacting the idle rpm screw.
13. Shut engine off.

 

[alldodge]

0L693979.
650 rpm at idle.

No reason for it to diesel unless its overheating

 

[Scott06]

How do the spark plugs look compared to other engine? Does color look lean on this one? I suspect you're running a tad lean on this engine vs other one and the combustion chamber gets hotter. If this is the case despite the fact carb was rebuilt I would look into fuel delivery.

 

[newfiez]

I haven’t looked at the plugs since I’ve put them in, but I did swap carbs (from the other engine), which was also rebuilt at the same time as the other carb. If I turn out the idle mixture screw (to make it rich) any further, let’s say backed out 2 turns, it’ll start to smoke at idle. I assume the float is set right because the other engine doesn’t run hot. Now that it’s mentioned, this engine is running hotter then the other, almost like the thermostat is opening up late. It’s a new one I installed last year when I replaced the water pump impeller and water pump. I’ll swap it out and check the water pump impeller again and go from there.

 

[Lightwin 3]

I haven’t looked at the plugs since I’ve put them in, but I did swap carbs (from the other engine), which was also rebuilt at the same time as the other carb. If I turn out the idle mixture screw (to make it rich) any further, let’s say backed out 2 turns, it’ll start to smoke at idle. I assume the float is set right because the other engine doesn’t run hot. Now that it’s mentioned, this engine is running hotter then the other, almost like the thermostat is opening up late. It’s a new one I installed last year when I replaced the water pump impeller and water pump. I’ll swap it out and check the water pump impeller again and go from there.

Hot means lean. No reason the twins should not run at the same temp.

 

[newfiez]

Hot means lean. No reason the twins should not run at the same temp.

What would cause the lean (hot) condition in this case?

 

[Lightwin 3]

What would cause the lean (hot) condition in this case?

As stated, it overheated once due to a failed impeller. IMHO, it's still overheating. Were all the impeller pieces found? Was there any further damage?

 

[newfiez]

As stated, it overheated once due to a failed impeller. IMHO, it's still overheating. Were all the impeller pieces found? Was there any further damage?

The impeller mostly melted. I did find impeller pieces along with the shutters. The only other item that failed (a year after the overheat) was the water circulation pump. There was no water in the oil or cylinders a year later. However, there was condensation in the flame arrestor for that engine (before I started it) the other day when I was at the boat troubleshooting.
I’m still a bit confused about the overheating you’re talking about. You said above it was running hot due to a lean condition and now due to lack of water. I don’t see how it could be running lean, but I do see how it could have a lack of water. I will look at the impeller and maybe take out the thermostat to see if it gets hot still. I don’t think there’s any impeller pieces left inside the engine.

 

[newfiez]

Okay, back on the boat.
I checked the impeller, showed some signs of overheating but not bad. I replaced it anyways. Thermostat looked good, replaced it for troubleshooting. This engine does warm up faster than the other. The good engine maintains 175° while the trouble engine climbs to 200-210°ish, holds for a few seconds, then drops down to 160° and repeat.
If I shut off the engine when the temp is 160°, there’s no dieseling. Anything above, it’ll diesel. So I removed the thermostat, took the boat out for a run, the temp was cold (for obvious reasons) and no diesel after shutdown. I even tried shutting it down at 2000 rpm, no diesel.
I guess for some reason the cylinder heads must be getting hot and causing the diesel. Problem now is, I’m not sure why. Maybe there’s gunk in the cooling passages, or something else is going on.
Another obvseration is the breather cover was soaked in condensation again. After the run, there was no water present, but small droplets were noticed on the valves in the carb.

 

[Lightwin 3]

What do your spark plugs look like? Are they real clean, like new? If so, you have water in the combustion chambers..

I am not understanding how you could have water in the carb throat. If that's where it is. Do you have water spraying around in the engine compartment? It could be a VERY small leak.

Your temp varying like that mays me also wonder if you have something floating around in the cooling system

 

[newfiez]

I spoke to the owner, he said he washed out the bilge, would explain the water (I hope). Pulled all spark plugs, all are the same, a little rich, but nothing abnormal. Doesn’t have the washed out look of water or spraying methanol.
I’m wondering if I can bandaid it for now and drill a 1/8” hole in the thermostat to let it run slightly cooler?