Couple of knocks and a hiss (1 Viewer)

[wchevron]

1977 with 2F engine. A couple of times now I've found if I'm on the highway for 1/2 hr or so and have the truck running around max speed 55ish, when I get home or work and turn it off, the engine gives a couple of knocks then a loud hiss for about 2 seconds. Has only happened 2-3 times. One day it did it twice in a row, two different trips, one soon after the other. It was about 90* and high humidity. Engine was running a little warm, had just filled the gas tank. Thought maybe driving it hard and the heat/humidity was causing the issue. That was about 2-3 weeks ago. Today, about 80* light humidity. Was on the highway for 30-40 min. Engine temp stayed right at mid gauge. Was at 55mph most of the time. Got back to the office, turned off, engine gave 2-3 knocks and the loud hiss. When I look underneath there is are no fluids anywhere. Not sure if I should be concerned or not or what I should be looking for, if anything.

 

[mrboatman]

Sounds like it’s dieseling

 

[Mark W]

Agreed. Sounds like dieseling.

What carb do you have on it?

Mark...

 

[charliemeyer007]

Check your air filter. Look at your spark plugs. Make sure your timing is on. Put a bottle of fuel system cleaner in the tank.

 

[wchevron]

Carb is a Weber 61249.

What is dieseling?

 

[Grayscale]

Dieseling means it is igniting with no spark from the coil, just glowing carbon, somewhere. Pre-ignition is related phenomenon, and it is a bad thing for sure, so don't hesitate on fixing it.

My '73 had an original carburetor. Back then, I would always shut-down with a foot on the brake, transmission in gear and stop the engine with the clutch. The funny thing is that I'm pretty sure that the idle circuit solenoid wasn't wired to the ignition-circuit, as part of a simplification process the p.o. did in the 90's, or Pirate 4x4 dial-up days. When I bought it from the paper-ad, it had brake lights, starter, and the coil wired, nothing else, like 30-feet of wire total in the harness - trail rig with a tag and title that was seriously sketchy.

However, my beater truck sports an aftermarket (Weber) carb, and when it diesels, it is really rare, but it likely has some form of glowing carbon, probably plugs. A Weber can be outfitted with an idle cut solenoid on the idle circuit(s). Circuits on carbs aren't really circuits, just one-way paths, btw.

 

[Mark W]

Carb is a Weber 61249.

What is dieseling?

Basically when you turn the key off, the engine is still spinning and trying to suck fuel and air in. When the engine is hot, particularly if there is carbon buildup or also iof the fuel mixture is overly rich, the air fuel mixture pulled in as the engine is turned off will ignite from the heat of the engine/heat of compression/hot spots in a cylinder(s). It is a not uncommon problem with old school carburetor technology.

Most likely your carb is jetted rich, or you are getting unmetered fuel flow due to misadjustment/flaw in the carb.

The OEM carb had a solenoid that shut off the fuel through the idle circuit when the ignition was killed. Since the idle circuit feeds in below the throttle plate, this is, in a properly functioning factory carb, where 99.99999% of the fuel that is drawn in on shut down feeds from. Pretty much kills dieseling in its tracks.

It may be that you have either the primary or secondary throttle plates open to far at idle compensating for other carb or tuning problems that you may not have picked up on. Or one of them may be sticking sometimes. This can increase the likelihood of dieseling on shutdown.

Mark...

 

[Mark Jennings]

A little dieseling. It’s generally due to the gas you’re burning, leaving carbon on the combustion chambers and maybe a bit to much advance on the timing….on older rigs there is a small knob on the side of the distributer that can be used as a timing fine adjustment for different gas grades of fuel. It’s got a veneer scale with the letters A and R (advance and retard). If you have the knob just turn toward retard….maybe 1/4 turn. Then get some Techtralene fuel additive……and find a gas station with non-ethanol and burn a tank of that….

 

[Living in the Past]

Circuit on carbs aren't really circuits, just one-way paths, btw.

Sure they are. The Carburetor is ground and that completes the circuit. That's the reason for only a single wire. DC is a one way circuit, AC travels back and forth. On a 12 volt DC grounded chassis 12 volts positive has to go thru a load and then to ground to work. Anyone of the three missing you've got a problem. By passing the load being the biggest. Direct shorts like to burn things up.

 

[Mark W]

Sure they are. The Carburetor is ground and that completes the circuit. That's the reason for only a single wire. DC is a one way circuit, AC travels back and forth. On a 12 volt DC grounded chassis 12 volts positive has to go thru a load and then to ground to work. Anyone of the three missing you've got a problem. By passing the load being the biggest. Direct shorts like to burn things up.

I think he meant the fuel "circuits". Idle. Primary and Secondary.

Mark...

 

[wchevron]

Just put some fuel system cleaner in the tank. Will see if I notice a difference. If not I'll bring it to the shop and have them check the mix on the carb. Thanks for all the replies.

 

[Grayscale]

Clean or replace your spark plugs. It makes all the difference on these old rigs. I clean them about three times a year on the 40, and almost as often on my other vehicle.

The Weber(s) really can't be dialed-in. However, is it the 32/36-progressive or the 38/38-synchronous? I ran a Weber on the 2F for an afternoon, and I might have some advice on what to look for.

 

[wchevron]

Not sure. Out of town until Monday. I'll take a look then.

 

[charliemeyer007]

Back in my aviation days my dad taught me to make a spark plug cleaning tool from a broken hack saw blade. You grind off the teeth so it doesn't eat your hand. Then grind down a finger (scraper) about 3/4" long and maybe 3/16" wide at the fat end and 1/8" at the point. You can then scrape all the crud out between the center electrode and the steel wall and bottom.

A square edge on both the center electrode and the bent over ground is important. I spent a lot of time adjusting plugs with the aid of a break down tester. Rounded edges could blow out of fire at 60 psi even tho gapped correctly - while square ones would continue to fire at 150 psi, the limit of our shop air.

 

[DeskJock]

Not that this is a fix, because it isn’t. But - leave it in gear when you turn off the engine and slide the clutch out to stop the motor spin.

A OEM carb has a solenoid that I think the Weber doesn’t have to stop fuel flow.

I’d make sure the plugs are carbon free for sure and then the rest of the advise above.

Had a 64 Buick Special that dieseled in the summer. Left it in drive when I turned off the motor -when it stopped - I put in park. My 74 TR6 dieseled ( rarely) with the stromberg side draft carbs on super hot days too. Spark plugs were my first go to.