Hi All,
Kind of a newbie here, thanks in advance for your help.

I've got a mid-60's F600, happens to be a dump truck, with a big V8 gas engine (couldn't tell you what exactly), and an Autolite 4100 Carb on it. I believe the carb has been replaced from the original since the original had a manual choke (disconnected **** on dash). It also has been fitted with an electric fuel pump (adjacent to the old one).

Anyway, the truck starts/runs fine when cold, but when it's been running for a bit it won't restart after shutting off. Reading around here and there online, it appears to be some sort of fuel boiling/vapor lock/flooding sort of issue.

The choke is open when it's hot. Might even be when cold... The carb was rebuilt a couple years ago but has been mostly sitting. No adjustments have been made since I got it except for the idle. All new fuel filters.

Any tips? Some sort of insulation scheme? I'd be game for replacing the carb with something else. I need it running reliably, don't care so much about how.

Pic attached.

Thanks!


Autolite 4100

 

I can see one thing that will help right off the hop. You look to have a heater plate under the carb. I would disconnect it and bypass it. Both inlet and outlet. See below pic.

 

what are you calling a "heater plate", I see a bypass hose???

 

The choke is an automatic choke. According to the line on the black adjusting cover, it's adjusted to the aggressive side. The choke may be closing early while the engine is still warm. Adjust the fine marker line that is lined up with the rear most smaller mark on the carb with the bigger one in the middle of the lines. Slightly loosen the three screws around the round black housing and move it clockwise until the lines I mentioned line up, then tighten back down, but not too tight.

Also, there is a heater hose coming out of the rear of the carb plate, most likely the rear orange/red one on the right, that matches the one on the front.

Remove both of those hoses and run a new one from the intake to the heater core where the one off the rear goes to. Just leave the plate without any of the water hoses. That plate with the hoses attached is causing the carb to have 180 to 200+ degree hot water under it while sitting after you stop.

 

Great, thanks all! I'll try both the choke adjustment and bypass the heater. I hadn't noticed the heater plate but taking a look, I can just remove the short front hose and run the rear one, which is longer, to point on the water pump where the short one went to bypass the plate.

Was this plate a legacy thing from whatever older carb was in there? i.e. what's the point of it? Did the intake mixture need to be warmed for some reason?

 

It was there to mitigate carb icing in cold weather. But today fuels are far less heat tolerant and lighter than they were decades ago. So most of these heater plates cause more problems than they solve today.

 

Update:

I disconnected the heater plate.

I adjusted the carb, except I did it so that it starts cold. i.e. it wouldn't really start when cold because the choke wasn't closed _enough_. However, it wouldn't hot start still, even with the choke wide open.

However, it does smell like fuel, which of course suggests a spark problem (or less so a timing problem) when hot. Is there anything that could be in that realm to look for?

I didn't diagnose it thoroughly yet but it seemed like there was an intermittent weak spark when hot. I did connect a plug direct to the coil to test that while jiggering the points, and it was sparking fine. I'm getting a new cap/rotor/condenser, though all that stuff including the wires looks pretty new.

 

Update #2:

I installed a new distributor cap and rotor, though wasn't expecting much in terms of results. I also borrowed a buddy's spark tester, a little "adjustable" spark plug thing with a viewing glass. I get decent spark from the coil to the cap, but not so much at the plugs when it's hot and I'm cranking it.

Why is this? Bad plug wires? They look pretty new, but can wires get hot and not perform?

Just for eliminating things, I connected a spare battery directly to the coil and ground, but same result.

Also, when the thing cooled a little after sitting a few hours, I tried it all again, and I got one or two sparks on the tester and a cough, but then nothing again.

 

I assume you are still on the points ignition? Points don't really have the juice to drive those devices, unlike HEI setups.

 

This one is just an adjustable spark gap. It works fine when the engine is firing.

Also,the last test I did with a plug actually, not with the tester. I didn't mention that.

​​​​​

 

Also, can anyone recommend a very heavy duty/oversized coil? What's in there is getting pretty hot. I guess we're dissipating close to 50 watts if it's
​​~3 ohm primary resistance

 

Don't need heavy-duty oversized coil for running W/points.
And double-check your voltage at the coil.

https://www.rockauto.com/en/catalog/...tion+coil,7060 Pick your poison.
The Standard units are of OEM design.

 

Okay! Problem solved and rationalized. For documentation purposes:

Today I pulled a plug and with a long plug wire ran it straight off the coil. When I cranked it, I noticed that I was in fact not getting even sparking, i.e. should get 8 sparks evenly for every 2 revs. Ah ha!

So I adjusted the points and she started running fine again, hot, cold, whatever.

What was happening is that the points, when warmed up by the engine, weren't opening, at least on some of the lobes, and on the others, only barely, perhaps so that there was some fairly high resistance across the points but it wasn't an open circuit, causing the coil to not spark as aggressively, if at all. For the nerds out there, it's V = L di/dt, and if di/dt (the rate of change in the current) isn't as high (because the points aren't quite opening), the coil voltage causing the spark isn't either.

So... what about the timing now, given that I monkied with the points? I threw a gun on it and there's a timing mark on the crank shaft pulley that lines up with a sheet metal pointer. I suppose I can look this up on YouTube or whatnot, but if anyone wants to weigh in, the following are my questions:

1) Is this timing setting okay? I didn't see any sort of "scale" indicating the angle before TDC, etc.

2) She's running rich. I suppose this could be attributed to timing, but more likely the carb mixture? It's an Autolite 4100 as noted. What are people's favorite tricks for tuning the mixture and where on that carb is the screw(s)?

Thanks again

 

Without knowing what engine is the truck you are not going to know the point gap/dwell setting.

Yes changing the point gap will change the base timing but not usually enough to worry too much about it as for the base timing setting ditto on needing to know what engine is in the truck. It is an FE but what displacement?

 

Hi All, another question.

It's been suggested that:

Which I did. However, I see there is a huge vacuum port in that heater block that provides vacuum to the valve cover. How much of a vacuum "leak" does this impose? Point being I'm trying to get the vacuum brake booster to work better. I do have a small standalone air filter/oil fill cap attached to the other valve cover as opposed to the original that has a hose that ran to the original air filter housing. Should that matter? What sort of vac should I expect? (Lining up a vac gauge imminently).

Thanks