Understanding the idle circuit and the main circuit in a carburetor and the venturi effect are key in understanding how a carburetor works. Like has been mentioned above vaccum leaks undermine the basic functions of a carb. It is important to understand why that is also.
Give this a read

 

https://www.onallcylinders.com/2016/...auge-readings/

 

Thanks! I just skimmed it and I'll give it a thorough read after dinner.

I just can seem to wrap my head around how there could be such a big leak that the choke has to be almost completely shut but still be rich, and foul the plugs for it to sort of run

 

According to my gauge when "idling" at 1500-1700 rpm, I am steady just above 20 and when I rev it, it doesn't drop all the way to 0 before bouncing back, it drops to 3-5 and up to 25 or so before returning to 21ish

 

After reading through it, I understand how a vacuum leak can cause problems with running and so on, however, after looking at the gauge guide you posted, my vacuum reading is not indicative of a leak as it reads steady at just over 20. Would idling it so high trick the vacuum gauge into reading a "normal" amount of vacuum?

I'm certainly not saying there is no vacuum leak, I'm just still unsure because of the two diagnostic measures I've taken, both have been negative (carb spray around carb base, and vacuum gauge connected to intake while running).

I assume if there were a large leak, I'd be able to hear a hissing/sucking when revving it. The only hissing I hear is through the tiny opening left on the choke (about 1/8 inch). If I open the choke and keep revving it, the sound goes away.

 

I should also note that when it's about to die, if you blip the throttle, it saves itself until the gas from the accelerator pump is used up. The way it sounds when it does die is like it's running out of gas. Yet it smells so rich and it's carbon fouling the plugs. That's why originally I thought the idle circuit was plugged and rebuilt the carb.

 

I would recommend checking/regapping spark plugs, seafoam/mmo, etc. Anything to get the idle smoother and therefore able to be lower for the time being.

I would then check for vac leak everywhere in the engine bay with either water/carb cleaner/propane. (many videos out there to show you how)

If smoothing and lowering the idle proves successful, use a timing light and time it. Then look at tuning the carb with a vacuum gauge.

 

Thanks for the reply!

I will test those all again. I do have some seafoam in the gas can I'm using but I'll add a little more.

Between carb spray, smoke, and propane, which is the most effective at finding a leak?

 

You can also put seafoam in the intake which is what I would recommend for you right now (also other videos on it).

For the vacuum leak any if them are fine, but I would go with carb cleaner cause propane is sketchy to me ngl.

 

Thanks! I'll look into the seafoam in the intake and stick to carb spray

 

Check the EGR plate. Those things were known to burn through causing a huge vacuum leak. And the EGR valve could be stuck open,causing the same problem. If you don't need EGR for emissions, get rid of the whole mess.

 

When you rebuilt the carb, did you replace the power valve?

 

I'll look more into getting rid of it. I have read that the distributor would have to be re curved and the carb jets bumped up a few sizes.

When I had the carb and egr off before, the egr was full of carbon but it looked intact as far as I could tell.

 

Yes. The one in the rebuild kit looked exactly like the one that was in there before so I just swapped them.

 

My rpm gauge timing light combo got here early. I was off slightly on my estimate of where it was "idling" with the choke fully closed. I estimated 1500-1700 rpm. According to the gauge, it was 1400 rpm. The timing was almost spot on. I was aiming for 12 degrees but it ended up being at 14 at 750 rpm so I backed it off 2 degrees. (Read below to see how I got it down to 750 rpm)

Today, I wrenched down as hard as I could without stripping the carb bolts. All 4 moved a VERY SLIGHT amount. I added more seafoam to the gas can. And I backed the idle screws out to 3 full turns.

After all of that, I started it up and held the idle with my hand until it came up to temperature (not quite open thermostat). The choke was still shut all the way but I got it to idle down to 750. It was stumbling a bit but otherwise ran well enough that I could let go of the throttle and set the timing.

After setting the timing, I shut it off, backed the idle screws out 4 full turns, fully opened the choke (by now the thermostat had opened), gave it 2 pumps of gas and it idled around 750. There was still a stumble or miss but not too bad. It still smells slightly rich but maybe that's just how these older cars smell.

After that, I sprayed carb spray around the carb. The vacuum gauge was reading steady at 18hg. (Lower than the 21ish from before but I assume that's because of the lower idle and richer idle mixture screws). Anyway, I think the idle slightly jumped when I sprayed around the throttle on the carb. It also slightly jumped when I used the little red straw and sprayed under the egr plate at the back. The rpm didn't shoot way up like I was expecting but it was noticeable enough that it's clearly been the main problem all along and the high idle was hiding it and making it hard to find the vacuum leak because of the fan blowing too fast to get carb spray to specific areas.

I ordered one of those $80 2150 carbs from Amazon just to test it out. It has a 1.14 venturi (opposed to the 1.08 I have now). It may seem kind of dumb but I was told by a friend that they work just as well but don't use the same jet threads so if I need more flow, I'll have to drill them out.

When I was rebuilding the carb, I noticed that the idle mixture screws were "loose" meaning no or little tension. Also, one of the jets was pretty loose so I think the carb body jet threads are stripped. I got these issues somewhat worked out during the rebuild but this core may be toast regardless.

I'll keep everyone updated until this truck gets running good enough that I'd feel comfortable driving it. Until then, any advice would be appreciated!