testing for vacuum leak (1 Viewer)

[albert110]

Am trying to track down the source of a vacuum leak in my 60, still has all the smog equipment that I replaced with silicone hose about 5 years ago. The truck was running fine until last spring, 2023 when it went under a car cover and was left over the Texas summer (No AC).
a month or so ago I started driving it again, and immediately ran into stalling issues and found that it wasn't holding more than 10-12mmHg at fast idle. I adjusted timing and thought I'd fixed it, not sure what happened but it came right back.
I bought a smoke machine and hooked it up to the system through one of the manifold inlets. I put cellophane over the top of the carb and turned it on, and immediately got smoke coming out of the rubber boot over the accelerator pump. I'm wondering if the heat of last summer was enough to crack the seals... I figure it's also totally possible I'm using the smoke machine incorrectly, first time... I assumed that covering the top of the carb would seal it, is there a separate idle air inlet that isn't routed through the air cleaner?

I appreciate any input, thanks.

 

[2mbb]

I suppose if the boot on the accelerator pump (#1 in the schematic) is broken and the pump seals have dried up, you could have air leaking into the venturis. But this would not effect your manifold vacuum, IMO. The "leak" would be above the throttle plates and manifold vacuum is measured below. Also I think the amount of air coming in through the accelerator pump channels would be very minor compared to what is going through the venturis that I don't think this would even cause a lean condition. However, you should make sure your accelerator pump is working. With the engine off and the air cleaner top removed you should see fuel squirting into the primary throttle bore when you manipulate the accelerator linkage.

Check sources like the PCV valve and brake booster.

The brake booster union is screwed into the intake manifold near the firewall.

 

[Borebrotherbore]

A trick I was always taught to find a vacuum leak. Is to take ether spray and just slowly go from one vacuum line to another. When you hear the engine start to rev higher, bingo, found the leak.

 

[albert110]

Thanks both to @2mbb and @Borebrotherbore. The insight about the accelerator plunger leak being above the throttle plate either way makes sense, so I'll need to figure that whole issue out separately. As far as the brake cleaner trick, I'm up for trying it, although it's a little annoying that I can only get the engine to run with the choke fully out, so it's a bit tough and I was hoping the smoke machine would give me specific results quickly so that's a bit of a bummer... I'll keep trying and report back either way, thanks again!

 

[2mbb]

Do you know your FCS/ICS is working. It is the electrical device mounted on the top side of the carburetor on the firewall side.

 

[2mbb]

This green plug circled in blue should be connected:

 

[albert110]

@2mbb I haven't checked either of those solenoids yet recently but will put it on the list. I just spent a bit more time with the smoke machine and found two more issues. The first was a large tear in the hose that (I think) routes to the VCV. It's the larger rubber hose that routes off the carb straight back to the firewall and then disappears. The tear was right at the nipple so I cut the hose and was able to get the shortened hose back onto the line. I ran the smoke machine again and after a minute or two noticed smoke emanating from the distributor, so I assume I have another leak at the vacuum advancer as well that I should repair before I move onto electrical.
My big question now is that I've been thinking about switching over to mechanical advance electronic ignition, and I'm wondering if this is a good time to take the plunge, and what my options are.
Another issue that while I was trying to fire the truck up just now I think my choke cable gave up the ghost because I get no resistance and I don't see anything at the carb move when I pull the cable... ah Sunday funday....

 

[OSS]

If the EGR valve sticks open, that scenario will create a mega vacuum leak.
Mine would get stuck open every once in a while.

 

[albert110]

If the EGR valve sticks open, that scenario will create a mega vacuum leak.
Mine would get stuck open every once in a while.

I'll look into this, thanks! 99.5% of the time I bypass the EGR system, but I suppose since I never use it, I also never check it... I'll inspect it along with the ICS/FCS

 

[albert110]

OK, status update is that I located a big vacuum leak at the distributor VCV. If I plug the hose leading to the distributor VCV along with fixing the other torn hose I fixed I'm now getting a little over 15mmHg when I run the engine, a little more at higher RPMs. I don't want to idle for too long since I'm not venting the distributor until I have the new VCV, but it looks like I still have one or two more vacuum leaks to track down before I have a clean bill of health anyway. Next step will be to fix the accelerator pump, since that's dumping so much smoke it's hard to diagnose anything around the carb, and then test the EGR valve. Man these Texas summers are brutal, this truck was running like a champ up until she spent a few months under a cover in the heat...