Zero vacuum at idle. Need help.

[misterpepper]

Idle screw is currently 2.5 turns out, although I feel it runs pretty similar from 2-4 turns out. I did set it using the lean drop method. Timing is bang on the BB, so 7 degrees. I jumped back and forth between distributor and carb when I tuned it last. I could try advancing the timing some but I figured it was best to tune things by the book until I got it running reliably.

I feel like it has to be something other than the carburetor, distributor, coil, plugs/wires or PCV valve, since I've tried multiple of each of those and the symptoms don't really change. My best guess is a vacuum leak, but I can't find it. I've tried using carb cleaner, starting fluid and an unlit propane torch. I've only really checked around the engine bay. I worry that maybe my rear main seal is bad enough that it is letting air in. I'm not sure if that's a thing on these engines, but I've had the rear main seal go out on my Audi and the vacuum leak it caused really messed with how it ran. I do have an oil leak, which I assume is my rear main seal.

I'll try again to identify if there is a vacuum leak in the engine bay using the short bursts of starter fluid method. I was maybe a little too liberal with my application of starter fluid earlier. I'll also check how it runs if I introduce an intentional vacuum leak.

 

[Pighead]

I could try advancing the timing some

You could. Some people find that advancing the timing increases the vacuum (to a point). There have been many posts over the years about this. Back in the day, before fancy digital timing lights, we would time by vacuum. Give it another 5 or 10 or 15 degrees advance, it won't hurt anything, probably need to turn down your idle speed screw, and check your vacuum.

 

[LAMBCRUSHER]

I forgot to ask if the valves have been adjusted?

If the rings are stuck or bad, or the cylinder walls are scored, you could have low vacuum. But that’s a bridge to cross last.

Definitely adjust the valves, bring up the compression on #1, do another comp test. If you still have low pressure, drop in a teaspoon or two of oil, and retest. If the pressure comes up, you have ring issues.

The idea that you don’t get any change in idle from 2-4 turns on the mix screw sticks in my mind. Almost like the idle circuit might be bypassed by a weeping carb.

Next time you have the carb off, check the bottom of the intake for cracks.

 

[misterpepper]

I just tried a couple of things with the vacuum retard distributor that is on there currently. I haven't tried swapping to the vacuum advance carb yet.

I checked all over the engine bay for vacuum leaks again by doing small squirts of starter fluid and listening for any change to the engine. I couldn't really find anything. I paid extra attention to the base of the carb, underneath the intake manifold at the carb, where the intake runners meet the block and on the carb body itself. I even pulled the oil fill cap and the dipstick, but nothing really changed anything.

Next I introduced a vacuum leak by unhooking one of the legs of the T fitting that is currently between the distributor and the "gas filter" on the intake. So the intake and distributor were basically open to atmosphere at that point, albeit with the intake still muffled by the gas filter. No real change in how things ran. I could get the engine to speed up a little change by spraying starter fluid directly into the open T fitting.

I also adjusted the timing. Increasing by 3 degrees (to 10 deg) improved the vacuum by about 1/2" to just under 15" and sped the idle up a little. Increasing timing to 15 degrees got me to 15.5" of vacuum. I went all the way to almost 40 degrees. While there was marginal improvement to throttle response at 10 degrees it is still poor and I am still getting a lot of backfiring, about 1 per second.

I did spot what looks to be maybe a missing plug in the intake, directly between the carb and the block. You can see it directly below the idle adjust screw in the picture. Should this be plugged? I tried putting my finger over it but it didn't seem to change how the engine ran and I couldn't feel a lot of vacuum there. Nothing like at the PCV valve hose.

And to answer LAMBCRUSHER, yes, I did adjust the valves recently. Along with changing to the PCV to a new Toyota unit and replacing the PCV grommet, valve cover gasket and the 4x seals for the valve cover screws.

Edit: That hole near the base of the carb doesn't seem to be anything. It just goes straight through. It must be some sort of mounting hole or for drainage or something.

 

[misterpepper]

I now have the vacuum advance distributor on. Still popping and backfiring and running rough with very poor throttle response. Still getting about 14.5" of vacuum. Throttle response might be a little better, but only maybe just enough to pull it out of the garage to work on another project. I'm feeling pretty defeated by this thing.

I don't know if this is relevant, but it doesn't have an exhaust on it right now. It has headers, but nothing beyond that.

 

[Pighead]

Edit: That hole near the base of the carb doesn't seem to be anything. It just goes straight through. It must be some sort of mounting hole or for drainage or something.

That hole was for the bolt that went through the intake and threaded into the exhaust manifold.

 

[aztoyman]

Do you get spark at every cylinder? Any chance a couple plug wires got switched around?

 

[LAMBCRUSHER]

Some back pressure from the exhaust could be helpful.

Is the needle on your gauge steady low, or bouncing low?

 

[LAMBCRUSHER]

Do you know if it has a cam? I mean, is the cam stock, or is it a hot cam. My delta KC mild grind cam leaves me around 16-17 (hehe) inches of vacuum at idle. A taller lobe will make that even lower.

Are any of the carbs you have tried freshly rebuilt? If they are all slightly used, and each has a chance to sit for a while with fuel left in them to varnish, you might just be banging your head against clogged air bleed jets, which will cause it to pull more fuel. The idea that you do t get a change in idle from 2-4 turns on the mix points me to the idle circuit is bypassed…

 

[misterpepper]

Do you get spark at every cylinder? Any chance a couple plug wires got switched around?

Yes, I'm getting spark at every cylinder. I've verified by both spark gauge and by pulling the plug wires at each cylinder while the engine is running. The order of the plug wires is correct.

Is the needle on your gauge steady low, or bouncing low?

I wouldn't call it bouncing. There is always some amount of bounciness to a vacuum needle but it stays within about 1/2" or so of the reading.

Do you know if it has a cam? I mean, is the cam stock, or is it a hot cam. My delta KC mild grind cam leaves me around 16-17 (hehe) inches of vacuum at idle. A taller lobe will make that even lower.

No idea as to the guts of the engine. I don't suspect it has been modified in any way, though.

Are any of the carbs you have tried freshly rebuilt? If they are all slightly used, and each has a chance to sit for a while with fuel left in them to varnish, you might just be banging your head against clogged air bleed jets, which will cause it to pull more fuel. The idea that you do t get a change in idle from 2-4 turns on the mix points me to the idle circuit is bypassed…

Here's what I know about the carbs I've tried:
I first tried the trollhole carb that was on when I purchased the truck. The truck had sat for about 10 years and that carb definitely had some varnish and clogged jets. I couldn't get the engine to run at all until I cleaned it out a bit. Over 2-3 attempts to "rebuild" that carb by following the PigHead videos on youtube and not having luck getting it to run well I moved on from that carb to the '69-'71 Aisin 2bbl. But by the time I was done tinkering with it all jets and ports were clean and everything inside was moving freely.

The '69-'71 was given to me years ago by a neighbor and was told that it had been freshly rebuilt. Based on the rubber boot over the accelerator pump shaft I'm inclined to believe that, but I didn't verify for myself. I don't have a throttle cable suitable for that carb so I haven't put a lot of effort into getting it to run with it. I originally was able to hook up the choke cable and got the engine started and even idling w/o choke using that carb. The throttle response seemed a little better, so I bought the Amazon carb that is similar to the trollhole. In all of the switching back and forth between carbs (I've probably had carbs on and off more than 20 times now) I have gone through that carb to set the 4 adjusting screws up according to my Haynes manual. After doing that I was able to get the engine running about as well as with the other two, newer style carbs which I do have a throttle cable for. That said, I have a rebuild kit for that carb on hand and a throttle cable coming.

The Amazon carb was brand new out of the box. I have been inside of it enough to adjust the float, check the flatness of the mating surfaces and gasket alignment for the big gasket above the bowl, verify that the idle solenoid is working and that the o-ring looks correct, and verify that the seal where the vacuum secondary diaphragm unit attaches to the carb is sealing and that the diaphragm holds vacuum. I have not rebuilt it, but I don't think it needs it.
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So with all of that, today's project will be to build a homemade smoke tester and try smoke testing the intake. I doodled up a 3D printed plate to mount onto the intake, in place of the carb, to inject the smoke. I assume the best way to do it will be to remove and plug the hose at the PCV end, so that I'm not feeding smoke into the block that way. Any other tips for smoke testing the intake?

 

[rlong]

 

[LAMBCRUSHER]

Ok. Good work on the smoke adapter!

I feel like you shouldn’t have that much bounce on the vacuum gauge. Quite possibly valve or ring issues, based on everything you’ve done. Eliminating big vacuum leaks will definitely help narrow down. Let us know how that smoke test goes. I’d suggest a leakdown test as a possible next step if no smoke comes out.

 

[DustyFJ]

I had two intake manifolds crack under the carb. (But I am running a 2f). It might be worth pulling the carb and looking down in there. Could be a hairline crack.

 

[misterpepper]

Intake manifold under the carb seems fine but I did find a small leak where the hard tube for the hose to the PCV meets the phenolic-like carb riser. I'll put some JB Weld on it. Unfortunately that means no more testing until tomorrow while the JB Weld cures.

 

[misterpepper]

Carb riser leak wasn't the source of my problems. Not only did I seal the leak with JB Weld, but I also sanded flat both sides of the carb riser and created new gaskets for top and bottom. Still runs about as crappy as ever.

I have a leak down tester on order since I'm becoming more and more convinced that my problems are internal.

 

[aztoyman]

The positive side of this is that you are learning a LOT about your engine and you won't forget any of it. I'm looking forward to the solution.

 

[LAMBCRUSHER]

Having fully built out a 2F over a decade ago, if I had to get into one again, I'd do the least amount of work needed to get it back into the operational window. Down to replacing only a single bearing, ring set, or piston.

 

[misterpepper]

Amazon is amazing. I don't even live in a town with a distribution center but I ordered a leak down tester last night and it got here by 10:30 this morning. So I did a little testing.

First, the spark plugs. They all look pretty equally covered in soot. None were wet. This is coming out of a cold engine that hadn't been run since yesterday, so any unburnt fuel on them would have evaporated, but at least there wasn't any oil on them. Here they are in order, #1 on the left, #6 on the right. Not sure why #4 looks shiny in that picture but it was just as dry as the others. Maybe with just a little less soot I guess. These plugs only have maybe an hour of run time on them, and basically all of that at idle. They are NGK BPR5ES gapped to 0.035".

Cold, dry compression numbers 1-6 are 65/122/121/118/120/121. Obviously #1 is the outlier.

Cold, wet compression numbers, after putting a teaspoon of oil down each cylinder, are 93/131/131/122/128/130. Cylinder #1 came up almost 30psi and cylinder #4 barely changed.

Leak down results by cylinder are:
#1 High (~80%), I could hear air coming from the oil fill cap, but not the radiator, exhaust, carburetor or cylinder #2.
#2 Low (~17%), air coming from the oil fill cap only.
#3 Low (~18%), air coming from the oil fill cap and the #4 cylinder plug hole.
#4 Low (~23%), air coming from the oil fill cap and the #3 cylinder and #6 cylinders.
#5 Low (~20%), air coming from the oil fill cap and the #6 cylinder.
#6 Low (~30%), air coming from the oil fill cap only.

So the good news is I didn't have any air bubbles coming up through the radiator from any cylinders. Nor did I have air coming up through the carburetor or out the exhaust, as best I could tell. I put rubber gloves over the carburetor, the exhaust and the oil fill cap. To check adjacent cylinders I used a lighter and watched the flame.

I am little concerned about the air leaking through the adjacent cylinder plug holes on a few cylinders, but I don't know if that's just due to certain valves being open or if that signifies a bad head gasket. The fact that #3 and #4 both leak into each other's cylinders would suggest a bad head gasket, but #5 leaks into #6 but not visa versa, and #4 also leaks into #6, which isn't really possible, so I'm guessing that has more to do with whatever air is getting past the rings and into the block being allowed to circulate back into those cylinders somehow.

I briefly checked inside some of the cylinders using my bore scope before beginning the leak down test. The only thing that looked out of place to me was a streak along the #1 cylinder wall. It doesn't look very deep. especially at the top, but it doesn't look good either.

As always, thoughts and suggestions are welcome.

 

[Pighead]

I am little concerned about the air leaking through the adjacent cylinder plug holes on a few cylinders, but I don't know if that's just due to certain valves being open or if that signifies a bad head gasket.

I wonder, if you backed out the valve adjusting screws far enough so that all the valves were closed, does the air still escape from adjacent cylinders?

 

[misterpepper]

So take the valve cover off and totally back off all the valves so that everything is always shut, then redo the leak down test? I guess that would tell me for sure if it was the head gasket. Sounds like a lot of work.

By the way, it turns out the engine is from a '65, not a '69. Not sure if that makes any difference.