Zero vacuum at idle. Need help.

[rlong]

The scoring on cylinder #1 and wet compression figure almost 50% higher are both signs of broken / damaged piston rings on #1. #3 and #4 do indicate likely head gasket.

 

[charliemeyer007]

With the engine warmed up and running, you could strike the #1 rockers at the valve with a mallet, several times each - I use a lead hammer. The goal is to bump the valves open under compression to try and blow away/dislodge any crud on the seating surfaces that could be causing the low compression.

That scuff looks like the engine was overheated at some point.

 

[pb4ugo]

Your plugs show a rich condition. It appears your rings are worn. Was the pic of the piston after you put oil in the cylinder? The top of the piston appears wet with fuel or oil or both. I think your compression #'s are on the low end.

 

[misterpepper]

Pic was after the wet compression test. The oil on the piston is from that.

Does anybody have reference numbers for compression by year? The pistons look stock to me so assuming the cam is also stock that could be helpful.

Considering the fact that all plugs are roughly equally sooty does that point back to the carb being the problem, rather than something in the block? Shouldn't either #1 (low compression, bad rings) or #'s 3&4 (possible head gasket leak between cylinders) be significantly different than the other cylinders?

 

[DIRDY]

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.


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.


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


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?
View attachment 4111101

I have done everything you have with no luck that's why I'm going with a small block Chevy

 

[LAMBCRUSHER]

I’ve heard Seafoam can loosen up stuck rings, but it seems by the numbers and pictures that you’re in for at least an in chassis ring job/ head gasket. Depending on your time, might be worth doing a shade tree hone, replace rings in #1, disassemble head, lap valves to verify seat margins are good, reassemble and send it. Obviously, if valves/ guides need more attention, this is the time.
If you get into a full rebuild, I cannot and will not recommend anything other than a stock grind cam, some decently ground OEM or new OEM lifters, and whatever you do, do not let your machinist “do you a favor” by increasing the valves you spec from 1.82” / 1.50” to 1.92/ 1.60”. It does not help…

 

[misterpepper]

I had to take a break from working on it because it developed a pretty bad coolant leak so I ordered all new hoses, which will take some time to get here. I did go through and clean about half of the electrical connections between the ignition and the coil which seemed to help a little. I have brand new plug wires and a new coil to install. I'll do that, plus finish cleaning the rest of the electrical connections.