Normally I can find the answer for just about anything car related on the internet but I can’t seem to get any positive direction for this vacuum reading anywhere…
you can see it at https://vimeo.com/244265209
Most mechanics I’ve taken it to refuse it.
It’s a newly rebuilt stock v6 in a 1974 FJ40 , running points, everything is new. No leaks anywhere on manifold. It runs great sometimes, but always starts spitting at some point.
hmm … well the intake manifold vacuum is created when a piston moves downward while the intake valve is open, and the exhaust valve is closed. It’s more or less constant b/c the next piston takes over for the one that moves from going down to going up on it’s own compression stroke. So something must be wrong with all that.
Exhaust valve not closing completely, check valve clearances
Intake valve not opening complete, check valve clearances
Camshaft timing incorrect w/respect to crankshaft, check valve timing
Piston rings or valves not sealing, check compression, both wet and dry
Lifter problem, checked for damaged or collapsed lifters
I recall a few Toyota engines of that period with loose valve seats which would certainly cause the vacuum to bounce. Find the misfiring cylinder and run a leakdown test.
As for that old Toyota running with points, you might check for an igniter attached to the coil or near it. If you find an igniter the points rarely need replacement or service because they only carry enough current to trigger the transistorized igniter. and can outlast the engine.
edit; on second thought the 1974 seems to have the inline 6 engine with a cast iron cylinder head which is unlikely to use pressed in valve seats.The inline 6 was somewhat of a clone of an early GMC engine.
I learned elsewhere that the vacuum reading can actually be caused by the gauge itself not having a “dampner”…
My distributor has now quit making consistent sparks. Spark is intermittently lost on all the plugs. Basically the points are occasionally failing. No amount of adjusting, checking grounds, wires, etc. changes anything. It’s an aftermarket distributor so I am going to replace it with an OEM one. Hopefully that solves the problem!
I’m not saying it doesn’t happen but my gauge is a cheap one from Target about 35 years ago and has been pretty reliable and never pulsated. Sure might be a few percent off but they are pretty simple devices.
No igniter. Engine had one before it was desmogged. But not anymore. I’m replacing with a OEM distributor that doesn’t need an igniter for sure. But I’m not sure if one was necessary for this or not. I’ve managed to drive it almost 200 miles on the rebuilt engine. It was working well initially. Manual says that if you pull apart the points with your finger and see blue spark, that indicates bad condenser. There’s definitely blue spark. Going to try a new condenser tomorrow just for kicks.