intake manifold vacuum measurement

[Zed Head]

20 hours ago, kenward1000 said:

Unclear on your suggestion about vacuum advance; suck on what?  Bought the car last year and it's been disconnected all this time. 

You want to see if the diaphragm inside the vacuum advance canister has a hole in it.  You can suck on the hose nipple to be sure.  Also, it's not a bad idea to remove the cap and make sure the vacuum advance is free to move.  They tend to get rusted bearings and lock up.

[kenward1000]

On 12/3/2020 at 2:28 PM, AK260 said:

Great job!

OK, first up the disconnected hose you are showing is a Positive Crankcase Vent (PCV) tube and when connected to the balance bar (the bit that is plugged) blow by gasses and hot oil fumes from the crankcase are sucked into the intake. You must connect that into a catch tank otherwise 1) it will eventually make a mess of your engine bay 2) it really is anti-social! 😉 It’s not supposed to vent directly to the atmosphere My 0.3L catch tank was £20ish off ebay and works perfectly.

Next up, you will find there to be a take off on the front carb that should connect to the distributor vac canister in your photo. It looks like this ...



If that isn’t plugged you will be causing a massive lean condition at part throttle or idle - it may also be whistling loudly - reducing manifold vacuum by quite a margin.

I was suggesting that you suck on the tube that normally would have been connected to the canister on your dizzy, but you don’t appear to have one.

Finally - cue[mention=19495]Captain[/mention] Obvious to tell me I’m at the wrong end of the car or saying something that is “obviously” wrong! 😛


Edit: now this I’ve never seen before - is it some kind of USDM emissions type thing!?



 

So I figured out the part number for the missing PCV hose barb for the intake manifold, ordered a replacement, arriving Friday.  The hose barb coming off the PCV fitting at the block, now has a black rubber cap instead of the green air filter.

You were correct about the open hose barb on the front carb, it was open to the atmosphere.  Figured out by looking at a 71 240z that vacuum line from the front carb goes to the vacuum advance on the distributor.

Checked to see if I could pull suction from the vacuum advance hose barb, could not.

Have a Pertronix ignition installed, does the vacuum advance mechanism still have any functionality?

[AK260]

Excellent, you’re getting there!! Those open carb barbs would have caused no end of issues. You will of course have to retune your carbs once you have all this sorted out.

I’ve never seen a vacuum feed off the carb insulator before (see my mark-up). That looks homemade.



The vac advance I’m a big fan of. In short it’s a closed loop load sensing mechanism. But yes you can leave it disconnected at the distributor and plugged at the carb end. It’s good that you can’t suck air through the canister. I had two that were completely open to the atmosphere!

When idling, off throttle or part throttle, the mix becomes lean. A lean mix takes longer to burn. Think of it as a line of gun powder - the more spaced apart the particles the longer it takes to burn all the way. So the manifold vac is used to drive a diaphragm inside the canister on your dizzy to add timing in low load / no load conditions. This ensures you get as full a burn as possible which has many subtle but positive outcomes:

1. better fuel economy
2. smoother transition from part to wide open throttle
3. The combustion completes inside the cylinder and not down the primary exhaust pipes, reducing under-bonnet temps directly below your carbs.
4. Cleaner plugs when idling in traffic
5. Lower emissions and less unburnt fuel smells.

My old 280zx distributor used to add as much as 20 degrees @ 3.5k rpm! That is probably OK with stock engines having 8ish static compression but on my modified engine with 10.5:1, it made it difficult to go to sleep at night! ;)

Here is a great table showing what advance which dizzy adds ....



Keep at it, sounds like you are making great progress!! [emoji106]

[kenward1000]

On 12/9/2020 at 1:21 AM, AK260 said:

I’ve never seen a vacuum feed off the carb insulator before (see my mark-up). That looks homemade.




 

Vacuum system is now complete and good vacuum is measured.  Thanks for all the tips!  Really appreciate it!

The manifold hose barbs are installed on each insulator, for my TwinMax carb balance/synch tool.  https://www.amazon.com/TMX-A005-Twinmax/dp/B004K5MH5M  It works great on all my vintage twin motorcycles and works great on my 240.  I'd previously tried my mercury column balance/sync tools, but the 240 pulls so much more vacuum that a bike, that they're useless.