Manifold vacuum

[IrishAv8or]

Hi all, recent owner of a Chim 500. My car has an ACT twin plenum with no existing spare vacuum ports on the plenum base (just the two for the brake servo and carbon canister). As I don't want to disturb the plenum, for fear of introducing a vacuum leak, to access the fuel pressure regulator vacuum hose, I'm thinking aout teeing into either the brake servo or carbon canister hose to get manifold vacuum to improve the idle and shunting. Is there any benefit of using one against the other?

[PabloGee]

Hi mate, saw your post on FB too.

No difference between those two for this application - they are both open routes to the manifold.

You could even go a further step to see if the carbon can is still working (by applying 12V to its connectors and listen for a solenoid/valve opening sound), which might provide a reason to strip it out and use that port for dizzy advance.

Then for MOT, simply disconnect it, and run the car with no vacuum assisted advance for a day.
It might be a bit sluggish, but should pass the emissions test.

[IrishAv8or]

My test is later today so I'll T the vacuum in later on. I'm aware that manifold vacuum would likely fail emissions. I'm getting no vacuum from my plenum port (for some reason!), so removing the manifold vacuum won't make any difference to the current performance :-)

[pmc_3]

The port on my plenum was blocked, from memory I managed to clear it with a bit of welding wire.

[PabloGee]

I've heard of it being blocked too.
Depends if you want ported vacuum signal or full manifold vacuum - which is arguably better for the engine running through lower revs.

But if you can unblock it, then (without having to do anything special) you will at least learn how the engine responds to having vacuum bringing in advance

[IrishAv8or]

It's strange, I can blow through the tube when connected to the plenum (albeit it's a very light hiss). I am changing over to manifold vacuum (teeing into carbon can hose) tomorrow and will just plug that vacuum for the test allowing it back to 12 BTDC and emissions-friendly.

[Belle427]

Maybe the take off point is blocked as said, may just be from when it was moulded rather than dirt.

[PabloGee]

IrishAv8or said:

It's strange, I can blow through the tube when connected to the plenum (albeit it's a very light hiss). I am changing over to manifold vacuum (teeing into carbon can hose) tomorrow and will just plug that vacuum for the test allowing it back to 12 BTDC and emissions-friendly.

I set mine to a base of 8° BTDC with no vacuum advance.
With full manifold vacuum, it pulls up to 18-20° BTDC.
I believe the engine will take more, but expect around a 10° advance to be added, clearly a decent timing light is your best friend in this plight!