Vacuum or not on the 34 ADF

[saidja]

The vacuum advance seems to be connected to the 34ADF but to my opinion this is not a real vacuum advance port on the carburetor. Any suggestion on running the distributor without vacuum and leave the port of the carb open or closed?

[Nut124]

I have always run manifold vac for dizzy vac connection for all my cars incl Fiats.

I have yet to work on and engine that does not run better with manifold vac advance. This can be a touchy subject thought among Fiat purists as in the late -70's Fiat used "ported" vacuum, which I think did just about nothing at all. IMO, ported vac was a short lived anomaly resulting from desperate attempts to comply with US emissions regs with carbs.

The key is: Set dizzy based on total advance at above 4000rpm, not to exceed 38deg. Target 36. This total, mech advance should be in by 3800-4000. The vacuum advance disappears as soon as you step on it. Listen to knock at 2500-4000.

In mine, idle timing is about 10deg w/o vacuum and 20-24 with vac at idle.

The vac advance only affects idle and low speed cruising though.

If not clear, manifold vac comes from the manifold, where the brake booster connection is.

[18Fiatsandcounting]

Just curious: What engine is in the picture? It has the block mounted distributor used on the early (1438cc) engines, but then it has a rather odd crankshaft pulley, water pump pulley, and oil filter mounting flange. Is the engine from a spider, or possibly something else like a 131?

Also, I wasn't aware that any Fiat block-mounted distributors had a vacuum advance. Later cam housing-mounted distributors had vacuum advance, at least in the later years.

-Bryan

[saidja]

Indeed took me a while to figure out what engine it was.
It was fitted in my 1978 CS1 1800 sport spider.

The block and the head is a 2 liter from a Fiat 131.
The distributor marelli SM810fx, original donor probably a Fiat Regata.
The carburator an 34 ADF Weber.

Anyhow the rebuild went smooth, and while reviewing recently the distributor, I noticed i never had the VAC connected.

[Nut124]

I would plug the vac port on the carb and use manifold vac for the dizzy, unless you must pass emission testing.

[jon8christine]

I've read about not exceeding mid 30s advance. I assume you use a timing light that allows you to set the advance on the light to read that far? I currently have my distributors vacuum hooked to the port on my 32 36 dfev and it doesn't seem like it's pulling anything (need to borrow a gauge and check). When I rev the engine timing stays about the same. Is the carb vacuum port ported? I didn't get too far with my timing setup before the engine overheated due to a failed fan relay.

[18Fiatsandcounting]

I assume you use a timing light that allows you to set the advance on the light to read that far?

Yes, that's what I use. A timing light with a knob on the back, and you turn the knob until the 0 degree (TDC) mark on the crankshaft pulley lines up, and then read the advance from the knob pointer. I have to admit to wondering how accurate this is...

Optimal ignition advance depends on a number of things, primarily engine speed, but also air/fuel ratio, gasoline blend, temperature, whether you have EGR or not, etc. I just set my engines so that the maximum advance at 3000 or so rpm is around 36 or 37 degrees, and then "ignore" the advance at idle although it's generally around 15 degrees. I have centrifugal advance only, and that centrifugal advance is limited to 22 degrees. Each engine is different, so you have to find what works for you.

-Bryan