Vacuum signal for distributor

[jimrat]

Hi

I took the car for a rolling road session to get the idle, timing and mixture dialled in. Very happy with the results - a mighty 91 bhp - it's a stock UK car except for K&N filter and one size up on the main and idle jets.

I have swapped the factory Marelli S135 points distributor (no vac advance) with an electronic Marelli SE100CX unit off a later Uno 1.3, which has vacuum advance. My question: where do I get the vacuum signal for the distributor?

I was using the port directly below the primary throttle plate on the 34 DATR carb. The tuner measured the vacuum at this port and found that it sits at 1.5 bar vacuum regardless of the throttle position. He said the vacuum signal for the distributor should drop as the throttle plate opens. As a result, using the SE100CX distributor, I'm getting too much advance, so he has left the vacuum disconnected.

Where is the correct place - the inlet manifold? He was talking about drilling a hole in the side of the carb to pick up a vaccum signal - anyone know if that is possible?

Cheers
Jim

 

[Alex(Tenerife)]

Great result, not surprised you are happy with that output.

I am sure on my original engine there was vacuum for the distributor and it came from the stock carb. I never got a chance to measure it as you did but I felt it sucked more on my finger as I reved the engine.

I don't remember where it was I am afraid, hope someone can help but I seem to think it was somewhere near the carb base. Is this where you have yours?

 

[Steve Thomas]

a mighty 91 bhp

Nope. That is some sort of guess. Better to be certain and say 55 hp at the wheels. Stock UK on carby = 85 hp at 6000 RPM at the flywheel. You saw 55 at the wheels but at about 5500 RPM. That suggest you actually have less than stock, not more. Bigger jets will not give much if any as nothing has changed in terms of moving fuel/air through the engine. The wheel power curve is a bit flat and fluttery above 5000 RPM [real data] but the projected [guess] is nice and smooth - cos its a guess. Dyno places love to give customers a warm feeling

 

[ng_randolph]

I have swapped the factory Marelli S135 points distributor (no vac advance) with an electronic Marelli SE100CX unit off a later Uno 1.3, which has vacuum advance. My question: where do I get the vacuum signal for the distributor?

I was using the port directly below the primary throttle plate on the 34 DATR carb. The tuner measured the vacuum at this port and found that it sits at 1.5 bar vacuum regardless of the throttle position. He said the vacuum signal for the distributor should drop as the throttle plate opens. As a result, using the SE100CX distributor, I'm getting too much advance, so he has left the vacuum disconnected.

Where is the correct place - the inlet manifold? He was talking about drilling a hole in the side of the carb to pick up a vaccum signal - anyone know if that is possible?

It is common to use ported vacuum for the distributor vacuum advance, This is vacuum taken from just above the throttle plate, so at idle there is no vacuum (and thus no ignition advance).