L-Jet going full Rich on Deceleration Options

[Brian Fuerbach]

1974 1.8 with L- jet. Been playing with a wide band air fuel meter and noticed that when I lift the throttle the AFR goes to max value on the gauge. I thought it was supposed to go lean. I checked the throttle position switch and it checks out fine. Going to check the wiring harness next.

What else should I check? Decel valve?

[Brian Fuerbach]

Wonkipop,
I did read the thread on on the cap. I came to the conclusion that the cap must function as a crankcase valve of some sort years ago while changing the seal and o-rings. I had tested my cap to see if the membrane was operational and it checked out.

emerygt350,
The crankcase vents into the rubber intake tube in between the Air box/AFM and throttle body. The AAR valve and Decel valves also enter in that area.

[wonkipop]

Wonkipop,
I did read the thread on on the cap. I came to the conclusion that the cap must function as a crankcase valve of some sort years ago while changing the seal and o-rings. I had tested my cap to see if the membrane was operational and it checked out.

emerygt350,
The crankcase vents into the rubber intake tube in between the Air box/AFM and throttle body. The AAR valve and Decel valves also enter in that area.

emery, here is a photo of mine.
brian's will be the same.

i was having the thought because of the location where the decel hose connects.
its between the crankcase vent hose and the afm. it could sort of want to draw air from either direction.

but brian's report of the condition of his pcv valve/cap kind of puts paid to my theory.
so unless his cap is misbehaving since he tested it, its not a valid thought.



i did come across descriptions of momentary full rich as an afr reading.
happening immediately after closing the throttle.
if fuel has condensed on the inlet manifold and port areas.
closing the throttle and high vac moment causes condensed fuel to suddenly boil off the surfaces and be drawn into the combustion chamber and expelled. it doesn't last for long and is transitory.
depends a bit on the the design of the inlet system.

then there is always the question of the gauge and probe behaving correctly.
came across people describing some gauges actually measuring full lean, but having maxed out indicating this as full rich. ???? ie unreliable indications!

[emerygt350]

interesting. I assume that PCV system is like the 73 where it is vacuum sensitive. At high vacuum it actually closes up (well, it has a little metered breathing available), opening under low vacuum when you actually need it.


That initial rich is definitely a thing, but the going full lean on full rich is obvious on my autometer at least. The gauge actually kind of says "well, if we are going to be like that, screw you" and pegs itself to full lean and stays there for maybe a couple minutes. Mine is analog so there is normally movement in the arm at all times. When the computer gives you the finger, you can tell because it stops moving at full lean. It only does that when it gets too rich of a signal. Stupid. My digital AEM in the mustang doesn't do any sort of stupid thing like that.

[wonkipop]

interesting. I assume that PCV system is like the 73 where it is vacuum sensitive. At high vacuum it actually closes up (well, it has a little metered breathing available), opening under low vacuum when you actually need it.


That initial rich is definitely a thing, but the going full lean on full rich is obvious on my autometer at least. The gauge actually kind of says "well, if we are going to be like that, screw you" and pegs itself to full lean and stays there for maybe a couple minutes. Mine is analog so there is normally movement in the arm at all times. When the computer gives you the finger, you can tell because it stops moving at full lean. It only does that when it gets too rich of a signal. Stupid. My digital AEM in the mustang doesn't do any sort of stupid thing like that.

yes emery - am sure thats how it works on the 74 L jet too.
might also do a little balancing act between crankcase and inlet pressure differences at other times.

guess i was trying to think of the next thing to look at to be sure the info telling brian this was happening was 100% reliable.

is this a new thing brian? never done it before with the probe hooked up?