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?

[wonkipop]

i am doing some more reading on just how the L jets handled backing off on decel.

i feel pretty sure that mine, without its decel valve connected would also be appearing to go very rich when i come off gas. doing a fuel dump more or less. probably like yours.
but maybe for slightly different reason. ie no decel valve.

there is no fuel cut off in these early L jets.

making sure the fuel pressure regulator is being operated by the vacuum line at both idle and coming off the gas is part of confirming the system.

but that i think is the way they are.

the distributor is definitely doing something to help clean up the hydrocarbons.
so it may well be that there is nothing here of concern if your afr etc are fine everywhere else.

by looking for info on your afr condition i now understand something about mine when its cold and i give it a rev and it almost dies and comes back to life. i'll post that up when i can summarise it simply and clearly. but its down to what the distributor is doing in that instance on my car.

[Brian Fuerbach]

by looking for info on your afr condition i now understand something about mine when its cold and i give it a rev and it almost dies and comes back to life. i'll post that up when i can summarise it simply and clearly. but its down to what the distributor is doing in that instance on my car.

While adjusting my decel valve I was able to simulate the same die and comeback to life situation as you. I think without the valve you will always have this issue.

[wonkipop]

by looking for info on your afr condition i now understand something about mine when its cold and i give it a rev and it almost dies and comes back to life. i'll post that up when i can summarise it simply and clearly. but its down to what the distributor is doing in that instance on my car.

While adjusting my decel valve I was able to simulate the same die and comeback to life situation as you. I think without the valve you will always have this issue.

only does it to me when engine is cold and in warm up.
not been an issue for 30 years that i have to put up with in normal driving.
goes away once car is warm.

i've always thought, probably simplistically, it was that the afm flap bouncing back and briefly cutting the fuel. at the same time the distributor taken all the way to full retard (about 3-4 BTDC) snaps back off retard (engine almost stops and vac drops off) and goes back to 7.5 - which is the initial start firing position. after the two things happen and it all stabilises the car fires again and normal warm up resumes. lasts about 1 second in real time.

guess a decel valve while doing other things on deaccleration stops the afm flap closing so fast it travels past fuel cut off and then does the bounce back up, just as the engine catches again with the timing off retard.

why it does not do it warm, i have no explanation. perhaps it does but its recovery is so fast its not noticeable.

that afm flap going back to full rest position is about the only thing i believe that ever cuts fuel off. it does it in a real dumb way - just turns off the fuel pump.
but you would think it has enough fuel pressure to avoid scenario i outline above.
i dunno sometimes.

anyway i think confirm operation of fpr and knock it off the list.

lets us talk bs and read more stuff.

the only other thought i am having is that its pulling crankcase fumes at high vacuum after throttle closes because the pcv valve in the oil cap is not working. but we should leave that after fuel pressure drop on vacuum is ticked off.

i am sure some others will pipe in with that info in hand.


(IMG:style_emoticons/default/beerchug.gif)

[Brian Fuerbach]

by looking for info on your afr condition i now understand something about mine when its cold and i give it a rev and it almost dies and comes back to life. i'll post that up when i can summarise it simply and clearly. but its down to what the distributor is doing in that instance on my car.

While adjusting my decel valve I was able to simulate the same die and comeback to life situation as you. I think without the valve you will always have this issue.

only does it to me when engine is cold and in warm up.
not been an issue for 30 years that i have to put up with in normal driving.
goes away once car is warm.

i've always thought, probably simplistically, it was that the afm flap bouncing back and briefly cutting the fuel. at the same time the distributor taken all the way to full retard (about 3-4 BTDC) snaps back off retard (engine almost stops and vac drops off) and goes back to 7.5 - which is the initial start firing position. after the two things happen and it all stabilises the car fires again and normal warm up resumes. lasts about 1 second in real time.

guess a decel valve while doing other things on deaccleration stops the afm flap closing so fast it travels past fuel cut off and then does the bounce back up, just as the engine catches again with the timing off retard.

why it does not do it warm, i have no explanation. perhaps it does but its recovery is so fast its not noticeable.

that afm flap going back to full rest position is about the only thing i believe that ever cuts fuel off. it does it in a real dumb way - just turns off the fuel pump.
but you would think it has enough fuel pressure to avoid scenario i outline above.
i dunno sometimes.

anyway i think confirm operation of fpr and knock it off the list.

lets us talk bs and read more stuff.

the only other thought i am having is that its pulling crankcase fumes at high vacuum after throttle closes because the pcv valve in the oil cap is not working. but we should leave that after fuel pressure drop on vacuum is ticked off.

i am sure some others will pipe in with that info in hand.


(IMG:style_emoticons/default/beerchug.gif)

I believe that the decel valve has alot to do with your dipping return to idle. I could adjust the level of return to idle from "long hang time" to the "dip and stall" with a turn of the adjuster. When I finally got it where I wanted it checked out at the specified 20hg. Go figure. This is not to say that you would not be able to tune around it but the valve certainly serves a purpose.

Checking the fuel press tonight and maybe a little drive too.