Surging Idle L-Jet m30b32

[Charlesm3]

Group,
I have a 1977 euro m30b32 with L-jet built by Metric Mechanic to Sport ST 3500 spec many years ago. Recently when the motor is warm while driving normally the car will idle surge when I roll to a stop. This pulsing (800 to 1500 rpm) may happen anywhere from 6 to 15 times before it settles back into a normal idle. I have changed out the idle control valve to no avail. Now I’m stuck.
Thoughts?
Best,
-Charles

[Brucey]

most likely there is a big air leak and/or fixed air bypass setting is wrong, and when the engine is warm, there is too much air getting into the engine, such that the overrun fuel cutoff starts to kick in (which happens anytime you have a closed throttle and the engine speed is above a certain speed. The fuelling kicks back in at about 800rpm (to prevent stalling).

cheers

[Charlesm3]

Thank you!
-Charles

[Charlesm3]

@Brucey...can you explain how to adjust the fixed air bypass system as mentioned in your reply to me?
Thnx
Charles

[Brucey]

set the throttle body to the factory specs, then set the TPS, then set the fixed air bypass. You should (in the absence of other air leaks) find that the wrong setting causes wrong idle speed and/or 'hunting'. You need to check the setting with both a cold/loaded engine (which needs more air) and a hot/unloaded engine (which needs less). The idle control valve should cope with both these situations, but only if the fixed air bypass is set correctly.

NB fixed air bypass valve is absent on later motronic systems; a wider range ICV is used instead.

There is probably a more long winded (and accurate) spiel in the workshop manual.

cheers