When you close the throttle WITHOUT the speed switch cutting off the fuel, it's going so rich that it's overloading your sensor. I bet you have fire coming out of the tail pipe. The only other answer is that you've got the idle adjustment way too lean and the main rack way too rich and it's transitioning between the two.

The lean AFR numbers on decel are to be expected when you remove the decel enriching system. That microswitch controls the large solenoid on the pump, which on throttle closed decel enriches so you don't get backfiring.

Hmmm.

What's throwing me is that when on load, at higher revs, AFR is good. Its only at lower revs and no throttle that it goes way lean.

I thought the speed switch & no throttle caused it to cut fuel, to stop the flames out the tailpipe, not enrichen it ?

Could be wrong, thats what I was told. Then again, I do get confused easily.

declang, You are correct. The speed switch/RPM transducer/solenoid are set up to cut off the fuel on decel.

Perfectly normal and nothing at all to worry about. Decell AFR's don't mean a thing. You actually want it to read very lean.

Absolutely correct. It's called a "shutoff solenoid", not an "enrichment solenoid".

As you are discovering, however, you really don't even need that pesky little devil, the speed switch, or the switch on the rack. The mix goes lean enough without all of that claptrap to accomplish what it is supposed to anyway, which is to minimize wasted fuel on decell and/or washing of the cylinder walls.

When you lift and completely close the throttles, the pump returns to its idle delivery position. It has no way of "telling" how much air is going into the motor. At higher than idle revs, despite the closed butterflies, the motor is simply drawing in more air. Hence the lean mix. And again, that is exactly what you want on decell. Don't touch a thing.

Thanks Jeff. That sounds much more promising.

I'll have to try a few more miles & see how it goes.

Cheers :)

Uhh, you do not want a 20:1 mixture under any circumstances. Given the design of the engine, that mixture won't burn anyway- I stand behind my suspicion that if the engine is actually running and you're showing 20:1 it's false informaiton.

Bosch designed the MFI system and wouldn't have included the speed switch and overrun shutoff if they didn't think it was important to keeping the plugs clear and the fuel consumption low. When you snap off the throttle, the rack doesn't instantaneously return to idle, it takes time. Slamming the rack forward with the solenoid was their workaround to an excessively rich mixture given the behavior of the pump.

I'm not disagreeing with you Jeff, I don't have a speed switch on my race car, I find that it's helpful to keep guys off my bumper at the end of the straights when fire shoots out of my M&K exhaust. But it is counterintuitive to believe that at closed throttle the mixture goes lean-- usually it goes lean at WOT in the upper rev range.

Declan: try this-- with the engine warmed up, turn the idle adjuster two clicks left and note the reading on the AFR. Then turn it two more clicks left and note that. I bet you that rather than seeing a mixture that goes from 20:1 to 25:1, you see a mixture that goes from 20:1 to 10:1 even though you are LEANING the idle mixture. . . If it doesn't it's easy to click everything back. . . and keep going with the idle mixture, clicking to the right, until you get to 14:1.

With no load on the motor (full decell) it could be sucking almost pure air and it would not hurt anything. The motor isn't really "running" per se under these conditions, it's being driven, and would never continue to run on its own on this lean of a mix.

The systems I have checked with my LM-1 that have the shutoff circuit intact do, in fact, go leaner than the ones without one, just like we would expect. Well over 20:1. The shutoff solenoid seems to force the rack even further forward than the normal idle position.

The flyweights remain splayed to various degrees with rpm, so the stylus arm does not return to full idle until revs drop, because the space cam is aft of the idle position when the flyweights are splayed. The shutoff solenoid lifts the stylus right up off of the space cam when it forces the rack forward past the idle position.

So, in the absence of the shutoff solenoid to do that, the stylus continues to track on the space cam. The decell, therefore, remains a bit richer than it would with the shutoff solenoid in place. Not much, though. Mine hits 17-18:1 or more when I lift. The shutoff solenoid pushes it up over 20:1 when it's working. That won't hurt a thing on the decell, under no load. There is no way it can detonate or anything like that.

Jeff is 100% correct and I would like to add that with the 019 RS pump the space cam at zero degree's, throttle closed, delivers less fuel as the RPM increases. 019 RS pump flow spec: 400 pump rpm= 13-14ml; 600 pump rpm= 10.5-11.5ml; 3000 pump rpm= 9-10.5ml. Max throttle and 3000 pump rpm delivers 57-58.5ml.

Thanks for the input folks :) Makes sense to me.