Idle speed stability
Discussion
Thought I'd share my findings on something that has bugged me for years...the idle speed is sooo inconsistent. Sometimes I stop at a junction and get 900 rpm, then after just a few mins driving and another stop I can get 1200 rpm sometimes even 1300 rpm.
Finally did something about it. Using a DTI on the throttle butterflies, the almost imperceivable last 50um (2 thou) of movement to "closed" is very random. And 50um is *a lot* where throttle closing is concerned. Even worse this doesnt really register on the pot readings...its just letting in more air, hence the adaptives creep to +ve, then the next time you happen to get fully closed, they'll creep back to -ve and so on...never really correct.
So I changed the throttle cable return spring so that I get about the same weight as the std one but I can set a bit more pre-load (i,e, a longer spring more compressed in the first place). After a bit of messing it returns to within 10um every time and still gets 94+ % fully open throttle i.e. the spring doesnt limit the travel.
Might be useful to someone
(NB: I have modified bodies with nice bushings etc etc and very low friction overall movement...but the final closed fractional movement is always a challenge when the return spring is almost out of ooomph)
Finally did something about it. Using a DTI on the throttle butterflies, the almost imperceivable last 50um (2 thou) of movement to "closed" is very random. And 50um is *a lot* where throttle closing is concerned. Even worse this doesnt really register on the pot readings...its just letting in more air, hence the adaptives creep to +ve, then the next time you happen to get fully closed, they'll creep back to -ve and so on...never really correct.
So I changed the throttle cable return spring so that I get about the same weight as the std one but I can set a bit more pre-load (i,e, a longer spring more compressed in the first place). After a bit of messing it returns to within 10um every time and still gets 94+ % fully open throttle i.e. the spring doesnt limit the travel.
Might be useful to someone
(NB: I have modified bodies with nice bushings etc etc and very low friction overall movement...but the final closed fractional movement is always a challenge when the return spring is almost out of ooomph)
Very interesting! The return spring on the cable returns... the cable. The return spring on the #3 butterfly returns all six throttles (and isn't shy about it!). You should be able to adjust the relationship between the cable return spring and the amount of tension that remains on the throttle by the cable locknuts where it passes through the bracket, and the clevis on the end of the cable, which is threaded and has a lock nut.
Or does that rob travel?
Or does that rob travel?
It robs travel...that's what I was fighting against. So I have ended up with an amount of pre-load by adjusting at the clevis pin/actuator fork i.e. set it just a bit higher than the hole so you have to press it down a couple of mm to put the pin in. The extra spring length allows more pre-load at that point because you have to pre-compress it to get it onto the cable behind the fork-part. Having driven it properly now for several days, the difference is remarkable...for such a microscopic movement.
PetrolHeadPete said:
It robs travel...that's what I was fighting against. So I have ended up with an amount of pre-load by adjusting at the clevis pin/actuator fork i.e. set it just a bit higher than the hole so you have to press it down a couple of mm to put the pin in. The extra spring length allows more pre-load at that point because you have to pre-compress it to get it onto the cable behind the fork-part. Having driven it properly now for several days, the difference is remarkable...for such a microscopic movement.
Pete - good shout! Where did you source the 'big' return spring?18 months ago, I had the devil's own job in balancing the air volume through the throttle bodies. The engine was running like a bag of spanners, and I could see on the diagnostics the AFR readings were massively high. After much looking/trying/frustration, I finally deduced that the air through the bodies was way too high. This was as a result of me disassembling the clevis/return spring assembly to clean grease and dirt accumulation. I then realised that the return spring was not exerting enough "closing pressure" on the linkage. I could get the syncrometer reading down if I physically pushed the linkages (hence closing the butterflies) 'up'.
Adjustment of the fork of the clevis' height to need a bit of pressure to get the pin through the linkage was all that was needed. With all the butterflies now going back to the "stop" position, it was simple to balance the TB's.
Nick
Hi Nick
That sounds familiar !
I ended up having to use 2x springs (I have a lathe so can easily make the seat/caps and a joiner. Each one is 1.5mm wire, 51mm long, 13mm OD and I think they were 12 coils each give or take with flat ground ends...these ones in fact:
https://www.ebay.co.uk/itm/322550086026
I'm sure with more searching I could have found a 1 piece 100mm 24 coil but I gave up and just put 2 in series.
That sounds familiar !
I ended up having to use 2x springs (I have a lathe so can easily make the seat/caps and a joiner. Each one is 1.5mm wire, 51mm long, 13mm OD and I think they were 12 coils each give or take with flat ground ends...these ones in fact:
https://www.ebay.co.uk/itm/322550086026
I'm sure with more searching I could have found a 1 piece 100mm 24 coil but I gave up and just put 2 in series.
^^Cheers!
I'm guessing, but I reckon my throttle cable clevis/spring/bracket assembly are still original, so after 14+ years the spring is probably getting a bit tired.
I've nearly run out of adjustment on the cable outer at the bracket...so maybe a slightly longer spring will get that back for me. I had to renew the cable last year....think you can get 2 types/lengths?
Nick
I'm guessing, but I reckon my throttle cable clevis/spring/bracket assembly are still original, so after 14+ years the spring is probably getting a bit tired.
I've nearly run out of adjustment on the cable outer at the bracket...so maybe a slightly longer spring will get that back for me. I had to renew the cable last year....think you can get 2 types/lengths?
Nick
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