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V8 saloon Idle speed

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Posts: 3
Joined: Sat Jul 23, 2016 9:46 am

V8 saloon Idle speed

Post by Aberdeendart »

Hi, looking for some advise with my V8 saloon, was out in a reasonably warm day and sat in traffic for 20 minutes or so, after that the Daimler was spluttery and lumpy but eventually cleared. I put this down to it running rich and had a little play with the carbs...after trying to lift the pots slightly I found one caused the engine to speed a little then start to die (as it should be I believe) and the other when lifted caused the engine to speed up, I understand this meant it was running rich and proceeded to wind the adjustment back a little which seemed to help but I still had issues with idle so....

I opened up the air filter and found it was pretty dirty, I then opened up both carbs in turn, cleaned and reassembled then ran the car until hot after which I checked the pots etc and seemed ok and after a run seemed good.

What I did notice and would like feedback on is the idle speed, I managed to get it around 800 rpm but on occasion when blipping the throttle it seems to reset to a different rpm, generally lower and on another note when I select drive (auto box) it drops further, mostly to the extent that it’s ready to stall (ignition warning light flickers)....any thoughts?


Posts: 656
Joined: Wed Sep 05, 2018 9:55 am
Location: Ridgewood - East Sussex

Re: V8 saloon Idle speed

Post by classiclife »

Hello Steve,

From your comment on the air filter, it gives the impression that the induction side of your car has been neglected to some extent or a greater extent.

I suspect the carbs are completely out of tune and may well be suffering from gumming up which in turn will cause the carb piston to struggle achieving a smooth operation which is a must for acceleration and correct idle. Additionally, you need to ensure that the carb dashpots are filled with the correct oil as this also has a bearing on how the piston moves via vacuum resistance.

I would suggest stripping the carbs right down to component parts and inspect for wear - especially any shoulder wear on the needles, you also need to check you have the correct type needles fitted.

When you lift the carb piston fully does it slide down smoothly with a metallic clunk on hitting the bottom ?? If not, this could also suggest that the needle is not centred correctly.

The above is assuming the ignition and rocker clearances are set correctly as you cannot have one trying to compensate the other.

Ensure the fuel filter is clean as all these little bits and bobs add to efficient or non-efficient running as the case may be.

Often scoffed at by others, but I still pop a big glug of Redex in the fuel every month as this prevents gumming up and improves the overall operation of the fuel delivery.

As for idle you really should be down to 600rpm and when engaging a gear the revs may drop slightly but certainly not to the point that it almost stalls, this certainly indicates a fuel delivery issue and probably a weak mixture - despite your assessment of perhaps running too rich.

Hope that helps a "starter for ten" ??

Good luck.

1968 Daimler V8-250 Saloon
DLOC East Sussex Area Representative.

Southern Classics Society Events Co-ordinator.

Posts: 508
Joined: Wed Mar 23, 2016 7:01 pm

Re: V8 saloon Idle speed

Post by tjt77 »

800 rpm @ idle is about right.. if it changes when you blip the throttle , the most likely issue is worn throttle shafts ... this will cause inconsistencies as to where the butterfly(s) close.. and always gives an unpredictable idle... easy too check.. grab the end of one of the throttle shafts firmly where it protrudes from side of carb, and see if you can detect any sideways play.. there should be none whatsoever.. replacing throttle shafts on the SU HD carburetor is best contracted to a specialist .. reason being they have drive pins holding the throttle stops, the position if which is critical and requires a jig to get them 'right' for drilling the new shafts ..plus the carb bodies will need to be re-bushed as well ..

Posts: 242
Joined: Sun Mar 06, 2016 7:05 pm
Location: Cambridgeshire

Re: V8 saloon Idle speed

Post by KV8 »

Firstly as was demonstrated to me nearly 5 years ago the use of a "Carbalancer" or similar device is vital and easy to use in setting the equalisation between the 2 carbs.
My (Carpenter rebuilt) engine will idle happily at 600 rpm but on blipping the throttle or driving will not always return to this level until a foot lift under the accelerator pedal. Whilst every joint in the somewhat convoluted throttle activation mechanism seems free and well lubricated my conclusion is that the throttle return spring is not quite man enough for the job. One with just a bit more return oomph is whats needed. I did find one spring that did the job however it was so stiff it caused a pain in the right foot!

Any suggestions for a suitable spring gratefully received.


armstrong whitworth
Posts: 15
Joined: Wed Mar 16, 2016 10:59 pm

Re: V8 saloon Idle speed

Post by armstrong whitworth »

I had exactly the same (wandering idle) problem as KV8; having set the idle speed several times, it would settle at a notably higher value in use. I too then noticed that the throttle was sticking slightly open, no matter that I'd cleaned and lubricated the linkages. I eventually found a heavy duty return spring from Classic Car Accessories of Castleton, Greater Manchester and the problem disappeared instantly (https://secure.classic-car-accessories. ... l#aCXE0252). When at the recent Bicester Super Scramble I saw a metallic green V8 250 in the JEC area and noted that it had a different but notably large return spring fitted, so perhaps this is a common issue.

Ian Slade
Posts: 531
Joined: Sat Feb 13, 2016 9:54 am
Location: Scotland

Re: V8 saloon Idle speed

Post by Ian Slade »

Idle speed on the 2.5 V8 engine should be 500rpm, if you think about it that equates to 1000rpm on a 4 cylinder engine as the number of firing strokes/ min, at 500rpm there is no centrifugal advance ( see the topic on advance weight springs).
Owner since the 70's, Genghis is slightly to my left.

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