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Ignition Timing Settings for Unleaded - 2.25 Petrol

bluesuit

bluesuit

Overdrive!
Hi All,

My '72 S3 is prone to over-run on shut down, it was suggested I should check and adjust the ignition timing. Looking at my trusty Haynes Manual it says 8.0:1 85 Octane 3 degress after TDC, and for 90 Octane: TDC.

I think my old book is pre-unleaded fuel which is 95-98 Octane. My wagon is standard - not modded for running unleaded, so I run with Redex. Can anyone tell me what ignition timing I should use?

Thanks in anticipation

Ian
 
Set static to 6 BTDC...and fine tune around that - with a brand new ignition system mine was running best at about 4to5 BTDC on un-leaded, with lots of power and no over-run.

I've yet to see a convincing reason why Haynes state ign timing settings as after tdc...why would you fire the mixture when the piston is already on its way down the barrel ??? Strangeness
 
If you mean running on then that has nothing to do with timing or fuel octane. Your petrol is unlikely to be low octane in the UK but you need to confirm what it is before altering the timing. If it runs ok otherwise then it's probably fine and set a few degrees before TDC, if you get pinking then it's too far advanced or the distributor needs servicing. Running on is caused by glowing material or parts in the cylinder head and is usually cured with a carb fitted with a fuel cut off solenoid or stalling. Standard Redex may contribute. Low octane fuel as is only avaliable in some countries and what used to be called pool petrol or military supplies years ago requires a timing to be after TDC to avoid detonation (pinking).
 
It still seems a bit wrong to ignite the mix when the piston is on the downstroke and maximum compression is passed...I'd have thought that a lower octane fuel - i.e slower to burn - would need to be set alight slightly sooner, perhaps with a more meaty spark (24v ignition system?...and or shorter duration spark ??? ) ... Surely that way would compensate for the lower power caused through fewer octanes in the fuel ??? ...to burn already poor fuel when max compression has passed seems like a great way to make it even weaker ! ?? :)
 
Before you even think about the word distributor , like any engine chain or belt drive cam - you need to check the mechanical timing. The chain may have stretch (archilles heel) as well as sprockets not worth putting a new chain on. Check the tensioner mechanism and if early engine the rubber damper pad. Get this lot correct before "advancing".

You will not get good engine starting/stopping or running if your mech timing is not good - get assurance the very basics are correct !!!

A word of warning - only use a genuine source crankshaft sproclet,

;)
 
Last edited by a moderator:
freqmikey said:
It still seems a bit wrong to ignite the mix when the piston is on the downstroke and maximum compression is passed...I'd have thought that a lower octane fuel - i.e slower to burn - would need to be set alight slightly sooner, perhaps with a more meaty spark (24v ignition system?...and or shorter duration spark ??? ) ... Surely that way would compensate for the lower power caused through fewer octanes in the fuel ??? ...to burn already poor fuel when max compression has passed seems like a great way to make it even weaker ! ?? :)
Click to expand...

Maximum combustion pressure is just after TDC and higher octane fuel is more resistant to detonation which is uncontrolled combustion so the timing can be more advanced to take more advantage of this than low octane fuel can. A bigger spark makes no difference as combustion produces a progressive flame front, if it did you'd be getting detonation.
 
Thanks for that guys.

My engine's only done 54,000 and is still nice and sound, so I'm happy with mechanicals, the PO seems to have replced the carburettor too.

Freqmickey - your suggestion of about 3 degrees before TDC would chime with the Haynes figures, which seem advance the timing by 3 degrees for each 5 octane increase over 85 octane. Oddly though they list SII 90 Octane at 6 degrees BTDC - a bit random! However, as cheap unleaded is 95 octane and super unleaded is 98, your 4-5 BTDC sounds like a reasonable setting, I'll see if I can find my other manuals to see what they say.

I have yet to see what mine is currently set at, it seems to run fine if a little rich, I leaned it off but then it didn't run quite so smooth, and has a tendancy to chugger on when the ignition is switched off, that might also suggest it is set at either TDC or ATDC.

Will report back after further diagnosis, meanwhile I must finish putting in the floor in my mates' horsebox! ho hum....

Thanks guys
 
If your engine is original 1972 set-up - then your idle speed should be set at 500 rpm and no more , . Later engines were set up with a higher idle speed (600/700rpm but could be as high as 800 rpm) and this is why carburettors were fitted with a anti-diesel solenoid valve - because some did run on (they were an option). Much later engines are best having the dizzy set up on a dwell basis as per the later green bible and Rover Service Guides. Generally all Rover published timings (copied by Haynes) were static.
Any carbon build up and you will have to retard. Slightest wear on the drive train and you can't maintain any settings ;)
 
For modern fuel in the uk you can ignore what the manual says. I set mine to 6 btdc static and then got the timing light and set it to 8 degrees btdc at idle (500rpm). She runs perfect but is a new Turner engine. I think it would run even more advance but i'm not going to push it. Other engines might not though.
I think a timing light is essential but others might disagree.
 
matt d said:
For modern fuel in the uk you can ignore what the manual says. I set mine to 6 btdc static and then got the timing light and set it to 8 degrees btdc at idle (500rpm). She runs perfect but is a new Turner engine. I think it would run even more advance but i'm not going to push it. Other engines might not though.
I think a timing light is essential but others might disagree.
Click to expand...

I agree with the timing light thing, best way to set it IMO
 
The big push for maximum advance is for power and economy BUT the last thing you need is getting it wrong - err for a bit of retard , lower winter engine temp. means you need less advance and you need to consider for when you are under full load / max. throttle, static timing is just to get you somewhere near and up and running. Petrol now is a guessing game that a good engine management can cope with, but these engines can't. At least with dynamic - you have taken up the chain & dizzy backlash - but not if it is considerable. I have used a cheap Gunsons 240 volt power strobe and dwell meter for almost 40 years and it is adequate (other than in very bright sunlight). This Gunsen "Tachostrobe" is not so good as a rev. counter - so a seperate rev. counter - that is a good investment . I don't think I have found a pulley TDC notch well out but I always check. At the end of the day - the plug colour / conditiuon reveals all ;)
 
RUXY said:
Petrol now is a guessing game that a good engine management can cope with, but these engines can't.
Click to expand...

Is'nt it a shame we cant play the same guessing game in return when it comes to paying for it ?
And thanks along the way for my better understanding of why the figures are quoted as after tdc... I'll be the first to admit any knowledge I have is based on old-skool stuff...which is why I also keep an eye on plug colour... :)
 
Thanks guys, I hope to have time next week-end to have a closer look at this, but all your inputs have given me a good deal to think about in the meantime. Must dust off my strobe light. I did try leaning off the mixture and dropping the idle speed, but couldn't find a decent compromise that ran smoothly when warming up from cold and also when fully warm, that and the run-on prompted me to think about ignition timing.

Will report back my findings next week-end

cheers, Ian
 
Lovely day today!:cool:

When eventually released from chores and let out to play, I decided to investigate the ignition timing set up on the SIII based on all your suggestions. The morning chores had helped warm the engine fully - the mild weather also gave it a chance to register Normal on the temperature gauge for a change. I started by checking the dwell first and that tied up nicely with the book. I then reset the idle to 800 rpm, it was running a bit fast to overcome the slightly rough running. I then located and cleaned the timing mark on the crackshaft pulley and dusted off my strobe light, which happily still works. Initially the timing was running around TDC. I then advanced it to about 4degrees BTDC as suggested and using the vernier adjuster played around either side until I was happy it was running smoothly. I then checked the mixture to make sure that was leaned off until it ran smoothly and adjusted the idle until it was happy around 800 rpm.

As a result of that it now seems to run and respond smoothly to the throttle, and happily when I shut it off it no longer runs on.:)

I will continue to monitor it's behaviour in different temperature situations but I think I've cracked it!! Thanks for the advice everyone. :)

Cheers

Ian
 
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