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MG MGB Technical - Correct Dynamic Timing
|A bit of help needed. I've got a 1972 MGB Gt with high compression engine number 18v.... the timing needs looking at as it hasn't been done since the electronic ignition module was installed in the distributor, there is a slight 'pink' on the engine under load and occasional run-on after the engine is switched off, when hot. The engineers notes, that came with the car, says he set the timing at 12 dg BTDC, to run on unleaded as the valve seats had been replaced. The workshop manual says dynamic timing 13 dg BTDC at 600 rpm. |
I've had a quick look at the marks with a timing strobe, at 600 with the vacuum pipe disconnected, the timing looks around the 15 dg BTDC.
Should I just adjust it a degree or two at a time and see how the engine performs, what is the best figure to head for?
|Elliot its been found that 32 deg total advance (with vac disconnected) is usually very close to right with an MGB|
So run it up till it stops advancing and take a reading. You will need a dial back timing light or measure a mark on the pulley. You can ten check what the reading is at idle for reference. Denis
|By taking the car out for a drive and backing off the timing a bit when "pinking" is encountered, you should be able to reach a happy medium between performance and economy. Ford actually recommended this procedure back in the '60s as no two engines were identical. RAY|
|don't be too slavish to figures go by what sounds and goes right for your car|
to set fully unless they've been done fairly recently you'll want to start by checking/setting the tappets
set-up/check - tappets, (CB points), plugs, timing and carb mixture in that order, if you have to adjust any item in that chain then you’ll also need to check, and adjust if required, all the items that follow it in the chain
Could you expand on your reply. 32 dgs advance seems a lot !! At what RPM are you saying this amount of advance should occur. I'm assuming 20 of the 32 dgs is due to the centrifugal advance mechanism, any idea of what RPM those weights start to 'slide out' and start further advance of the timing.
The 32 degrees refers to maximum advance. This is different from the idle figure you refer to because the amount of advance increases as engine speed increases, up to a certain point.
For example, in B's made before approx 1973, the original advance spec as engine speed increased started at 13 deg at 600 RPM (as you noted above), then moves on to 19-20 deg at 1000 RPM, 22-26 deg at 2000 RPM and on to a maximum of 30 deg at 2300 RPM. After that there is no further increase in advance.
Better fuels can take a bit more advance, hence 32 deg.
However, the usual problem which causes pinking is that the advance mechanism wears with age and may no longer conform to the original spec and the additional advance happens sooner than intended as a result.
I have explained it more fully and how to address it at http://www.mgparts.co.nz/advice/technical-notes/distributor-advance-rate-adjustments-compensating-for-high-mileage-wear/
|Elliot around 3000 revs your total advance should be close to all in. Total advance does not include vacuum advance as that only comes in under light load (cruise) situations. The mechanical advance should start above a fast idle. All motors vary but but 32* will get you close and safe, and you could play around a little from there. Some will say otherwise but with advance more is not always better. Denis|
|"with advance more is not always better." Definitely. too much advance and the buring fuel is fighting the piston on the way up. A wate of its energy and the piston might come off second best.|
|>>the advance mechanism wears with age and may no longer conform to the original spec and the additional advance happens sooner than intended as a result.<<|
although you've got an electronic top with the bottom end of the dissy you can't be too exact, anything that's 3 years or so away from original or rebuilt will probably be worn and out let alone ones that have decades of use and wear
I'm glad Paul also mentioned fuel, you want to set up with what ever fuel you normally use in the tank
|You don't give your engine prefix, which would give the timing figures, but a UK 1972 would normally have had an 18V 581 or 582 with a 25D4 41288 distributor. You can see the vacuum and centrifugal advance specs for that here. |
The only way you can do it in this day and age - for a high compression engine - is to advance just short of pinking at any combination of throttle opening, load and revs. In fact my Haynes, dated 1989, on page 104 step 17, says "Small readjustments under running conditions can be beneficial. Start the engine, allow to warm up to normal temperature, accelerate in top gear from 30 to 50 mph and listen for heavy pinking. If this occurs, retard the ignition slightly until just the faintest trace of pinking can be heard under these conditions."
Today's fuels are very different to when the figures were first published, and if you use 95 octane instead of 97/98/99 you will almost certainly get pinking at the book figures. As said every engine was different, and the original figures took into account a worst-case scenario, plus a safety factor, to avoid warranty claims. That meant that many engines could run quite safely at *more* than the book value, and give you the benefits of better performance and economy, as I discovered in the 70s. At the other end of the scale some engines struggle at the book values even on 99, like my present engine.
You should have a 25D4 distributor and the beauty of that is the vernier adjustment makes small adjustments to timing very quick and easy. This may be necessary touring the wilds of the UK and elsewhere if you can only get 95 octane and usually use higher grades, or if you set it up for Norfolk then go touring in hilly parts.
Exactly what your distributor is doing is also factor, as the springs age they weaken and the advance comes in faster, which is also likely to bring on pinking.
|Thanks for the detailed replies, especially to Paul in NZ and Denis in Australia. I tried the timing light this morning and achieved figures very close to what Paul suggested i.e. 14 at 700, 22 at 1000, and 30 at 2000. All a bit high, so possibly weak springs is causing the weights to have more of an advance than they should have at higher revs. Not surprising for something over 40 years old... It still works, so I'll keep it as a working spare. I've ordered a new Powerspark electronic dizzy, so will be interested to see how the above figures compare. |
Paul, the engine number is 18V 583F-H433, where on this site is this timing info kept?
Thanks to all for their replies.
|Elliot, the extra advance in the figures you have is a significant amount. The 2000RPM figure is 4 deg over advanced from the upper end of the book range (30 v 22-26) or 6 deg from the middle of it. This is certainly enough to cause your problem. If you tighten the hook of the heavier spring so it shortens it by about 1/2 mm then it will correct this.|
Paul, Good point about fuels and octane. We only ever had 96 here for years so became well used to tuning to suit. Road testing, as you suggest, was the best way to get that intermediate advance figure right. Typically this meant setting the 2000 RPM advance back to the bottom end of the range. On engines with the compression raised to around 9.5-10:1 then 20 at 2000 was the figure to aim for.
However, on the rolling road 30-32 deg full advance still gave the best result even with the 96 octane, while at idle less than 14 deg at 700 was found to be unhelpful for run-on. Thus we had to reprofile the advance curve to make the most of the fuel we had.
When the premium fuel moved back a point to 95 a decade or so ago it became even more so. Then fortunately 98 became an option a couple of years later, luxurious!
Though it's surprising how many owners here run on 91 ... they are used to putting it in their Jappa other car and don't think past it.
|Elliot - sorry, forgot to post the link: http://www.mgb-stuff.org.uk/curvestext.htm#41288|
At the end of the day, unless you want to take a lot of trouble recurving your (either) distributor to suit your engine and fuels, just tweak it for no pinking.
This thread was discussed between 22/04/2014 and 24/04/2014
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