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MG MGB Technical - vac gauge ignition timing

Well i have read a lot about how you should ignore manufacturers settings which was based on a totally different 1970s leaded fuel and use a vacuum gauge to set timing and not a strobe. I connected my Gunsons vac gauge up after slackening the distributor clamp and as soon as started engine the gauge was showing retarded timing (even though it was set at the prescribed 10deg). I advanced the timing bit by bit. Vac went up and so did tickover speed until it levelled out at 20" so retarded it 1" on the scale which measures in inches. Disconnected and went for a run and a massive difference in acceleration and sounds smoother. On return out of interest i connected my timing strobe and found it was now 20 deg advanced. Seemed a lot to me but will run it and monitor for any pinking.
steve livesley

Have you checked your maximum advance?
Dave O'Neill 2

and then there was a noise, then
silence
William Revit

I used to set my timing by vacuum gauge from the 70s onwards when I had our era of cars from new. Always made a big difference, and always came back running like a dog after the dealer service as of course they always put it back to book.

The instructions for my Redex (attached) say to adjust the gauge for increased sensitivity, advance until there are very slight downward ticks, retard until they stop, then retard a further 3/4 of a degree. As the instructions say this takes account of engine condition and fuel grade used. Haynes says much the same thing except they go by pinking when accelerating at full throttle in 4th from 30 to 50.

The reason this works is because mechanically our engines are pretty crude and not built to tight tolerances, and the book figures take account of the worst case plus a safety factor. That means that some engines can run perfectly safely with more advance, as did mine in the 70s. However even on leaded I found that my roadster bought in 1989 and set up with the vacuum gauge ended up almost exactly the same as book - i.e. the tolerances were working against me, so I didn't bother using it after that, just going by pinking. Unleaded has been even worse, even higher grade 97/98/99.

The limitation of the vacuum gauge is that it sets timing at idle or just above, so depending on what your centrifugal curve is like today you could end up getting too much advance at higher revs i.e. if the springs have stretched or got weak and that could be damaging, as Dave O'Neil implies. Personally I go by pinking under any combination of throttle, revs and load and back off until that just stops, as I have found pinking can occur at part throttle but not full throttle.


paulh4

I assume using a vacuum gauge might be a problem on a V8? Where would you get a vacuum take off which represented both inlet tracts? Presumably one take off on each leg of "the pants" and a "T"?
Allan Reeling

I've never investigated it fully but my carb adapter seems to have a very small passage between the two ports (attached), and a threaded hole with a blanking plug above (next post), as if that might have been for just such a vacuum port.

paulh4

I've not seen that blanking plug on other examples, but where the through-passage is could be original.

Failing that you could always drill and tap the two square bosses, but it's questionable whether it is worth it.

paulh4

Interesting Paul. I've no intention of going down that road and neither of mine have that blanking plug, but the through holes are there...........I think!!
Allan Reeling

Pinking on part throttle which disappears on full throttle is usually down to too much or too soon vacuum advance.
Commonly, I found it when people fitted a 3-10-10 Vac unit from a Mini instead of the original 5-13-10 unit for one 18V780 etc engines.
Easy to diagnose: disconnect the vac line and see if pinking remains.
I have also found that higher cr engines can give this effect and seem to dislike too much vac advance, the std unit giving too much. This was not quite what I would have expected, so I'd be interested in what Peter and Chris have found on higher cr engines (su h as 10:1 on std cam)
Paul Walbran

Or just too much advance full-stop. Fitting a capsule with more advance noticeably improved part-throttle acceleration, I just backed off the timing a little in compensation but kept the improvement. I also have a vacuum 'switch' on mine (modified TCSA unit) so I can turn it off altogether if need be.

Tuning distributors with vacuum advance involves a three-dimensional map, which is one of the reasons why so-called competition distributors don't have it. The trouble is that the unwary think that by buying one they are going to get more performance over a comparable standard distributor.
paulh4

In my world vac advance only comes into play on part throttle - as soon as you drop manifold vacuum below the vac capsule's start point it may as well not be there.

So when the engine is under load you are back to the mechanical advance curve - I always found the 25D4 41234/41391 with its long advance curve gave the best results with 1868cc fast road engines - set at 8 static, 13 strobe at 1000rpm, maxed out at 38 at 6000 rpm.

Chris at Octarine Services

Paul W
Did some work to a mate's Targa Midget here a while back 1275 around 10.5:1 and a bit of a cam it was on 1 1/4" SUs and went quite well without any pinging issues at all--Thought we'd try some 1 1/2" SUs on it and found, although it went significantly better it had a pinging issue at 3000-3500 rpm on just above cruise throttle (light acc.) Decided that it was being caused by the fact that the larger carbs didn't have the throttle open as far as the smaller ones did at this rev/load and that the vac advance was still sucking hard whereas with the smaller carbs vac advance would have been decaying at this load
Removed the vac. line and all was well-just left it like that- As you know in Targa type events there are usually plenty of high speed part throttle sections so vac. advance isn't really a good idea anyway
Paul H
So you fitted a larger vac pot, it pinged, then pulled the base timing back to compensate then fitted a switch to turn it off if needed anyway,it all sounds a bit messy to me
willy
William Revit

Willy - it all depends on what roads you are travelling. Round here it is relatively flat, but we frequently are touring in hillier country. No pinking at home, it can pink on hills when away. Far messier to keep altering the timing although I do now have a 25D4 fitted. I originally got the valve for an experiment switching between carb vacuum and manifold vacuum to see how they varied on a vacuum gauge in driving conditions and just left it installed.

As for the experiment, apart from at idle where carb vacuum is zero and manifold is at or near max, as soon as you start opening the throttle carb vacuum rapidly rises to meet manifold vacuum, which is just starting to fall gradually. After that, which is at any normal driving throttle opening, the two are exactly the same.
paulh4

Paul- All good, Just out of interest, on the RV8s the vac for the dist. is connected to the throttle body and works normally like a carburettor connected unit with no vac at idle---but --the Japanese import version of the same car had the vac direct onto the manifold plenum with full vac at idle
Driving both versions-
The UK car drives as you would expect with a bit of vac advance coming in above idle to help that urge forward feeling
The Jap. car, having full advance already is ok and on the road drives exactly the same
except that if you really concentrate when first moving off you can just notice that there's no increasing advance to give you that extra urge feeling, it just drives slightly different
A friend here has a few of them and he likes the feel of the UK car better but thinks the Jap. car could be a tiddle more economical around town
Cheers
willy
William Revit

We tend to get rid of vac advance with higher CR engines, esp over 10:1. The higher CR seems to give much better part throttle fuel efficiency. The same can be said of increasing cylinder filling efficiency of poor A and B series cylinder heads. Combine better flow and higher CR and the vac seems redundant to me. We have never utilised vac advance on 123 distributors etc. I reckon the road would be the only place to try and set it up. I have never run vac advance on my own tuned cars. Mind you, do folk remember the horrible vac retard on some Range Rover V8s?
Peter Burgess Tuning

Belated thanks for comments above. Still no sounds of pinking - checked plugs and look ok. 20deg on the strobe does seem a lot so knocked it back a bit to 18 - got a decent run coming up this coming weekend to Notts classic car show at Thoresby park so will see how she runs. A higher advance i think is better for higher cr at 9:1 - a PO bored engine to 1844cc and skimmed head 55 thou and added k&n filters plus aaa needles. It has a 41610 dist which does not fully advance till about 5500 rpm which is way above driving style - these days!
steve livesley

Once you've got the whole car fully serviced and maintained a modest investment in a visit to Peter Burgess's rolling road tuning will no doubt pay great dividends.

He knows a thing or two about MGB engines and tuning, even wrote a book on it.

http://www.peter-burgess.com/indexstart.html
Nigel Atkins

In theory the higher the CR the more likely you are to get pinking, which is why with American LC engines you can advance the timing without pinking until it stalls the starter. Even with higher CR the combustion chambers can be modified to take out hot-spots which improves things, and standard heads were modified in that way over the years, possibly 1970 and 1972. However my 73 with the correct Gold Seal engine has the pre-72 shape and is very prone.

From the distributor the engine would be an 18V-846/847 i.e. all UK rubber bumper production, and as you say has a very lazy curve compared to earlier types. Oddly someone elsewhere has just had a 41610 refurbed by Distributor Doctor and the paperwork that came back with it shows it as all-in 25 degrees at 3500 i.e. significantly more aggressive. When asked he said it was designed for 95 octane, which is even more strange as lower octane pinks sooner. The purchases said this change wasn't discussed or agreed beforehand.

If your Mk1 ear is up to spec you should be fine if there is no pinking.
paulh4

This thread was discussed between 10/04/2019 and 30/04/2019

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