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MG MGB Technical - Correct Timing for 18V 847 HC engine

Hi

I am confused!!!

My Haynes""
manual states 10 Deg BTDC at 1000rpm which I assumed was with the vacuum diconnected!

I now read in my MGB Workshop Manual (CD from EBay) that the CENTRIFUGAL timing with vacuum pipe disconnected =

1 - 5 Degs at 16oo rpm

10 - 14 Deg at 3400 rpm

and

20 - 24 Deg BTDC at 5400

Do I need to adjust from 10 to 3 Deg BTDC at 1600rpm and then reconnect vacuum tube?

Also I must check to see if Plugs are set at 35" for 45D4 DIZZY.

Rees
RH MARSHALL

Yet another Q

is my engine an 18V847F if engine-plate is stamped

18V 847 - H029313?

Thanks

Rees
RH MARSHALL

Rees. You seem to be confusing the dynamic timing of the engine with the mechanical advance curve of the distributor.

The basic mechanical advance curve is what you are showing in your post.

The dynamic timing is a combination of the distributor's mechanical advance plus some "static" (additional) advance which gives you the actual overall advance--e.g. ten degrees of static advance plus 1 to 5 degrees of mechanical advance gives a reading of 11 to 16 degrees of total dynamic advance at 1,600 engine rpms.

As to the engine, Clausager, page 64, shows only the 18V-847-F-H and notes it is "As above (i.e. Home/Export, not North America), bit with overdrive (overdrive became standard on these cars in 1976). So, I would assume that your engine is of this type as no other engine with an 18V-847 prefix is shown. Were it a replacement engine, the prefix would be BHM 1111.

Les
Les Bengtson

Thanks, I sense you are right, but I need to explore this a little further.

The Distributor's Mechanical advance is determined by the Centrifugal weights and for my car will be between 1 - 5 degrees at 1600 rpm!

Static advance can be set with a lamp and is determined by turning the crank to say 10 BTDC No 1 cylinder and turning the DIZZY until the lamp illuminates. This is the most basic form of timing and excludes mechanical advance.

If I set 10 Deg by strobe at 1000 rpm I have no idea of how much or any centrifugal advance has occured.

The figure of 10 Deg BTDC by strobe assumes that the centrifugal advance is working correctly and that the static and centifugal combined = 10 Deg, 7 Static + 3 Centrifugal

I thought one ignors the Static if you have a timing light to set 10 Deg with vacuum pipe disconnected?

I missed the fact that my engine is designed to have 7 Deg BTDC Static? Hence without vacuum I should see 10 Deg at 1000 rpm and 11-15 Degs at 1600 RPM.

Thanks Les

Rees
RH MARSHALL

Timing *has* to be set with the vacuum disconnected if the source is the manifold. Usually carb vacuum doesn't need to be disconnected as the port is closed at idle, but if the dynamic rpms are high enough it could start to open and affect things. That's why even carb vacuum has to be disconnected when measuring the centrifugal advance curve at higher rpms.

The static figure is for setting the initial timing on an engine that has some work done and lost its timing, and allows the engine to start and run so that dynamic timing can be performed.

The specs for a dizzie should quote the *additional* centrifugal advance when running, that is over the static advance. This is because a distributor could be used on any engine type, with different static timing requirements, but the additional advance per 1000 rpm will be the same regardless. In the early distributors this works, for example the 25D4 40897 specifies a static of 10 degrees, a dynamic of 14 degrees at 600 rpm, and the advance curve specifies 4 degrees at 600 rpm. What that means is that the static of 10 degrees for *this* engine, plus the additional at 600 rpm of 4 degrees for *any* engine using this distributor, gives the dynamic setting of 14 degrees at 600 rpm for *this* engine. A number of distributors follow that rule, but then you get some where the curve figures seem to be taking the static figure into account anyway, and some where it seems to be half-way between the two.

"my car will be between 1 - 5 degrees at 1600 rpm!"
That's the *additional* advance, not the measured advance. You set the timing to 10 degrees at 1000 rpm, then as you open the throttle further you will see the effect of the advance curve adding to that figure. Subtracting your static from each of the measured figures gives you your actual curve for that distributor. In theory with a static of 7, and a dynamic of 10 at 1000, indicates there is 3 additional at 1000. However the book specifies it's 3 (mid-way between the 1 to 5 quoted) additional at 1600 rpm, so this is one of those where it doesn't quite stack up. Taking the dynamic figure the additional advance at 1600 rpm should really be 3 to 7 degrees or a nominal 5 degrees. But taking the curve figure the dynamic figure should really be 8 or 9 degrees.

It's all academic anyway. The original figures took individual component tolerances into account, plus a safety factor, and produced a figure that was 'safe' for an engine that was more likely to suffer from pre-ignition than any other. In practice most engines could be run with more advance that that and give better performance and economy. My 73 A-series did in 1973, but always came back from a service (to keep the warranty) running like a dog as they had set it to the book figure again. However my 73 B-series in 1990 couldn't be set higher than the book figure without pinking. When unleaded came in it pinked even easier, even on 97 octane, so I had to retard it from the book figure slightly. If I could only get 95 octane it was awful, timed to not pink it wouldn't pull the skin off a rice pudding and ran noticeably hotter on the gauge.

The best you can do is advance it just short of pinking at any combination of throttle opening, revs and load for your petrol in your geographical environment. If you live in Norfolk then you may find it pinks in Wales and Scotland.

Ironically that makes the 25D4 preferable to the 45D4 as with it's vernier adjustment you can quite easily fine tune to timing for different environments.
PaulH Solihull

This thread was discussed between 21/06/2012 and 22/06/2012

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