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MG MGB Technical - Haynes and Dynamic Timing

A word of caution for those of you using Haynes as a data reference for dynamic/stroboscopic timing. I have a '71 roadster, with the GK engine, high compression model. Totally stock, but de-smogged, apart from the standard pollution control advance curved 25D4 #41339. Haynes quotes 14 degrees BTDC at 600 RPM. Late last season I set this figure, and noticed an unhealthy increase in running temps, despite the cooler fall temps, and a consistent misfire coming in around 3000RPM. After three changes of points and condenser, a coil change, careful valve clearance setting and many SU tuning hours to no avail, I bought a Leyland Factory Manual reprint for the MGB, the one with the specific reference appendix section for North American cars, and noticed immediately that Leyland quoted 15 degrees BTDC at 1500 RPM. Vacuum disconnected and plugged for both manuals.

Frustrated and concerned, because my previously "dry head" engine had started weeping, then spraying small amounts of coolant, which rapidly got worse,( and which lead to my head gasket job this past winter) I emailed Chris Betson and asked his advice and professional opinion. He return emailed me, saying that Leyland was correct and Haynes incorrect, and that Leyland figures was the way my car left the factory.

After the head gasket job I started at ground zero again- Valves, then points, then initial SU set up to get the car running and warmed up. Then I hooked up the timing light, and was shocked to see the timing mark way off the scale- reading probably about 25 degrees! (I did not notice pinging, because I tend to run the car rich, and at this time of the year, I was only using the car on short flat commutes from our village to down town and the gym, and only then if there were dry roads, and this is the start of the rainy season, so the car was not often used.) I cranked it back to Chris's suggestion of 1 or 2 degrees less than Leyland figures on account of the lead -free head/fuel, and got the SU carbs tuned in about five mins. Absolutely smooth idle, which would run as smooth/slow as 600- I like about 900-1000, and set it there. Smooth power increase, running like a top at 4500 RPM, with no hint of a miss. Took it for a quick( short!) test run- it was well below zero air temps, and bloody cold on the ears, and it seemed a new car.
Did this bad data from Haynes cause or contribute to all my problems? I can't prove it, but in my mind it did, and I advise everybody using Haynes to check the Haynes figures against the Leyland figures in Section 6 of AKD957, 5TH EDITION, Workshop Manual Supplement. It covers the full range of North American models. I also wonder just how many people who complain of overheating are using Haynes as a reference for timing data. Be warned. All the foregoing my humble opinion, and ready to be shown the fallacies in my thinking/practice.
Ken R
Ken Rich

Ken. Get a copy of the Bentley reprint of the factory workshop manual. It shows, in the back, the correct tuning specifications for US/NA cars, including the 15 deg BTDC at 1500 for the 71. Haynes, and other non-factory manuals, do have their place. Often, they may explain something better to the non-professional than the factory manual (designed to be used by a factory trained professional) does. But, the basic law of research applies--start your research with the source document whenever possible. The source, in this case, is the factory workshop manual or an exact reprint of same. Sorry you had to learn the hard way, but, by pointing out the problem, you may have saved others from trouble. Thanks. Les
Les Bengtson

Ken and Les,

The Haynes books are good because they actually take a car to pieces to write it and take the photos - just like you and I - the factory manuals ( I have the originals - jealously guarded!) were designed for service stations to use when repairing the cars.

The factory manuals therefore assume knowledge that the Haynes ones give explicitly.

Even the factory manuals are not infallible - the torque settings for late cars is muddled between the conrod bolts and the head bolts!



Chris Betson

Les, I think you and I are talking about the same manual. Yours is a blue, Bentley reprint of the official BMC MGB manual; mine is a green reprint of the official British Leyland Manual of 1976- published by British Leyland UK limited, 1976. I am sure it incorporates all the material from the days of BMC. I see no words or date referring to reprint, but it is new- at the bottom of this page it says, "Leyland Cars- Service, Cowley, Oxford, England," along with the post code and that Leyland logo we hate so much. and on the back page it says "printed by Brooklands Books ltd, Cobham, Surrey,England." It must be a reprint, because I find it difficult to believe that Octagon Motors in Vancouver has had them sitting around 20-25 years. (In 1967 I bought a BMC MG workshop manual, but it was three ring binder, with a supplement of loose leaved pages to file and add to replace, add to the original binder. Sadly, I gave it away long ago.) This new one is called MG B Workshop Manual, publication Part # AKD 3259, 15th edition, with Engine Emission Control Supplement, Publication Part # AKD 4957, 5th edition. ( this is the section to which we are both referring, I am sure.

Yes, I do not intend to toss Haynes out, just ignore the dynamic timing info. And any other data I learn is incorrect. I like the "do it yourself" approach of Haynes- it suits my mind set perfectly. I just don't like serious errors- and it could be that Leyland has errors also- I pointed one out re the carb drawings in an earlier thread, though those are not serious, just wrong. My Haynes is dog- eared, oil smeared and spends most of its time out by my car or on the rad or front wing; my Leyland is pristine, and stays in the house, on the kitchen table, but still only a door away from the shop. And as you noted, I have been checking one against the other, whenever I attack a job, since I bought the Leyland ( how I hate that name- it reminds me of a truck or bread van.)
Chris, I hope Leyland is correct re torque settings for rocker bracket studs- 25 lbs, and head studs, 50 lbs, because that is what I set, mine being the dimpled studs.
Thanks once again, Chris, Les.
Ken R
Ken Rich

Yes Ken, 25 ftlbs is OK for the rocker studs but the dimpled studs will take 55 ftlbs - the figure used by the factory but which never made it into the manual!

The current manuals are reprints of the last factory manuals and contain much which was originally contained in supplements and other booklets - they are good value for money.

The info is the same as the original - it is just nice to have the original - I never give away any of my old manuals! My 1974 one is the 13th edition of AKD 3259 and is in a 4 ring blue plastic binder. I also have an MGB Repair Operation Manual AKM 4070 dated 1977 which is White card covered with a black plastic binding comb.


Chris Betson

This thread was discussed between 28/02/2002 and 01/03/2002

MG MGB Technical index

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