Strobe timing at tickover or high revs

[GeoffK]

Hi. Several ‘official’ Workshop Manuals quote that dynamic timing for an 18V engine should be 13 degrees btdc @ 600 rpm. But I have read a number of articles which suggest that it should be set @ 32 degrees btdc at approximately 3000rpm. My gut feeling is to go with the manuals but has anyone any views on the alternative? Geoff

[Ian F]

The usual advice is to set circa 13 BTDC at 600 rpm with the vacuum disconnected. The advance after that varies according to the advance curve. Around 30 BTDC at max advance is usually considered to be around optimum.

You should get Peter Burgess's book on MGB tuning. Of course all of this is subject to some changes depending on which engine/distributor you have and how much tuning you have done.

Hope this helps!

Ian F

[GeoffK]

Hi Ian. No idea of the state of tune of the engine only had the car for 2 months with no history whatsoever. The engine does not even have number plate, just the letter C stamped on the block where the plate should have been. Sounds like 13 @ 600 is the best way to go. Thanks

[Ian F]

Hi Geoff. Assuming the advance curve is approximately right for your engine and the distributor is in good order, then try hard acceleration from low rpm in top gear and listen for pinking. If excessive then gradually retard the ignition using the distributor Vernier until it's just gone.

That shouldn't be too far away from 13 BTDC. If it is then you may need a distributor rebuild or an engine decoke!!

Ian F

[Statler]

I've read about these verniers on distributors.
My original and the replacement Powerspark distributor don't have such a thing.

I've found with running Unleaded fuel that the timing changes from the workshop manual as that is 95 RON.
The MGB was designed to run on 98-100 RON 4 Star so I'm planning to swap to Super Unleaded and see how that goes, now that I'm lead free in the head

[GeoffK]

Hi. I think that the distributor is quite new as are the coil, cap and leads. They all look very clean and the previous owner gave me an old distributor plus coil cap and leads implying that they had been replaced. I run the car on super unleaded as did the previous owner and currently there is no sign of pinking. I think that I will reset @ 13 btdc @ 600rpm and then ultimately go for electronic ignition to remove the variability of setting the points. Thanks for your input

[Ian F]

Hi Geoff. I was a little concerned when I read that your distributor was new. The following tale just recounts my experiences (described here before) and may be totally irrelevant to your situation.

When I finally rebuilt my 1800cc MGB engine about 10 years ago, I decided to treat it to one of the new distributors being widely advertised from the usual outlets. They were very competitively priced (read cheap.....).
This gave me endless trouble unfortunately. I found that it was impossible to set the points gap AND the dwell angle to the correct values. The cam was completely the wrong profile. On a long trip down to the South West we had several "breakdowns" where I had to readjust the points gap to get the B to run. I eventually returned the distributor and got a full refund, no questions asked. These distributors have generally received a very bad press.
I then dug out my original Lucas distributor and had it fully rebuilt by the Distributor Doctor. This transformed the performance, and it does not go out of tune on an annual cycle of servicing. Can't recommend this option highly enough.
With any luck Geoff, you will not have one of these newly manufactured distributors on your B.

Ian F

[Paul Hollingworth]

I always find that 600 rpm is too slow for a smooth idle. You end up with the strobe fluttering too much. I would recommend that you find out what the advance curve of your distributor is a try and set at 1000 rpm. This also tends to reduce the variation in timing you get from distributor wear. For example my 18GG engine with 25D4 distributor has advance curve 40897 which is 13 deg at 600 rpm but 20 deg at 1000 rpm. All with vacuum disconnected. The advance curves are all available on Paul Hunts Bee and Vee website. If you are not sure which distributor you have the advance curve should be stamped on the body. Early 18V (72/73) had the 25D4 but most have the 45D4. In his book Peter Burgess makes no secret of his dislike of the 45D. You can tell the difference straight away because the 45D doesn't have a Vernier adjustment. (knurled wheel on the bottom of the Vacuum module) It depends how much you want to spend on electronic ignition. I have a Luminition Magnatronic unit to replace the points but my friend and I fitted a 123 fully electronic distributer to his B. They are a really nice job but expensive.

Link to Pauls site:
http://www.mgb-stuff.org.uk/curvestext.htm

[GeoffK]

Must admit that setting the timing at 600rpm seemed a bit low especially when the manual that I have recommends a normal tickover speed of 750-800rpm. I have established from the V5 form that the engine should be an 18V 582FH and it does have a 25D4 distributor fitted, but with the engine number plate missing and the base colour of the cylinder block in the area of the oil filter being a sort of metallic tourqoiusy blue I am wondering whether the engine has been exchanged at some period. Can’t remember that colour being a normal/original colour. Current mileage is 118000 so who knows! The ‘old’ distributor has the number 41288 on it with 07 72 underneath which I suspect is the date of manufacture (July 1972). Can’t see the number on the current distributor without removing it but externally it looks identical to the old one - lets hope that it is not a cheap copy. I will study the info provided and try to establish how many degrees btdc matches with 1000rpm. Thanks

[Ian F]

Hi Geoff. I think the low rpm for setting the timing is just to make sure that the Bob weights inside the distributor are "at rest" and not advancing the base plate. I think your engine (an 18V of that vintage) should be black, a semi Matt colour in my experience.
Hang onto that old Lucas 25D4! It may come in useful.

Ian F