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MG MGB Technical - BP270 cam and 123 dissy
|I have a 67MGB with 1868cc engine fitted with a BP270 cam, 1.5in SU's and 123 MG4RV dissy. which is the best curve to use for this cam, any advice please.|
|unless that's a hot cam I'd start at the lowest setting in the range for your model and year of car|
|Depends primarily on the combination of compression ration and inlet closing point.|
BP270 is listed as inlet closing at 65 deg. As a starting point: with a CR of about 9.5:1 that would match the characteristics of the standard curve very well. If CR higher than this, go for one with less initial advance. If lower, to the standard curve should suffice.
Set max advance initially to 30 deg and then fine tune to actual engine and fuel used.
|CR is 9.5:1, so am I right in using curve 2 as the standard curve. Also should the tickover be lumpy with a BP270, because it is at present.|
|The 123 curves aren't really relevant with today's fuels even with a totally original and in-spec engine. If you modify it you are on your own, and will have to find which one suits your engine and grade of fuel, the amount of advance you can run is usually noticeably less with 95 than with 99, for example. The 123 setting procedure is to adjust the distributor so the light goes on (or off) at TDC, so that when running the 'correct' static advance and hence starting point for the centrifugal and vacuum advances comes in to play. But those figures are based on 4-star leaded, not 95 octane unleaded.|
|Peter Burgess is your man to set that up on his rolling road for excellent value (unlike most other RR)|
|I intend to go to Peter when the engine is run in, I have bedded the cam in at 2500rpm for 20 minutes, but am having trouble getting the engine to idle very well, its very rough at present and I can't work out why. Ive set the mixture using colortune and the 123 dissy as the instructions say, but the engine will hardly run, any good ideas please?|
|all I know about setting up is to follow the order of the chain and if you adjust one item then you need to check the items that follow in the chain|
if it's engine rebuild or head off then you might start with check/re-torque head
usual chain - tappets, (CB points), plugs, timing, mixture
I'm not a fan of those Colourtune but I know others are
are you sure the idle isn't something stupid like filter or gasket upside down or other minor things that I can't think of, dissy in 180 out, order of leads
perhaps a photo might help
|How did you time the camshaft?|
What sort of tickover RPM are you trying to achieve?
|Its a complete rebuild inc pistons, cam kit, pump etc. Ive gone through everything methodically. The cam was timed at 107deg. 800rpm would be nice. One thing I had to do when I got the engine back, was alter the dissy drive by one tooth to allow fitment of the 123 without the vac adv hitting the starter, but that shouldn't affect timing, as the 123 is set at TDC, it just moves the dissy round slightly. Checked the tappets, they were OK at 014 inlet 016 exh. I'm coming to the conclusion it must be timing, but don't know why.|
Your setup sounds similar to mine +.060" bp270 cam, Peter burgess econotune head and 123
I did have the crank plus everything attached balanced, its idling between 600-700rpm which i feel is a bit low but this is how Aldon automotive set it up after a rr session.
As for the setting on the dizzy im pretty sure peter burgess advised setting B
Its been 3 years since i brought the cam and head off Peter so would have to pull the dizzy to make sure.
The car does run lovely
|Graham you need to set the max advance at say 4000 rpm with vac disconnected, at 32 degrees, try curve C.|
|Peter Burgess Tuning|
|Checked everything over this morning and can't see anything wrong, dissy is in correct position with no1 at TDC on firing stroke, fuelling was OK according to colortune. One thing I noticed on the paperwork from the rebuilder was "cam timed at 107deg BTDC" surely that is a typo error and should be ATDC. Can't imagine it would run at all otherwise. Will try curve C next Thanks Peter.|
|Graham, firing at TDC is irrelevant, you must set the dynamic timing or you are wasting your time, 32 degrees at 4000rpm if curve C, set without vac.|
|Peter Burgess Tuning|
|"firing at TDC is irrelevant"|
As said before setting the timing at TDC on the firing stroke of No.1 is SOP for the 123. One uses the little light to set this, then the distributor automatically applies the correct static and dynamic advances for the curve you have selected when the engine is running.
|Dissy is set to curve C now and the engine is running much better. I understand what Peter is saying, setting the timing dynamically doesn't equate to the standard set up of the 123 using TDC and the LED. Using an adv strobe to time the 123 gives an exact level of advance at any chosen rpm.|
Thank you to everyone who posted, I am now a little happier and will press on with putting some miles on the clock to run the engine in, before a tune up.
We have fitted at least 40 of the 123s, the light on/off method is fine to get it running but not accurate in any way,the only way we can do the job properly is check the dynamic timing and set it to what it should be. If you check the curves they are not always exactly what you would expect or as advertised.
|Peter Burgess Tuning|
|I doubt that dynamic timing to the original specs could be used on most cars, especially with 95 octane. That's one of the thing that in my mind makes the 123 too expensive - one still has to adjust for no pinking.|
|? Doesn't every ignition system need setting not to pink? 16 different curves gives one a chance to stop pinking yet still retain a good level of performance.|
|Peter Burgess Tuning|
|Most do now, which is my point about paying a lot of money for the original curves and special setting-up procedure of the 123 which you then can't use. Originally i.e. with leaded 4-star the original curves shouldn't have pinked as they catered for a range of tolerances plus a safety factor that was intended to protect the engine against damage from ignored pinking. Those engines could often run with more advance than the book said, to give you better performance and economy, as I found in the 70s. With this 73 roadster I couldn't run with any more than the book value even on leaded 4-star, and I'm not alone.|
To go back to something you said earlier "If you check the curves they are not always exactly what you would expect or as advertised." which makes splashing-out for a 123 even more pointless. I had assumed, naively perhaps, that one did get the curves that were advertised. If one really insists on using an electronic trigger then get a points-replacement type for a tenth of the cost of the 123.
|The beauty of the 123 is the multiple curves allowing one to find a curve which might well suit, whereas a 'boggo' distributor has a like it or lump it single curve. With the best will in the world one does not know until one tunes the car/bike what advance is required at different rpms. The difference is maybe between measuring what is happening and seat of the pants tuning as I know you do not like rolling road tuning.|
|Peter Burgess Tuning|
|I've had electronic igniter heads in most of my classics and that includes at least three of the four "MGs" but it's putting new onto old, the rest of the mechanical dissy can still be wibbly-wobbly (technical term) to varying degrees|
on my present midget I originally had points which I disliked even before the poor quality modern ones were about because points had let me down whereas in many, many tens of thousands of all year round miles I'd never been let down by the various electronic ignitions fitted to my different classics
as soon as I could I swapped to electronic igniter head on my present Midget as the 123 wasn't available for right angle 1275s at the time so I had an Aldon igniter head and it was good
Sod's Law not much later the right angle 123 became available
so I swapped to a 123, much better still thorough the whole rev range
I then had the car set up properly by visiting Peter Burgess and his excellent value rolling road tuning - even better still
the car wasn't set as well without the rolling road even by a very experience chap doing it nor was it set up so well on another A-series tuning expert's rollers
the car was set best with a 123 set up by Peter Burgess on his rolling road
I've 20+ years experience of using classic cars as dailies and almost as long rolling road tuning at various places
I keep meaning to check for improved mpg but the car is still too much fun for me to have the discipline to do so
This thread was discussed between 11/10/2013 and 15/10/2013
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