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MG MGB Technical - Euro-spec distributor - Ported or Manifold vacuum?

It seems like it's been 100 years since my last post, but I've resolved most of my struggles with the B, and am driving it regularly again.

The only thing that I'm fighting with now is pinking/detonation on acceleration. I currently have HS4 carbs and am using ported/carb vacuum, which is the source of my problem since the vacuum advances under partial throttle leading to knock.

I'm just curious what type of vacuum source others that have the Brit-tek Euro-spec distributor are using. Thanks!
SteveO

Steve. That particular distributor was used with both ported and vacuum manifold, assuming it is of the later model MGB, UK spec application. You need to note the specification number on distributor, if it has one--not all of them did. Paul Hunt's website, The Pages of Bee and Vee, has the information on the "standard--Euro-Spec" distributor.

Might be worth playing with the source of vacuum advance and the basic ignition timing to see what can be done. I find that ambient air temperature seems to effect run on and pinging. Higher temperatures, i.e. lower air density, result in more run-on and pinging than lower temperatures due to mixtures being weaker at higher temperatures. Something not commonly addressed, but, very obvious here in Arizona with our elevated summer temperatures. (It is 108 deg F just before noon and my car tends to run on when it did not at 90 deg F.)

Les
Les Bengtson

Once off-idle the result of the two vacuum sources is exactly the same.

Low-octane fuel is the biggest factor in pinking, although I can't speak for the effects of altitude. The only thing you can do is back off the timing degree by degree until it no longer happens at any combination of throttle opening, load and rpm. But even then, if you live in flat country at sea level then go touring in mountains it is likely to start again. I get part-throttle pinking as well, and paradoxically it ceases (to be audible anyway) by opening the throttle *more* (as well as less). Whether this is due to the wider throttle giving lower vacuum and less advance, or the bigger charge cooling the cylinder more I don't know. I've also installed a TCSA solenoid controlled by a manual switch so I can switch vacuum advance off altogether, which does stop it under certain conditions. However it is also very dependant on driving conditions, and gentle touring round windy and hilly country lanes in warm weather makes it worse than higher speed stuff even with the same sorts of inclines. I suppose this is due to the higher engine bay temps at lower road speeds. I suppose I should take my own advice and back the timing off a bit, but then I'd reduce performance and economy *all* the time, at the moment it's easier to modulate the throttle or turn off vacuum on the few occasions it happens.

If you have a choice of source then carb is best as it makes starting easier, but it isn't worth switching (unless you already have both carb and inlet manifold sources available) in order to try and stop pinking when running. If you already have ported then it is relatively easy to get manifold as well by modifying a spare manifold bolt, if you can't get one from a scrapper.
Paul Hunt 2

Thanks for the input guys! I have access to both ported and manifold vacuum, and was trying to figure out what most people were connecting to with this specific distributor.

I've retarded the timing about as much as I really want to, but I still have it. I switched to manifold vacuum, and it only seemed to make the pinking worse. Totally disconnecting the vacuum advance has eliminated the majority of pinking, now I'm just waiting to see if it has any effect on my gas mileage.

I've got an accurate wideband air/fuel gauge, and I can eliminate the knock simply by going wide open throttle which results in an air/fuel ration of 12-1. However, when my wife drives it, she isn't quite as sensitive to the painful sounds the engine is making, and I worry about the damage being done.

Paul, I'm not convinced that 'Once off-idle the result of the two vacuum sources is exactly the same.', but I'll try a test tonight before I stick both feet in my mouth.
SteveO

Steve, Most if not all of the Euro-spec distributors offer 20 crank degrees of vacuum advance. You'll want to use ported vacuum or your idle will be quite high!
Jeff Schlemmer

I am about to do this too. I plan on connecting to a non ported source (on the man., not the carb). This will allow me to bump the initial timing down.
So you end up with increased timing at idle but as soon as you place a load on the engine (past part throttle) the vacuum advance will cut out dropping off to where the initial/centrifugal is.

This is actually supposed to improve the mpg too because of the time spent at idle and partial throttle in the city.

That's the great thing about the vacuum advance. As soon as the engine is under a significant load, the engine vacuum drops causing the vacuum advance to drop out. I'm really big on vacuum advance for the street, and even more so for performance street cars with a cam.


The fact that yours is still pinking with the vacuum advance disconnected indicates that you have another problem, so dont even bother messing with your timing yet.
Maybe one or more of your cylinders are not operating efficiently for one reason or another. This places an unusual load on the remaining cylinders thereby promoting detonation.

Check to make sure the plugs, plug wires and cap and rotor are ok. Its important to get a bright and consistent spark to all cylinders.
If that checks out, the next suspect would be a vacuum leak at the intake man or carbs. If that doesn't do it, check the throttle synchronization and jets.
If that doesn't do it, valve adjustment or head gasket would be the next place to look, unless I missed something else.
BurgerCokeFries

That's true - more often than not you'll find a valave that's either leaking or mis-adjusted when you have a major pinking issue. It has to do with a diluted mixture causing a hot cylinder - excess air being allowed in.

Another problem is the Euro-spec distributor itself. More often than not, the advance curve is drastically wrong for these engines! The timing is all-in at 6000 rpms rather than 3000, AND the curve starts before idle, sometimes causing an unstable idle.

Be cautious with manifold vacuum to a vacuum advance that offers 20 crank degrees. That's WAY too much advance for performance or economy. Its fine with ported vacuum, but a timing swing of 20 degrees on/off throttle takes time, and that time lapse is enough to promote pinging! Just check out the vacuummodules used in the GOOD distributros designed fo ruse with ported vacuum - they ALL offer 10 crank degrees of advance! Only the frequently discarded Opus distributors offered more, and that was to help them pass smog tests for import!!!
Jeff Schlemmer

I mistrust this 'Eurospec' distributor, unless a reference number is given it could be anything. 'European' distributors used vacuum capsules giving 14, 16 and 20 degrees of advance, and the majority of them will have been for HC engines not LC. The centrifugal advance curves vary even more. That's not to say that North American engines won't benefit from a different curve, I'm sure they will, after all from 1967 they were increasing setup for emissions and not performance, unlike UK spec engines. But the original curves aren't even that valid for UK engines anymore, given the very different fuels and other changes over the years. Unless you spend a lot of time on a rolling road to determine what the ideal curve is for *your* engine and the fuel *you* use, and be prepared to do away with crusing economy, anything else is poking and hoping, and you might just as well retard the ignition until the pinking stops. Talking to someone recently he couldn't get his LC engine to pink no matter how much he advanced it, and it seems to be running at 31 degrees BTDC at 900 rpm! If other LC owners pink no matter what, then maybe it *is* a symptom of a mechanical defect.

SteveO - I rigged my UK 73 up with a modified TCSA solenoid so I could switch between carb and manifold vacuum while driving and monitor the results on a vacuum gauge. Results at http://www.mgb-stuff.org.uk/wn_ignitionframe.htm and click on 'Vacuum Advance - Carb vs Manifold'.
Paul Hunt 2

This thread was discussed between 12/08/2007 and 15/08/2007

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