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MG MGB Technical - 45D search

My friends '75 B is equipped with a 25D dizzy that does not belong there. This might be the reason why we can't get it to run correctly. I would like to try a 45D but we are not willing to pay for the new thing which we maybe can use. Is there any other british (or other) car which has a 45D that can be exchanged?
Carl
C. J. Gaasbeek

I do have a 45D in my 74, which, oddly enough, shoud have a 25D. I just bought a used 25D for it. The 45D will be surplus, but it would require rebulding as the shaft bushing is awfull! Contact me by email if you are interested.

Dave
Dave Whitehead

Unless the 25D is faulty you should have similar if not better performance to a 45D, in what way is it not running correctly? But each engine has a prefix (e.g. 18V 581, 582, 583, 846 or 847) which determines which type of 25D or 45D it should have denoted by the serial number on the dizzie body. The wrong serial number can result in poor running. Check your engine number, correct and actual dizzie serial numbers, specs and curves at http://www.mgb-stuff.org.uk/wn_ignitionframe.htm and click on 'Distributor Curves'.
Paul Hunt

Interesting charts Paul. I haven't checked my 45D yet to see what the model was, but I am hoping the UK '73 25D I have bought will be an appropriate match for my soon to be installed 9:1 CR pistons and piper 270 (18V-672). Apart from the idle avance difference, do you think the manifold vs. ported vacuum advance will make for a big difference? I realize that the manifold vacuum will also be stronger than the ported, and it will hit max advance earlier. I suppose if it is a big problem, I can modify the carb to provide a port as discussed in the archives.

Dave
Dave Whitehead

There is a big difference between manifold and ported vacuum sources. A ported source will give you minimal vacuum at idle and maximum at full throttle. Manifold however is just the opposite, max at idle and zero at full throttle. Both have their applications but it's important to understand the pros and cons. Generally speaking ported has been considered a better choice for performance as the increased advance when rolling on the throttle will help acceleration. Manifold was introduced durring the emission years and supposedly isn't as suited to performance applications. However in my 67 I am running manifold vacuum. Why? I have an oil burning problem that likes to cause some pinging at full throttle, usually up hill(which is where maximum advance was.) Now at full throttle vacuum advance drops to zero to help avoid this. I can't say that I noticed any difference in performance between the two. I'm not sure that I fall into the train of thought that a performance dizzy has to run on ported vacuum. I've read several article(albeit concerning americam V8 engines) that highly suggest running manifold vacuum is benificial in many ways. After all, consider full race dizzys are mechanical advance only, add manifold vacuum to that and you have the same thing at WOT and better fuel economy at partial throttle cruising.

I'll try to find some of those articles on ported vs manifold.

-Jared
Jared Snider

Well, I was going to say that the difference between manifold and carb vacuum is minimal, but I would be referring to driving differences. It is quite true that manifold vacuum applies maximum vacuum advance at idle whereas carb applies none, but as the throttle is opened the manifold vacuum falls and the carb vacuum rises, they become the same very soon, and are virtually identical for the remainder of the throttle travel. At light-throttle cruising speeds the vacuum with either will be the same - relatively high, under hard accelleration both will be virtually zero.

The only reason for manifold vacuum I am aware of is that, if all other variables are the same, it gives a higher idle speed. You can see this by sucking on the pipe of a carb vacuum system - the idle speed will increase noticeably, or simply by disconnecting the manifold vacuum and sealing the port, which will probably cause the engine to stall. This effect allows the idle screws to be unscrewed slightly with manifold, which reduces the fuel consumption and hence emissions. But this is all kidology since off-idle they are both pretty-much the same. The down-side of manifold is that it applies some vacuum advance during cranking which can make starting more difficult, and because the vacuum capsule is held at maximum longer and more often it will probably puncture sooner. I have known of people modifying carbs to produce a vacuum port, but personally I wouldn't bother.
Paul Hunt

And at WOT there little or no vacuum. That is the reason most race cars do not use a vacuum advance. I learned that many years ago when windscreen wipers were operated with vacuum motors. The wipers would stop any time you went up a hill or passed a slower moving vehicle. Some deluxe cars had a vacuum reservoir tank to supply the wipem motor under WOT conditions. FWIW, Clifton
Clifton Gordon

Paul,

The problem is that the engine in the car has it's engine tag removed. In it's place there is a number stamped in DEK 0198c 366. I posted the question who is stamping in DEK numbers on rebuild engines and got no usefull reply except from someone from the UK that also has a DEK number instead of the original tag. So up until now I'm not shure what kind of engine it is (18V/18GB)and certainly not what number or if it's low or high compression etc.

All I know is that both the dizzy and type of SU's used point in a pre '74 engine which is now in a late 75 car.
We showed the car to an "expert" and he made an estimate of what it would cost to make the engine run smoothly. Within his estimate (which was too high for our taste anyway) there was the replacement of the 25D for a 45D a job we can do by ourselves as well. Since we are not sure this will solve the problem my friend is not planning to spend a lot of money tp buy parts that we are not sure of in the first place.

Is there any other obvious and easy way to determine what engine we are working with?

Carl
C. J. Gaasbeek

As with any engine, you start with the basics.
1. Air.
2. Fuel
3. Compression
4. Ignition/spark.

An engine not running right? Well start with is it running just poorly, or a misfire?
If it's a misfire, then pull the spark plugs, check their condition, go to the wires, cap and rotor. next check for available voltage from the coil.
Then check engine compression, and adjust the valves to spec. If compression is low on all cylinders, then it may be it's just worn out. If it's low on one cylinder, than perhaps a burnt valve or broken ring.

Still everything checks out? Check for ignition and or camshaft timing. Worn timing chain, mechanical advance stuck in the dizzy, etc.

Got enough fuel up to and into the carbs.?
air filters pugged? If you're not up to some of these procedures, then find a willing auto technician, preferably one who knows MG's
I'd volunteer, if you weren't too far from Minnesota.
Safety [hope you find the trouble] Fast
Dwight McCullough
Dwight McCullough

CJ - can't help with identifying an engine from external clues, but with a rebuild they will be useless anyway as you will have no idea what changes have been amde internally. Any distributor will run correctly with any engine at some point in its operating range, the correct distributor will run correctly at (hopefully) a wide range of operating conditions. In your initial post you say you cannot get it to run 'correctly', later on you say 'smoothly'. As I asked originally, what exactly is the problem?
Paul Hunt

This thread was discussed between 04/08/2003 and 11/08/2003

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