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MG MGB GT V8 Factory Originals Technical - Carb replacement update

Hi

Well, I finished installing the Edelbrock carburettor at the weekend. Unbelievably, it started first time. (I'll have pictures soon.)

It was very straight-forward, with just a bit of mucking about to plumb the heating.

It idles smoothly and goes well under full throttle. However, cruising or part throttle acceleration is very 'jerky'.

Chris at RPI suggests that the ignition is poor. I've got a Lucas 35D8 distributor, fitted with a Lumention optical trigger kit.

I'm a bit reluctant to replace it with a Mallory unit as:
1) It seems a retrograde step to go back to points.
2) I've just paid fifty quid (yes, really) for a vacuum capsule.
3) It's going to cost me another two hundred, odd pounds.

On the other hand, if I go out and buy a cap and rotor arm for the Lucas I'll be well on my way to the cost of the Mallory.

What do you think, chaps?

How can I verify that my ignition is poor? (Without spending money on it - as that would be wasted in the event that I replace the distributor.)

[I've taken it apart and greased the weights' spindles and where they ride on their base. There seems to be no play in the shaft, the gear on the end looks new (black, not worn silver) I don't see what else could be wrong with it??]

Oh. And another figure in the equation is that a new 35D8 is an eye-watering 270 + VAT.

I look forward to hearing the group's coments.

Thanks


Nick


Nick Wilson

Hi Nick,

I had a tired Lucas distributor and replaced it with a Mallory on my first V8. I found that when i was tying to set the timing on the lucas the timing would be right on average but you could see with the timing light that it was actually varying +/- about 1 1/2 degrees. When I got the Mallory the +/- variation went away, the engine ran much smoother and fuel consumption improved. When you consider it now costs £45 to fill the tank on a V8 £130 for a new distributor starts to sound cheap.

Phil

Philip Shingler

A 35D8 on points will show igition varaition even when new. As it wears then you get more variation. replace the points with an optical or magnetic trigger for the coil and a large degree of that variation is removed. Only excess wear in the dizzy will start to show significant varaition again.

Therefore the answer is to not buy anything yet, but to actualkly go out and measure the degree of timing 'float' you have with the current system. If there is a significant amount then going to a Mallory with points will improve things over what you have now.

There is one saving grace for the V8 which has been applicable for years and applies to standard and modified engines. That is the V8's tolerance to ignition timing variation and the small degree of reaction (in terms of power loss and fuel used) with quite wide variations.

One common failing though is the vacuum advance diphragm which hates fuel vapours and gives up when it is subject to this. Ideally you want to fit a fuel trap into the line before the dizzy vac advance capsule to catch this vapour. To do this properly get one of the many that are fitted to many other Rover cars. Make sure that the trap is mounted HIGHER than the point of contact on the carb. This to allow easy draining of condensed fuel back into the carb/manifold. The extra length of pipe between the carb and dizzy will have no bearing on performance but corrctly mounted it will save you another £50 on another new capsule. (These traps have come in as the same problem affects ECU pressure transducer diaphragms and they are much more expensive to replace/repair.)

Rog
Roger Parker

Roger,

Thanks for your reply. I'll try to get one of those fuel vapour traps.

Nick
Nick Wilson

Edelbrock (carter) carbs, though one of the best carbs going have very little writen on tuning them. Most Know how to tune for WOT. ie. accelerator pump, and final jetting.
Surging/jerky at part throttle usually means LEAN. Mixture needles and springs are cheap and way to easy to change.
For the money an air/fuel meter w/ an O2 sensor mounted in the car is invaluable.
Vincebo

Nick and others,
Are new distributors available for the Factory GT V-8? I only have one, and it is somewhat rough. It would be nice to find a new unit and keep my current one all set up in the boot as a spare.

Cheers,
Paul Kile
Paul Kile

Roger - I can vouch for the timing tolerance you mention. Although my V8 was running well I checked the advances one day and found the vacuum capsule punctured and the centrifugal siezed. Spoke to Clive Wheatley about a replacement capsule who said if it's running OK leave it alone. However I did both jobs (having swallowed at the price of the capsule) and it made no detectable difference.

PaulK - Clive has told me on a couple of occasions that the 35D8 is unobtainable and says he fits SD1 electronic units when the originals fail. Can Nick Wilson tell us where the ones he mentions are from?

PaulH.
Paul Hunt

> For the money an air/fuel meter w/ an O2 sensor mounted in the car is invaluable.

Vincebo, I second that ... they've gotten incredibly cheap ... I paid $30 for an analog Intellitronix unit at Summit (had to get a generic GM O2 sensor and have a bung welded into the exhaust ... took two minutes). I mounted the gauge in the glove box so that it doesn't look silly. You don't end up looking at it every minute. It certainly allows even an advanced-beginner mechanic like me to get the carb pretty darn dialed in. The nice thing about having it set up right is that there's no stumble or anything else when you hit the gas ... you're gone!

Don't waste money on the Edelbrock, Halmeter or MSD A/F meters. I was told that they are exactly the same thing (by definition; they just read the voltage coming off the 02 sensor), just way more expensive.

You might want to add a heated sensor rather than a 1-wire type. Supposedly they are a little more robust, I guess because they burn off contaminants.
Chris

Hi Paul(s)

I bought the vacuum module in my local part shop. (He had to order it.) At the same time I asked him to get the price of a replacement dizzy. He phoned someone and quoted 296 pounds. It could well be that it's one of the later electronic units - we did have commumication problems. It took a while to explain what the vacuum advance module is.


Cheers,

Nick

Nick Wilson

Roger,

How much 'float' as acceptable? I've had a look whth the timing light. It seems to have about 2 degrees of jitter at 1500 rpm. (Couldn't measure at idle - not enough light form the strobe.)

I don't really see how that could cause the car to 'kangaroo' up the road.

Also, my distributor only provides 13 degrees of mechanical advance. This can't be right as several postings in the archives mention 35 degrees of mechanical advance at high revs. Have I got a 'strange' distributor, or do I misunderstand something?

Regards,

Nick
Nick Wilson

The Workshop Manual supplement quotes up to 37 degrees of advance at 4900 rpm, the 8 degrees stroboscopic will be in addition to that, plus up to 16 degrees of vacuum advance! I suspect a mechanical problem inside the dizzie preventing full travel of the cam wrt the drive shaft.

PaulH.
Paul Hunt

Thanks Paul,

I was wondering if I misunderstood and the total advance was 35. I.e static + mechanical + vacuum. But you have re-assured me that it's the dizzy that's wrong and not me!

It's not a mechanical problem though. I've taken it apart and had a good look. There is a 'thingmy' that limits the amount of advance and it is stamped '13 degrees' (well, it's actually 13 then a small flying circle :-)

I thought that was unlikely so I measured the available movement. I got 6mm of rotation at a radius of 21mm. That works out at 16 degrees. So, given the rough measuremnts the 13 degrees is quite believable.

Once I got it all back together I measured the advance with a timing light and get 20 - 22 (ish) at high revs. (Don't have a separate tacho or a helper to look at the one in the dash, so I don't know what RPM but I revved it 'til the timing stopped advancing.) That's consistent with 8 degrees of static advance + 13 degrees of mechanical advance.

What's going on here? What vehicle is the dizzy from, anyone?

Can I just file about 4mm off the stop to get the missing advance?

Or should I just pay the money and get the Mallory? (225 + VAT is not insignificant, though.)

Thanks

Nick
Nick Wilson

The 13 you saw on the weights is only half the total mechanical advance. Remember the cam/crank relationship. What you should do is set it for about 8 deg static - with no vaccuum and go for 28 deg max advance at 5500 rpm.which will save any pinking.
bob p

Thanks Bob,

I guess it _is_ sticking, then.

Nick
Nick Wilson

That cam doesn't sound like the correct one. Stamped with '13' (the can, not the weights) means 13 distributor degrees maximum advance, which equates to 26 crankshaft degrees. But the book quotes 37 crankshaft degrees which should mean 15 or 16 distributor degrees so that is what the cam should give and be stamped with. Could also be incorrect springs i.e. stronger than should be present - that would limit the advance. On the other hand it could be weak springs which means the cam is much further through its travel than it should be at 1000 rpm (the stribe setting point), which means there is much less left than there should be.

Physically twist the rotor arm as far anti-clockwise as it will go then let it go slowly. It should go all the way back i.e. you should not have to manually twist it to get it fully clockwise again. If you can you have weak springs.

You should see 8 degrees at the marks at 1000 rpm, and 26 to 30 degrees at 2000 rpm. There should be no advance below 600rpm.

PaulH.
Paul Hunt

Paul,

I think your theory about weak springs is on the money. The springs have no effect 'til I've turned the rotor (say) 5 degrees (maybe more). Also, should the two springs be the same? I've got a thin one and a thick one.

Anyway, I've got the car working properly now. I just gave up on the scientific approach. I loosened the dizzy and turned it 'til it sounded 'happy'. Thinking about it now, I suppose I chose the setting that gave the fastest idle.

I could tell it was fixed as I backed it out of the drive. During a short test drive it ran fine.

I suppose that since the advance curve is ballsed-up the only place where the timing is right is at low revs / small throttle opening. I'll have to sort this out, but it's a great relief to get this far.

Cheers,

Nick


Nick Wilson

Nick,
regardless of springs being weak or any thing else. 26 deg of advance from 8 static is generally regarded as all you need giving you a total advance of 34 deg. Ask Peter Burgess, or Auto Techniques or Roger P or whoever.
There should be one weak and one strong spring. The weak one allows you more initial advance per increase in revs, the stronger one doesnt come in until later. Get it set up on a rolling road . I suggest Sigma Engineering in Gillingham Dorset. Its near Shaftesbury, and say I sent you. Both my cars have been set up on their road and I highly reccomend them. When I used to live in Gt Missenden I used to trek across to Auto Techniques in Luton , but thats too far to go from Wareham in Dorset when Im only home for two months in Summer. If the initial spring is too weak you will get pinking and it may need changing.
Go for it.
bob p

Hi Bob

Gt. Missenden? I live just up the road in Wendover, so Auto Techniques in Luton would not be far. But now that I've got an idea what's wrong it should not be hard to get the dizzy to produce the advance curve I want.

Regards,

Nick
Nick Wilson

Nick - one thin and one thick is correct, and the thick one should be loose on its pins. This means that the first bit of advance is on the thin spring only i.e. relatively steep. After that the thick spring slows things up and it doesn't rise as steeply.

I have just posted the V8 dizzie specs and curve on the above site - select 'Spanners', 'V8 Stuff' and 'Dizzie Curve'. You can see quite clearly the effect of the two different springs.

PaulH.
Paul Hunt

Thanks Paul.

Actually, I'd been to your site and found the 4 cyl ones, which gave me the idea of plotting the curve myself. On a scrap of paper, not at all hi tech :-)

You and others had posted the specs in the past. So I was able to draw the curve and can clearly see the change in rate of advance that you mention.

Incidentally, is your published curve measured, or calculated? It seems too good to be measured, yet not straight enough to be calculated.

In my dizzy, both springs sit loosely on the pins, so a bit of juditious bending is called for, I think.

(The car will be good when it's finished, and it'll be finished when it's good.)

Cheers,

Nick
Nick Wilson

The curve (and the remainder of the specs) is straight out of the factory V8 Workshop Manual Supplement, as are the 4-cyl ones.

PaulH.
Paul Hunt

Paul,

Why's it a bit wobbly, then?

Finally sorted out some pictures:

www.bluebonnet.demon.co.uk/mg

Cheers,

Nick
Nick Wilson

Ho ho!
Paul Hunt

This thread was discussed between 02/11/2000 and 10/11/2000

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