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MG MGB GT V8 Factory Originals Technical - Vacuum Advance

I have converted my GT many years ago with a 3.5 SD1 engine, LT77 and a 3.3 rear axle. The engine is fitted with a JWR dual plane manifold and a Holly 390 carb.
The dizzy is still the 35DE8 Lucas with an optical trigger system and blackbox (Newtronic/Piranha).

When excellerating hard at WOT, the engine starts missfiring at 4500+ rpm. Excellerating slowly there is no missfiring and it goes well over 6000 rpm.
I checked the fuel pumps (double Mikuni setup) but there is no starvation in delivery.

I have checked the ignition wires (copper core) and plugs without any sign of faults.
The mechanical advance is working fine.
The sparks have nearly white isolators (runnung very lean), so i think the problen is somwhere in the vacuum advance system giving exessive advance at WOT. The capsule of the dizzy is conneted to the timed spark vacuum source on the front metering block of the Holley.
Has anyone tried manifold vacuum with this type of manifold? What results were noticed then?

Any ideas?


I'm inclined to believe that you need to richen (at least) the secondary side of the carb. What you are doing by accelerating slowly is reducing the load on the engine which is allowing combustion of the lean mixture.

I don't think that the ignition timing is the problem although that carb has a full vacuum port and it would be easy to test.

Wayne Pearson

I don't see how vacuum advance could be at fault if you have diagnosed lean mixture. At full throttle there is no vacuum advance anyway, only at cruising speeds (this is irrespective of whether you use carb or manifold vacuum). The vacuum *capsule* could be at fault if it is punctured as that will allow additional air into the vacuum advance port so weakening the mixture. I don't know where your port is, but on the factory V8 HIFs it is on the bottom of the carb throat. This allows fuel to run down into the vacuum capsule, which rots the diaphragm. I had to replace it twice at considerable expense before I did a mod. This is to provide a small separation chamber mounted above the bottom of the carb throat, with ports installed such that any fuel that collects in the chamber automatically runs back to the carb and not the distributor, see and click on 'Vacuum Module' and 'V8'. That was many years ago, and it hasn't failed (so far!) since.
Paul Hunt

@ Wayne

Hello Wayne,

first this was my idea too. I tested the CO with a 12 Volt tester driving the car in different situations of excellerating normal and with WOT.
It allways showed readings between 4.5 and 5.5 CO volume.
Sparks read OK when driving normal (BP 6 E, light brown).
Excellerating with WOT and fast runs on the highway at 120 Mph seem to weaken the mixture. May be the dual plane JWR manifold tends to built up to much vacuum or, the now used timed port at the metering block, gives to much advance.
Normally there should be 36 at 3500 to 4000 rpm. Tested with a strobe in the garage, everything seems perfect until the car is pushed hard on the road.

@ Paul

Hello Paul,

i read the details on your page yeasterday allready. The setup is so much different that the discribed problem with the works setup could not be a reason for the missfiring in my installation.

I know the HIF 6 carbs well and run them on my stage 6 roadster too, but it seems to me that the V8 gets to much advance due to uncontrolled vacuum. The dizzy itself works fine on my test bench, so the fault seems to be in the induction system.

You have reported about the faulty vacuum advance on your V8, not seeing any difference between the defective item and the new one. The conclusion could be to disconnect the vacuum and drive it. As we have no speed limits in here, the car would not perform as i want it to do.

Any ideas where to start at?


- I will be online again on Friday 12th -

I would be looking at the secondaries on the carb. I'm certainly no expert on Holleys, but at the speeds you're talking about the secondaries will be in full operation. From memory, the secondary metering block has a sort of generic jet arrangement & for serious performance should be jetted like the primaries. Also all the secondary controls should be checked out. Barrie E
Barrie Egerton


If you haven't yet solved the problem and are convinced that it is vacuum relsated why not disconnect the vaccum pipe, plug the carb and give a try.

Personally I agree with others that it is a lean situation on the secondaries, is it possible that one of the secondary jets is blocked or the fuel filter is resricting flow?


Kevin Jackson

Thank you for your ideas.

I checked the Co with an 12 Volt gas tester and therE seems to be no reason for running lean on the secondaries. Not at engine speeds up to 3500 rpm nore at wot.

I will chech the ported vac this weekend and the manifold vac too. May be i should connet manifold vac to the inner side pic up of the D35DE vac capsule to comensate excessive advance.

I will try it out and report upon my impression if there has been any difference.
I think it is a design problem with the JWR dual plane manifold on my car. Wait and see...



On my 3.5 I have the webber 500 with a dual plane JWR manifold.

When we set up the carb we found over 50 deg of adavance at 4500 rpm.

With the vacuum disconnected the advance was exactly as the book stated , if I remember about 28 deg at 4500rpm.

My friend who drag races his 4.6 MGB GT (11 sec 1/4 mile) said he had a 3.5 with the same problem!!


D M Tetlow

This thread was discussed between 08/09/2008 and 14/10/2008

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