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MG MGB GT V8 Factory Originals Technical - Inital Weber Set Up

Can anyone give me a few pointers here? Sorry if this is bit dense.

I am having problems setting an inital idle on my weber 500.

I have backed out the idle mixture screws two turns like da man said to. Then set the idle screw (throttle stop) to about 800 rpm

But as the engine gets hot it wants to stall, suggesting I have it too rich, so I back off the mix screws, which slows it down even more, so I increase the idle screw but have to get it up to 1500 rpm before it smooths out.

So - I increase the mix again, abd back off the idle speed screw, and now its racing. No matter which method I choose to slow it down it get rough as soon as it gets near 800 rpm.

The choke has no effect whatsoever - it fires right up cold without it, even with the mix screws set (I think!) to lean.

I can't seem to work out which adjustment screws to use to set the idle speed - the idle mix screws, or the idle speed screw - they seem to fight with each other (the psychometrician in me would call it ipsative).

I can't get it to a rolling road, there are no seats in it!, but I do have a vacum gauge connected which sits in the 17-21 idle range when I have it set to a fast idle using the idle speed screw.

Some advice would be great.

Cheers, Liam

Liam H


Firstly did you buy the carb rejetted for a 3.5/3.9 if not the primary rods and needles will be far too rich, the rods you need are 1441 and jets 1421.

Notwithstanding that, my own car would start and idle ok with the original jets but was obviously running too rich, are you sure that the choke is in the fully off position?

I have had a look at the manual and it describes a slightly different procedure for setting the slow running, on page 11 it states that once the engine is fully warmed up the choke should be fully open which could be a little confusing as it means no choke at all.

Try going through the procedure on page 11 again and if there is no improvement there must be another problem.

What fuel pump are you using? it should not provide more than a max of 6.0 psi at idle or it could cause over fueling.

Also take note of the caution on page 10

Let us know how you get on.

Kevin Jackson

What ignition system are you using?

You need a damn good spark to use that webber. Idle can be erattic if the timing chain is streched also.
JC Moulds

Hi Guys,

Well, I went out today and set it diligently as per the manual (hard with a low profile filter on).

Fuel pump is stock, the ignition is in good nick and timed properly, I have new magnacor leads and plugs.

When it gets hot - its lumpy, it's OK when cool. Alos, fires up with no choke at all.

Time for new rods and jets methinks.

Cheers, Liam
Liam H


Forgot to mention you can get the specific rods and jets from RPI or Real Steel, and you do not have to buy a complete set.

Kevin Jackson

They are on their way Kevin. I asked for the ones you recommended, plus based on some reading of the archives.

What is the procedure for fitting them? The users manual is a bit dense - or perhaps thats just me.

On a positive note - I managed to halve the tacho reading today - just found a small yellow screw in the rear of the tach and turned it - worked like a dream.

Plus stopped a few water leaks and got my suspension (FINALLY!!) down to a normal level and torqued up.

All those months of worry.

Liam H

This thread was discussed between 17/04/2007 and 21/04/2007

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