Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - Leaning off a weber 500 carb

Hi all

Having driven my car (MGB V8 conversion)now for about a year i am starting to get a bit of a backfire when she gets a bit warm, having had it rolling road tuned about 3 months ago and being told that it is just right not sure what to do.

It is running very rich and i believe it would benefit from just a bit of leaning, what is the best way to do this to a weber 500? do i have to turn both screws in or out the same amount or should i only adjust one of the screws?

The car goes well for about 25 miles before this is noticeable, i have gone through all the basic checks and all seems well, have put it on a machine and Hydro-carbon reading was about 1500, was 2700 until i found out i was running on 7 cylinders!!!(my fault recently changed plugs and put a plug in with a closed gap)

Thank you in adavnce for your help.

Graham Lavis

Something else is going on and my first suspicion is that the secondary butterflies are not closing all the way. The linkage could be fouled, or one butterfly plate is barely loose on its shaft. Not sure, but there may be a screw on this carb that prevents the secondaries from closing tight - could be holding them open too far. Of course, there's always the possibility that a small piece of crud is in the float needle and seat allowing the carb to flood slightly. Or the float has taken on some fuel (if it is a hollow one).

Carbs just don't go out of adjustment that much in a short time without having an underlying problem.

Wayne Pearson


If the carb was fitted out of the box, the primary jets will need changing.

The rods are 1441 and jets 1421. and available from RPI. Ypu don't need a complete jetting kit.If you have the handbook and nimble fingers it's not a difficult job to do.

The adjustment screws on the front of the carb are purely for setting the idle and have no effect once off idle.

Kevin Jackson


I'm assuming that your engine is 3.5 or 3,9 if not disregard the above advice. I also agree with wayne that there may be other problems but until the carb is correctly jetted it will be dificult to diagose the problem. May be worthwhile to check that the float levels are correct.

Kevin Jackson


You need to change both in the same direction by the same amount.

It took a chap 3 hours to do mine on a rolling road with a gas anaylyser. He got the two sides balanced/matched and in harm,ony with the choke/idle speed in a way I never could. Getting the combination right is a skill I think.

He told me the only thing I should change now should be the choke idle speed and leave the rest alone.

He left it on the rich side, which I agreed with -

1. To prevent overheating.

2. To keep the revs up a tad.

Be very careful, altering those screws changes idle speed. Get one wrong and you have a rough running engine. I couldn't believe the difference once he finished with it. And he was a motor bike bloke, not trained on webers at all - he just knew how to sort the problem from root cause.

I occasionally get a crack when starting from hot, but hey it sounds good!

I'd leave well alone, or get it back on the rolling road.

One more thought - check your choke hasn't come slightly closed - mine can vibrate out a 1/4 inch or so after a while.

Liam H


Just for clarity it's great that your guy achieved a nice smooth idle for you, but the idle jets have no effect on mixture once you open the throttles, the only way they could effect general running is if they were set very rich giving a lumpy tickover which could foul the plugs if you were driving in traffic and the engine was on tickover for extended periods.

For a 3.5 in fast road spec the jets and rods for the primaries are definitely, Rods 1441 and jets 1421 as confirmed by Dan Jones in his post under Best Carb now in the archives.


Kevin Jackson

I have spoken to RPI and said that my carb was sent out set up for a 3500 so has the correct rods and jets in, they need some more info from me on the ignition system i am using.

i will check the choke, i do check it is pushed in a few times on a journey but need to look at the other end.

At idle it is running pretty smooth, i will do the other checks recommended here and post the result.

Thanks for all your help.(still cannot load a picture!!!!)

Graham Lavis

First of all I assume you have a 1404 Edeelbrock.
I have it on a 3.5 with a 3.9 cam and stage 2 heads.
It has taken a lot of fiddling to get the mixture right, through the rev range. I too had it on a rolling road and was told everything was right, but the plugs were always sooty. All the books say the engine is extremely tolerant of variations in mixture and timing is! As supplied by RPI it was too lean, but the float heights were all over the place. After setting these it ran too rich I now have it just right on 6552 rods, 083 primaries and 092 secondaries. All engines, even with ostensibly the same spec, do vary in their demands. The idle mixture DOES affect the transition from idle and also the "off idle" throttle positions. Consider that 1000rpm is about 30mph (depending on axle, wheels and gearbox), you spend a lot of time in town, just off idle! So getting the idle mixture right IS important!Also check all the other variables, timing, vacuum advance mechanism. Backfiring through the carb could be a weak mixture caused by an air leak at high vacuums, (trailing throttle)either a holed diaphram, or leaking inlet gasket,OR over advanced ignition. Try blanking off the vacuum advance at the carb and see if things improve.
Also if you have the the large chrome air filter supplied by RPI, BEWARE if you over tighten the centre screw the "lid" will foul the choke flap!
Allan Reeling

This thread was discussed between 27/08/2007 and 01/09/2007

MG MGB GT V8 Factory Originals Technical index

This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.