MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

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 Technical - Weber problems

HI THERE

I CHANGED MY 2 SU CARBS FOR A WEBER FIRST QUESTION IF EMMISION CONTROLS ARE NOT HOOKED UP PROPERLY COULD THAT CAUSE A BACK FIRE AND SECONDLY DOING 60 ENGINE RPMS ARE AROUND 3000 IS THAT TOO HIGH I HAVE STOCK GEARS IN REAR END.

JUST GOT CAR BACK ON THE ROAD AFTER 32 YEARS MAYBE IT JUST NEEDS A GOOD RUN???

ANY HELP WOOD BE BE GREATLY APPRECIATED.

THANKS
KEN
Ken Kuss

Well you can work it out, from axle ratio and gearbox output ratio, but you don't say whether overdrive or not and what size wheels and tyres.
Allan Reeling

A bit more clarity needed Ken. What gear and rpm are you doing when it backfires? Is it when you take your foot off the throttle? I imagine your car no longer meets your state’s emission standards with the Weber setup, do you care?
Webers are highly tuneable but you may need to run it on a rolling road dyno to get the optimum power or emissions depending on your objective.
Mike
Mike Ellsmore

Backfire in the exhaust or the intake? Is the tach (assuming you have one and not the early rev counter) jumping with the back-fire? Could be almost anything without further diagnosis.

Standard speed/revs was 18mpg per 1000 rpm in straight 4th (i.e. 1:1 in the gearbox) and 22mph per 1000 in OD 4th. 3000 rpm at 60mph is 20mph per 1000 so exactly in the middle of those two. Even with standard wheels tyre size has a big effect on gearing - as the width goes up you need to reduce the aspect ratio, or the speed per 1000 rpm goes up i.e. the speedo under-reads. Speedo gearing is also a factor, you need to compare with sat nav i.e. GPS.
paulh4

Backfire is more common WITH the emission control connected than without, especially at altitude. There is no provision with the Weber for a gulp valve connection so if the air pump is operational and the gulp valve circuit plugged, the car would naturally backfire on deceleration.
Glenn Mallory

Ken
Is it a side draft Weber (DCOE)or the little twin barrel down draft DGV (E)
Is your backfire during acceleration or on the overrun
If it's in top gear and you've got 3000 revs then that's it , these cars are more buzzier than modern (higher) geared cars
Cheers
willy
William Revit

RE: Paul's post:

"Standard speed/revs was 18mpg per 1000 rpm in straight 4th (i.e. 1:1 in the gearbox) and 22mph per 1000 in OD 4th. 3000 rpm at 60mph is 20mph per 1000 so exactly in the middle of those two. Even with standard wheels tyre size has a big effect on gearing - as the width goes up you need to reduce the aspect ratio, or the speed per 1000 rpm goes up i.e. the speedo under-reads. Speedo gearing is also a factor, you need to compare with sat nav i.e. GPS."

My 79 is a straight 4-speed (no O/D) with my tire size 185x70/R14 is spot on. Speed alert flashes on the roads matches the speedo in the car. Prior tire set was 165x70/14. The speedo was way off.

Cheers

Gary
79 MGB

Gary Hansen

This thread was discussed between 28/08/2020 and 29/08/2020

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now