British-Cars.org

Welcome to the DMR Site for British 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 - 1974 BGT Carbs

Hi All:

I am new to newer MGs as I have always had T-series and do not know how the carbs on my “new” 1974 MGBGT work. I have looked at the archives and can not find a problem similar to what I am experiencing.

Above 55 MPH the car surges slightly. Pulling the choke on by about 3/4 of an inch eliminates the problem. Any suggestions of how to fix this?

Yours in Safety Fast
Jc
John Crawley

Does the problem occur at a lower speed when in a higher gear? It "sounds" like fuel starvation, pulling the choke out drops the jet nearer to the fuel level and exposes a richer section of the needle.But do the basics first. Float chamber levels, needle size correst and fitted correctly. Damper spring weight correct (easily assessed even if the colour code has disappeared). Do the pistons rise and fall at the same rate or stick, when lifting with a finger and allowing to drop, damper out? Oil in the dashpots? Is the fuel pump delivering enough. Tank cap venting?
Allan Reeling

Allen:
Thanks for your logical approach. I looked at the carbs did not recognize anything and went aaaaaa!!! And then forgot all I that I Knew about SUs. You are correct . . . back to basics. It has been raining here for the past few days so will try to get at it tomorrow. And no, the only time it surges is in 4th or 4th overdrive at speed.

Jc
John Crawley

I believe that a 74 MGB has HIF carbs which have a rotary choke valve feature rather than dropping the the jets. This site has some good photos http://www.mgexperience.net/article/hif4.html whilst the vidoe from John Twist gives a good description of how to tune http://www.youtube.com/watch?v=ASeMfXfjNpw

Good luck!

Barry
BJ Quartermaine

'74 is on the cusp of the change, but North America could be different.
I assume from the fact that the problem doesn't show in intermediate gears that it is related to a "trailing" thottle situation. i.e., cruising, therefore high manifold vaccuum. Check for air leaks on the inlet manifold, throttle spindles (besides the obvious "do they rattle" try dribbling some thin oil or redex on the spindles at tickover, you might see it disappearing and see a bit of smoke from the exhaust)and check the distributor vacuum unit, including the vacuum pipe and rubber ends. Two effects here; 1. the mixture will be leaner than the normal trailing situation as the engine pulls in air from wherever it can get it, and 2. if there is a vucuum unit problem, the ignition won't be advancing to compensate for the weak mixture.
Allan Reeling

72-74 US/NA cars are HIF, unless somebody changed them.
The described condition is "lean surge", and Allan's recommendations are mostly good. The throttle shafts are not likely the reason here, since under these operating conditions there will be little to no air leakage there, because the throttles are open far enough to let far more air in than the leak. Gum build up on needles and jets is a possibility, especially if the car has been sitting for long periods - run some "fuel injection system cleaner" through it. It is possible that the vac chambers need to be cleaned. Or it may just have been tuned too lean. Worn needles will result in need to lean the adjustment excessively to get idle mix right, which leaves it too lean everywhere else.

FRM
FR Millmore

This thread was discussed between 23/05/2011 and 25/05/2011

MG MGB Technical index

This thread is from the archives. Join this live forum now