MGB Register Forum

The revs of my '79 BGT are very slow to drop when I take my foot off the accelerator in neutral, Indeed sometimes it idles at 2000rpm for several seconds before it slowly comes down. It is NOT the cable catching anywhere so must be a fault in the HIF carb(s) . Any one had a similar problem any ideas what is the cause and the cure?

Take the dashpots off and give the inside a clean with carb cleaner. Check the pistons rise and fall without sticking. Failing this you might have an air leak into the inlet manifold somewhere. Check the thin pipe to the distributor is attached and the rubber ends are ok (both ends) and also the vacuum take off to the servo, check this hasn't perished. Finally check all the nuts holding the carbs to the manifold and the manifold to the head are tight.
Hope this helps
Pete

I've just had another thought!
Check the breather pipe from the front tappet cover to the inlet manifold, in particular the plastic Y piece which is infront of the front carb. The Y piece can crack with age.
Hope you find your problem,

Thanks, Peter. Hoses and Y piece relatively new all look OK and tight. Have cleaned piston and chamber with carb cleaner as you advised- it looked pretty grubby. The manual I have is not very detailed on these HIF carbs- Should the dash pots be filled with engine oil (10/40 in my case) to 12mm above piston tube as recommended for the earlier carbs?

Hi John,

Bit late replying to your revs query - internet connection been out of action due to Black Friday and Cyber Monday surge. May I tentatively suggest you could just possibly have a split diaphragm in the advance retard unit on the distributor. This happened to me a long time ago and it took me ages to track down. A colleague at work gave me a complete 25D unit from which I removed the good vacuum unit - nice inexpensive repair - doesn't happen very often these days.

As has already been suggested , my first move would have been to have the dashpots off and clean then - then look for leaks.

Good luck

Rog

Hi John,

I don't know if you ever solved your "revs slow to drop" problem. If your carbs are to the original spec, the butterfly valves incorporated a small valve for anti emission reasons, so that all the fuel was burnt off BUT it meant that the revs dropped very slowly - dangerous in my view!! With my 1979 GT I originally super-glued the valves shut and later replaced with plain butterfly valves. Just a thought.

Geoff

ROGER COOPER wrote:
> Hi John,
>
> Bit late replying to your revs query - internet connection been out of
> action due to Black Friday and Cyber Monday surge. May I tentatively
> suggest you could just possibly have a split diaphragm in the advance
> retard unit on the distributor. This happened to me a long time ago and it
> took me ages to track down. A colleague at work gave me a complete 25D
> unit from which I removed the good vacuum unit - nice inexpensive repair -
> doesn't happen very often these days.
>
> As has already been suggested , my first move would have been to have the
> dashpots off and clean then - then look for leaks.
>
> Good luck
>
> Rog

Roger and John,
A stand the recent Ricoh Arena Restoration Show was selling the vacuum advance units separately for £10. I'm sure they will be attending the MG and Triumph show at Stoneleigh on March 1st.
Remember there are several different designs for the various Lucas distributors so make sure you know which you need.

Geoff Edwards wrote:
> Hi John,
> I don't know if you ever solved your "revs slow to drop" problem.
> If your carbs are to the original spec, the butterfly valves incorporated a
> small valve for anti emission reasons, so that all the fuel was burnt off
> BUT it meant that the revs dropped very slowly - dangerous in my view!!
> With my 1979 GT I originally super-glued the valves shut and later
> replaced with plain butterfly valves. Just a thought. Geoff
Dear everyone thanks for your help - what a wonderful forum this is!
I have checked and re-checked many times for any air leaks and am convinced all is OK on that score. I have taken the dash pots off and cleaned them ( they were grubby) The car has been in hibernation for the past 6 weeks so don't know if the problem is still present. BUT... Geoff I am so glad to read this tip from you as it confirms the suggestion I received at a recent local natter. So if the problem's still present, it's off with the carbs and stick the small valves closed. ( Intrestingly I can "pull" the revs down at idle by putting it in gear, slowly letting the clutch pedal up until the revs drop as though about to stall. Once the revs are down they stay down- do you think this is a pointer to confirming your diagnosis?)

Hi John,
Excuse the slow response.
I'm glad that you found my note interesting and yes the valves do eventually close down - but it takes time AND there is the problem. So I think if you have checked other avenues, it can do little harm to glue up the butterfly valves or replace as I suggested and you never know it may cure your problem. The cheap method only requires a little time and some super glue!!!
Geoff