MGB Register Forum

I have a 1979 B and no history, I assume the engine is standard.
I have recently set the HIF carbs up, but I cannot get the engine to idle below 1000rpm and the adjustment screws are now off the physical stops.

I believe the jet height is set by the mixture screw, the mixture seems about right, as the car runs well on the road (save for the occasional splutter...) the plug colour seems good (cylinders 2 and 3 are darker than 1 and 4, but I believe that to be normal) can I even adjust the jet height via another means, or is the mixture screw the only way to do that.

The linkage all seems good and well set-up, the cable is adjusted correctly and has slack when at full idle. Pushing down on the adjustment screwss will force the butterflies tight against the body and you can feel it all tighten up... so I think ots all there but the idle speed is too high.

your assistance is welcomed.

Could the fast idle screws be holding the idle speed high?
In other words, is there play in the choke cable when the choke handle is fully in?

Also, is the accelerator cable properly seated in its ferrule?

Thanks... the cable is well seated, I would need to chek the fast idle screws and the choke cable, I'm pretty sure the choke cable is loose when closed off, thought it has to be. As for the fast idle screws I need to review where they are. It's a good point thanks.

An obvious one, but: check the tachometer with an independent machine.

Unlikely to be the cause, but worth checking.

With the throttle completely closed it should be impossible to start the engine, so perhaps an air leak somewhere? Do the butterflies have the holes/springs?

Ian F

Have the carbs been refurbished if so check that when sleeves have been fitted to the throttle spindles that the bodies weren't drilled right through so leaving a way for air to flow round the throttle flap when closed.

As always, lots of useful bits to be looking for. I have no history of the car, or work done to the engine, I have not had the carbs off the car.
With regard fast idle, if the fast idle adjustment (HIF carbs) is related to the small screw that follows the choke cam, then as it sits at idle the cam is not in contact with the adjustable screw, as the choke is activated the cam comes in contact and lifts the screw to alter the throttle position adding rpm. That seems to be set-up correctly.
The various hoses for the breather system appear to have seen better days... perhaps I should replace them and the distributer vaccuum line, to be certain of no leaks.
As it is the car is quite driveable, reducing the idle from 1000 to 800 is just a personal preferance.

It just seems strange that the idle adjustment screws are off the physical stops and with plenty of slack on the cable, throttles held back with reasonable spring pressure, it appears a physical stop is affecting the full travel. If needs be I will pull the carbs and rebuild them.

Thanks everyone, I shall update if I find the problem.

If the throttle discs have that useless spring loaded valve in them these valves tend to jsm and that affects the tickover. The solution is to get a pair of plain throttle discs from BFS in Salisbury and ditch the existing ones. It was a common fault .
Before I ditched the HIF4's in favour of HS6's I had the same problem. With care the discs can be fitted in situ, you just need to remove the dash pots and air filters for access and stuff a small piece of rag behind the disc to block off the inlet just in case the screws escape. The discs fit only one way. New retaining screws will be needed.

I'm with Vic in ditching the throttle discs with poppet valves. Useless items in my opinion and they interfere with a smooth air flow. Fitting new discs is a bit fiddly because they have to be exactly positioned before tightening the screws or they will leak air when closed and you will not be able to lower the idle speed. Is that what's happening now? New screws have to be spread after fitting to prevent them working loose and entering the engine.

Whatever is wrong, the most likely cause to me is an air leak somewhere. I have heard that you can spray aerosol carb cleaner round the various gaskets and joints. If it sprays onto a leak point the engine revs will rise as the extra fuel is sucked in. I've never had cause to try this, but the theory sounds good.

Before changing any components of the HIF carbs , it may be worth looking at John Twist's / University Motors videos on u-tube where he goes through the whole process of setting up HIF carbs on an MGB if you haven't already done so.
Cheers
Charles