MG-Cars.net

Welcome to our resource for MG 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 - Be Careful What You wish For! HIF Problems

Due to the lack of posts, I thought I'd start with this: So I'm pretty happy with the ignition timing & the car starts & idles'OK' The front carb (HIF4) mixture is as per the tuning instructions, lifting pin up, rise in revs then back to idle. However the rear carb is still too rich, black plugs & starts from cold with no choke. I'm starting to think that the choke is stuck on. The linkage is fine with no sticking but the carbs were rebuilt about 4 years ago, with little or no use since (yes I know!) I have seen other posts about the washers on the choke mechanism failing, and with the problems I've had with fuel hoses disintegrating, could sitting fuel have caused the same problem internally to the carburettor? Also just a query really, lots of posts about ensuring the float height is correct, if the flow is regulated by the jet & needle why is the float height so critical? Surely as long as fuel is covering the jet intake then that will suffice?
Pat Gregory

I'll start the replies then.

My pet check - making sure the dashpot and piston are paired correctly, see JT vid below.

Also, if (the inside of) the dashpots and pistons have been cleaned that only the piston rod has been lightly oiled and not the rest.

Another obviously thing with dashpots is to check both have oil in and the same sort of oil and level.

If you change the needles & seats, or any parts, never assume new parts can't be fault (or piss-poor manufacture quality) see JT vid below.

41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA

232 MG Carburetter Needles and Seats (odd sizes) - http://www.youtube.com/watch?v=pVX_JClyeDQ
Nigel Atkins

Ignore me just seen HIF, rushing too much.

Hope my reply with encourage better replies than mine.

Try this anyway -
35 Tuning HIF Carburetters - http://www.youtube.com/watch?v=ASeMfXfjNpw
Nigel Atkins

As a test, can you loosen the linkage so the choke mechanism for the rear carb is non operational.If no change the choke is stuck. (The front carb mechanism will still raise the revs for cold starting). Also I'd check the choke is fully returning -ie the return springs are doing what they should

Fuel is getting blamed for all sorts of things now-usually with reference to Ethanol. FWIW I have yet to find a single instance where ethanol here or in the US (where they had have strange brews for years) as being proved as being the cause of a problem. Lots of opinion-no fact.

When you first start the car from cold , the fuel pump chatters as it fills the float bowl. This is because heatsoak has caused the fuel to evaporate. So fuel hoses and carb internal parts are not likely to be immersed in petrol-maybe a bit of vapour but I'd doubt that.

Modern hoses /rubber bits are not "like they used to be" but as a big a problem is that we forget how long ago it was that we fitted said bits!

There are O rings in the HIF choke system (they are easy to change).

I'd agree the float height needs to be correct but is not as hyper critical as is usually made out. Having said that it is as easy to set it right as not!
Michael Beswick

The simple answer to your last query; the needle valve does indeed control the fuel flow but, more importantly the level of the fuel in the float chamber. This in turn sets the height of the fuel in the jet and therefore it's "availability".
Besides John Twists matching check, (do it first), you also need to check the fall of the pistons in situ, with the filters and dampers removed. Lift both pistons with fingers, all the way up, then let them drop. They should hit the venturi bridge at more or less the same time, with a "clunk". If this doesn't happen, check the dashpots and pistons for dirt, also check the location surfaces on the carb bodies which can slightly twist the dashpot causing the piston to bind on the carb body.
Measure the height of the jet, i.e., the distance it is below the venturi bridge, They are usually more or less the same, but tuning often leaves them slightly different.
Also check the needles. The needle spring carrier should be flush with the piston body and the securing screw located in the groove of the needle carrier.
Also the springs could be odd. Check their free height, i.e., the same, and it's likely if they have had little use since the re-build, that their colour code will still be visible. RED. Although the cheapskates often don't bother replacing them!!!
Also as a rough check observe the butterflies while on the stops, see if they look similar in their opening, then pull the throttle cable to see if they are lifting together, i.e., in sync!
Allan Reeling

Hi, I had almost the same problem.After trying everything I removed the butterflies and replaced them with new plain ones. Problem solved. It was one of the poppet valves. On inspection it looked closed but probably the spring was not strong enough anymore.
Other solution is to solder them closed.
Herman
H. Jorens

Thanks all, I have Done the Twist drop & swapped the Pistons so that they drop evenly (quite satisfying, strangely) The rear carb cant go leaner as the screw wont adjust anymore, which is why i think that fuel is still entering elsewhere. I had thought of the poppets, & was going to solder them when I rebuilt last time, but was in too much of a rush to get them finished & back in. I'll pull the pair this weekend & do a full inspection. Less haste etc.
Pat Gregory

Pat, I have also puzzled over why the fuel level was quite so critical. As Alan says it not only determines the level in the float chamber but also in the jet. My (imperfect) analogy is as follows: If you have a straw in a glass full of orange the level in the straw is the same as that of the surrounding liquid. When you suck you get orange/fuel quickly. When the glass is 3/4 empty, and you suck, you get a slug of air and the orange fuel takes longer to reach your mouth!

When you consider the minute differences along the length of the needle that determine the mixture, I can see the need for the orange/fuel level to be (a) constant) and (b) as near to the jet orifice as possible.
But, as I say that is just my understanding.....
Michael Beswick

The poppet valves opening when they shouldn't results in weak mixture so I doubt that is the cause.

The "choke" mechanism is notorious on HIF carbs for leaking additional fuel into the engine.

Poorly sealing "O" rings or fuel erosion of the tapered barrel are likely causes.

I once did a running repair to one of mine by wrapping a bit of teflon tape round the barrel and refitting it to cut off the flow!
Chris at Octarine Services

Michael - the needle height in the jet determines the size of the orifice through which the fuel is sucked by the venturi effect of the carb throat - fuel level does not affect that.

Also, as the sucking is relatively constant, there won't be slugs of air.
Chris at Octarine Services

Chris,
Whist my brain is freewheeling I sometimes go down this fuel height issue and my view has mostly been similar to yours. But it would be an interesting experiment to take a carb, drastically alter the fuel height from very low to high and compare the amount of fuel picked up on each setting. I can imagine an initial lean spot on the low setting due to fuel travel distance, but would there be constant weakening due to weight of fuel and distance lifted...............i.e., work?
One of these days....maybe!!!!
Allan Reeling

Another vote for choke o-ring dealing. Often overlooked when carbs are overhauled, and a common
cause of HIF running unaccountably rich.
Paul Walbran

It really can only be the enrichment mechanism that is at fault - IF the jets are at the same height with the piston covers and pistons removed. If one is much lower than the other, and can't be altered by the mixture screw, then that is the cause.

Float height will affect mixture, but that can be compensated for in the normal set-up procedure with the mixture screw. However you may then get an incorrect mixture balance across the throttle range, which is harder to determine, and may well be why the float height has quite a small tolerance.

In the temps we have at the moment it's quite likely that a hot switch-off will evaporate a significant amount of fuel from the float chambers - I've heard it boiling in the HIFs on the V8 and seen vapour pouring out of the vent hose. But under cooler conditions the fuel level won't drop anywhere near as much, and hoses and carb parts will contain liquid fuel.

Faulty poppet valves/butterflies will cause a racing idle, they won't affect the mixture as such.
paulh4

Just an update on this, rebuilt both on Saturday, but stripped the thread on the throttle adjustment screw, then broke a 'replacement' M3 screw in the lever arm, so another contribution to Moss's pension fund. I'll refit tonight, the over-rich appears to have gone as they are both backfiring now.
Pat Gregory

This thread was discussed between 05/07/2018 and 12/07/2018

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.