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MG MGB Technical - HS4 Jet pipe seal.

Chasing a "failing to start" yesterday, I had the 73 BGT firing and revving well for a minute or so, but each time, it then spluttered and died. I'm going through all fuel delivery aspects from rear to front as a matter of good practice; one thing I came across was missing seals (AUD 2194 I think) on each jet supply tube. The front carb has a washer, presumably to retain the plastic tube, but nothing else: the rear carb (pic attached) has nothing at all!
Could anyone advise: what is the effect of little or no seal at the base of the float bowl? I have to say I have not been aware of any fuel leak, which amazes me - but could the absence of a gland or seal lead to "failure to keep running"?
Thanks for any advice.
John.


J P Hall

I would have thought that petrol would just pour out if there was no seal on it.

It must be there somewhere.
Dave O'Neill 2

It's probably inside and so sealing when it's refitted. If it's decomposing it could be partially blocking the jet tube, but I'd expect that to affect running within seconds.

Other than that if it isn't leaking it's fine and won't be a cause of this problem.

When it splutters and dies if fuel starvation was the problem the pump may well chatter a bit when turning on the ignition immediately before it restarts. Otherwise take the lids off the float chambers before turning on the ignition and check/compare the fuel levels with before a start.

Also check fuel delivery by taking a pipe off a carb, directing it into a container and turning on the ignition. You should get a minimum of one Imperial pint per minute (or pro-rata) and in practice at least two, in a continual stream of pulses with minimal bubbling.
paulh4

As Paul says, the seal is probably stuck up inside the hole with the washer.
Mike Howlett

The fact that it has come out suggests a problem.
The assembly is a "gland" . Similar to a gland on the rotating shaft of a tap.
The rubber/neoprene washer is compressed by the brass nut and washer to grip and seal the delivery pipe.
Fish out the washer and seal and replace the latter.
If there was no seal you would be wading in fuel.
But if the delivery pipe came free without slackening the gland nut, suspect the pump and/or filter for not delivering enough pressure to blow the joint apart.
Allan Reeling

Good advice all, thank you.
Tomorrow I'll poke inside and probably find something; fortunately I have replacement washers and seals in my "B cupboard". Hoping to not have to disturb the jet assemblies if I can get away with it, but if needs must I'll go the herl herg ( remember the great Peter Sellers ?).
John.
J P Hall

We must have seen 'A Shot in the Dark' half a dozen times, made my sides ache every time.
Even the first scenes as soon as he pulls up beside the fountain sets me off.
paulh4

Dave, Paul, Mike, Allan - spot on gentlemen; shrivelled up rubber seals hiding behind little washer up in the entry to the float bowl. Prised out, entries cleaned, new jet tubes inserted. Fuel delivery good - I forgot to mention I have an American Carter pump, it feeds exactly the right 1.5 litres per minute;spent time today moving the carb settings from ridiculously lean, to about right by the "lift the piston with your little screwdriver" and the "length of hose" methods for richness and balance.
She runs not too bad but still an electrical issue to chase, because at about 1000 rpm, she decides - when she's ready - to fluctuate the tacho briefly, and die. I have Lumenition, with Megaspark coil and separate ballast resistor, so will need to check that plus timing, distributor function and anything else I can think of.
Any thoughts gratefully received. I have an auto eleccy coming round in the morning to help. That is, I'll help him!
John.
J P Hall

Fluctuating tach indicates a problem in the ignition LT circuit, especially if it drops to zero while the engine is still spinning down. A known cause of that is older original coils with riveted spades (BT, DT). Later coils even during production had threaded studs and nuts.

However faults in the HT side can cause a lesser effect on the tach, but it will continue to register as the engine spins down.

If the ignition warning light comes on while the engine is spinning down, instead of only when it comes to a stop, then the ignition fed from switch to coil has failed.

With electronic ignition the only diagnosis is to replace it - with points and condenser if you can!
paulh4

John.
More important when running SU's is the pressure the Carter delivers.
The carbs can handle about 3 psi. Above that there is danger of the carbs flooding when the float chamber needle valve is "defeated".

You say external ballast resistor, why? What Ohm coil? Not all Megaspark coils require the ballast resistor.
Most "drop in" points replacements require a 3 ohm coil to function properly.

Tach fluctuation does indicate a Low Tension problem. Check connections starting at the coil going back through the bullets and fuse spades to the ignition switch.

Does the ignition light come on as it stalls?

Allan Reeling

Useful, thanks Paul. Much to do tomorrow.
I'm quietly suspicious of the coil, as I noticed it hot to the touch after 20mins today; the ballast resistor was hotter again - but perhaps it's supposed to be, I've not felt one before.
J P Hall

You're lucky you can still feel anything after grabbing hold of the ballast---they run hot

Something to look at if you're running a ballast resistor and electronic ignition---The power feed to the electronic ignition needs to be 12v so has to be from the ign. switch(input) end of the ballast, not the coil (output)end

Your auto elect. can test the output of the electronic ign. unit with his scope or use duty cycle on his multimeter to pick up an intermittent dropping out fault like you appear to have

willy
William Revit

Depends on the values but original coils and ballast run equally hot as they are much the same resistance with the same current. But the factory ballast is in the harness so spread out over a few feet, and the coil has quite a large surface area. An after-market discrete ballast resistor is usually much smaller so will get hotter.

Again it depends on the values but most factory systems used about 1.5 ohms for each, although factory coil with the later North American 45D4 was only 0.8 ohms as it is a variable dwell ignition system so except at the highest rpm the coil is not being energised for as long as with points or with the earlier 45DE electronic system. If after-market coils and ballast are lower resistance they will get hotter.

Unless you have the later starter with the ballast bypass contact there is no point in a ballasted system, just something else to go wrong, but if the after-market coil is lower than about 2 ohms (Lucas 'sport' coils are about 2.4 ohms) then it will need an appropriate ballast.

The surface temperature of the coil will depend on the ambient temperature as it can't start dissipating any heat until rises above that, and how long it has been carrying current of course. In 8 to 10C mine are only 40C which is barely warm to the touch. But in near 30C it can be over 60C.

That's with points, and usually when the engine stops they are closed so you shouldn't leave the ignition on for long periods without running the engine unless you disconnect the LT side of the coil as the coil will overheat. But electronic systems these days as well as being variable dwell only energise the coil when the system detects cranking so it shouldn't even get warm with the ignition left on.
paulh4

Paul, it was the 45DM4, a reluctor distributor using an AB14 CEI amplifier, which used the 0.8 or 1 ohm coil at full battery voltage.

The GM HEI module in the AB14 not only limited dwell at low rpm, it also limited current. Hence the coil didn't overheat, which is how it could run such a low primary and secondary resistance coil. What can suffer from heat stress is the GM module itself, which is why it's best not to go less than a 1 ohm coil on a 4 cylinder, and provide it with a good heat sink mounted in a cool place.

Because the module received, basically an alternating current from the reluctor, it had amplitude information in order to adjust dwell.

At high rpm it actually increased the dwell.

You're right, since activating the coil is a product of distributor rotation, it receives no current until the engine is spun.


Allan Reeling

Slip of the digit, that's what I was supposed to type, likewise missing the '4' off '45DE'.

Not many people realise that many (if not all) electronic ignition systems reduce coil current to protect the electronics, compared to a good set of points, so fitting such a system with the standard coil has the effect of reducing spark energy.

North American systems using the earlier 45DE4 system used a coil with slightly lower primary resistance compared to RHD points coils, and a higher secondary resistance i.e. more turns, both of which give a higher output in compensation.
paulh4

This thread was discussed between 13/02/2021 and 16/02/2021

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