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MG Midget and Sprite Technical - HIF44 vent

Hi,
I have a HIF44 fitted to my 1275 midget. It is running well, but there is one thing I would like to understand.
When I fitted it, I fitted an open pipe to the float vent that hung down to somewhere near the chassis leg.
All was well.
When I took it to a well known rolling road, the well known (and well respected) rolling road operator removed the pipe saying that it can cause problems hanging in the air stream under the car. I assume this would be a partial vacuum caused by air rushing past the open end of the pipe.
The car still runs well without the pipe but I have noticed that starting from cold takes longer now than it used to. The engine spins for some time before it fires.I suppose this is not such a bad thing as it allows some time for oil pressure to build before it fires.
When starting from warm it starts much quicker.
I assume this is because, over time, the fuel in the float chamber evaporates through the vent. This sounds reasonable to me apart from the fact that even if I leave the ignition on with the fuel pump running filling the float chamber, starting from cold takes just as long.

What should I do with the float vent hole?

Does my assumption sound reasonable?

Has anyone experienced the same or have any advice?

Thanks

Dave
Dave Brown

I ran my vent down to just above the chassis leg, so it wasnt in the airflow below but was well away from the manifold. I dont think it was especially by design, more than likely that was how long my spare bit of hose was ;-)

has run well for 14K Miles over a number of years without any noticable issue starting (I have had starting problems related to weak battery + poor earth strap)

If you suspect the floatbowl isnt full - you should be able to wait until the pump stops ticking before you try to start to eliminate that as the cause. if you have an SU pump and it doesn't stop ticking maybe you do have a small leak ?
Dean Smith ('73 RWA)

Hi Dean,
I don't have an SU, I think it is an Eccu - it is from New Zealand, I can't remember the make. It doesn't tick.

I have tried leaving the ignition on for a while but it doesn't seam to make any difference.
Perhaps only the volatile elements of the petrol evaporate leaving the float full of degraded fuel and so the engine needs to get through some fuel before the float chamber delivers good quality fuel that the spark plugs can ignite.

Dave
Dave Brown

Not sure if the issue of the vent pipe and starting are related. I too have a HIF 44 on 1275 vented to a bit of pipe exiting near chassis. I simply cannot believe any pressure drop so created by pipe being in the way of the airflow would affect the carb - besides which, is it really in any airflow anyways considering there is a dirty great radiator in front of it?

As regards cold starting - check eveywhere for fuel and air leaks, ensure mix is correctly set, check vac advance is pulling, choke operation - especially the cam which also opens the throttle a smidge etc ....

Mark O

My HIF just has that coil of tube, wrapped around the choke cable, and the remainder laying close to the w/screen wiper motor. Never had any problem with it.

Why dangle it downwards?
Lawrence Slater

I'm with Mark in that I don't think the starting problem is necessarily related to the vent, I'd start with checking the battery condition and fully recharge it for drain on problem solving if nothing else, then I'd check battery posts, clamps main leads and earths are clean, secure and well protected
Nigel Atkins

Hi Nigel,
Battery is good, no problem there. It turns the engine over quickly and does not fade.

Hi Mark,
I don't think there is any issue with the tune, air leaks etc. as warm starting is just fine.

To prove what is going on here, I will reattach the vent tube and run it back down to where it originally was and run the car like that for a while. Then, I will place it up near the windscreen wiper motor and run it like that for a while to see if airflow/vacuum does affect running.

Thanks

Dave
Dave Brown

Dave

No, just because it is OK warm does not mean to say carb is not at fault. Cold start requires a richer mix - provided by the carb.....

Regards

Mark
Mark O

Hi Mark,
Point taken, I should check the carb.
I am confident there are no fuel leaks and that the choke mechanism is working correctly and activating the fast idle control. I changed the plugs last weekend and they looked a good colour, perhaps a little darker than light straw so perhaps a touch rich, but not too bad.
Vac advance and air leaks will be checked at the weekend.
Thanks

Dave
Dave Brown

My Hif44 vents out over the valley of the front inner wheel arch (it's a frogeye). No issues with starting whatsoever.
G Lazarus

This thread was discussed between 14/01/2015 and 15/01/2015

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