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MG Midget and Sprite Technical - Carburetter Overflow

HI,

attached is a picture of HIF44 carb. When the fuel pump is on fuel comes up the exposed pipe marked with blue arrow. I assume this an overflow.

I generally understand how these work, but cant find a diagram that shows why this would happen. From what Ive read so far HIF carburetors have a seperate fuel cicuit.

I think this would due to the float chamber. The carb was serviced with an su kit a last year and has been running fine.

Can anyone suggest where I start on this?

Also is there a book that covers this. I have the SU Carb high perfomance manual, but this doesnt cover step 1 of general maintaince etc.

Thanks

BruceBurrowes

You forgot the image. I can send you the carb manual pages for the HIF44 if you send me an email. Dominic AT clancy DOT ch
Dominic Clancy

Bruce, Service and adjustment sheets are available online from the Burlen website, though whether they cover your specific issue I cannot say.
GuyW

Bruce. Generally speaking, an overflow of fuel can be traced to one of two conditions--the valve or the float. The float can develop a minor hole or crack and fill with fuel resulting in its never rising upwards sufficiently to close the fuel shutoff valve. I have seen this a single time in something over 50 years.

More common is some form of problem with the valve that the float rises up to shutoff. It is not uncommon for the valve to either get a small piece of some form of "gunk" in it that does not allow it to seat properly and shut down the fuel input, or the valve deteriorates in some fashion, allowing the fuel to continue to flow.

If the valve or float has completely failed, there will be a steady stream of excess fuel flowing out of the overflow vent, a very dangerous situation and one which should be corrected immediately.

The HIF carb is not as well documented as the H and HS series, both of which had much longer production runs. The HIF series was a result of emissions controls required by various governments (mostly the US) and is somewhat more complex that the earlier forms of the SU carb. In addition to the information on the Burlen website, some information on the HIF carbs is contained in the factory workshop manual, and there are several aftermarket books which focus exclusively on the various SU carbs or contain sections dealing with the SU carbs. I find all of them have been useful over the years.

Hope you get it sorted. Les
Les Bengtson

SU Berlen (not always the best place to source info and parts) - HIF (click 'Next>>' at bottom of page to continue) -
https://sucarb.co.uk/technical-hif-type-carburetter-dismantling

Bigger carbs but general idea -
https://www.v8register.net/FilesV8WN/SUbookletsAUC9939AHIFtuningandservicing.pdf
Nigel Atkins

I had a similar problem years ago. In my case, I had installed a Grose Jet float valve, and it simply wasn’t sealing. Going back to the original type cured the problem for good.

And yes, everything suggested above is also valid.

Gryf
Gryf Ketcherside

I know it's on the other side of the world, but we have had quite a run lately of HIF floats sinking
Usually it's a very gradual thing and the mixtures have been adjusted to make up for the higher fuel level but eventually there gets to be a limit and they flood up
If your float has sunk ,it will pay to check and possibly adjust the idle mixture after the float has been adjusted

We had quite a good discussion about hif carbs on here earlier in the year which would be worth your while having a read through

willy
William Revit

probably the second half of this but worth a read anyway

https://mg-cars.org.uk/cgi-bin/or17?runprog=mgbbs&mode=archiveth&archiveyear=71_2020.dat&access=&subject=71&subjectar=71&source=T&thread=2016080709320631662
William Revit

Nigel

Interesting info from the V8 register.

V8 carbs aren’t actually any bigger - HIF6 is the imperial equivalent of the HIF44
Dave O'Neill 2

Blue arrow marks the overflow


BruceBurrowes

thanks for the links, and manuals, some light reading.

Carb is on the bench ready for inspection. If the issue is a needle valve, Im assuming its just a case of renew?
BruceBurrowes

sorted, the needle valce seat and come lose and was stopping the float, float. Thanks for your help.
BruceBurrowes

Well done, thanks for reporting back.

If only other issues were sorted so easily.
Nigel Atkins

Hi Dave,
hard to believe I know but my mind wandered off the fact it was a single HIF and on to (smaller) twin HIF (probably just been reading a MGB thread).

From earlier days of watching John Twist videos I discovered the code, H = 1" and then the following digit was a multiplication of 1/8", so HS2 1.25", HS4 1.5" and then it all went metric and I have to check the mm on a tape with inches.

(Yet I still have to stop myself thinking HS2 is inch and a 'alf.)
Nigel Atkins

Bruce. Glad you got it sorted. Les
Les Bengtson

so after sorting the problem, I now need to impove the thing (until I break it again) and a have a couple of questions.

1. As you can see I have no flexible hose attached on mine, it spills right onto the exhaust manifold. What does everyone else do? Im considering a fuel hose leading down and out, anywhere but on top of the exhuast.

2. Heat plate is fitted, but do people have any thoughts on heat wrapping the exhaust.

3. Im my reading I notice people reduce the butterfly screws and the profile of the retaining support to improve airflow. Any got an experience of this?

thanks again
BruceBurrowes

Bruce,

On mine I run the overflow as you noted in fuel hose and take it into the open triangular stiffeners of the chassis just below (old tiewrap to keep in position.

I see your engine breather is blocked off with a bit of hose and a bolt - does your engine run one ?
On mine I run the breather from the front timing case canister and as I do not use a heated manifold, I use the pipe through the manifold to pipe the breather to the other side of the carb, with another short bit of pipe onto the carb breather tube.

R.
richard b

Bruce,
is this going to be a track car or road car - either way fine tuning fiddling is only done after all the basic servicing, maintenance and repairs on the whole car, with regular use over reasonable length journeys for a good period of time, has been done.

Messing with the carb's form will get you (tiny fractions of) increase performance only if everything else is fully functioning and at peak/optimum performance already. These types of fiddling around give very diminished returns and can only be effective at the end of all other standard servicing and tuning.

Having the whole car properly set will give and retain very noticeable performance improvement, the carb fiddling you're looking at will give you nothing without what's required before it.

From what I've read exhaust heat wrap potentially could do more damage than any good.

Engine and carbs is the bottom of the list for attention, always more important are brakes -absolute number one, (inc. tyres), steering (inc. tyres), suspension (inc. tyres), lights, horns, glass - see and be seen.

Last is engine and carbs, very last is carb fiddling and reforming (if it's required(?)).
Nigel Atkins

Bruce
My thoughts are-
1--Yes it needs a hose away from the exhaust
2--Your heat shield is a tiddle small, it needs to be a bit wider/deeper to protect the float bowl from the exhaust----exhaust wrap is a waste of time it will reduce your power output---it's a known fact that an unwrapped exhaust pulls more horsepower than a wrapped one
3--The butterfly shaft mod is a personal choice thing, but will only give minimal advantage at full throttle only, so unless you are on absolute full throttle all the time it's basically going to do nothing

willy
William Revit

Wrapping an exhaust can also cause corrosion which you can't until its too late!!
Bob Beaumont

I route my overflow down by the wing.
Without the pipe, starting from cold took much longer, I assume the fuel evaporates without a pipe attached. With it, the engine starts first turn of the key. I was told not to have the end dangling in air flow, i.e. underneath the car, as this can affect air/fuel ratios when driving at speed.
I have not heard that exhaust wrap impairs performance. My car has been on several rolling roads and none of the operators (with many, many years of experience tuning all sorts of cars) have suggested I take the wrap off my exhaust manifold, but, yes it can trap moisture and cause corrosion.

Your research into power reductions with exhaust wrap would be most welcome, Willy.

I have always wondered about the tiny heat shield on the HIF44. The one you have is OEM and BL must have thought it 'did the job'. Perhaps wrap the float bowl in exhaust wrap?

Hope that helps.

Dave
Dave Brown

Dave, I don't have anything in writing but-
I can see if you have a car with high underbonnet temps then exhaust wrapping 'might' give a possible power increase by decreasing the intake temp--
My experience with exhaust wrap was on a 350 chev racer with 180 deg. headers and an open slightly muffled exhaust - After wrapping the headers it lost noticeable horsepower in the high rev range from 6500-7800(limit)
All the theories say that wrapping keeps the temp inside the exhaust and increases the pressue/velocity and increased scavenging by doing so
Not so in our case, exhaust temp was sky high further down the exhaust creating higher exhaust pressure (more backpressure) and therefore reduced scavenging and rich mixture,we had to cut nearly 3 feet off the exhaust system to reduce the pressure to get the power back to race ready level --
Therefore I suggest that without a tune to suit, if your car has a full exhaust system it will possibly have less power compared to an identical car without wrap, unless it was out of tune and the extra back pressure has corrected an incorrect mixture setting

That's all i have to offer Dave, hope that's been of interest for you
willy


William Revit

I didnt realise wrapping the exhuase would so contenious :)!

Thanks for the comments on the overlow hose, one will be fitted. Not very happy with the garage that fitted it and surely this would be the sensible thing at the time.

Has anyone got a picture of alternative (ir larger) heat sheild? This seems a sensible mofication.

Richard B, you asked about the engine breather being blocked off. When it was fitted this is how it came, and I didnt question it. I'd be intertesed to see a pciture of how yours is fitted?
The HiF was fitted when the engine was rebored to 1380. It has an oil breather in the rocker cover nothing under the pistons. Even with the current breather (into a oil resorvoir), the presuure forces oil through the oil filler cap (it had a small vent).

BruceBurrowes

The bling chrome oil filler caps have a small hole which don't help the engine breather as well as the original factory plastic vented and filtered cap. I'm not sure what difference it'd make on your engine if you have oil coming out of the cap and usually its air in from the cap.



Nigel Atkins

It's only air in through the cap on engines designed with an active extraction system. That's why the correct plastic cap has a filter. Without the active suction, whichever route it is applied to, then there will be a positive pressure at the oil filler cap, varying with engine revs.
GuyW

Yes my thought (not mentioned unfortunately because of rushing) was if the present system doesn't work it might be changed or altered or amended, as we know there as many variations applied to how the system is operates (or doesn't operate).
Nigel Atkins

Bill
Here is a shot of where my overflow ends up - into the triangular chassis hole,sorry about the poor pic quality.


richard b

Bill,

As Guy noted an active breather system - a ventilated oil filler cap and piping to carb, this is what mine has.Some incorporate a pcv as well.

Some people vent to catch tanks etc etc.

I run mine from the timing case filter unit to the carb and as I don't use the heated manifold facility on the Titan manifold, I have used the manifold pipe to plumb the hose across the engine enroute to the carb.

R.

Again sorry the pics are not the best !





richard b

many thanks for the pictures, sorry I wasnt specific in my request. Do you have a picture of where the tube ends up, so I can see it coming out the Titan manifold, its where this ends up on the timing case filter unit.

On the filler cap, I appreciate its meant to take are in, but before (I went back to the plastic) the inside of my bonnet was coated. I'm now concerned my engine is building too much pressure as it seems to have got a bit more leakly than usual on the rocker cover gasket.
BruceBurrowes

Bruce,

Is this what you were asking ?

The hose fits onto the outlet spigot of the cylinder attached to the timing cover (its filled with a mesh/wirewool stuff to restrict oil - only mist leaving the timing case - well thats the theory !) - that is standard for 1275's.


richard b

This thread was discussed between 04/07/2020 and 20/07/2020

MG Midget and Sprite Technical index

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