Rough running, petrol leaking onto inlet manifold.

[Mr. V]

1275 Midget engine HIF 6/44 carburettor (still haven't worked out which one it is).
The engine has been rough and jerky accelerating from low revs and smoothing out as the revs increase, becoming normal at cruising speed. The situation has been getting worse, the engine becoming increasingly difficult to start, so I took the car out to check the plugs.
Up a longish hill, switched off and coasted to a halt. All plugs normal. However, noticed fuel dripping from float chamber gasket onto hot manifold (not happened before). Oh dear! The four float chamber screws were tight and the overflow pipe was dry.
A mechanic from a nearby garage, that coincidentally had experience of carburettors, upon being told my story, grabbed a hammer, accompanied me to the car and told me to turn the ignition on. No clicking from the fuel pump. He tapped the float chamber a few times and then told me to start the engine. The carb, though weeping was now not dripping so I went on my way, having been advised to replace the gasket. On the short journey home I drove reasonably slowly and noticed that the engine was now picking up smoothly from low revs and behaving normally.

Can someone tell me what has been happening, given such apparently contradictory symptoms?

[svenedin]

I think the float or more specifically the float needle valve has been sticking. In such a situation the movement of the float upwards as the fuel pump is pumping gets stuck. Too much fuel is pumped into the float chamber (the fuel level is too high) and then fuel starts to leak out of the overflow. The float needle valve can also stick shut so there is too little or no fuel in the float bowl. I do note though that you say the overflow pipe was dry but presumably it leaked out of a bad gasket instead. The mechanic hit the float part of the carb which freed the float for the time being.

The solution is to remove the float bowl lid (make sure you have a new gasket), remove the float and the needle valve. Clean the valve and its seat. Replace the needle if worn and occasionally also the seat. Check the float is not leaking. If it is the float will have fuel in it.

The engine will run rough if the fuel level in the float chamber is much too high especially when the revs are low. When the engine is running fast it can presumably use enough fuel to compensate

It is very simple to check the float and needle valve with an HS type carburettor but less so with a HIF type. Since you have a HIF type it will have to come out of the car to check the float chamber because the float chamber is in the bottom of the carburettor rather than by the side as it is on an HS. The SU Carburettor Workshop Manual has excellent diagrams and instructions for all of the common SU carburettors and there are also some diagrams on the Burlen website.

If you suspect that your carburettor is well overdue a full overhaul rather than just a little bit of routine maintenance then there are various companies that will fully overhaul your carburettor to perfect running condition or you can buy a full service kit from Burlen and do it yourself.

I believe HIF6 and HIF44 are basically just the Imperial (1 and 6 eights of an inch=1.75") and Metric designations of the same size of inlet 1.75"=44.5mm. According to the Burlen Workshop Manual there are differences in these carbs over the years so it would be necessary to know the model according to the tag which should be on the carb somewhere

Stephen

[oliver90owner]

My first thought would be “Is it a proper gasket or a failing home-made version?”

[svenedin]

My first thought would be “Is it a proper gasket or a failing home-made version?”

Whatever type of gasket it is, it is probably quite elderly now! Since the bottom plate of the carb has to come off to fit a new gasket, might as well check the float and needle.

Stephen

[Mr. V]

Thanks for that. Time to strip, check and possibly rebuild that pesky carb.

[svenedin]

Various service kits are available from Burlen (www.sucarb.co.uk)

The cheapest is just a gasket kit. Then there is a service kit and the most comprehensive set of new parts is the rebuild kit.

Confusingly, there are multiple different rebuild kits offered. I would suggest trying to definitively identify your carburettor and phoning Burlen for advice before ordering parts.

Basic gasket kit is here:

https://sucarb.co.uk/carburettor-kits/g ... -pack.html


Diagrams are available here: https://sucarb.co.uk/technical/category ... rburetters

Or if you think it may be too much of an undertaking I can recommend Mr Glen Watson who did a beautiful job of rebuilding my HS2 for me. https://www.sucarbs.co.uk. As well as rebuilding carburettors for a living, Glen's hobby is watchmaking and his attention to detail really shows (my hobby is also watchmaking so I noticed!)

Stephen

[Mr. V]

Thanks for that. Very useful.

[Mr. V]

An update to the above: I replaced a worn float valve and seat, set the float height, fixed the leak and reset the mixture and am getting the same result. I've checked for wear and don't think the carb's in too bad a state.

I can't help thinking of the improvement the mechanic brought about by tapping the carb with his hammer. For those few miles there was no hesitation or stuttering at low revs/part throttle. I thought this was a result of the float valve being reseated. Obviously not. And why did the fault manifest itself over the course of not many miles in the first place?

The plugs look OK, by the way. Is it a timing thing? The distributor doesn't appear to have moved.

[geoberni]

I've checked for wear and don't think the carb's in too bad a state.

I'm no Carburettor expert by any means; however, I can see no correlation between the mechanic hitting the carburettor with a hammer, the problems described and the car working for a few miles.
Unless the Carb is actually in serious need of an overhaul, not only leaking from a faulty float, but having a sticking main piston and probably other parts worn...

[rocco]

Maybe I'm missing something here but the mechanic said the fuel pump wasn't clicking and then hit the carb. I was expecting him to hit the pump. Does it click periodically when the engine is running?

So if the needle valve is now clean, moving freely and there is no leak because you have fitted new gaskets but you have the same problem - does the fuel pump actually work?

Is the choke cable in good order, not sticking? If it isn't fully closing then an over-fueling issue?

[myoldjalopy]

It is possible that the leaking float chamber was not the cause of your running problems if they persist after fixing the leak. Unlikely a timing issue if you have not touched it but it could be an ignition issue - rotor arm, cap, condensor, coil connections etc. When was the dizzy last serviced?

[Mr. V]

I assumed when he tapped the carb he caused the float valve to reseal, but then the fuel pump (in this case working and, strangely, under the floor above the rear axle) would have been clicking beforehand, so it's a mystery to me!

I haven't investigated the timing because the distributor's electronic and I don't have a timing light. I'm assuming it's a carb issue, but then why did it occur in the space of a few miles after running satisfactorily?

Maybe I'll have to do some resistance testing and on the road trial and error timing investigation. Thanks for the help so far.

Any more advice/ideas?

[philipkearney]

Sound so like your car has been fitted with a 'pusher' type SU pump where the pump is mounted at the tank and pushes the full to the carb rather than the standard suction SU which sits in the engine bay. I doubt that will have any impact on the rough running but it's a curious modification when suction pumps are easily available. It worked fine on cars like the mini and the A40 anyway.

[geoberni]

I assumed when he tapped the carb he caused the float valve to reseal, but then the fuel pump (in this case working and, strangely, under the floor above the rear axle) would have been clicking beforehand, so it's a mystery to me!

Sound so like your car has been fitted with a 'pusher' type SU pump where the pump is mounted at the tank and pushes the full to the carb rather than the standard suction SU which sits in the engine bay. I doubt that will have any impact on the rough running but it's a curious modification when suction pumps are easily available. It worked fine on cars like the mini and the A40 anyway.

All well and good,..... so long as it's actually got an HP pump instead of an LP.
If a past owner doesn't understand the difference, they could have got the 'Bright idea' of rear mounting it and not changed the pump type.
We all know that some people can get strange ideas.....
Unless it's still got the Alloy Tag fitted to get the pump part number, the only way to confirm it is to dismantle it and see the size of spring that it fitted.

I'd be disconnecting the fuel line from the carb and seeing just how the pump is coping with pumping the fuel through....

Pump Differences.JPG (28.81 KiB) Viewed 602 times

[philipkearney]

Agreed Bernie. Not everyone knows that there is a difference between the pumps, so if it was fitted with a LP pump, it could be struggling to deliver sufficient fuel. Definitely worth starting at the beginning and checking this before moving to other areas.

[Mr Spigot]

"I believe HIF6 and HIF44 are basically just the Imperial (1 and 6 eights of an inch=1.75") and Metric designations of the same size of inlet 1.75"=44.5mm."

While basically true, there is a small difference which can cause problems. The HIF44 has (as the name suggests) an inlet diameter of 44mm. The HIF6 is 1.75", which is 44.45mm. That 0.45mm, while small, either results in a larger butterfly valve not fitting or closing, or a smaller one leaving a small air gap all the way round. Both will cause problems.
I found out the hard way, having bought what I thought was an HIF44 carb and then found that my new butterfly valve did not close properly. Having contacted Burlen and given them the casting number on the carb body, they confirmed that it was a HIF6 carb. As far as I know, they are otherwise identical.