66 Steve Rothwell answers more of your technical questions, ranging from a wonky Cresta to a slow Cortina clock.
GENERAL SU CARBURETTOR Q
Your series on various makes and models are brilliant – keep up the good work. Can we have an article on the SU HIF 44 carb? I have one on an MG Midget 1275 engine in a special that I have built which keeps on flooding. Although having played around with the old SU carbs a great deal on Imps and Minis, I do not understand the HIF carb and cannot stop the flooding. I am sure many would be pleased to see an article on them.
David Price
AThe HIF (horizontal integral float chamber) SU carburettor was designed with a view to reducing emissions, and it operates in much the same way as the earlier unit, although the jet operation and adjustment is slightly different. Instead of being beside the carb, the float chamber was incorporated into the main body with the float shaped around the main jet. The fuel route into the carburettor is still controlled by a needle valve operated by the float, which is lightly spring loaded. Flooding of this carburettor is quite a common occurrence, and can be caused by a couple of different conditions.
The first check is to ensure that the float level is correct. If this is set correctly and the valve is closed off when the float is at the top of its travel, then one common problem is that the needle valve has become contaminated with tiny particles of rust or dirt and is not sealing off correctly. Removing the valve and ensuring it is clean should eliminate this problem – it is always advisable to fit an online filter to classic vehicles which did not have the benefit of one when first produced. The fuel pressure should also be checked, as the carburettors were designed to run at a pressure of around 4psi, and it is not unknown for modern electric pumps to be fitted which can run at much higher pressures.
Another common problem which can occur is that an air lock forms in the float chamber, preventing the float from rising and closing the valve fully. This air lock can be caused by the vent in the float chamber becoming blocked. This may take a little investigation to discover, but when all the other points have been covered, then this is the most likely source of the HIF carburettor flooding.
In a follow-up to his initial enquiry, David wrote:
What has confused me is that the pipe into the carb which has the fuel inlet connected ends just inside the float chamber and does not connect to the needle valve on the other side of the float chamber. I do not know if this is right or if somebody has changed it.
To which Steve replied:
If the pipe ends inside the float chamber and does not go via the needle valve, this sounds as though the inlet has been put onto the breather. It’s not surprising that the carb would flood if there is no regulation into the float chamber. I think that you should check out the inlet going to the needle valve and connect the feed there, but if things have been altered it is going to be difficult to know why such a modification has been done.
The result? David wrote to tell us:
Just to let you know that Steve was spot on with his advice. I changed the vent pipe and fuel supply pipe around and the flooding has stopped.