General Chat

Air Filters

I first removed my Longflo air filters.  These looked OK but when I picked one off my bench I noticed black dust left behind.  The foam has started to become brittle, break apart and dented when pressed.  New foam has now been ordered from Amazon which I will cut to fit.

Carburetors

I have a 1962 TR4 in which a previous owner has installed a TR4 top end, so as a result have TR4A SU HS6 carbs, model AUD209 (R&F), which look very shiny, and I am fairly sure have been a new replacement set. 

On initial inspection all looked good, no visible wear although I could see a broken a fuel line to front jet (poor storage by me) and the other fuel line was missing the metal cover, so two new jets required. 

I examining float bowls which had brown crud inside, but looked ok as did the white plastic floats, but I will replace with the new improved black Nitrophyl ones which can’t crack and are easier to adjust. I was pleased to discover the float bowl needle valves had both been upgraded to Viton tipped ones so hopefully there will be no sticking needle valves in my future.

Needles

I removed the front piston and chamber assembly and all looked Ok, but the needle was loose, which was a worry.  On removal the needle did not match the diagram in the Moss catalogue.  I was expecting a single needle but actually found a needle, spring and collar (guide) which confused me greatly.

I brought out my magnifying loupe and eventually spotted TW stamped on the needle.  So I confirmed the needle is the standard setting, although it looks very different to photos on line. I searched in multiple websites and catalogues and wondered if I even have the correct carbs.  I placed a posting onto the TR4 Facebook group and got a reply within hours which suggested I have what are called Bias needles.  I then did some google searching but could not see the needles, spring or collars available anywhere, but I did learn more about bias needles.

Apparently sprung bias needles were introduced in the late 60s to help with emissions. The idea is that because the needle is sprung and can flex, it does not need centered like fixed needles, so easier to fit and gives a move consistent setup car to car.  It does mean the needle is lightly touching the jet so will wear it eventually, but feedback from the net is that it lasts a long time.   Apparently, all new carbs are supplied with bias needles now, hence why my newer set has them.  It also explained the needle movement I had assumed was due to a loose needle.

I called Joe Curto at British Superior Parts for advice on my setup and if I needed special jets for bias needles, but was advised std jets are fine.  He also advised that bias needles are difficult to obtain, so likely need to get them from the UK if required, but it is possible to purchase a ‘bias to fixed needle adaptor’ should you wish to convert to a more conventional setup.   He has these adaptors in stock.  I also found it on SU carbs website.  Part # M8050

Bias Needle installation

Joe advised installation can vary depending on the type of guide, but in my case, I have a metal guide with a groove in one side.  The spring is clipped to the end of the needle and is pushed into the hole in the piston and the guide is slid over the needle until it is flush with the face of the piston.  The securing screw is screwed into the groove in the guide, which sets the direction that the slightly angled needle will face.

I do highly recommend Joe Curto for your SU carburetor needs.

I am hoping this article will spread the knowledge of bias needles and speed up the learning curve for anyone else as easily confused as me.

Robin Thomson

1962 TR4

Useful Information

Joe Curto   British Superior | Vintage Carburetor Parts and Restoration | United States

SU Carbs Uk Burlen Ltd | SU Carburetters

Air filter foam ordered : Amazon.com: cyclingcolors Universal air filter foam 300x210mm car motorbike moped mofa lawnmower, 15mm : Automotive