Carburettors

Carburetor Cross Section

Flow through a carburetor

The Stromberg carburettors are disliked by a lot of people, historically there were problems with the diaphrams but modern materials have solved this. The Strombergs are easier to tune and set up and are more fuel efficient, their air flow capacity is similar to the Webers on 38mm chokes. As they do not have an accelerator pump they can have a slightly slower throttle response, however with the right needle you can run them with thinner or no damper oil which will sharpen things up, I use automatic transmission fluid for mine. Strombergs are essentially a copy of the more familiar SU carburettors thus any tuning or setup guides for SU can be applied to Strombergs, Veloce Publishing has a great book on tweeking SU also see Ancillaries for ram pipes and air filters. There is a greater range of needles available for the SU range but this should not be a major problem as there are still many profiles available for the Strombergs. Burlen Fuel Systems still make both SU and Stromberg carburettors and are an excellent source for spare parts as well as new units. Any modifications you make, even just using a different air filter, will require adjustment to mixture. This may only be adjusting the mixture on the jet or needle or by changing to a different needle profile. My modified S4 is now running a B2Y profile.

Air Fuel Ratio

Mixture power vs economy

To ensure the carburettors are balanced (i.e. both drawing the same ammount of air) use a synchrometer, personally I have never been able to balanced carburetors by listening to the hiss via a length of hose. Check the balance at tickover and higher rpm. The only way to be sure that the carburettor mixture is set properly is to use a wide band lambda sensor to check the AFR (Air Fuel Ratio) across the rev range and under different load conditions. I use the LM-1 from Innovate Motorsports and I have the engine running between 12.3 and 13.1 AFR across all rpm and loads which I am pretty pleased with. The Lotus twin cam wants to be run a little rich and with a slightly hotter sparkplug than standard, I use NGK BPR6ES.

I am not sure that the Strombergs fitted to my Elan are of the original type as fitted by Lotus as they have fixed needles and adjustable jets. These give a greater range of adjustment of the mixture but do not take the later B2AR needle profile. I strongly recommend that new needles and jets are fitted if you are unsure of when this was last done, these items wear and will make it difficult to get a reliable mixture also check for wear on the throttle spindles.

Click here to download a spreadsheet of available needles. The spreadsheet also allows you to compare two different profiles.

The recommended needles are B2AR as fitted to the later European specification cars. Profiles close to this are B1DG B1DF