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Classic Engines, Modern Fuel

Comments on Topic: Carburettors with fixed jets

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Submitted by Dave

Hi - the marque of car I have has prewar Solex carbs (BSA - but these were widely used both pre and post war). Mixture is primarily controlled by a submerged jet under the float chamber so you have to drain the carb and get the spanners out to change the jet.

One of our recent club rallies ran through mid-Wales where E5 fuel was largely unavailable (small filling stations only have two pumps - diesel and petrol - E10). I note that apart from corrosive issues, E10 can cause enleanment and weak mixtures can lead to overheating etc. At the moment I suspect that levels of Ethanol in E10 fall far below 10% as our production level seem to have collapsed over the past 2 years, however this may become more serious if things change and levels do indeed reach 10%.

Is there any accessible information about how much enleanment occurs? You can still get sets of jets for the Solex carb but of you are going to counter possible enleanment, how much larger jet should you fit or will the bleed valve provided be enough to adjust it?

Any advice gratefully received



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Submitted by The Author

Hi -- sorry for taking so long to get back have been touring in my TC (about 2,000 miles).

Firstly, your suspicion about levels of ethanol is correct. E5 and E10 are MAXIMUM levels rather than actual percentages. Indeed one garage admitted they had put the E10 label on their pumps even though they had not received a new petrol delivery for some months! What was in their tanks was definitely less than 5% ethanol. Also I understand the refiners are finding it difficult to source ethanol at the moment. We had two batches of 95 octane fuel from the same filling station in Manchester for the tests. One had ethanol, the other had non. All this goes to prove that you do not really know.

While adjusting the SU carburettors is realtively easy, as you say, with SOLEX carburettors running on E10 becomes more of an issue. However, when touring in rural areas, I fill the tank with a super grade when I can (the TC has about a 12 gallon tank capacity). When the tank is down to 1/2 full, If I cannot get super grade, I usually top-up with just a couple of gallons. In that way the remaining 6 gallons "dilutes" the lower grade or E10 petrol.

Your second question about jet sizes as a little harder to answer. At Manchester with E10, we had to drop the jets by 1/3 to 2/3 of a turn of the jet adjusting nut, still running with the standard needle. However, I realise this is not very helpful for SOLEX cars. I will do some more research and publish some update figures in the near future.


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Submitted by The Author

Tuning Engines to run with E10

The terms Stoichiometric ratio and AFR are defined in the book, Classic Engines, Modern Fuel.

The Stoichiometric ratios are for:

  • Pure petrol: 14.7 : 1
  • E10 petrol: 14.08 : 1

That is you need less air inducted into the engine when running on E10 or 4.4% MORE petrol compared to an engine running on Pure petrol. The engine management systems in modern cars are able to adjust the mixture and, all else being equal, running on E10 will reduce their MPG or Km / L by 4.4%. On SU carburetors this can be achieved by dropping the jet by 1/2 to 1 turn of the adjusting nut or screw. It may be possible to adjust fixed jet carburetors sufficiently using the slow running bypass but that will depend on model and design.

For normal use the mixture is set slightly rich with AFR values of:

  • 0.85 - 0.9 for road use
  • 0.9 - 0.95 for maximum power

Hence for an engine running on pure petrol and tuned with an AFR of 0.85, the Air / Fuel mixture is 12.49 : 1. With this tuning the AFR when running on E10 is 0.89. (calculated from 12.49 / 14.08).

For an engine running on pure petrol and tuned with an AFR or 0.9, the Air / Fuel mixture is 13.23 : 1. With this tuning the AFR when running on E10 is 0.93.

Both these values are WEAK compared to running on pure petrol but not excessively so. Hence providing your engine is tuned on the rich side (AFR 0.85) then it should run OK on E10.

However, A WARNING for people trying to tune their engines to run on E10. Meters used to measure Air / Fuel ratio usually use a Stoichiometric ratio if 14.7 - for pure petrol. If you are tuning an engine to run on E10, this will give the wrong ratio. For example, when the engine is running on the Stoichiometric ratio for E10 (14.08) the meter will show an AFR of (14.08 / 14.7) or 0.957 NOT 1.0. Hence to achieve an actual AFR of 0.85, it is necessary to tune the engine so the measured AFR of 0.814.

This is calculated by: {Required AFR} * 14.08 / 14.7

Also it is worth remembering that bot E5 and E10 are MAXIMUM values of ethanol. This means you may fill up with petrol one day with NO ethanol in it, worth bearing in mind when retuning your engine.

Hope this helps.

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