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Classic Engines, Modern Fuel

Comments on Topic: A different engine

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Submitted by Ernie

Great book. However, somewhat SU centric. I am interested in Amal carbs as fitted to my motorcycle - Vincent 1000 V twin.

So with regard to CV, I wonder to what degree piston flutter in the SU may be contributing? Amal carbs also use jet/needle combination but the slide holds it’s position as set by the throttle.

On combustion I would like your opinion of the applicability of your conclusions to the Vincent. 500cc cylinders, same stroke but bigger bore. C/R about 7.5:1. Cross-flow hemispherical combustion chamber. Some swirl generated by offset inlet and exhaust ports. 32mm carbs. Lucas magneto ignition which works fine. Automatic Timing Device (ATD) also uses spring loaded bob weights. No advance curves available but we reckon it’s an on/off switch. Full advance is about 38 BTDC.

I was most interested in your conclusions about adjustments to ignition timing. Commonly held belief in our forums is to retard by 2 or 3 degrees to accommodate - wait for it - slower burning fuel! Oh and of course the story about ethanol is that it is the fruit of the devil and the ruination of all we hold dear! I exaggerate but your results are very refreshing.


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Submitted by The Author


Thanks for the comments about the book. Glad you found it interesting.

Carburetors: One advantage of using the SUs for the tests is that not only is it easy to adjust the mixture, it is possible to estimate the volume of air flowing through them by measuring the suction piston heights. However, to do that I had to remove the oil dampers these would have stopped any flutter. The oil dampers were fitted to SU carbs from about 1945.

As you said, with an AMAL carb you will not get any flutter. The question is: will they suffer in the same way as SUs? Basically, any back pressure in the inlet manifold will upset the air flow through the carbs. It was just that with the SU it appeared as flutter in the suction pistons. Have a look at this post it is about my son's Lotus Europa. This is fitted with two twin choke Weber carbs. Unlike the SU and AMAL carbs these do not use a needle to control the mixture, they have a fixed choke and an emulsion tube that bleeds air into the petrol to control the mixture as the throttle opens. There is no piston or indeed any mechanical component to flutter. When tested on the rolling road, this engine suffered enrichment in the same way as the XPAG with the SUs. Advancing the engine over this RPM range resolved the enrichment problem suggesting the Webers were suffering the same problems as the SUs. That is back pressure was affecting the mixture. Given this, I would expect AMALs to be affected in the same way, even though their piston will not flutter.

Slower burning fuel: I am afraid that even if the fuel did burned slower, retarding your ignition is the worst thing you could do. I needs to be advanced. However, as I say in the book, the fuel does NOT actually burn any slower than classic petrol. What is of real interest is that owners of your motorcycles, which have with cross-flow hemispherical heads a very different configuration than many classic cars, have the same "view" of modern petrol, i.e.. it burns more slowly, as classic car owners. This confirms my view that the problems described in my book affect all older carburetted engines.


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