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mdodds #879162 05/01/22 2:24 am
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Instead of saying that petrol starts to boil about 38 C, it would be better to say that about 1 or 2 percent of the petrol volume will vaporize at about that temperature with the rest remaining as liquid. ACEA has a specification for gasoline that requires that at 70 C no more than 20% to 45% by volume is allowed to evaporate in ambient areas above 15 C. At 100 C, ACEA's spec allows 50% to 65% to evaporate. At 180 C, 90% is allowed to evaporate.

Iso-octane, a component of petrol boils at 99.3 C. At 100 C its all gone.

Last edited by Bustednukel; 05/01/22 2:26 am. Reason: punctuation
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mdodds #879175 05/01/22 9:42 am
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There is some good information on this thread concerning the O ring and carb flange and it's made me think carefully about tightening the nuts etc.

I believe that the fuel charge should be as cool as possible when drawn into the intake tract?

Last edited by semprini; 05/01/22 9:43 am. Reason: spelling

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mdodds #879181 05/01/22 11:36 am
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Yep tighten carefully, it is much loser than feels "right".

as far as the coolness of the charge ......... if we are being accurate, the AIR should be as cool as possible (to be more dense) but the fuel should be as hot as possible to vaporize completely. Of course as it vaporizes it cools ............

My point is that maybe we are overthinking the whole carb temp thing and we should just stick to the thin Tufnol spacer BSA used, and accept that when starting after a long hot ride you MAY have to press the tickler to make sure the float bowl is full. Personally I never have the problem as I never turn the petcock off so float chamber is always full.

Dave Martin #879184 05/01/22 12:03 pm
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Originally Posted by Dave Martin
Yep tighten carefully, it is much loser than feels "right".

as far as the coolness of the charge ......... if we are being accurate, the AIR should be as cool as possible (to be more dense) but the fuel should be as hot as possible to vaporize completely. Of course as it vaporizes it cools ............

My point is that maybe we are overthinking the whole carb temp thing and we should just stick to the thin Tufnol spacer BSA used, and accept that when starting after a long hot ride you MAY have to press the tickler to make sure the float bowl is full. Personally I never have the problem as I never turn the petcock off so float chamber is always full.

If you’ve never had the problem then you’ll never know why you need the cure. On a baking hot day the bike won’t want to idle, starting it is the least of your worries. Even if the bowl is full the fuel will still boil.

I watched A video yesterday with a bike shop showing about fitting spacers or something, the video was incomplete. One thing I did notice was oif taps fitted to a pre oif a65, that plus the straight down spigots meant the fuel pipes ran directly next to the engine. The two taps used on the pre oif export tanks originally directed the fuel away from the head, so you weren’t pre-heating the fresh charge before it entered the bowl.

The 6mm spacer/insulator cured the problems for me (infact when I took the bike to Denmark a few years ago and the trip through Holland was way up in the 30’s (Celsius) it had a thicker insulator still. The bike gave me no problems. It was so hot that it was almost too hot to ride.

It’s the light ends of the fuel, the components in the fuel which aid starting and are crucial to slow running which evaporate when the fuel gets hot.

The fuel and air flow through the port is cool. The fuel itself atomises, it’s the effect of the air pressure below atmosphere through the port that draws it through the jet. It’s that vacuum and high volume of air which causes the fuel to atomise.


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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