Float level advice, please.

[Mark Maras]

 True, unless your Merlin float chamber was upgraded to a "Miss Tilly's Orifice. 

Edited August 13, 2016 by Mark Maras

[siteunseen]

3 hours ago, Blue said:

If you are in a Hurricane chasing an ME-109E, you can't push negative G's to give chase because the floats lift and shut off fuel.

 

19 minutes ago, Mark Maras said:

 True, unless your Merlin float chamber was upgraded to a "Miss Tilly's Orifice. 

History lesson here?  It is for me. 

Also a spelling lesson. One T in the US and Canada, two TTs in Europe.    carburetor carburettor

According to this it was fuel starvation at first then overly rich causing them to flood out. 

The early versions of the Rolls-Royce Merlin engine came equipped with a SU carburettor. When these aeroplanes performed a negative G force manoeuvre (pitching the nose hard down), fuel was forced upwards to the top of the float chamber of the carburettor rather than into the engine, leading to loss of power. If the negative G continued, the fuel would collect in the top of the float chamber, forcing the float to the floor of the chamber. This would in turn open the needle valve to maximum, flooding the carburettor with fuel and drowning the supercharger with an over-rich mixture. This would lead to a rich mixture cut-out, which would shut down the engine completely, a serious drawback in combat.^[1]

Negative G commonly occurs when manoeuvring to fire on an enemy aircraft in a dogfight. Moving the stick forward would starve the engine of fuel, producing a sudden loss of power. This would let the enemy get away, and if continued the manoeuvre would cause the carburettor valve to open, provide far too rich a mixture and stop the engine. During the Battles of France and Britain, the German fighter aeroplanes had fuel injected engines and therefore did not suffer from this problem as the fuel injection pumps kept the fuel at a constant pressure whatever manoeuvres were made. The German pilots could exploit this by pitching steeply forward while pushing the throttle wide open, the pursuing British aircraft being left flat footed since trying to emulate the manoeuvre would result in loss of power, or fuel flooding and engine shutdown. The British countermeasure, a half roll so the aircraft would only be subjected to positive G as they followed German aircraft into a dive, could take enough time to let the enemy escape.

[Mark Maras]

4 hours ago, Blue said:

 If you are in a Hurricane chasing an ME-109E, you can't push the negative Gs to give chase because the floats lift and shut off the fuel.

   Could Cliff be correct? Oh Boy, More discussion. As I understand it, the Miss Shilling's (Tilly's) Orifice was a restrictor orifice in the fuel system that restricted the total amount of fuel the carb. could deliver to match the maximum amount the engine could use and no more. Thus preventing flooding at negative Gs. At least that was the way my Dad explained it to me. WW II, Army Air Corps, Biak Island, South Pacific. The pilots, back then, would roll the plane over when going into a dive to temporarily counter the effects of the negative Gs. Although they couldn't sustain the dive. You see WW II fighters doing that rollover-dive maneuver in the movies and documentaries.

 

 

[siteunseen]

More discussion equals more thoughts.  The brain, use it or lose it.  That's why I love this forum, you gotta think a little. With the exception of our "tell me what I want" members.

[Stanley]

Besides, the rollover dive looks way cool.

[jonathanrussell]

Attached is a scan of TS73-10.

 

TechnicalSupportBulletin-TS73-10.PDF

[Stanley]

Thanks, good researching. My numbers might be a little off, used harbor freight angle meter and rounded off, so Nissan probably more correct.

[jalexquijano]

On ‎08‎/‎14‎/‎2016 at 6:26 AM, jonathanrussell said:

Attached is a scan of TS73-10.

 

TechnicalSupportBulletin-TS73-10.PDF

Where do you got this?? I  have been looking for this measures for a long time. I tried getting the gap with a 9/16 inch drill bit and a digital caliper and have not been successful in the contrary to others.  My floats are still too lean. I think everyone has a different method.

[jonathanrussell]

Others can fill in the gaps if I get details wrong but every year Datsun / Nissan published a Technical Service Bulletin book for dealership service departments. Each book contains multiple bulletins or instructions on various topics where the factory service manual needed correction or elaboration or explanation based on new knowledge once models were out and being driven. Each yearly book covers all Datsun / Nissan models....in other words is not exclusive to the zcar models. Each book may contain bulletins pertaining to current or recent prior model years. So, the 1974 TSB book may have bulletins that pertain to 240z cars. You can find the TSB books from time to time on Ebay. I have collected pretty much all of them through about 1984. I find them to be really informative. The TSB I cited above is from the 1973 book. For 240z owners, if I were to only own one TSB book it would be the 1973 book. 

Here is an example of a TSB book on ebay.

1978 TSB book 

Regarding adjusting floats....personally I think it is the most important and tedious step in getting the carbs working right. Bending the little tabs one way and then the other in an effort to get the float set just right is insanely frustrating but worth the effort. My personal preference for setting the floats is to remove the bowls, turn down the mixture screws 10 turns, and set the fuel level so that the convex meniscus fuel bubble sits perfectly even with the top of the nozzle. Once you get this accomplished, in my experience, you are able to freely adjust the mixture down to 2.5 turns and then detail adjust using Color Tune or similar. This technique has been documented multiple times on this site and others, fyi.

 

Edited February 24, 2018 by jonathanrussell

[Captain Obvious]

The 1972 round tops (3-screw) used different float needle valves in the front an rear carbs. Does anyone have the length measurements for the original long and short 72 needle valves?

Here's a pic for (what I believe are) the earlier 70-71 round tops. The length of the valve (just the valve without the sealing washer) is 18.5mm (0.729 in😞

After all that I've read, it is my belief that the two 72 valves should be on either side of the valve measured above. In other words, I'm thinking the front valve should be longer than 18.5mm and the rear should be shorter than 18.5mm. But without having the parts here, that's speculation.

So anyone have some original 72 long and short ear valves handy they could measure. Or anyone have any old ones they replaced before that they would send me to poke around with?

[Terrapin Z]

I need to get some carb parts out for another guy, while I have the carb box out, I will look at the 3 screw valves and see what's in there. 

[Terrapin Z]

Short one in the front carb, tall one in the rear. 

I measured just the hex portion without the sealing washer.  (Is your measurement up on the round part near the top of the valve?)