SU float bowl lids

[siteunseen]

Anybody know of a reason I should NOT use two rear lids? 

I've read until my eyes are red. 

I want the front and rear to use the short valves like I bought from Ztherapy. Tired of fooling with them and was about to drill the longer fronts when I remembered my extra set of carbs. Switched everything around and I can't see any difference except they both have "R" stamped on them.

Please tell me if I'm overlooking something.

[siteunseen]

I kept reading the same advice, "make two as one".  So I made two rears.

[Stanley]

Front carb needs fuel level to be about 2 mm lower than rear carb to get equal fuel height at the nozzles, due to tilt of the engine, so they used longer valves. Otherwise the front float has to be bent oddly (P.O. did that to my car). which could make setting the floats even harder than it already is.  The float needs to be sitting just right in the bowl or it can hang up on the side. IIRC, the ZTherapy video mentions using washers if you don't have the long valve, haven't tried that though.

But if it works, it works.

[Stanley]

I seem to remember reading that the earlier SU's, the 4-screws, had equal length valves and ears on both carbs. If so, you'd just be going back to that.

[jonathanrussell]

@siteunseen I stopped trying to make the long ear / long needle valve float lids work in my 3 screw carbs. I use two short lids (borrowed from an extra set of carbs) so they end up working like the 4 screw carbs. I spent an enormous amount of time about two years ago really focusing on getting the floats set right. With the long ear lid, I was never able to get the float set so that the meniscus hit the top of the jet at 10 turns down. BTW, I don't pay any attention to where the fuel level sits in the bowl. For me, removing the domes and setting the levels at the jet tops (10 turns down) makes more sense and gives me more repeatable results- ie: both carbs end up adjusting in the 2-2.5 mixture screw turn range. I use a Color Tune also to help set the mixture screws.  

I am sure others have figured out how to make the long ear lids work but for me, it just feels like a design flaw. Essentially, with the long ear lids, I am never able to achieve the 10mm mark and the angle of the float is at such a severe angle that I believe the float is hitting the wall of the float chamber. 

[madkaw]

I drilled the long ear to match short - done


Sent from my iPhone using Classic Zcar Club mobile

[jonathanrussell]

@madkaw- I agree regarding drilling new holes and had thoughts of doing the same but managed to break one of my long lid ears. They are cast and very brittle.

 

@siteunseen I want to clarify my post above based on morning memory rather than what I was remembering late last night. I tried two things with the long ear lid. Z Therapy ships (at least as of a year or so ago) their 3 screw carbs with long ears but short needle valves. When I try to adjust using this configuration, the angle of the float is extreme and causes me at least to not be able to achieve the 10mm level. Not sure whether it is because the float is hitting the chamber wall or because the extreme angle causes the float to not engage with the modern (and more cheaply made) needle valves smoothly. When I look at old original needle valves vs new needle valves, the new ones move less smoothly, less precisely, have thinner needles, and just seem to always want to catch on the metal tab on the float. unless you carefully manage the angle of the tab. Alternatively, when I use the long ear lids with longer needle valves (not sourced from Z Therapy), I find that the floats start getting limited in terms of their ability to rotate because they hit the float chamber wall. My recollection is that the float chamber seems tapered a bit as you move from top to bottom. Since the longer ears push the float down a bit, the float ends up hitting the narrower portion of the chamber wall. I even tried shaving the shape of the float but was never able to achieve the 10mm level. Hope this clarifies and is helpful. 

Edited December 30, 2016 by jonathanrussell

[siteunseen]

Thank you all!  

I'm joining this crowd, and am DONE fooling with long ears versus short ears.  Now I'll set them in small glasses the size of the float chamber and get them at 23mm down.  Once that is set, I'll do 10 turns down and see where the fuel is.  This is the most talked about method for getting them dead on. 

[siteunseen]

3 minutes ago, jonathanrussell said:

@madkaw- I agree regarding drilling new holes and had thoughts of doing the same but managed to break one of my long lid ears. They are cast and very brittle.

 

I want to clarify my post above based on morning memory rather than what I was remembering late last night. I tried two things with the long ear lid. Z Therapy ships (at least as of a year or so ago) their 3 screw carbs with long ears but short needle valves. When I try to adjust using this configuration, the angle of the float is extreme and causes me at least to not be able to achieve the 10mm level. Not sure whether it is because the float is hitting the chamber wall or because the extreme angle causes the float to not engage with the modern (and more cheaply made) needle valves smoothly. When I look at old original needle valves vs new needle valves, the new ones move less smoothly, less precisely, have thinner needles, and just seem to always want to catch on the metal tab on the float. unless you carefully manage the angle of the tab. Alternatively, when I use the long ear lids with longer needle valves (not sourced from Z Therapy), I find that the floats start getting limited in terms of their ability to rotate because they hit the float chamber wall. My recollection is that the float chamber seems tapered a bit as you move from top to bottom. Since the longer ears push the float down a bit, the float ends up hitting the narrower portion of the chamber wall. I even tried shaving the shape of the float but was never able to achieve the 10mm level. Hope this clarifies and is helpful. 

That's exactly what mine was doing, hanging on the flimsy needle.  I had thought of trying to find a different float that wouldn't have to be bent to such an angle as mine was.  I got so frustrated a couple of months ago I put the OE Nissans back in.  Long valve in front, short in the rear.  Drove it until the cold weather finally got here and started back trying the ztherapy valves.  The rubber tip on the 42 year old Nissans were squashed out around the edge.

Thanks again, Cliff.

[240260280]

For CV carbs, only venturi vacuum from air flowing in the throat of the carb between the piston and bridge is what draws fuel. 

The height that the fuel has to be lifted from the liquid surface in the fuel bowl to the carb throat opening at the bridge is what must be approximately the same for both carbs.

How the fuel bowl maintains the correct level under all driving conditions is what the float and needle valve have to do as a team. Fuel pressure, variations in fuel pressure (pumping impulses), acceleration, braking, turning, bumps, and engine vibrations are the primary forces affecting how well the float and needle valve function to constantly maintain a constant fuel level.

If you can figure out why the float stand-offs are different in one carb for one year then I'll be glad to know. Engine tilt is moot as it is static. Dynamics of refilling under acceleration out of a turn when racing is my guess... a fuel bowl going empty is a bad thing.  It does not matter when braking as the fuel sloshes forward as there is no engine demand for fuel but it seems to be very important to adequately feed fuel when accelerating. (when the fuel sloshes backwards). The fact that the hinges on the fuel bowls are on different fore-aft sides seems to be why the stand-off heights were changed.  When accelerating, in one fuel bowl the fuel sloshes away from the hinge point  but in the other it sloshes towards it. This seems to be the reason for the attempt to mitigate by altering the stand-off height in one carb.

Edited December 30, 2016 by 240260280

[siteunseen]

On acceleration, briefly, I understand why they would do it and I never had a problem but pulling a big hill is a different story.

One beautiful sunny day I got about a fourth of the way up Alabama's highest mountain and the car started running on one carb.  I had to roll backwards and cut the wheel to level the car while the bowl filled back up to go further up the mountain.  I did this 4 times and knew what was happening so no big deal to me but the sweet young lady that was sitting next to me got worried then a little scared she was gonna be walking home.  Now if she was a little older, she would've known we could coast back down the mountain.  Trade-offs, at my age I'll do anything to get up that mountain. 

[240260280]

Interesting!  I read in a post from KATS they did hill testing in 1969 in San Francisco.  I guess it would be a side-hill-gouger modification

https://en.wikipedia.org/wiki/Sidehill_gouger