SU carbs will not sustain Heavy load

[MY1PATH]

❓Does anybody know what size the small hole in the float needle valve is supposed to be?
Maybe I got the wrong one? Or maybe folks on performance builds are running a larger size?
I've seen them go all the way up to 0.125" on other SU's for other engines (race builds)
 

12 hours ago, grannyknot said:

You could try a length of thin gauge aircraft cable, stiff enough to be able to push on but flexible enough to go around the corners, it may also clean out some of the accumulated crud.

I got it out. Much shimmying in each direction with compressed air helping each direction. Something came out of the vapor line, I just didn't see it but the string was able to slide freely and drag a mop through when I was done.

8 hours ago, 240260280 said:

Vapour line does little.  You seem to be running out of fuel in the bowls or have some sort of strange air leak into the manifold that increases with rpms.

Not an air leak, the problem does not come with RPM, it comes with sustained load and we've already proven the the carbs are running out of fuel when this happens.

[Captain Obvious]

I'm not sure there is a "standard" size for the needle valve hole. I wouldn't be surprised if different manufacturers might have different sized holes there. What's the hole size in yours? I've got a couple around here that I can compare when I get the chance. Are your valves original, or aftermarket?

And as for the vapor line potentially causing the problem, I doubt it, but it certainly wouldn't hurt to check. I was going to suggest a run with the gas cap off or loose, but I really don't see it. And even if there is a problem with the venting, it's not caused by the vapor line. The gas cap itself is supposed to have a check valve built into it such that it allows air into the tank to replace the volume of fuel removed.

If that valve in the gas cap is blocked and doesn't work, you could draw a vacuum on the tank and make it hard to pull fuel out. Although, I'm not sure you would see that stark of a difference between WOT and just cruising around normally. Nor would I expect it to be that fast acting or repeatable.

[MY1PATH]

1 hour ago, Captain Obvious said:

I'm not sure there is a "standard" size for the needle valve hole. I wouldn't be surprised if different manufacturers might have different sized holes there. What's the hole size in yours? I've got a couple around here that I can compare when I get the chance. Are your valves original, or aftermarket?

Mine are aftermarket. I can't remember where I got them from but it was a kit with lid gaskets, needle valves and hoses (I used Tygon hose instead). Tomorrow I will take them out and measure them (forgot to today).
From looking around I think S30's and other small HS carbs got .070" from Skinner Union but I might try to order some .096" ones...

Edited June 3, 2018 by MY1PATH

[Captain Obvious]

I will start digging around here to see what I have. Of course, I have no idea if what I have is factory or aftermarket. And I've also got a number of float valves from flat tops... I wonder if they are the same. I never looked into that.

I would be really surprised to find out that the difference going from a L24 to an L28 is enough of a change to push you over an edge like that though. I guess I've seen stranger things, but it would just be surprising. Makes me still wonder if there's something wrong with the delivery system somewhere upstream of the needle valves.

Finding needle valves with a larger through hole might fix the symptom, but still might not be reversing the root problem, just accommodating it. Although, if that's the case... does it really matter? Fixed is fixed?     :)

[240260280]

240z needle valve is 2.0mm (below).

I can not find data for 260z.

 

[MY1PATH]

 

2 hours ago, 240260280 said:

240z needle valve is 2.0mm (below).

I can not find data for 260z.

Thanks! This is exactly what I need, 2.0mm/0.0787"

3 hours ago, Captain Obvious said:

I will start digging around here to see what I have. Of course, I have no idea if what I have is factory or aftermarket. And I've also got a number of float valves from flat tops... I wonder if they are the same. I never looked into that.

I would be really surprised to find out that the difference going from a L24 to an L28 is enough of a change to push you over an edge like that though. I guess I've seen stranger things, but it would just be surprising. Makes me still wonder if there's something wrong with the delivery system somewhere upstream of the needle valves.

Finding needle valves with a larger through hole might fix the symptom, but still might not be reversing the root problem, just accommodating it. Although, if that's the case... does it really matter? Fixed is fixed?     :)

I totally forgot I had a set of Flat Tops... If the valves are similar I might try them or even try to ream them out without worry of ruining a good set. Better yet I wish I knew where my original valves were as well as the new spares (kits came with one long and one short for each carb)
To some extent it may matter... IF The problem is fixed stopped one way but if the root cause worsens does the problem come back?

Just for fun: High Flow 0.156" Needle Valve for SU H8 used on 3.4L Jaguar straight 6...
 

 

Edited June 3, 2018 by MY1PATH
numbers

[MY1PATH]

At a glance Flat Top valves are nothing close so I stopped right there.
Just measured my round top valves at #52 or 0.0635" (1.61mm, side is stamped "1.7" which is only .0025" difference)
That's over .0152" (0.39mm) too small. If I was within 0.010" I would not be so concerned but I would rather be bigger than smaller.
The more I think about it, this actually MAKES A LOT OF SENSE!!! L28 and carb rebuild happened at the same time... 
On the L26 I took the carbs right off Ebay, installed them and they ran ok so that was that... When I got ready to install the L28 I took the carbs off and rebuilt them, to include viton O-rings on the shafts (I think I copied someone Obvious) and of course the new Needle Valves. If I can find my old parts I will measure them too I think they were original.

The Viton Tip is just under 0.150" (not going quite that big) so if I find my spare set will will see if I can successfully ream and debur them without having them leak afterwards.
If I can't find spares new ones are going to be $30+ per carb and I'll have extra viton needles too, just don't lose them this time lol.

Edited June 3, 2018 by MY1PATH

[Stanley]

Still using stock N-27's ?

[MY1PATH]

9 minutes ago, Stanley said:

Still using stock N-27's ?

I actually don't know what needles I have, there are no visible markings but I suspect they are N54? I read somewhere that those are really Fat idle and lean up top.
I think the SU's came of 1972.
I have some damaged needles out of a flat top that I have heard are the opposite, I might try to clone those...

Edited June 3, 2018 by MY1PATH

[Captain Obvious]

I found one round top needle valve. I have no idea if it's factory or aftermarket, but it is the old style with the hardened steel seat and the regulating spring built into it.

It measures .079 through hole, so it's at least what the book says is correct.

I'm pretty sure I've got a couple others around here somewhere, but I don't know when they will turn up. I will keep an eye open for them though and let you know if something turns up.

[Captain Obvious]

Ooop... I got mail... 

[MY1PATH]

Sorry for the silence, I have opened up the flow buy reaming the needle valve seats to 0.0995" and adding side exit ports. This helped the problem but it did not completely eliminate it. (Power cuts out more softly and recovers quicker)
Solving this problem has slipped back down on the priority list a few notches but my next steps will be to try deadheading and then pull the fuel tank...
Got a lot on my plate right now and other things to catch up on but I will revisit this thread when I get back to troubleshooting.