Modified SU carb needles for L28

[Stanley]

I don't get it either but last summer I tried richening stations 1 & 2 on the SM's thinking it would allow me to raise the mix nuts and have a good idle without going too rich at mid-range. Didn't work, it ran way too rich; don't know why.

[Captain Obvious]

4 hours ago, Stanley said:

why are SM's and similar British needles, listed as .100 which refers to the nozzles, all listed as 0.099 at station 1, while the N-27's are 0.095 (miked by Captain Obvious and verified by me and my Japanese micrometer ? I expect the other stock Hitachi needles, like N-54's and N-58 (US spec. emissions) are also .095 at station 1.

All of those needles are called ".100 needles" because they are "designed to be used with .100 nozzles." That's just the way they are classified.

There are ".090 needles", ".100 needles", and ".125 needles." That's just the family characteristic.

[Patcon]

11 hours ago, Stanley said:

Here's part of the needle profile book, just the .100 needles so is easy to print. It's an old book so there might be newer needles that aren't listed.

Cool. I guess the real problem with selecting needles is knowing what needle station is producing what AFR reading. I don't know how you know at what height the piston is in the dome at any given RPM?

[Mark Maras]

 Another one of those old SU articles talked about removing the dampers (screw top with the jiggly bits), inserting rods (pencils?) with 1/8" graduations into the piston stems, starting the engine and monitor the height of the pistons at any given RPM. The part that confuses me is, will the pistons go higher than normal with no dampener. The article didn't mention it but t seems like it would due to the fact that different wt. oils will change the mixture over the entire throttle range. A possible way around this, IMO, would be to mark the side of the piston that is visible with 1/8" graduated lines (Sharpie pen) and watch the piston as it moves up through the RPM range. The article also mentioned that if the rods are equal length, the carbs are balanced when the part of the rods are equal ht. Makes sense.

[Captain Obvious]

37 minutes ago, Mark Maras said:

The part that confuses me is, will the pistons go higher than normal with no dampener.

Maybe this will help... They wouldn't go any higher. They would just get there faster. The dampers don't control the height of the pistons, they just slow down the speed of travel.

Wood sticks temporarily in place of the dampers would give you a good idea of what's going on inside the carb, but I've got two issues with that technique. First, when you remove the caps, you create a small vacuum leak. The same vacuum that pulls the dampers up is now vented to atmosphere. Might be OK above idle, but I'd be worried about that leak at idle. And second... You won't really get much useful info with the car sitting still under no load conditions. What you really need to do is drive the car with sticks in place and see what the pistons do with a load on the engine.

Since the amount of piston rise is linked to engine output, with no load you won't ever get much piston rise. A transient when you blip the throttle yes, but no steady state synchronization info to be gleaned from that scenario.

[240260280]

1 hour ago, Patcon said:

Cool. I guess the real problem with selecting needles is knowing what needle station is producing what AFR reading. I don't know how you know at what height the piston is in the dome at any given RPM?

Camera or measure vacuum from nipple drilled into top of plastic plunger .....remember our discussion many moons ago of mapping vacuum in carb dome against load and rpm then replicating on a flow bench to "find station"?   We have been there and almost done that

 

https://www.youtube.com/watch?v=i2I5QKDDEDc

 

Another:

 

 

[Mark Maras]

 Nice. If the pistons or the intake throat were marked in 1/8" increments, one could accurately determine the the needle stage at any RPM under a load.

[240260280]

I think the vacuum method is important  as it can be replicated on a test bench.

One can wet-flow water safely on the bench with the reference needle and old carb then replace against the modified needle as it is worked to measure differences in fuel flow for each station(vacuum reference).  As mentioned, CO and I talked about these methods many years ago

Another method is to have non-tapered needles of different diameters then drive with these to map a/f against rpm and vacuum.  This will work nicely for optimizing the higher stations of idle and cruise. Again... this was from many moons ago. Note that each straight needle will suck in most load/rpms except for some sweet spots so the goal would be to find these sweet spot stations and assign the diameter of the test needle to these.

 

 

Edited December 14, 2017 by 240260280

[Stanley]

So yes, the math in post #1 was useless (except at station 1). The smaller the stock needle area at a certain station, compared to nozzle area, the more would need to be be removed from the modded needle to make 1/6 more gas surface area. The graph in post #4 shows this. Eventually the needles get very pointy and getting 1/6 more gas area than stock becomes impossible.

There's another problem with the math though. The larger engine makes more vacuum at a steady rpm, raising the piston and putting the needle at a richer station as discussed above. Maybe stock needles would work on a larger modified engine, except for acceleration.

 Bought a pair of stock SM's and a pair of ZH's (slightly leaner than SM's) today. Also have the modded SM's and N-27's. Don't know if I should stay with the 4 ounce red springs or move up to 8 ounce yellow. Nothing in between. Got to think about fuel economy. Yellow springs might make it a real guzzler.

 

 

Edited December 16, 2017 by Stanley

[jitenshakun]

Springs would only effect acceleration right?

What was the final consensus on the same needle for L24 vs L28?

[siteunseen]

The thickness of the oil also effects acceleration response time.

[Mark Maras]

My early 71 came with yellow springs. I think they all did. I'm not an expert on springs but logic tells me that a stiffer spring would raise the piston a bit at idle and richen the mixture unless one raised the nozzle to compensate. Thicker oil will increase the mixture during acceleration. It acts like an accelerator pump on a normal carb.