How lean is too lean?

I’ve been having this issue for a while, and not yet been able to solve. I have O2 sensors in each downpipes, and 2x HD8s on a high compression 4.2. I have a custom air plenum and cold air intake the car gets more air than stock. The front carb is mixed perfect, about 13.5 at idle and anywhere from 12-13.5 under acceleration, with a UB needle. The issue is the rear carb goes considerably leaner under heavy throttle. I raised the float thinking maybe it’s running dry under load. That helped a bit. I also swapped out just the rear needle for a UE which also helped, now at idle and cruise the rear carb is also in the 13.5 range. But when I step on it the rear carb goes into the 14’s and even pushes low 15’s. I can control it with my foot, keeping the rear carb in the 13’s but obviously am losing power and can’t go all out when that carb runs into that range. I don’t want to blow a piston. Not sure what else to try? I already have 20w50 in the dampers so not sure I can go heavier there. Maybe a heavier spring? I know the air flow being not stock is an issue but it’s the only way to fit the engine in the MK2. Any more ideas?

If you have a spare needle why not try richening up the area where it is going lean?

Have you considered G force effect on the rear carb float bowl under acceleration,
just a thought. !
The only way to find out is to move the rear float chamber to the forward position. I am aware of the complications but as you have been troubled by this weak reading for some time TRY IT
Peter B

I follow this with great attention: I just wonder when the “good enough” occurs…:grimacing:

Have you tried swapping the o2 sensors and whatever gauge you are using to the other side to compare?
just a thought!

Have you tested that both springs have the same tension? Even if marked the same sometimes they can be different (wear or PO shenanigans). Easily checked with a small, kitchen digital scale and a modified toilet paper tube or cardboard - to keep the springs straight and give you a gauge when you compress them on the scale.

~ Mike

I have Des Hammil’s books on power tuning and on SU carbs… I will look at it again to see how he explains the process. You measure the piston rise to the rpm where you need to modify the needles?

I have no idea how to even attempt this, I would have to remove the starting carb and it looks like change the linkage around? Seems like a big job!

Not chasing perfection… just don’t want to be consistent too lean under load. Thus the question… is 14.5-15 too lean? Or is it safe?

I could do that but I doubt its the sensor as they’re both new and running digital gauges, but anything is possible.

I haven’t done this but it seems like a good next step. I was poking around on Burlen’s website but it wasn’t clear which spring was standard for HD8’s and which were stronger.

There is a soft cover book titled Tuning SU Carburetors that includes a chart detailing the standard springs and needles for various SU setups. When I converted my Series 2 xj6 to dual hd8’s many years ago it was running too lean at cruising speed. The chart indicated that the correct spring for the 2 carb setup was different from the standard spring installed in the 3 carb setup that I sourced the carbs from. Changing the springs solved the problem. If your problem occurs only in your rear carb you could try adding a bit of weight to that piston to simulate a stronger spring and then retest. A washer with a large centre hole would safely sit in the piston under the spring. If the situation improves then order a stronger spring.

Again: in a street scenario, the time the engine is under full accelerative load is short: 15:1 would be a little lean (14.7:1, being stoich) but not destructive.

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I will relook at my book, I have that one. I want to experiment with the washer, my question is, how much weight is the right amount? Is a bigger washer that fits around the damper fluid tube? or more of a smaller washer that sits in the side? I can obviously experiment just wondering how much weight makes a difference vs. too much.

Trying to follow the logic here. Lowering the piston (by adding weight to it) should make the mixture leaner as the tapered needle is now lower in the jet. I would think that adding weight would make your issue worse. Am I missing something here?
Pat H

On acceleration, keeping the needle further down in the jet enriches the mixture: thing is, all the pistons must see identical “spring” pressures to work properly, at cruise.

Large washer around the tube. I just drilled out a fender washer (big round washers from the hardware store) when I did it. Get a few thin ones and experiment adding one at a time while monitoring your o2 sensor response.

Think of it as the way damper oil is used to keep the mixture rich on acceleration by slowing the rise of the needle. Seems counterintuitive but it is the way it works. Probably something to do with different flow characteristics of fluid vs gas but that is above my pay grade.

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@Ray_Livingston posted, years ago, the best description of the physics on how this works.

I cannot find it, but perhaps he can lay hands on it quickly.

I would be interested to see that.