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L28 w/ SU Carb intake manifold vacuum leak

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A little late to the party, but here is my 2 pennies worth:

It can’t be your cam as it’s only affecting 3 cyls not all six. So IMHO it’s definitely fuelling / carb / intake related.

The only time my headers have been visibly glowing is when I’ve been heating the car up at night on choke at around 2-2.5k rpm with an AFR of around 12 (so read very rich). The camera does make it look 10x hotter than it was to the naked eye!!!


4k is high indeed for a leak but also do we know how advanced your idle timing is? Just advancing the distributor I’ve had the engine at 2k rpm, let alone adding in a leak.

Either way, I would focus on the carb serving the bank of 3 not glowing.

Good luck, I’m confident you will find something simple soon as the culprit!

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I'll get a picture of my ceramic coated ones maybe tomorrow night if it doesn't rain here. I'm going to get both cars out, weather permitting. New oil and valves adjusted. Now to wear the brake pads down and heat the tires up.

Big plans for Saturday, if no rain comes around.

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6 hours ago, AK260 said:

4k is high indeed for a leak but also do we know how advanced your idle timing is? Just advancing the distributor I’ve had the engine at 2k rpm, let alone adding in a leak.

I haven't retried finding TDC on my new damper, but you can kind of see what's going on here: https://youtu.be/5KwKMwHtFMI?t=45


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5 hours ago, heyitsrama said:

@Awk34 can you post pictures of the throttle shaft? Part number 13 and 14 in this diagram.


Here's some photos. As you can see I don't have any of the vacuum diaphragm / valve stuff on the balance tube, and all the barbs are blocked off with caps.




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Here's a video to help: 

It shows the front carb when the engine "idles" at 4000 / 3000 RPM. The piston is definitely shooting up, so there has to be a vacuum leak somewhere between the cylinder and the carb, right? I've attached photos of the front (0007) & rear (0010) butterflies.



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OK, MEGA dumb question - are you certain both carbs have the depression springs installed on top of their pistons?

Also that gap looks a little wide to me - could be just a trick of the camera. I will take a photo of mine later today for comparison. If memory serves, mine look almost completely shut.

A vac leak between carb and intake does not make the piston rise - in fact quite the opposite. Air flow THROUGH the carb does.

[mention=23570]siteunseen[/mention] - no tricks just the iPhone camera. The CCD being infrared sensitive picks up the hot manifold and makes it look way hotter. To the naked eye it was just a very slight dark red glow / barely visible but enough to make me want to take a photo!

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Well, I’m forever stunned at how little air / fuel a healthy engine needs to rev up without load. So not too sure chap. I will get you a photo of mine shortly.

The good news is, you can see one piston shooting up more than the other, so now you have something to work from.

Forgive me if I’m going over things already discussed / suggested, I’m just trying to go back to basics and eliminate possibilities in my head.

The simple fact is, at the most basic level an engine only needs timing, fuel and air to rev up.

So, we’ve determined your timing is OK, let’s park that.

As for fuel, I know you’ve checked fuel bowl levels and I’m assuming your jets aren’t set overly rich and are equal. So for now let’s also ignore them.

I am also assuming you don’t have a sticking piston and have done the “drop test”, so I’m making an assumption that your pistons move freely.

The only reason a piston rises is, sufficient airflow across the bridge that with a pressure differential overcomes the effects of gravity and the carb spring. So either you have one carb with little vacuum on the inlet side compared the other or not enough resistance against the piston. You have a balance tube - so at idle, theoretically, you have equal vacuum across the manifold side of both carbs which should raise both pistons equally. An inlet manifold vac leak would be shared across the carbs - do we agree so far?

Would you please do me 2 favours:

1. Test your spring rates - I did mine very simply using books and kitchen scales to prove they were pushing equally at the same height (67mm). Let’s prove you have equal resistance between your carbs so as to eliminate that.


2. While you have the domes off, check that the hole(s) in one of the pistons isn’t blocked (red markup). The one that “communicates” manifold vac to the piston. Also check the small carb inlet holes that “communicate” atmospheric pressure (yellow markup). These are the only mechanisms by which the piston is lifted. If blocked, or restricted, the rise will be affected. Give them a good squirt of carb / throttle body cleaner and a small brush / wire through the holes. Also ensure that the washer at the bottom of the spring isn’t blocking the piston holes when reassembled.


Once we prove the above:

Let’s try something a little different to prove you have or haven’t an air leak. I want to separate your carbs from each other.

I want you to block the balance tube with a home made rubber gasket at both ends such that we eliminate any possibility of any leaks through it and prevent the equalisation of any manifold side vac leaks between the carbs.

After that, you can disconnect the balance “bar” such that both carbs work independently of each other.

Now you can measure air flow with an airflow meter through each carb and adjust idle screws until the airflow is equal. This will very quickly show you a difference between the two. The one with a vac leak would have the larger opening.

Then we can start to work on that side of the manifold and investigate any false air sources.


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The plates look too open to me two.

You removed these from the shafts didn't you?

They only go in one way. Are you sure they're installed correctly?

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It took a while but I got to finally say hello to the car today, so here is mine for comparison


Bear in mind with my cam it has a high idle at 1k revs and 16 degrees timing.

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Posted (edited)

@AK260 One spring measured at 2.2oz, the other 2.1oz, so probably within margin-of-error. I confirmed that the holes in the pistons (two on each piston) are clear of debris.

Apologies if the context for the carb video was unclear. The footage of the carb running was all from the front carb. The footage with the borescope to the butterfly was of the rear carb.

By "drop test" if you mean I lifted the piston and heard it slowly creep back down to make a light *thunk* then yes.

Attached is a picture of the front carb's butterfly when it's off the car. It seems like it completely closes off. The second is when it's installed. It appears that the flats on the throttle linkage push the butterfly open slightly. Might I need to bend these back a bit? They do look slightly more open than yours.

I might be able to cut some flat rubber to fit between both spots where the balance bar meets up with the intake runners.

@Patcon I have not taken the butterflies apart. Is there a seal which might need to be replaced?



Edited by Awk34
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Good work there.

So it’s good that we’ve eliminated the possibilities of blocked holes etc. The drop test is exactly that and it should be a relative quick drop and thunk!

Your last borescope photo shows me it’s way too open for idling at sensible revs. If I put yours and mine side by side below, and we use the throttle shaft butterfly screw as our scaling datum in each photo, you will see yours is way more open than mine!!!


Now it’s interesting that putting them back on is causing them to open - you’re now starting to point to a potential linkage issue. Are you able to put the carbs on and back off all the linkage screws such that both carbs are independent of each other. And to be very clear - no choke cable connected either!

You then want to back off completely on the idle screws such that the butterflies are completely shut when on the car. Inspect visually.

Now, feeling the butterfly with fingers on one hand, you can turn the idle screw with the other until you feel it “just” start to move. Do the same on both sides and inspect visually to ensure they are equal and not more than a hair width open.

Now try to start the car and let’s see what happens!

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For clarification here's the rear carb: 


Yeah, I can see visually that there is WAY WAY too much airflow for idle right there. I think we’re starting to get somewhere. You definitely need to back off those idle screws a lot and or figure out why your butterfly is being held open.
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I was able to get it running at around 1200 RPM. Some attempts around there just killed the engine.

I shouldn't have to use the choke at all to get it started when it's this warm out (60-70℉), right? Or do I still need to choke it to get it up to temp?

I also wonder if some combo of my cam & distributor make it not want to idle lower. The engine sounds pretty good now. I have a Kameari distributor and the unknown PO's cam.

The trouble with tuning now is that since it still wants to idle so high, my flow meter is always off the charts.

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Fantastic progress, well done.

That butterfly position is FAR more like it. It’s been frustrating for you but you’ve learned a lot about your car at least, so next time you are better able to troubleshoot it.

There is nothing complex or magical about any of it - remember what I said about timing, fuel and air is what makes engines rev?

Apply that same principle again. The great news is that your flow meter is off the charts showing that you don’t have an air or vac leak down stream, making the rest of it easy. Unless you have a full race cam you absolutely should be able to get the idle down a bit. I can get mine to idle at 700 rpm but it would be rough and won’t be good for pulling off the line smoothly, hence it’s dialled up to 1000. I actually vary mine with timing as I’m happy with the mix.

If the idle is higher than normal you typically close the idle screws, if that makes it die then you can richen the mixture a “tab” or two at a time and try again. Or alternatively dial back a degree or two of “static” timing if the mix is good.

BUT .... you must bring the engine up to full operating temp before doing this. On a well adjusted carb without an over rich idle you will still need a little bit of extra fuel until at operating temp (notice how your modern car will still smell rich for the first few mins even on a warm day).

Simple way to adjust carbs is: on the left front side of each carb, you will find from the underside a “lifting pin”. This allows you to lift the piston up a little. If at idle you raise the pin by say 1-2mm and the engine dies, you are too lean.

If it speeds up and settles down (while you hold the pin in the same place) you’re good.

If the engine speeds up and stays high then you’re too rich.


Have a good read of this ...


In your case you have good flow, but the engine dies - to me that suggests too lean at idle.

But of course changing the mix at idle affects the mix elsewhere so it’s a balancing act.

Do you have an AFR gauge / UEGO? It makes tuning aooooo much easier!

Hang in there - you’re nearly there now!!

EDIT: one last thought - what idle timing do you have and is vac advance connected when you check?

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One very dumb last question, have you dropped the jets down a couple of turns since taking the photos of them? If not, that may well be the reason as to why it dies if you reduce butterfly opening further.

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@AK260 Yes, I turned them up all the way, then backed them off two turns like the tuning guide suggests. Here's some clips of my progress: https://youtu.be/oXMduC3bvT8

In one of the clips you can see me pull up on the rear carb's throttle with quite some force. It causes the RPMs to drop by a few hundred. The main throttle linkage isn't causing it to stay open by a bit. It seems like the butterfly itself has a little bit of room to move back closed.

I have a coolant sensor inline with my lower radiator hose. As I just realize, I probably should have put it on the upper radiator hose, as not it's reading coolant temp after it goes through the radiator. Silly me. What would full operating temp be? Around 200°F before the radiator, right?

I have an O2 sensor bung and wideband controller on the way. Once I get all that connected I'll have a better idea on AFR.

I tried to get my timing close to 10° but I haven't confirmed that the TDC mark on my new pulley is exact, so I'll check that when I can. I don't have a vac advance as I'm using the Kameari distributor which uses a transistor box (or whatever it's called).

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