L28 w/ SU Carb intake manifold vacuum leak

[Mark Maras]

There should be a very small gap (just enough air for idle) between the butterfly and the carb housing if the linkage is correct and the choke linkage isn't opening the butterfly a bit for cold start. The only way the pistons will lift is by the engine drawing air past the butterflies.

[Awk34]

Here's a bunch of photos I took last Fall when I did a bunch of engine work (head gasket, seals, etc)

[AK260]

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!

[heyitsrama]

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

[siteunseen]

Holy crap, that's amazing! No photo tricks, photoshop or anything? Just hot as hell and at night? Wow.

[siteunseen]

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.

[Awk34]

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

 

[Awk34]

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.

[Awk34]

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.

[Awk34]

@heyitsrama here's a couple more photos

[AK260]

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!

[Awk34]

 

@AK260 Yes there are springs on the pistons. Even the gap being a little too open like it might be shouldn't cause a 3000 - 4000 RPM idle, right?