L28 w/ SU Carb intake manifold vacuum leak

[AK260]

So chap, any news / progress? Intrigued to see what you’ve uncovered!

[Awk34]

On 4/25/2021 at 6:28 AM, AK260 said:

So chap, any news / progress? Intrigued to see what you’ve uncovered!

I had a mishap with my battery, so have been waiting for a new one to arrive by mail. I've mainly been working on some non-engine stuff with the car.

 

I'll also be picking up some welding gas today. I ordered some O2 sensor bungs on Ebay a few weeks ago, but they still aren't here, so I ordered some other ones on Amazon. I have a TinyWB which I'll use to hook up my Bosch LSU 4.9 to my Speeduino. Hopefully that will help me get some good diagnostics info.

[AK260]

There’s always “something” to be done on our cars!! :)
@jonbill has had great success with speeduino on his car! It seems like a great bit of kit.

I have to say, the AFR data will transform your tuning life and wreck your love love in equal measure! :p

[Alex240z]

Bit late to the party so I'm not quite sure where you are as of right now with high idling carbs. I just recently posted about my carburetor "run away" some weeks ago. Seeing your photos of the linkages they looked too depressed. Is your linkage allowing the butterflies to fully close when you completely back off the idle air screws located on the carbs? I had that issue where my linkage was too short and causing the butterflies to be too open and cause the engine to appear to run away. Also look for vacuum leaks. Spray starter fluid everywhere and I mean everywhere. Did you make sure your intake is clean where it meets the head and gasket? Is the gaskets between the carbs and manifold healthy? Do you have gaskets between the balance tube and manifold? Are you certain you covered all the holes in the balance tube? Check your PCV valve. Mine was leaking and caused a vacuum leak. Unfortunately I found way too many places where vacuum leaks can occur and in my case all these places had vacuum leaks. Just takes time and patiences to find them all out. 

 

EDIT: I just saw your video of the 4000 RPM idle YouTube video and I think it might be your linkage? My linkage connection from the bell crank throttle to the throttle piece that goes to the manifold connects "on top" not under. Here is a photo for reference. The bottom photo is yours. 

Edited April 28, 2021 by Alexg280z

[Alex240z]

It appears to me the connecting linkage that runs parallel to the firewall from the bell crank linkage to the longer one is too short. It's keeping the linkage from laying at rest. When I get the chance I'll send a photo of how I fixed it. 

[Alex240z]

[Awk34]

Quote

Is your linkage allowing the butterflies to fully close when you completely back off the idle air screws located on the carbs?

I improved it, but I have a feeling something with the linkage is still causing it to not close 100%.

Quote

Also look for vacuum leaks. Spray starter fluid everywhere and I mean everywhere.

Tried lots of that, found nothing.

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Is the gaskets between the carbs and manifold healthy? Do you have gaskets between the balance tube and manifold?

Yes, new gaskets in all those places.

Quote

Are you certain you covered all the holes in the balance tube?

Yes.

Quote

Check your PCV valve. Mine was leaking and caused a vacuum leak.

I ended up putting a plug with some teflon tape where the PCV valve goes.

Quote

Unfortunately I found way too many places where vacuum leaks can occur and in my case all these places had vacuum leaks.

I've replaced pretty much every rubber thing on there, and have sprayed every spot with carb cleaner while running, and haven't found any leaks.

Quote

EDIT: I just saw your video of the 4000 RPM idle YouTube video and I think it might be your linkage? My linkage connection from the bell crank throttle to the throttle piece that goes to the manifold connects "on top" not under. Here is a photo for reference. The bottom photo is yours. 

That was one of the issues. I've fixed that.

Quote

It appears to me the connecting linkage that runs parallel to the firewall from the bell crank linkage to the longer one is too short. It's keeping the linkage from laying at rest. When I get the chance I'll send a photo of how I fixed it. 

Thanks for the photo. I'll got check mine tomorrow.

[Alex240z]

45 minutes ago, Awk34 said:

I improved it, but I have a feeling something with the linkage is still causing it to not close 100%.

Tried lots of that, found nothing.

Yes, new gaskets in all those places.

Yes.

I ended up putting a plug with some teflon tape where the PCV valve goes.

I've replaced pretty much every rubber thing on there, and have sprayed every spot with carb cleaner while running, and haven't found any leaks.

That was one of the issues. I've fixed that.

Thanks for the photo. I'll got check mine tomorrow.

When you do check look into the carburetor. Push the piston up and use your phone to take a photo of the side or a video. Send that photo to us so we can see how the open the butterfly is. 

[Awk34]

I got my new battery so the car runs again 🙂. It was idling at around 1200. I'll make sure to get another picture of the butterflies tomorrow.

I welded on an O2 bung. Now I'm waiting for paint to dry; then I'll try to wire up the sensor to my ECU.

[Awk34]

@alex240z Here's a couple pics of the butterflies I just took. It was idling around 1200 - 1400 earlier today.

[Awk34]

Got my exhaust pipe painted and exhaust reinstalled. Wired up my new TinyWB wideband controller to my Speeduino, and it was showing I was actually running pretty lean. I'm assuming I want to be around 13 - 15 AFR. Ignore every gauge other than coolant, AFR, & MAP in TunerStudio. The other ones aren't hooked up.

I've also included a couple more photos of my throttle linkage for clarification.

[AK260]

Excellent! Well done you - now you are flying with instruments!

18.9 is obscenely lean! Ideally I would aim closer to 14.7 for idle if possible as a 16 idle is a hot one.

It explains why lifting the carb piston would make it stall as there would be too much air for the fuel going in.

Rule of thumb:

12-13 for power at WOT (racers like it as low as 11.5 for cooling effects)

14-15 idle

15-17 cruise

17+ on the overrun (foot off the throttle going downhill at speed, with lovely popcorn making noises in your exhaust system!)

Obviously every engine set up will be slightly different around these numbers but they will all be within the quoted ranges.


Here’s a table to help —




Your other limiting factor is that unlike EFi, your carbs will always have a compromise somewhere. For example, on mine I have a richer idle than I like (mid 13s) but it’s perfect for cruise / wot. That is, until I get my own design custom needles CNC-ed of course! I have a needle profile that I came up with to match my engine’s characteristics - based on HOURS of experimentation and AFR data analysis. I’m telling’ ya’ brah, swear to gwad, you just ruined your life hooking one of those up!!!!!!