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L28 w/ SU Carb intake manifold vacuum leak


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Okay, the issue with the rear butterfly was that the choke cable was mounted a little high.

I tried using the lift pin method you mentioned, but each time i tried, the engine lowered in RPM and almost died, which would indicate 'lean', but I cranked the mix screw all the way to 'rich' and it still stumbled and died. 🤔

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Right then chap, let’s take it one at a time

Operating temp: basically you need it to be at a temp where combustion happens efficiently. I speak Celsius and don’t fully comprende F. 200F I think is 93c and that is fine. As long at you are over 170F (70ish c) and below 95c you are in the range. And I’m talking water temp. Also remember that water temp is not the same as oil temp. Typically the oil takes another 5 or so mins to come up to temp.

There is in fact a raging debate on some sites about which end of the rad the sensor should be. Personally I like it at the hot end as I want to know what is coming out of the engine without taking any cooling effect into account. But if I could have both, then that would be fab! Then again, I’m a data junkie ;). A lot of modern cars measure both.

As mentioned and probably lost in my very verbose replies before - disconnect the choke cables when doing any of this stuff. Also wind back the “fast idle screw” so that you can see a gap and that it has no effect at idle. See markup below (not my car).

6b0926e0e4f88f47cfb38c8cfbd883a0.jpg

Or better still just disconnect the accelerator ball link while you are doing this job (the bent one in the photo above). We need to remove from the equation the things that could be interfering and focus on the carbs themselves.

The lift pin test is a subtle thing so it takes a little practice to get used to it. You only need to lift a tiny amount. Too rich will also stall your engine. So for now, assuming your carb needles are stock N27 or SM, just take the mix screws down 2 turns. That should be rich enough but not too rich. The AFR gauge will help a lot. Then start closing down the idle screws equally on each carb - very gently - until the revs come down to an acceptable idle. The butterfly adjustment screws only need to move a tiny amount for a large effect on idle speed.

Crank pulley marks can be off - how are you planning to determine TDC?






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And one last thought: that kameari distributor is supposed to be set at 35-38 degrees at 4k rpm!!!! Which is a lot of timing! I know people using it typically gun for 32 - 34 @4k to keep it safe.

So if you want to check your timing, check it / set it at 4k and then see what it drops to at idle.

http://www.kameariengineworks.co.jp/L6-racingdyisubihaisen-2.pdf

9edee6156b753e3c25cb005c5a23635b.png


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Also to be clear, all this testing I've done is with the fast idle screw completely backed off, and with both carb's idle screws completely backed off.

When you say the choke cables should be removed when testing, that is just to be extra sure they're not adding any richness, right?

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3 hours ago, Awk34 said:

I got one of these to get TDC without taking the head off: https://www.amazon.com/gp/product/B078WJN4YP

That would work great for a DOHC engine where the spark plug hole comes straight down from the top of the piston. Not sure how well it would work with the plug hole coming in on an angle like the Z. I'm picturing the shaft bending if the tip doesn't slide easy on the piston top.

All comes down to vectors and friction.

What you really need is a pivoting arm. Does such a device exist?

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I use one of these ....

https://www.ebay.co.uk/itm/Top-Dead-Center-TDC-tool-Timing-Gauge-14-mm-thread-high-quality-in-best-price-/201984173362?_trksid=p2349624.m46890.l49286

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For a stock engine with dished pistons and no head shave you may have to make a new longer rod for the end of it - I can’t be sure but good to be aware of it.


Not sure about your carbs but my choke cables not only move the jets down but they have a slight effect on the butterflies. You seem to be experiencing a lot of strange issues, so the idea is to remove anything that is potentially contributing, until you get to normal idle! Then connecting things back to how they should be “one at a time” and testing at each stage. That way if the revs rise when you put back, say the choke cables, you know it’s them to blame / sort out.

With these things it’s often just the simplest thing that you eventually discover, keep at it, you will find the culprit!

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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.

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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

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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. 

e773cef7ab21c28e7b62955834f6077f.jpg

Screen Shot 2021-04-27 at 6.28.41 PM.png

Edited by Alexg280z
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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. 

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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.

Quote

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.

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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. 

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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.

PXL_20210504_020157626.jpg

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

IMG_0001 (2).JPG

IMG_0002 (2).JPG

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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.

PXL_20210506_004606744.jpg

PXL_20210506_004600276.jpg

PXL_20210504_173633796.jpg

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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 —

f136308b6dfac2212aa587f64697eb14.jpg


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!!!!!!

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On 5/5/2021 at 7:57 PM, Awk34 said:

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

IMG_0001 (2).JPG

IMG_0002 (2).JPG

Im curious how it idles when you remove the center link. 

image.png 

 

It's going to be that bent one in the center. Remove that piece and screw in the idle air control screws on both carbs one and a half turns from when they start to affect the butterfly's position. The linkage will visibly move. It'll rev at like 2000 so don't be worried. But see if you can bring it down to 800-700 and then use a carbsync to sync the airflow. If you can it means the linkages are messed up. If not then I'm not sure. 

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