L28 w/ SU Carb intake manifold vacuum leak

[AK260]

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.

[Awk34]

@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).

[Awk34]

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. 🤔

[AK260]

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).



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?

[AK260]

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

[Awk34]

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

 

[siteunseen]

Let us know if it works good. That's cheap to me and all the stress I went through to get true TDC. I'm no mechanic.

 

[Patcon]

says currently unavailable

 

[Awk34]

1 hour ago, Patcon said:

says currently unavailable

 

I guess I bought just in time. It's also out-of-stock on Jegs' website.

[Awk34]

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?

[Captain Obvious]

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?

[AK260]

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



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!