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Datsun 240z with L28 & 3x 40 DCOE, MJLJ w/ TPS
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I'm still looking for the lean spot root cause.

I've been able to go WOT for many seconds in 4th gear whereas lean spot can be observed in 2nd gear. Fuel supply is not the root cause.

I've played also with timing, it hasn't changed anything so timing is also out of the way.

It remains carb tuning & choke too small.
First trials with carb tuning led to no improvement or very marginal. So I'm leading toward intake restriction, I need bigger chokes.

In the mean time, I've played with timing and I've made some progress today at low load.
Since timing backup at WOT did not change anything regarding performance (at least feeling wise), I've also kept 33 max timing (versus 35 before)
No knock to report with this map on my setup.






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Mathieu
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Correct me if I'm wrong, but chokes in a DCOE are inserts with a specific inner diameter that are placed between the air intake side of the carb and the butterflies, right? If you replace these with others with a larger inner diameter, doesn't that lean out the mixture even more if you leave the jetting unaltered?

Trying to understand the graph: the engine runs at almost 5000 rpm starting with afr around 14, then suddenly afr drops down and recovers, leans out for some more time and turns rich. Is my assumption correct? When the recording was made, was there any change in the throttle position?

Alex

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Hello Alex,

You're right, Chokes are inserts I can switch. Size range would depend on DCOE size (40 mm in my case)
You're also right regarding the mixture becoming lean when going bigger on chokes without altering other jets.

I'm my particular case, I've got tons of jets so I can play with them to make the AFR curve as good as possible. Problem is I don't have bigger chokes available right now. A friend of mine got some 35mm I should try instead of 32mm but we haven't been able to meet yet.

So as soon as I can switch my chokes to 35mm, I would have to go bigger also with main jets, maybe other jets would have to be changed also, I've got other air, idle, accel jets available.

Regarding AFR curve, I usually start at 2000rpm in 2nd or 3rd gear. I then slam the throttle to go WOT. As a result, carbs thru accel circuit richen the mixture (rich spot you can see at 2s on AFR chart), then accel circuit stops supplying extra mixture to the engine, hence the 14:1 afr between 2.5 & 3.5s where I'm most likely only using progression circuit, then engine revs up freely with main circuit after 3.5s.
The whole time, I'm at WOT (100% TPS), there's a bog to be felt at the very beginning. During normal driving conditions, where you go WOT in revs or in 1st gear, car is very smooth with no issue at all (except at high revs where there's no power)


Since the last AFR chart, I went back to 50f9 (richer jet than chart with 45f9) and I increase main jet from 125 to 130. Car smells definitely richer (raw gas) but pulls further up in the rpm range (not sure about my English here... Confused )


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Mathieu
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Hi,

there is a very interesting group on yahoo that deals with tuning the DCOE....

Have a look...

http://autos.groups.yahoo.com/group/sidedraft_central/

There is a lot of useful information. Unfortunately a lot to read through since Yahoo Groups is not very user friendly...

But people are very friendly there, so you can ask questions.

Klaus

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Hi Klaus,
Thanks for the tip Wink
I'm already following this group. There's some good knowledgeable people for sure in it.
I'm trying not to bug them as long as I haven't tried my solutions first.


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Mathieu
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