Running super lean off idle

[RS02]

I've been battling with my 71 240 recently to get it running right. I spent about 4 years working on the car and the engine, and this is the first time I got it to run somewhat right.

Here's what I have:

freshly rebuilt L28 Flat top engine with E31 head 

CB performance programmable box triggering a MSD unit

230/460 cam (IIRC)

"Rebello modified" carbs that I purchased from a form member a while ago. I think they are bored out to be bigger and I matched all intake ports.

Zstory street header to 2.5" pipes to Zstory muffler in the back

Now, here's the problem:

I have installed a O2 sensor after the final merge in the header. The carbs had got a set of unknown needles that are ground down. The previous owner of the carbs told me that they were for a 3.1 stroker. When I get the car started and set the idle at 900rpm with the mixture screw 1.5 turns down, the car runs rich at around 12.5:1 afr. When I tried to drive it on the street and give it more than 1/4 throttle, the car stumbles badly and wants to die. 

This would indicate that the carbs are running waaaay too rich right? But, my afr gauge tells me otherwise. It keeps climbing all the way to 15. I can hear exhaust backfiring at this point. 

I thought that the ground down needles would make the engine run super rich off idle, but I think the engine is doing the exact opposite. So I started diagnosing:

1. I checked the timing, it's at 14 idle 34 total.

2. I checked the plugs, they are dry fouled because of the idle mixture (brand new plugs before the run).

3. I checked the carbs, they are clean inside with piston return springs and 20wt oil.

4. I checked the fuel bow with float-sync, both level are correct and remain steady when I rev the engine.

5. I checked for  vacuum leaks, all vacuum ports are blocked off, only one is plumbed for MAP sensor.

I am at a loss here. I have searched the internet and this very helpful form for information regarding this problem. But I haven't yet found a solution to the problem I am experiencing. How can a needle that is supposed to be super rich cause a engine that is too small for it to lean out? 

(Attached is the timing map)

[Zed Head]

30 minutes ago, RS02 said:

"Rebello modified" carbs that I purchased from a form member a while ago.

This would indicate that the carbs are running waaaay too rich right? But, my afr gauge tells me otherwise. It keeps climbing all the way to 15. I can hear exhaust backfiring at this point. 

3. I checked the carbs, they are clean inside with piston return springs and 20wt oil.

How can a needle that is supposed to be super rich cause a engine that is too small for it to lean out? 

I would let go of the assumption about the needles.  Can you raise the needle to richen the mixture?  It's obviously lean.

There's a lot in the news these days about trusting the facts.  Your facts say that you need to make things richer.  No matter what you thought was supposed to happen.

[RS02]

22 hours ago, Zed Head said:

I would let go of the assumption about the needles.  Can you raise the needle to richen the mixture?  It's obviously lean.

There's a lot in the news these days about trusting the facts.  Your facts say that you need to make things richer.  No matter what you thought was supposed to happen.

Do you mean raise the needle by the set screw? That would mean that the idle mixture would be even richer than before right? 

[Zed Head]

I'm not sure how idle mixture is set.  Usually there's a separate circuit for that.  That's the carb tuning game, balancing the various fuel circuits.  But at 1/4 throttle you're probably "on the needle", so raising the needle will let more fuel get pulled though the main jet.

I'm just winging it from past carb memory.  But I know that there's a lot of back and forth and trial and error involved.  Good luck.

[RS02]

4 hours ago, Zed Head said:

I'm not sure how idle mixture is set.  Usually there's a separate circuit for that.  That's the carb tuning game, balancing the various fuel circuits.  But at 1/4 throttle you're probably "on the needle", so raising the needle will let more fuel get pulled though the main jet.

I'm just winging it from past carb memory.  But I know that there's a lot of back and forth and trial and error involved.  Good luck.

Well thanks for the reply. SU carbs only have 1 circuit though. It does everything including idle.

[jonbill]

I wouldn't really expect stumbling and backfiring at AFR 15. I'd expect more like 17 /18 for that.
It could be that your carbs aren't balanced, so you've got one at 17/18 and one at 12/13 averaging out on the o2 sensor around 15. The imbalance could be throttle opening or needle lift.
check and adjust so that they both flow the same air at idle and 2000 and 3000 rpm.
then check that the needles are being lifted equally at all 3 points.

[Zed Head]

17 hours ago, RS02 said:

Well thanks for the reply. SU carbs only have 1 circuit though. It does everything including idle.

Ooops.

 

 

Edited May 15, 2020 by Zed Head

[Mark Maras]

 IMO, the engine stumbling under acceleration indicates a lean condition. I'd start with the basics. Fuel pressure and volume test. Check the float levels. Balance the carbs. New needles. If you really want to learn the finer details about SUs, The "Just SUs" DVD from ZTherapy is an excellent source but any early Z service manual has the the carb tuning procedure.

[siteunseen]

I need dramamine to watch the dvd but it is excellent on the info.

My favorite is the "quick and dirty" Phillip put together.

 

[RS02]

8 hours ago, Zed Head said:

I'd start with reading the basics about SU's in the Nissan service manual. (No offense intended, but it describes things like an "idle hole" and secondary systems, etc.). These early service manuals are like school books.  You can learn a lot.  They've even created a troubleshooting table.  Your problem is probably in the "Power Shortage" area.  Good luck.  And don't put too much faith in the AFR gauge.  You can spend as much time making those work right as tuning the engine.

Well sir that diagram you posted was for a down draft carb. 

 

[RS02]

Thanks for all the input, so far I have checked float level, synced the carbs, set idle mixture, set off idle balance, The car is still too undrivable for me to really get it on the road to get it tuned right. 

 

[Zed Head]

You're right, I grabbed the wrong drawing.  Screwed that one completely.  Redo...