Nissan 720 Passing Emmisions

[TheProtoSmith]

Ok so problem was the fuel solenoid vacuum switch was caught in a feed back loop. After adjusting boost control it was solved. Switched out all the vac lines and am confident they are right. Switched out cap and rotor plugs and wires and definitely big improvements for rough idle I had before. I still feel like I'm running rich even with the idle mix screw all the way in. I read that there is another part that controls the mix but all the manual said was to have the dealer do this. What should I be looking for next?

 

Datsun Mike grace me with your vast wisdom 🙂

[datzenmike]

Does your engine have an O₂ sensor in the exhaust manifold?

 

The idle mix screw is set and then plugged at the factory. Are you sure you are adjusting the actual mixture screw??? It's under an aluminum plug here...

 

[TheProtoSmith]

Yeah I'm definitely adjusting that one. That the screw driver is sitting in. On my rebuilt carb there was no plug. It's the lower one right?  I'll have to check on o2 sensor in the morning. Also just found the ECU which po told me was not part of car. 

[datzenmike]

Look also for a round 6 wire electrical plug at the back of the carburetor, that and the O₂ sensor would make it an ECC feedback carburetor. The primary jet is replaced with a solenoid that vibrates open and closed to meter the fuel into the carburetor using the O₂ sensor in a feedback loop to constantly adjust the mixture. Does not work at idle, when not warmed up and at full throttle.

[TheProtoSmith]

So it looks like the only wires I have coming off the carb are the 2 wire Fuel Shut off solenoid and the Electric Thermal Choke. Also didn't see any connectors floating around back there 

 

[TheProtoSmith]

So i've been thinking and now i'm curious if my float bowl height might be too high. I don't have a viewer but it could explain the constant over rich condition right. 

[datzenmike]

3 hours ago, TheProtoSmith said:

So it looks like the only wires I have coming off the carb are the 2 wire Fuel Shut off solenoid and the Electric Thermal Choke. Also didn't see any connectors floating around back there 

 

 

That's good news. Carburetors run best when the fuel level is correct.

 

If the idle mix screw is all the way in it shouldn't idle. So something not right there. Is the idle at or around 750??? Too high and you won't be using the idle circuit.

[TheProtoSmith]

Thats what i was thinking. My tach is dead. i tried reflowing the board but wasn't able to get anything out of it so i'll pick up a photo tach later today and get a reading off of that. 

is there a good way to tune the float level when you don't have the window? (something like tighten the screw all the way then back off 1.5 turns) 

[datzenmike]

There should be a small round sight glass on the front of the carburetor. It will have a small white dot where the fuel level should be.

 

[TheProtoSmith]

My carb doesn't have a window. I spose i could add in a piece of glass if i try hard enough. but without that is there no way to tune?

[datzenmike]

Should have. Stock carburetor or Weber? Picture?

[TheProtoSmith]

It's this https://www.oreillyauto.com/detail/b/tomco-5051/fuel---emissions-16775/carburetors---parts-16801/carburetor-12234/840def96ca83/tomco-hitachi-2bbl-carburetor/10096/4746383/1984/nissan/720-pickup#WAR

Without the glass panel

Edited September 17, 2020 by TheProtoSmith

[TheProtoSmith]

So I just failed again, but it's the lowest scores across the board that I have gotten which is a win. Down to 130's in CO. The test shows that it's great during decel and horrible during acceleration leading me to think a new jet would be helpful but confirming my fears of having way too much fuel entering the system. 

[TheProtoSmith]

Ok so continuing to dig I was able to find the fuel air mix solenoid. But it's behind a Welch plug. I plan to drill that out and adjust it tomorrow. Hopefully that will offer me more options in terms of tuning... Any one say that this is a terrible idea? It's this part even adjustable? Diagrams are not as detailed as I would like. If it's not electrically controlled how is this actuated? 

[datzenmike]

This is not an ECC feedback carburetor which DO have a solenoid for the primary jet. It's also definitely NOT a stock carburetor. I have no idea what Orilley's sell.

 

The ECC is operated electrically through a 6 wire plug at the rear of the carburetor. You don't have this.

 

Under there would be close to the power valve.

 

The primary jet is under the fuel bowl left of your finger. probably a 107 jet, y ou could try something closer to 100 to lean it out. I got a dozen assorted motorcycle carburetor jets on Amazon for $10 and they fit Hitachi carbs.

[TheProtoSmith]

Any chance I can get a link? I don't do motorcycles but I'm sure there are different types of jets. It seems to really match the diagrams of the carb in my service manual . I just think it's missing the window but I'm no expert. Could also be a fully rebuilt ebay one. I opened the top off the carb and it def has something that interfaces with valve at the bottom 

[datzenmike]

 

 

The power valve is in the center. The plunger on a spring drops down from the carburetor top and is activated by vacuum. Drilling out that plug would probably be a bad idea.

[TheProtoSmith]

ok Cool so i think i found generic motorcycle carb jets. that are metric which should line up with it being a japanese carb. but they are for like 150cc motors so i'm a bit confused on that. 
unless i'm mistaken (because i haven't been able to take the carb apart today. it should be an M5. x .8 thread with a #100 diameter nozzle. although i might just run it even leaner than that given my insane numbers. also thinking of dialing back the pilot too to probably be the 107 instead of the larger bore. But thats just a thought right now. Ill lay off from drilling out the plug and see if i can get there with the jets 

if these parts look right some sort of confirmation would be great. 
Cheers,
Smith

https://www.amazon.com/gp/product/B07JZM493B/ref=crt_ewc_img_huc_2?ie=UTF8&psc=1&smid=A23MTL1LIY6YXM
https://www.amazon.com/gp/product/B07SL2JH4T/ref=crt_ewc_img_dp_1?ie=UTF8&psc=1&smid=AAQI0AA583SPQ

[datzenmike]

Get the ones farther down the page lower left that start at 90. The California jets start at 94 not that you woulld want to go that low. These range from 90 to 150. You might want to be able to go down into the high 90s.

 

[TheProtoSmith]

Alright old jets are out one was cracked, both were too big. Like #200 ish mix screw now makes a difference wooo. Did a 92 and 97 on the new jets.  Now I gotta tune this carb in. Runs pretty well when both coils are on and like ass when you full throttle. Which should mean the system it too far retarded right?

(Because of the full throttle second coil shut off circuit?(

 

Edited September 20, 2020 by TheProtoSmith

[datzenmike]

The distributor automatically advances the timing when switched to single plug mode. If it runs shitty on single plugs maybe the  wires are bad or coil tower is cracked and the spark escaping. Won't be noticed with dual spark.

[TheProtoSmith]

Hmm, I thought changes in timing were all handled by vacuum on my year, and I'm not sure if this helps but brand new cap and rotor.

[datzenmike]

There are 3 timing settings. All in the distributor. (technically there is 4)

 

Initial.  Is set with timing light and turning the distributor. Once set it is fixed and does not change. On the Z24 it's 5 degrees BTDC.

 

Mechanical. This is built into the distributor and increases advance as engine RPMs increase. Higher speed means less time for the fuel to burn so it is ignited sooner.

 

Vacuum. A part filled cylinder (light throttle) burns slower than a full cylinder (full open throttle) so a method has to be used that varied the advance that is load dependent. Intake vacuum is highest at low throttle speeds and least at full throttle so it is used to advance the timing at part throttle and almost no advance at full throttle.

 

 

Dual plugs, with two ignition sources, shorten the burn time so this is why the timing is 5 degrees. A similar single plug head would require a 10-12 degree timing advance to have enough time to get all the gas burnt. Designed into the EI module is a 'switch' to turn the exhaust plugs off during heavy throttle to reduce noise. It also advances the timing for single plug running so that the performance is comparable.

 

 

[TheProtoSmith]

Hmm, ok, so maybe I should look at the base timing. I know vacuum is good and based off your description of the timing adjust system; it shouldn't be activated under full throttle load. This issue is persistent across even low rpm so that would negate the mechanical advance as the issue aswell. Since my ignition system is all fresh (plugs, rotor, cap,and wire), it should rule out faulty ignition, guess I just gotta bite the bullet and pick up a timing light instead of just tuning by ear. In my conceptual understanding of the system if I want to effectively set the base timing, I should disconnect the vac advance, and the secondary ignition coil, and then add those systems back in after the 5d BTDC has been calibrated.

 

Sound good?

[datzenmike]

 Thought you said it runs like crap wjhen the exhaust side plugs shut off?

 

 

Your distributor should look like this one above. Has 3 wires bundled together and a forth one with it's own plug. Disconnect this plug and the exhaust side is disabled from shutting off. Now try it and see if the bad running is related to the plugs shutting off.