Choosing a carburetor for a built Z24?

[Spades]

I have had issues with timing and other things getting the most power out of my Z24, and after getting the timing set up to peak performance, I found that it is running lean. The gas analyzer on the shops scope shows it is running a little lean when the throttle is stabbed, so this explains why the motor wants obscene amounts of timing to have good off the line performance. I am going to do a fuel flow test on the new fuel pump just to be safe, but it looks like the little 32/36 just isn't cutting it. The engine runs very smooth and has plenty of power, but not as much as it should have.

 

This is what I have now: http://www.webercarbsdirect.com/32_36_DGV_p/22680.005.htm

 

 

These are the engine specs:

 

*2.4l NAPS Z24 (fully rebuilt motor from an 86 pickup) bored over w/ forged pistons

*Custom grind "Mystery Cam" from OCG(not sure the spec's, close to stock duration but a very high lift, would be the equivalent of a stage 2 or stage 3 circle track camshaft from what I can tell)

*Ported and polished intake and exhaust manifolds

*Ported, polished, and shaved head

*full exhaust

*Nissan electronic 8 plug distributor w/ dual flamethrower 40k volt coils

*stripped EGR & vacuum junk (running only an alternator& distributor)

*NAPS Z20S flywheel

 

I don't know what these motors need for CFM, and unfortunately there is much I do not know about the camshaft specifications...it is a fairly radical camshaft, and the PO that was paying to have this motor built(I bought it and finished assembling it) said it was the largest camshaft they could machine without the valves touching(which isn't saying much with how close the NAPS valves are to each other).

 

I have been reading that the 32/36 is OK for a stock Z24 but you won't get much more than stock performance from it, and I noticed that the symptoms I am getting after fixing the timing chain are the signs of a lean sag. I pulled the choke 1/4 of the way out, and it had better low RPM performance and the sag symptoms went away...which tells me it isn't getting enough fuel.

 

I have a couple of what I think are HS6 SU carburetors on a shelf(tag # AUD 33R, case # AUC 8490, I think from a Volvo 6 cylinder) that I could build a manifold for, but I am unsure if they would be too much or not enough CFM, or I could always kiss fuel economy goodbye and go with a 38 series, or a holly 390cfm 4bbl, or a quadrapuke from a 4.3 v6 (I hear they are highly tunable and some people put them on large 4cyls and smaller 6 cylinders). Any thoughts? I want low RPM torque, I don't need the motor to pull past 4500rpm, and it is a daily driver, so less tuning and better fuel economy is preferred.

[datsunaholic]

The 38DGAS won't kill your mileage. I get the same mileage with one on an L20B as I got with a Hitachi. It's all in the jetting. If you think your 32/36 is running too lean, upsize the main jets. If it's running out of air, then you have to go with more CFMs. More CFM only works at the extreme top end anyway.

 

Running the choke partially out doesn't tell you you have too small of a carb, it tells you you have way too small of jets. The analyzer saying it's lean confirms this. If it's lean, you're getting plenty of air. The choke REDUCED the CFM and made it run richer, so upping the primary/secondary main jets (and whatever else needs to be changed when doing that) will probably help.

 

Hell, I've seen 32/36s on 4.0L Jeeps. There's plenty of CFM for a 2.4L.

[flatcat19]

Want power out of a NAPS motor...ditch the head. Go L.

[Spades]

The 38DGAS won't kill your mileage. I get the same mileage with one on an L20B as I got with a Hitachi. It's all in the jetting. If you think your 32/36 is running too lean, upsize the main jets. If it's running out of air, then you have to go with more CFMs. More CFM only works at the extreme top end anyway.

 

Running the choke partially out doesn't tell you you have too small of a carb, it tells you you have way too small of jets. The analyzer saying it's lean confirms this. If it's lean, you're getting plenty of air. The choke REDUCED the CFM and made it run richer, so upping the primary/secondary main jets (and whatever else needs to be changed when doing that) will probably help.

 

Hell, I've seen 32/36s on 4.0L Jeeps. There's plenty of CFM for a 2.4L.

 

Thanks for the tips, I am still learning on this stuff...I haven't worked much with any cars older than the mid 90's, so carburetors are a new art to me. I will do a fuel flow test and make sure my secondary is working well, and then start reading up on carburetor tuning so I can learn about jetting and most likely by a kit with an assortment so I can do trial and error and learn in the process.

 

It doesn't help that my health problems have wiped out a good portion of my memory and I forget most of what I learn within 3 months of reading or doing it.

[DISLEXICDIME]

If you want power and want to keep it carbed you are going to nead side drafts plain and simple.

 

If you wanna stay down draft you are going to nead a 4 brl

[620racer]

I ran engine like that with 2bbl 350 cfm holly. flatcat19 is right L-head !

[Spades]

I am just trying to optimize what I already have. The 8 plugs, hemispherical and crossflow design, and low RPM high torque characteristics of the NAPS head make it a good choice for daily driving, if I decide I need much more power I will go with forced induction. I prefer low RPM torque vs high RPM horsepower. I will most likely try a different carburetor and jet setup and someday look into forced induction.

[datzenmike]

The Z24 displaces about 145.6 cu in, so if we assume a top RPM of say 6K and it takes two revolutions to complete one four stroke operation then divide by 2 to get 3000 actual 4 cylinder fills per minute or 436,945cu in per minute. A cubic foot is 1728cu in so we divide to get 252.8 cubic feet per minute. You would need a carb capable of allowing at least this amount. I think the 32/36 is rated at about 270cfm. The stock Z24 Hitachi is very restrictive and most who switch to the 32/36 notice an improvement.

[yello620]

Spades, you have a nice long write-up about all the motor specs, but absolutely nothing about the jetting of your 32/36. You know that it is running lean most of the time and worst when you stab the throttle.

 

Lets look at the jet numbers. Get yourself a new top gasket(about $5 at discount in tigard) just in case and open that carb up. It is easy to check them and get numbers.

 

What size idle jets do you have? dont even have to take the top off the carb to check these.

 

I bet it is still running stock jet sizes.

 

While in the long run, the 32/36 may not be the perfect carb for a cammed z24, you should be able to make it work very well for a daily driver.

 

 

Are you still living in the Newberg area? Need help opening up that weber, let me know.

[ggzilla]

It is lean when you "stab the throttle"? That indicates you need more accelerator pump, not more carburetor.

As said, 32/36 is plenty big for a built 2.4. It is a common carburetor on 2.8 and 4.0 liter engines.

[Spades]

Spades, you have a nice long write-up about all the motor specs, but absolutely nothing about the jetting of your 32/36. You know that it is running lean most of the time and worst when you stab the throttle.

 

Lets look at the jet numbers. Get yourself a new top gasket(about $5 at discount in tigard) just in case and open that carb up. It is easy to check them and get numbers.

 

What size idle jets do you have? dont even have to take the top off the carb to check these.

 

I bet it is still running stock jet sizes.

 

While in the long run, the 32/36 may not be the perfect carb for a cammed z24, you should be able to make it work very well for a daily driver.

 

 

Are you still living in the Newberg area? Need help opening up that weber, let me know.

 

I am going to look at jet sizes next, and yes, they are most likely stock jet sizes, I bought the 32/36 brand new, and I have never re-jetted it. I assumed there must be some magical limit to the jet sizes available to the 32/36 and that is why many people on the internet were saying the 32/36 was too small, so I was looking for something larger...if the 32/36 is enough, I may just look at a larger secondary jet and go with it.

 

I have had the top off before and will do some reading on carburetors to get a little more information on 32/36 jetting and setup, if I have any questions I will fire you a PM.

 

It is lean when you "stab the throttle"? That indicates you need more accelerator pump, not more carburetor.

As said, 32/36 is plenty big for a built 2.4. It is a common carburetor on 2.8 and 4.0 liter engines.

 

it seems to be running lean most of the time, and is the worst when I stab the throttle below 3500ish RPM. I am guessing this is because more timing is needed due to the lean mixture. Enough power to roast the tires below 3500RPM, but not as much power and torque as expected, and definitely no where near the power it has in the 3500-5500 RPM range.