Going from EFI to carbs? Perhaps this will help..

[datsunlover]

I've noticed quite a few posts asking about this, and thought maybe I could help out.. possibly avoid more of the same question(s) being asked again.. (Hint... perhaps a sticky is needed? )

 

I did a carb swap onto my 75 280z, which was originaly an FI car. (72 round top SU's) Very easy to do, and I'm quite happy with it. Assuming you get carbs/intake, this should work for any carb swap (Webber, mikuni,ect) although vacum is an issue with triples sometimes I believe...

 

Basically, just rip out (well, pull nicely I gues.. depends how angry you are with the FI not working..) all the sensors, FI wiring harnes (it's not spliced into the rest of the cars electrical for the most part..couple common grounds, a clip at the battery..) and take off the intake manifold.

 

*note*

Some heads don't have the threaded holes for the SU carb manifolds, so you have three options; Possibly get another head, Drill and tap them where you need them, or do what I did last year, and build 'finger clamps' from 3/16" flat bar and use the FI holes to hold the top. (I can explain that in more detail if neccesary...) The bottom will get held the same as the FI one, by the stock thick washers. For cars with 02 sensors, I think you could just leave it in the manifold and simply disconect it... heck, it's be nice if mine had one, just for tuning purposes..

 

Once the carbs/intake are on, you'll need to hook up proper fuel line. SU's are nice to work with here, as stock 240z fuel feed/return lines bolt right on. Next, replace the FI fuel pump (30psi AHHH TOO MUCH!!) with a low presure pump. (3-5 psi) I got a generic aftermarket pump intended for full size chev p/u trucks w/carb'd V8's.. mid 80's or so. I believe there is a Holly pump that bolts in close to stock.. the one I'm using took a little 'rigging' to fit/work. I'd get a new fuel filter as well, the big FI filter would probly work, but I figured a $5 clear plastic one was cheaper to begin with, and would probly work better..

 

Ignition;

I couldn't see a way to make the stock dizzy work with the carbs, so I did some searching.. On older z's, a 280zx distributor is a 'drop in' deal, and works great (what I'm using) and if the 'black box' dies, it's a simple wiring job to hook up an MSD box to it. I'm fuzzy on weather or not the 280zx's ECU controls or gets signals from the dizzy, but to prevent possible issues I assume it's possible to just re-wire it in a zx just like I have wired it in my 280z. (one conection from box to + coil, other to - coil. ) Anyone who has info to add, please do so. Hope this is helpfull..

[Guest DoTheDrew888]

Couple questions:

 

What head do you have on your engine? And could you explain the way you adapted the manifold to bolt to the head?

 

Also, did you have to get the 240Z fuel lines or is there a way to just run my own lines to the carbs?

 

Thanks

[akeizm]

You should be able to use the stock fuel lines in your car. I just went from carb to efi and my z had a return line factory installed but blocked off near the carbie. So just run some hose from the metal pipe in the engine up to the carbs and block off the return. That prolly didnt answer your question but I thought I'd give it a shot.

[Guest Phil1934]

I put a 280 engine in a 260. Manifold bolted right up. Only hitch was I had to add fuel pump eccentric from the 260 to the 280 engine.

[datsunlover]

Couple questions:

 

What head do you have on your engine? And could you explain the way you adapted the manifold to bolt to the head?

 

Also' date=' did you have to get the 240Z fuel lines or is there a way to just run my own lines to the carbs?

Thanks[/quote']

 

Easy stuff first: My car currently has an N42 head, which has all the holes (FI and carb manifolds) BUT for some reason, the N42 that was on it when I got the car, did not.. ? Hence my need to build clamps.. I'll explain in a sec.

 

On the fuel line; At first, I used various bit's and pieces of the stock fuel rail/lines from the factory FI setup and it worked.. but not well. The FI lines are larger diameter, and there is no restriction in the return line. If you look at a stock 240 setup (the two attached pieces running around the valve cover, that feed the carbs) you'll see the return is way smaller, AND has a rolled over end with a small pin hole in it.. I'm not positive, but all I can figure is there has to be some restriction there so the fuel can keep the float bowls full. On mine, (with the stock FI lines) it would idle fine, but if I got on it hard I'd get halfway through 2nd gear and it would sputter. I temporarily fixed the problem by using a small pair of Vice-grips to pinch off a section of the return line which was rubber. I found a stock 240 fuel/feed line, and it's been great ever since. Basicaly I have the stock 280 FI feed/return lines from tank to engine bay/fender, then rubber line conecting to stock 240 lines.

 

Ok.. the manifold clamps.. Well, if you take an SU carb manifold and place it on the side of the head, you will notice something interesting; The front and rear FI manifold holes (M10 I believe) end up in between the carb manifold holes/casting. The two middle FI manifold holes end up to the inside of the carb manifold. If the head you're using has the carb manifold holes, you just bolt it up easy.. if not, well.. I looked at the bottom of the manifold and figured if the intake and exhast can be held by single studs with a thick washer (basicaly clamping on the edges, like the machine shop 'finger' clamp principal) then it should work for the top!

 

Using 1" X 3/16" flat bar I built two pieces about 2 1/4" long with a single hole (clearance for the FI head bolts) in the center, and one piece 5.5" long, with two holes. The two holes are 3 1/8" apart, centered on the flat bar. Basicaly, you line up the manifolds using the bottom studs/washers and then used the stock FI manifold bolts through the flat bar, thus clamping the top of the manifold to the head. Theoreticaly I could have used one long piece stright across, but I didn't have anything to acuratly measure and get all 4 holes in the right spot. I'll see if I can dig up some pics of this..

 

Oh, Phil; thats funny.. my 280 has the eccentric, although it obviously doesn't/didn't have a mechanical pump.. strange..

[Nismo280zEd]

For Webers it's a little different. You'll need proper intake manifold TWM or Cannon or other. Different gasket, header might or might not work depending on what brand it is. I had to get a new "thick wall" header to work properly with my carbs. You could weld on ears on the header if you wanted. Also if you don't want rubber hose running rampid in your engine compartment you will need to run hard lines up to the carbs. I recomend running a return line. Also with webers you will have to run a cable to be able to use the chokes. I use a bicycle shifter i picked up used from a local bike shop 5bucks with 6' cable.. used a lawn mower sheeth to run to the carbs. Also.. you will need to cut and weld pieces to make your linkage attachment unless you buy in a kit, then adapters will be included.

Don't be surprised if you have to grind on the manifold to make things fit and work properly I gave TEP hell about that one. You will need an electric high volume low psi fuel pump. around 5psi. I highly recomend a regulator as webers are very sensitive to fuel pressure.

Correction to the above post.. you CAN run a stock dizzy with webers, however it's a nightmare. I run a vacuumn canister with a one way valve to keep vacumn for the dizzy and brakes. I'd recomend mechanical advance or try the ZX if you want.

 

Not to mention setting aside countless hours of learning about webers and jets. But hey.. they are alot of fun when you mash the gas pedal and you hear them suck in all that air

 

-Ed

[Guest DoTheDrew888]

Datsunlover,

 

Im about to make the swap over to roundtops and I wanted to know a couple things first... Was it worth it? What kind of benefits did you gain when you did the swap? Pros and Cons???

 

Thanks again,

 

-Andrew

[Guest DoTheDrew888]

Anybody got any input on this?

[PeterZ]

What don't you like about the stock FI? The tiny intake runners and lack of HP or is there a driveability problem?

[hellraiser]

whats ur goal ?

[Guest DoTheDrew888]

My goal is to make power over 5500 rpm. The stock FI seems to have a problem with that. I also have an engine with a big cam running about 10:1 compression so stock FI isn't cutting it. Aside from that, I want something that I will be able to tune.

 

Basically my question was, what kind of results will I get by doing this conversion?

[ww]

My goal is to make power over 5500 rpm. The stock FI seems to have a problem with that. I also have an engine with a big cam running about 10:1 compression so stock FI isn't cutting it. Aside from that' date=' I want something that I will be able to tune.

 

Basically my question was, what kind of results will I get by doing this conversion?[/quote']

 

In my opinion, you're going in the wrong direction if you want to tune. You'll get much better tunability from FI than from carburetors.

 

You'd be better off keeping the FI and converting it to stand alone engine management. You'll get much better tunability.

[JMortensen]

From what I've experienced the SU's don't like to rev that high on an L28. The stock FI flapper door seems to be the limitation with the Z/ZX fuel injection system.

 

I was just reading a thread in the archives yesterday or the day before where Bastaad525 was saying he got a 20 hp increase on the dyno from swapping a motor from FI to ZTherapy SU's. I think he took the motor out of a ZX and swapped it into his 240. IIRC he still felt that it wouldn't smack the redline though.

 

In my case I went from SU's to 44 Mikunis, and I went from having a rev limiter that I could touch if I stood on the gas long enough, to having a rev limiter that I could hit really hard. If you want max revs, triple carbs or triple TB is the way to go IMO. SU's will be an improvement over stock FI, but you need to open up some of those restrictions to really get the revs IME.

 

Yes, triple carbs take some tuning skills, but they aren't rocket science, and a simple O2 sensor in the exhaust along with a good understanding of what is going on with each carb circuit is all you need to get them in tune ($$$ for jets helps). The triple TB setup is better, but people who have done it have spent big dollars on it. If it can be done inexpensively, triple TB injection is the way to go.

[Guest DoTheDrew888]

I would love to do triple carbs, triple TB, or stand alone but I just dont have the money for it at this point. With SUs I should get a power increase of some sort and they are much more tunable than the stock FI. I also plan to bore the SUs out in order to have the "Big Bore SUs". I'll have to look into that and get more information though. Another question also, isn't it true that by putting on carbs my torque curve should move lower down in the rpm range? Maybe im wrong. Thanks for the info guys.

[Nismo280zEd]

If you don't have the money.. stay with your FI.

[ZONKER]

This is for do888 I don't know if I have a weard head but my P-79 head on a '82ZX engine I put in my 240 has only four holes threaded to bolt down the intake along the top. I am going to run SUs also. I made spacers for the two center bolts so that the washers would be supported from all sides . I had asked other Z drivers and no one else had come accross the same problem. The rest of the bolt pattern is the same as for all Z heads . Gary

[ww]

Stock fuel injection with MSnS stand alone engine management and an AFM to MAF upgrade from a 300ZX can be cheaper than building up a carb'd motor and will give you infinitely more tuneability, better gas mileage and equivalent power with none of the headaches of trying to keep carbs in sync.

[Nismo280zEd]

This is actually what i'm doing... going back to EFI for the driveability and milege. Gonna get a microtech prolly and junk the AFM.

-Ed

[clarkspeed]

You would be amazed at what you can get out of the stock EFI system with some "tweaking". It is tunable, just not very easily. Tricks include adjusting the AFM spring tension and potentiometer needle location. I've also added a variable resistor pot to the temp sensor to richen the mixture under all conditions. Higher fuel pressures and different injectors can also be used.

 

The problem is it is trial and error for the most part. Stop, make an adjustment, drive, repeat. Using a wide band O2 helps. And of course, you will never get it as close as a fully programmable system.

[phantaz]

Clarkspeed...and anyone else with Z FI experience

im not trying to jack this thread but ive run into a prob with the aparent hp killer in the FI zcars.. the AFM. ive recently been told that the FI inthe z is pretty much as is because of the inabilityof the afm to cooperate with things such as cold air intakes or portingthe TB intake manifold, or larger fuel injectors.. ifyou have a "check list" of things you can tune or help i would really love to have that info.. i ahve a 77 with a mild street cam and a msd coil with 7mm msd wires and denso plugs and teh afore mentioned cold air intake that i have been told is useless...no headers yet ive heard of putting mustang injectors will help but am confused on hhow it helps if the system will not increase aor flow to keep up with higher fuel injector rates. is there a way to bypass teh afm without gonig to the expensive stand alone systems which im not sure i could install myself anyways. anything to help me get a few more hp out of my beloved z would be great because im assuming here that FI tips for my 77 z will work in my 79 zx ..

FI frustration is gonna kill me i think lol