I am going to get a 4" spoa fabbed locally and I plan to put 31's on also.  My question is:  Do I have to swap gears to maintain 55 mph?   And what are the other pitfalls?

I am going to get a 4" spoa fabbed locally and I plan to put 31's on also.  My question is:  Do I have to swap gears to maintain 55 mph?   And what are the other pitfalls?

If you still have the 1 liter engine, 55 is going to be difficult. Matter of fact, isn't 55 pretty hard to do anyway, even being stock? My ol' SJ410s were lucky to hit 50 mph.  

I'd suggest t-case gearing change.

If you do the SPOA, start looking for a traction bar setup you like for the rear or you'll have warped springs soon. YMMV

HTH -Eric

I can go the 65+, no kidding.  It will also start in -30 degrees weather (F)─tested it today.  Should I get new springs for the rear anyhow?  

Yes, and it has nothing to do with the lift. RPM's of an engine is based upon the rotations of the entire system. You'll have to compensate for the larger tires some place within the system. Most do in the differential or the t-case as the engine and trans are normally not interchangeable with separate gears. A combination of the gears is what you are looking for to compensate for the larger tires.

Lol, I followed an SPOA 410 once at 65mph down the M1 motorway and it was towing a Jimny at the time!! It did have an SU carb and a big bore exhaust though. I was reluctant to try and keep up with an SJ on an A frame behind my Vit, just glad I wasn't the one on the end of the rope.
Edit: When he sold the 410 I bought the carb and exhaust from him for mine.

Lol, I followed an SPOA 410 once at 65mph down the M1 motorway and it was towing a Jimny at the time!! It did have an SU carb and a big bore exhaust though. I was reluctant to try and keep up with an SJ on an A frame behind my Vit, just glad I wasn't the one on the end of the rope.
Edit: When he sold the 410 I bought the carb and exhaust from him for mine.

What's a SU carb?  Was it a 1 ltr?  where did get these things?  

SU carbs are a sidedraft design.  They have a piston in them that is in a chamber that allows the piston to rise based on vacuum conditions.  When they raise up this opens up the throat of the carb and makes for a bigger venturi capable of more air flow.  Also attached to the bottom of the piston is a needle that meters fuel.  As the piston rises with higher RPM conditions, the metering needle is pulled up with it which allows more fuel to be supplied.

Sounds complex but they're actually pretty elegant in their simplicity.  They're more popular on British cars.  Another source is either a Datsun 240Z, not a 260Z and not the much more common 1975+ 280Z and 280ZX as they're EFI.

Preferrably you want the ones off of a 1972 or older 240Z as the 73's were more emissions oriented than the earlier designs were.  (The 260Z version of the SU is an emissions oriented pile of crap if I may say so and should be avoided like the black plague).  You can also find them on some older Datsun 2000 roadsters which predate the Z cars.

Realistically though finding a 240Z or a 2000 Roadster in a junkyard is very tough these days.  I've got 2 pairs of em for my 240Z and basically it's not likely I could be talked into selling them because they're that hard to find anymore.  (And that's even factoring in that I'm planning to convert my 240Z to EFI).

If you find a 240z, one carb off of that would probably be enough for a 1.3 or a 1.6 but you'd have to modify the manifold.  The 240 carbs were originally in a pair with one feeding the front 3 cylinders and one feeding the back three on the inline 6 cyl.  Since the 240z is a 2.4 liter engine and unmodified SU's are adequate even on a swapped in 2.8 liter plus version of the engine from a 280Z I think 1 SU would probably be fine for the 1.3 or 1.6.  Running them as a dual setup on the 1.3 or 1.6 would be overdoing it by a ways, but I don't think you'd be pushing it past it's limits running a single on a zuke engine.

Try some british cars if you want to find one.  They're really a pretty good carb all in all but they do require occasional tuning if they're the kind that are tunable.  If you're only running a single that's pretty easy, with duals it's a bit more involved of course.

I should also mention that the ones on the Datsuns are actually Hitachi made SU's built under license.

If you want to see some pictures, go here:

http://www.ztherapy.com

I have a 1.0 ltr.  Can I still use one of these '72 240z carbs?  

I don't see why not, it might be slightly overcarbureted but you can adjust the mixture on the 72 and earlier SU's on the 240Z to compensate so it should be ok.  

I'd highly suggest spending the 35 bucks or so on a little gadget called a colortune if you do though.  Basically it's a sparkplug with a clear window part built in so you can adjust the mixture based on the color of the combustion when it fires.  They come with a chart for what's rich and what's lean.  It's kinda like a Wideband O2 sensor for tuning but a bit more low tech.

If you get an SU it really doesn't matter if it's the front or back one if it's off a 240Z, the only real difference is where the float bowl is attached.  But running a single on a 4 cyl as long as it fits ok that's all you need to worry about.  Well that and modifying an intake so it'll work.  If you can grab the intake off the 240 for the flange so you can weld that to your existing manifold.  There's also a phenolic spacer you'll want to get that's between the carb and the intake.  Plan on putting in a choke cable too, the 240Z SU's  have a manual choke.  If you can find a british car that has SU's some of those have water/electric chokes.

Also plan on buying one of the 6" round K&N filters for it.  If it's there, grab the airbox on the 240 and cut out one or both of the air horns.  Otherwise plan on buying one from somewhere like twm induction.  (www.twminduction.com).  The TWM ones are much prettier and probably work better, but running an SU without an air horn is not a good practice for performance as the sudden 90 degree edges really screw up air flow.  Honestly for the 60 or so bucks that one costs, you're far better off with one of the TWM pieces vs even the stock air horns as the TWM parts have a full radius and the stock air horns are only partial radius.  (Plus they look very sexy compared to the stock ones and that's what's really important.)  You could also build a plenum to go around the TWM one for better airflow than the K&N 6" rounds, those have a solid front face and I think opening it up so air can flow in from any direction would help considerably.

I will warn you that winter cold starts with an SU that's not adjusted right (and sometimes even if it is) are sometimes a pain in the butt so you'll want to keep a can of starting fluid for those mornings when you're in a hurry if it's a daily driver.

I think if you get one of the ones with the automatic choke that helps a great deal, but I would guess that most of those are also not tunable like the older manual choke ones off of a 240Z will be.  Also in the piston dome on the SU's there's an oil damper like a shock absorber to make the piston's rise more smooth and gradual than just slamming open like they will if there's no oil and you mash the throttle.  Normally 20 Wt oil is recommended although you might want to go lighter if it's colder.  What I found works good and is easy to put in (they do suck a small amount of it in over time so you'll have to top it off every few months depending how much you drive) is the silicone shock absorber oil for 1/10th scale Remote Control cars that you can buy at hobby stores.  It's pretty cheap at 3 or 4 bucks and the silicone oil is available in a bunch of different viscocities and keeps it's viscocity over a much wider temperature range.  They also sell regular oil if you prefer and it's a bit cheaper.  Never had any problems using the silicone though.  The reason I suggest this method is that the oil fill hole is pretty small (maybe 1/2") and the dropper style bottles these shock oils come in makes putting it in a whole lot less messy.

Hope that helps.