roundtop differences?

[clutchdust]

i pulled the carbs off my '73 (allegedly '72 roundtops) and also the carbs off the g/f '71. now i have questions. during disassembly i noticed a couple differences. the ones from the '73 had a four bolt flange for the damper and also had a black plastic jet assembly. the '71 had a three bolt flange for the damper and had a stamped steel jet assembly with some type of locking screw mechanism. otherwise, they appear to be identical.

so the questions are, what's the difference? is one better than the other? are the internals interchangable?

one other question, how much meat can be removed from the manifold and heads for a little better flow?

[SuperDave]

i pulled the carbs off my '73 (allegedly '72 roundtops) and also the carbs off the g/f '71. now i have questions. during disassembly i noticed a couple differences. the ones from the '73 had a four bolt flange for the damper and also had a black plastic jet assembly. the '71 had a three bolt flange for the damper and had a stamped steel jet assembly with some type of locking screw mechanism. otherwise, they appear to be identical.

so the questions are, what's the difference?

Go to http://ztherapy.com/ and click Z Car and read the "ZTherapy 240Z SU Carburetor Spotter's Guide"

[dogma420]

The round tops have a couple of differences 3 and 4 bolt for the piston housing, and the tab at the bottom of the intake....go to www.ztherapy.com to see the differences.

They are basically identical however other than those few differences.

[Bambikiller240]

ZTherapy are definitely the ones to ask, but I don't believe that all of the internals are interchangable regarding the 3-screw and four-screw versions

[clutchdust]

checked it out and just on the home page, it doesn't address those differences. since i plan on making an engine swap sometime in the future, i'm not going to spend alot of money on super zoomy stock carbs. just do the parts store rebuild.

so far, the only things i'm planning to do to the stock engine is have a bit milled off the head, do some minor porting and polishing on the ports and bowls and install a header.

i figure that will get me running enough so i can do the work i want on chassis and suspension for a good track car. once i have the rest of the car sorted out, i'm going to drop in a nice cheby v-6/t-5.

[Bruce Palmer]

The 3 screw carbs had provision for water from the intake manifold thru the insulator block to pre heat the carbs. Other than that there isn't a nickles worth of difference between the 3 and 4 screw carbs. The parts store kits have nothing of any consequence in them but they'll stab you like they do. I would suggest a set of SM needles for drivability issues and new fuel line from the float bowl to the nozzle. See many of these that are too old and brittle and about to become a major fire hazard.

After you do whatever you are going to do, we buy cores..............

[clutchdust]

thanks for the response, bruce. good to hear from someone with some authority in this matter. what are the SM needles? when the time comes for a 'heart transplant', i will definately keep you in mind with the roundtops.

[Bruce Palmer]

SM's are a profile that fatten things up in the upper RPM ranges on the early Z car. They tended to, with the stock Hitachi needle, lean out up top.

[dogma420]

SMs make a big difference!

Why a Chevy V6? T5 would be nice but what style of V6 would you go with that would make you wanna change from the Nissan motor? Just curious

I have the 3 bolt Sus, and the water tubes have been disconnected for a while. I thought that was the only diff between the 2 types....

[clutchdust]

since i will be running this as a track car, maybe i'll investigate those SM jets. definately don't want the thing falling flat on the top end.

dogma, probably a 4.3L odd-fire (if i can find an odd fire). a 4.3 is basically just the old 350 minus two cylinders so there is lots of cheap go-fast out there for them. you can easily build one up to over 300hp N/A and have it completely drivable and damn near bulletproof. not to mention i'm really familiar with the bowtie, it weighs less than the straight six, can be set farther back for better balance. oh, and if you've never heard an odd-fire 4.3 at 7000rpm :devious: just plain wicked.