What's the difference between 240z Carbys and 260z Carbys?

[stevo_gj]

Hey guys,

 

We get this question often enough. Might help if we centralized a bit of information on it.

 

240z carby's AKA Round Tops:

 

Made by Hitachi, they were a Jap licensed copy of British SUs. They came in 3 and 4 screw variations, which refers to the number of screws holding down the dome. They are good carbs and are easy to tune and work on. I put a set on my 260z.

 

 

 

260z carby's AKA Flat Tops:

 

Also made by Hitachi, they were still based on the British SU but were much more modified to pass emissions regulations. 260z carby's are known for being difficult to work on, poor for performance and generally ugly. It is very common practise for Z owners to replace them with British SUs or 240z Hitachi SUs.

 

 

Feel free to add more info guys - I'll update the main post with it.

 

Edit: Here is a good website with a lot of relevant information: http://www.zparts.com/zptech/tech_tips/izccposts/su_differences.htm

[sexual_sushi]

Just a technicality but "knock off" sounds a bit dodgy, the designs were actually licensed from SU. Parts are generally not interchangeable between British and Japanese though.

 

Might also be worth mentioning the differences in round tops as well, generally categorised as 3 or 4 screw (counting the screws holding down the "dome" lid)

 

More info here:

http://www.ztherapy.com/technical_stuff/spotterguides/zcar/240spotter.htm

[Scoota G]

The Flat Top's are commonly referred to as "Boat Anchors"

[Rock Doc]

Hi guys, I am new to the forum and to my 260 z 2+2 '75. 

My carbies seem to be flat tops but run round port not square ports on the intake from the air filter.  My understanding is that the flat tops have the square port intake (according to K&N) or did Australia get a halfway house carbie between the old round tops with the round port and the US' Smog flat tops with the square port?

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[1600dave]

There is a "halfway" carb - the smaller flat tops off later model 180B SSS's. They are only 38mm bore though, not really up to the job of supplying a 6 cylinder.

 

"Z" flat tops have the float chamber integrated into the carb, all other Hitachi SU's (including the 4 cylinder flat tops) have seperate float chambers off to the side of the carb.

[Gordon Dobie]

The 260Z SUs have had a lot of "bad press" mainly due to the fact that most mechanics don't understant the basic design pricipals or how to go about tuning them.

The first thing to check is the float level. A small window is provided in the front of each carbie for this purpose. The fuel should be level with the small dot in the centre of the window. If this is correct and assuming some "bush mechanic" has not been fiddling with the factory settings the carbs will do the job they where designed for. The fuel level is usualy too high due to worn neeldles and seats. Replacement of the needles and seats should bring the float level back to the original settings. Another possible cause of a high float level can be a sunken float. Solution, replace the float.

 

Gordon Dobie

[gav240z]

Thanks Gordon, hope you don't mind but I edited your post to remove the large blue font from your post.

[NZeder]

I too have heard that the 260z carbs are not a bad as many think and could actually be better depending on application/engine spec. Thanks for your input Gordon

[dazzed]

i was going to say something but i forgot what it was just as well  i did

[Xnke]

The reason I find the flat-top carbies superior is that they have a separate idle circuit. You can run a rich main needle and still lean the idle out so it doesn't smoke out the pipe in the driveway.

 

Set the float level, turn the idle screw 1.5 turns out, set the main jet nuts 1.5 turns out, sync carbs. Set idle mix. Set RPMS to 2500RPM, adjust main jet nuts. Return to idle, re-set idle mix, BAM carbs done.

 

Wheras the roundtop carbies tend to run fat at idle, or if idle mix is proper then they run lean up top. Needle adjustment is possible but adjusting the idle area only is a pain.

[Paulo]

Hi all,

 

I was going through my "How to tune Datsun" book, made mention of several needles/sizes and numbers appearing on the top most section where the grub screw holds it in place.

 

Neither carb (round top 240type) had a number, so went through the process of measuring at intervals (as per the table in the manual) and nothing matched - way leaner than specified.

 

My question, seeing as these are Datsun products, is it worth contacting SU England  for a new set, or Nissan or was there someone out there long, long ago that was fiddling with alternate needles?

 

My original fuel return 1600SSS carbs have the correct numbers, so I know something must be wrong with my set up.

 

Any help appreciated - I don't want to machine down the needles, just want an original set for tuning the engine.

[G]

Stewart Wilkins still has needles, or speak to ztherapy

 

http://www.ztherapy.com/