cheers dude!!

thats what i wanted to know. WEll thats settled it
SUs it is, and I will get the bigger ones sorted out to go onto the manififold that I have, then its all go!
The carbs that I have I bought second hand, and I dont know of their condition, so I think a strip down will be the go to make sue they work right and all the bits are there.
cheers will keep yo guys posted on their progress

SUs are the most simple carb in the world... once you understand how they work - only prob is not many people do!

Seriously, there's not much to rebuild. If they don't flood (sunk float or needle and seat probs) or leak fuel, you really don't need to 'rebuild' them. SUs don't have accellerator pumps that wear like conventional carbs, or power valves or anything.

*Strip, inspect and clean the piston chambers.
*Check that the needles are 'shouldered' in the piston - ie the shoulder of the needle should be level with the base of the piston.
*Check that the jet is centered.
*Check condition of the springs (ie not bent or stretched, and make sure they are the same - I've seen a few cases where they're weren't)
*Refill the piston damper with oil (about 2/3rds of the way). trans fluid or machine oil works well.
*Set the float levels
*Balance them, and get the mixtures right and you're all set.

That might sound like alot of work, but it's all very simple and quick. The workshop manuals have some good info about tuning SUs, and the usual exploded diagrams etc. They're a good resource when you're learning about them.

One common problem I've seen is people often connect the vents in the float chambers to each other via a hose - not a good idea. It has to be vented to atmosphere. Air cleaner is the ideal place, but if you have ram tubes you can just leave them open.

Its a myth (or common misconception at least) that SUs go out of tune all the time. You'll invariably find that it's only people who know nothing about them that say that. But its surprising how many mechanics know nothing about them as well...They are a good carb for a street car.

hmmmm well I think that I have the decsion made. I will go the twin SUs, get them rebuilt and tuned properly. I have found the my webber drops outa tune regularly so the SUs will be the go I think.

Another question....
are the SUs easy to rebuild, or should i go to a professional to get them done. and are replacement parts easy to get?

Quote: I can't speak about "genuine SU's," I can only speak about the Hitachi's that i have on my GX engine, & i used them for many years without any hassle. They just need to have the oil level in the dampers checked periodicly, & every now & then i had a look at the adjustment screw that ensures that the two carbs open at the same time. Other than that, they received no attention.
In fact, when i finally wore the engine out, i rebuilt it, & re-installed the manifold & fired it up. It did another full service life without any repair, or major tune up on the carbs. It just shows that the Brits may have come up with a good design, but the Japanese made it work.

Well that just throws out the sizing! Well I have three sidedraft carbs that are from a standard 1600 manifold(twon on + one spare) I think that these are the 1 3/4" variety. They alos have the dome tops.

and I also have a pair for smaller sidedrafts that came from a 1000. These are flat top type, and are about 2/3 the size of the other three, which makes them about 1 1/4 inch i guess. Unfortunatly I am at work and cannot view them first hand till next week when I get home. DOH!!!

Well seeing as you cant really have too large a carb, then the only decision is how hard they are to keep tune. The Teech pages here say that they are a set and forget item, whic is ideal!! I plan to get it all dyno tuned anyways when its all done, so that I know what is the best performance and if it drops then I know.

Hey Rueby_Tueasdy

Are the carbs you have Hitachi or SU?

SU carbs bore sizes are not metric (mm) they r in inches ( inch & three quater or 2 inch etc...) Have you got 2 different sized carbs?

No expert but,
Su's work on constant velocity. when u plant your foot down, u open the butterflies, but u do not raise the pistons inside the carbs. These pistons wait until there is more air entering the engine before they go up. In my opinion, it would be extremely hard to have su's which where to big.

with normal carbs, when u plant your foot, u increase the air passage way, and for a while, the air velocity enetering the engine slows down, this does not happen with su's.

Hey rueby, have a look at this site:

http://www.teglerizer.com/sucarbs/

go through the tuning guides for the SU's on this links page and you will find that they are quite easy to tune once you have the basics. There is one link there that is specifically from Nissan and goes through everything to help you keep those SU's purring.

Hope it helps with the decision

I found some pics of SUs on the web and stuff, and i think that they are the 38mm units. They are the standard ones off a 1600, thats what the bloke said when i boughht them. Ita also helps that they came bolted to a 1600 manifold.
I also have a smaller type Su with the flat top domes. They would have to be about 20mm i guess. They came of a 1000 from my friends grandfather!
Another question would twin 38s be too much for a slightly more than street 1200.
( ported/polished head, 2mm shaved block, really lumpy cam, headers, 2"straight thru exhaust)

Well i will keep a lookout for a gx head, but for the moment, I plan to use my ported standard head.
So I guess that the idea is that a webber is gonna be more user friendly than twin SUs, right?
If i decide that SUs are the go, the how muche xtra HP can I expect to get from the SUs compared to the webber?
basically would it be worth the extra misery of the constant tuning required with SUs?