impulse

I have the 3-screw dome-top SU's on the car, now I've been doing all my reading on this site here to try and find out how to adjust these properlly. Really all I need to do to finish up, is lessen the fuel.

Now I found the air-mixture 'screw' on the top side of the carbs, and apparently for fuel I'm looking for a mixture 'nut' on the bottom of the carbs.

Unfortunantly all I can find on the buttom of the carbs are 2 'dial' type finger screws (size of a nickle). One of them seems to be related to the choke. (has a spring between the dial and the carb, with 2 ball berrings in the dial itself) The other is a long-stem with a thinner nickle size dial on the bottom.

How do I lower the needle (and lower the fuel amount) ? Should be good to go for a test-drive this weekend, if I can stop it from running so extreamly eye-watering rich.

nismospek

Let me start off by saying i don't know how to adjust them either.
The thing is that i have some 4 screw carbs with those type of adjustments.
I thought it was something that only came on the 4 screws.

theramz

Before you start adjusting the mixture you need to make sure the choke cables are pushing the linkage all the way down. Even a partial opening will make it run extra rich. Unless there has been a significant change in altitude you shouldn't have to adjust the mixture. The earlier 3 screw carbs have a thumb wheel (pic 1) they are preset and have a tab that limits the fine tune adjustment. The 4 screw carbs have a knurled ****. On mine I turned the **** all the way in and backed out 3 turns to start with, then final adjust resetting the air balance and idle speed until I got it to run real good. Make sure the pistons rise and fall evenly. Observe them with the air cleaner off at different throttle speeds. Actually you should do that first because if their not even it will run like **** no matter what you do.

impulse

Thanks! Thats the info I needed.. So somehow I have a 3screw dometop, and the bottom of the 4 screw? wierd, but oh well. Thanks again.

theramz

I think there were some changes to the su during the years. Also MG and Jag used them. Things can get mixed and changed over 35 years. I am at 4500 elevation, so you may need 1/2 -1 more turn on the screws. Make sure you have clean 20 wt oil in the top bells. This controls the opening speed of the pistons. Be very careful with the long needle valves. You can try 10 wt to get them to open faster but you should have the basics done before you play with them. When I took mine apart I was surprised at how simple they are compared to a Holley or others.