SU spring and needle choice

Will Reeve · Post by **Will Reeve** » Sun Sep 12, 2010 9:50 am

Hi all,
Pulled apart the top of my Su's to set the mixture (level the jets up for a starting point!) and one of my springs has seen better days. Wondering what your recommendations are for the new ones. Standard SD1? If so which colour is that?
Engine is a first overbore 3.5l 9.35CR SD1 (non strengthened block). Running a Piper 270 cam, stock SD1 inlet manifold, SU's and a pair of K&N's. Exhaust manifold is tubular of unknown origin! Has about 500 miles under it's belt and plugs indicate it's running a little rich.

The needles in there at the moment seem to run the engine OK and pull fine so I am inclined to leave well alone, however a standard pair won't add much to the bill so a recommendation to try wouldn't go amiss please!

steveshaw · Post by **steveshaw** » Sun Sep 12, 2010 2:36 pm

Standard spring for SD1 is AUD4398 Yellow

kiwicar · Post by **kiwicar** » Sun Sep 12, 2010 4:53 pm

Hi
to get the right springs you need a rolling road, at full throttle you want the piston to reach the top of it's travel 500 revs before max rpm, with a near standard engine the standard springs should give you this, however as you go further from standard the only way to see this is with the engine running flat out on a dyno (or with your head under the bonnet on the open road

).
Once you have this set right you can then select the correct needles again on an RR with a lambda sensor up the exhaust and looking at the power comming from the engine, it is the cheepest way in the long run.
Best regards
Mike

Will Reeve · Post by **Will Reeve** » Mon Sep 13, 2010 6:46 pm

I just need her running OK, smooth idle, smooth power. I am very weak willed but want to spend more time driving around in the nice weather than tweaking and tuning

...this time! It was the main reason I went standard 3.5 instead of larger....I am still thinking I should have slipped in a stroker crank and pistons when the engine was in it's stand!

I love the engineering and simplicity of the SU. I've ordered a new pair of yellow springs from the SU people. I'll stick with my needles for the time being and keep an eye on the plugs. I've levelled up the jet tops. The plan this weekend is to stick some 20/50 oil in the carbs. Get them balanced with my new carb. balancer and adjust mixtures using the maximise rev technique (check with the push the button technique)...it will be a learning curve!

steveshaw · Post by **steveshaw** » Tue Sep 14, 2010 11:42 am

I'm in the middle of the same job of carb tuning, got nowhere untill I got a rebuild gasket set from Burlen.
The main problem I had was the o-rings in the cold start enrichment valve had got old an cracked, this was passing fuel from time to time, like the choke was out.
Things are a lot sweeter now, just waiting for a Gunson gas tester to set the mixture better.
I found 20w50 way too slow for me, liking 15w or 10w at the moment.
Be patient!

Will Reeve · Post by **Will Reeve** » Tue Sep 14, 2010 1:49 pm

...indeed. Didn't think of that. The carbs were taken off the car while an engine was built and they dried out, leaked like a good one from the seal at the bottom of the float chamber when re-assembled. If I can't get her idle speed smooth at the weekend it will have to be a rebuild I guess!

Will Reeve · Post by **Will Reeve** » Thu Sep 16, 2010 12:43 pm

Looks like a previous owner / tuner had stretched the springs in my SU's. I guess it's a tuning technique more "linear" than changing the needle. A stretched spring would presumably lean the mixture as the needle would be further down than 'standard'?

Will Reeve · Post by **Will Reeve** » Sat Sep 18, 2010 10:22 am

Pretty much sure it's carb. problem. Set up the linkages, one carb's mixture is set OK and the plunger test does nothing, the otherwise pushing the plunger always stalls the engine no matter what the mixture screw setting!
Idle is now 1.5k. I did mange once to get a perfect 700rpm idle, lovely but on a quick throttle open and snap shut the revs fell very slowly to stall!
The engine will also run on rough for a few revolutions when you kill the ignition..a sign of rich mixture?

Time for some rebuild parts I think!

DaveEFI · Post by **DaveEFI** » Sat Sep 18, 2010 10:46 am

Will Reeve wrote:Pretty much sure it's carb. problem. Set up the linkages, one carb's mixture is set OK and the plunger test does nothing, the otherwise pushing the plunger always stalls the engine no matter what the mixture screw setting!
Idle is now 1.5k. I did mange once to get a perfect 700rpm idle, lovely but on a quick throttle open and snap shut the revs fell very slowly to stall!
The engine will also run on rough for a few revolutions when you kill the ignition..a sign of rich mixture?

Time for some rebuild parts I think!

Have you checked the float levels? If adjusting the mixture screw makes no difference if might be flooding. Assuming the screw is actually moving the jet.

Will Reeve · Post by **Will Reeve** » Sun Sep 19, 2010 6:39 pm

I will check the float levels. I should have done it when they were off to repair a fuel leak from the bottom o-ring! My poor old brain didn't think that all the rubber components would probably be shot!
Here is the choke from one of the carbs, the o-ring doesn't look too bad but the end seal is shot!

Going to get a service kit and rebuild them both, also polish the dash-pots and clean the rest with good old Jizer and petrol!

I also found that the needle fitted to both carbs were a BCA which is very rich, that and the stretched spring wasn't helping my rich looking spark plugs!

Ordered some BBW (as recommended by Mr. Hardcastle for a standard engine with K&N's!). It's a start.

By the way I use winsu to graph needle profiles, good bit of software, although I've not tested out the "needle your carbs. via a tick chart feature" yet.

Will Reeve · Post by **Will Reeve** » Sat Sep 25, 2010 5:08 pm

Dave, the offending carb. had a sticky float, it would stick in the closed position (strange!) but it wasn't free to bobble about like the other carbs one. New one ordered.