SU Help

[twiss]

Yeah definitely block up the overrun valve, they are known to cause all sorts of problems... we all run plain throttle discs. (I was going to say "here" but I'm pretty sure I'm the only person in Malta running an SU carb )
If the engine revs are dropping off it's going to be creating a lot of vacuum very quickly when the throttle disc closes, and this will probably suck the overrun valve open and keep the revs up...
When you have the choke on it keeps the throttle open a little bit, so there will not be as much vacuum applied to the throttle disc valve

[3VILC]

Id say I'd be one of the few other than kiwizook running one here too.
My theory completely based on my own common sense is that running the SJ head on the G16 block (which is a whole different arguement altogether, I know it wont have any top end due to smaller valves and gains nothing but it was cheap and had all the boltons i needed) Is that the smaller valves and probably smaller ports and intake aswell will create a slightly higher than normal airflow with the throttle closed, since its trying to draw more air for the bigger displacement, which may be just enough to stop that valve springing closed again. Hopefully, fingers crossed haha. I just want to get the darn thing running so I can finish everything else off and go wheeling

[3VILC]

Ok so got chance to at least test a new return into a spare fuel container, idles a bit better but still high, but most importantly no fuel out the overflow, so thats a good start. Next to work on the idle.
If logic serves me correctly, the SU has no bypass for air at idle, so with the idle screw wound right off the throttle stop the butterfly should be completely closed and the engine should die right?
Sprayed some WD40 (messy, but all I had in an aerosol) around the manifold joints and throttle spindles etc no change so I suspect its not drawing any air in here (although I didnt drive it, a little hard with the return line hanging out the gaurd in a container haha) but was up to temp. So has to be that overrun valve surely. Cant (better not) be drawing anything though the valve seals those are all new, valves and seats all cleaned and re-lapped, were covered in carbon from the oil burning through the seals.
Next time, remind me to buy a running engine haha.
Today the max idle warm was 1200, so not as bad and can live with that in a offroader but still like to find the cause

[rodbourne]

i've got exactly the same problem. i've read somewhere that there are a couple of different jet sizes?!?!?!?!?!?!?!?!?. slightly less fuel should drop the revs a bit ??????
only got the sj back on the road today after it sitting for last 4-5 years. so it's all new to me again.

[rodbourne]

here is the info i've found so far

http://retrorides.proboards.com/thread/141403

According to my Haynes SU book there are 3 jet sizes in the SU family. 0.09", 0.1" and 0.125". They need to have matching needles.

The HIF carb series only uses 0.09" and 0.1" so don't go buying 0.125" needles for it! If you know the needle code letters I should be able to tell you what size jet it needs. If you know what car you carb came from I can also tell you what size jet and which needle it was born with. Also what spring it had fitted - there are 3 or 4 different spring weights fitted (and two different diameters) and this also matters as the spring weight also affects the fueling profile.

Cheers

Nick

Yeah. BDL is for a 0.100" jet.

[3VILC]

Cheers for that Nick. Yup its the .1 Jet so should be correct with BFM needle. Originally had from memory a BCP needle don.t know where its gone haha. Was brought for a Land Rover V8 (twin HIF's) so assume it would've been set up for the originally. Still yet to get out and solder the butterfly up see if that helps

[rodbourne]

mine had bcw needle and tickover was fine. now has bdl and i have 1750 rpm tickover. no leaks or tight cables, tickover screw isn't touching.

[3VILC]

As far as I know, the jet sizes only differed by carb size, and are not interchangable. Well, perhaps they are for extreme tuning but I don't think they were intended to be, HIF44/6 should always be .1"
Changing the needle shouldn't change the idle, since the idle portion of the needles is virtually the same across the board.
Since we are aiming to set the mixture where the engine just runs at the fastest speed, adding more fuel should have no effect on the idle speed either, so in theory, more air has the be getting into the system somewhere. As I mentioned, as far as I can see, maybe your book might tell you otherwise, theres no idle air passages on an SU so with the idle screw right off it 'should' stall. I haven't got my kids this weekend, so hopefully will get plenty of time to get out there and pull the carb again and investigate this over-run valve.. its about all I've got left to try before I start haha

[3VILC]

Some progress today..kindof.
Investigated the over run valve, spring seemed pretty strong but short of any other ideas I disabled it anyway. Decided I didnt have the equipment to solder it very well so I took it out, drilled out the center of the valve part and riveted it back over the hole. A bit primitive, but it does the job. Presto, now I can wind the idle down until it stalls.
Then the overfueling problem showed how bad it actually was, obviously the extra air flow thru the valve was enough to burn most of it. Dialed the fuel reg down a bit (supposedly factory set at 2PSI) and it seem to have stopped coming out the overflow and idles good, now it backfires out the carb at high revs but the idle mixture seems OK using the lifting pin to raise the piston. I'm about ready to start throwing spanners around so must be time to leave it again for now haha

[3VILC]

So after todays tinkering, I've got the mixture probably not far off being full rich, but still getting a high rpm backfire out the carb. Upto about 4000rpm it runs pretty good. Still wondering it its to do with the head struggling to breathe the extra 300cc of the 1.6 block. Wouldve thought that would just make it gutless up top not lean it out. Maybe I should chuck the BDL needle back in and see what happens, thats richer across the scale but slightly more so up top, at least it should put the jet adjustment back where it should be rather than right in.
Prehaps I should upload a video maybe someone can offer a suggestion listening to it?

Update: swapped the needle back to the BDL, re adjusted the idle mix, had to go slightly over rich but now it pulls hard and feels a little stangled up top as expected but no backfire. And I mean pulls to round 5500rpm now. Prehaps this is where people are finding the BBZ needle better thats hugely richer in the mid range