PDA

View Full Version : Tuning SU's



MacGD016
13-05-04, 05:42 PM
I am trying to set up my SUís but Iím a bit confused. Iíve balanced them OK and am about to start on the mixture, however, if I slightly lift either SU piston the engine dies which the book says indicates a lean mixture, good news as this might solve another problem I have (see post on backfiring), but all the plugs look a bit sooty, indicating rich mixture. I'm still at the chassis stage so cant take it for a run so I guess the soot could just be from the choke.

Any views before I start adjusting things.

Mac

russell_ram
13-05-04, 06:57 PM
Mac,

If they are balanced OK and the enginne starts and runs I would leave well alone until you can drive the car under load. you can't really set needles untill you can pull full load on the motor. The plugs will always look 'sooty' after running at idle even if the mixture is near perfect because they don't get hot enough to 'self clean'

As for you timing issue in the other post, 4 deg at idle vac disconnected is far to retarded. You actually want as much static advance at idle as you can get without the engine 'pinking' under full mech advance under load. The 4 deg Range Rover number is so you can put any old cr*p fuel in without danger of pinking. Advance it up until it runs ok, you say 12deg, which I think is fine until you can do it properly later when you can drive the car.

Russ

kdavies3
13-05-04, 06:58 PM
Hi Mac, you are probably right about the continual starting on choke causing the black plugs.
I would wait until you drive it and clear it's throat a little, then try to set it up prior to SVA.

http://www.woodoouk.eclipse.co.uk/Wales.gif

Kev Davies
Dax, 383ci Chevy Stroker, Tremec TKO
South Wales

MacGD016
13-05-04, 07:32 PM
Guyís

Many thanks for your helpful comments, I will do as you say and leave it alone for now and get on with getting the body ready.

Mac

MacGD016
26-06-04, 02:12 PM
I have just got my GD on the road and its great to drive it at last. Having got over the initial euphoria I have been giving some attention to the timing and tuning of the SUís. I have made a number of observations:

1. With static timing set as per the book at 4 BTDC for a 3.9 it struggles to rev on the road beyond 3500 rpm. At 12 BTDC it revs to 4500 and at 24 BTDC to 5000 but then it pinks under load. 12 BTDC would seem to be a good setting for now, interestingly in first it revs up to 5000 before stuttering but wont go beyond 4500 in 2nd.

2. The plugs look about right (as per the Haynes manual) but there is a strong smell of petrol when under heavy load up a steep hill. I have checked the mixture with a colour tune, it might be just on the rich side, orange with blue, and rich when the throttle is blipped but I think this is normal.

Not really sure where to go from here

Mac

kdavies3
28-06-04, 02:12 PM
I would have a suspicion that your cam timing may not be all it could be. :(

Assuming that your float levels in the carbs are correct.

As said I would check the cam timing. you may be a tooth out or something similar.

MacGD016
28-06-04, 09:16 PM
I would have a suspicion that your cam timing may not be all it could be. :(

Assuming that your float levels in the carbs are correct.

As said I would check the cam timing. you may be a tooth out or something similar.
Kev

I hope its not the cam timing but if it is it will have to wait until I take it of the road for the winter. It might be the float level, however, it pulls well as soon as I change up so I suspect its not that but it might well be something to do with the carbs, wrong needles maybe, as they are an unknown quantity. Does anyone know if std needles for a 3.5 are correct for a 3.9?

Mac

Grease Monkey
28-06-04, 10:09 PM
Mac,

Sounds like the advance springs are knackered if its running better at various speeds with different manual advances set.

Have you got the revs/advance documented anywhere?

maxGD059
29-06-04, 08:19 AM
Mac

Standard needles are too lean for a 3.9 -I used to run BAF ones I think, & even then after some mild polishing of the heads, there was still the tendancy to run weak. Somewhere in the depths of my garage there lurks my old SU's which were in excellent nick when they came off the car -you're welcome to them if it helps

MacGD016
29-06-04, 11:12 AM
Mac,

Sounds like the advance springs are knackered if its running better at various speeds with different manual advances set.

Have you got the revs/advance documented anywhere?

Thatís an interesting thought, I rebuilt the dis so I know that the springs are there and the posts are firm and that advance does take place but that about all. I will have to have a look with the timing light.

MacGD016
29-06-04, 11:14 AM
Mac

Standard needles are too lean for a 3.9 -I used to run BAF ones I think, & even then after some mild polishing of the heads, there was still the tendancy to run weak. Somewhere in the depths of my garage there lurks my old SU's which were in excellent nick when they came off the car -you're welcome to them if it helps

Richard, That a really good offer I may take you up on it, I will do some testing first to see if I can pin it down to the carbs.
Thanks

Mac

MacGD016
02-07-04, 02:06 PM
Well thereís an interesting thing! Having rechecked the float levels, re-balanced the carbs and checked the mixture again, Iíve just been out for a quick blast to see if any of this has improved things, NO is the answer BUT; I was on full throttle in third, 4000 rpm and major stuttering as usual, then just as I lifted slightly the engine roared back into life and rev-ed freely upwards. I have since tried accelerating on ĺ throttle and all is well. I am sure that this points squarely at the problem which is likely to be too little or too much fuel but I have no idea which!!!!!!!!

I feel that I am close to sorting this, someone please HELP.

Ref: earlier post from Richard, I have BAK needles which are richer than the std 3.5 ones but slightly leaner than your BAFís.

russell_ram
02-07-04, 02:22 PM
Not sure you can tell which it is very easily without experimenting.

However, you could raise the needles up in the slides by one groove and re-try provided their not already as high as they will go. If it's better, then obviously it's weak. Make it richer first!

The different needle designations only really effect part load tune. The taper on the needle is the only variable once you have a matched pair of jets and needles on maximum diameter basis.

Russ

kdavies3
05-07-04, 02:59 PM
Just a thought but what oil are you running in the damper pots?
If it's too thick or thin you could be having trouble there, if too thick then the piston won't be able to lift so you'll be lean. And of course vice versa for too thinner oil.

MacGD016
06-07-04, 07:41 PM
I have 15/40 in them which should be ok but I have noticed that one piston is much easier to lift than the other?

kdavies3
06-07-04, 08:41 PM
They should be the same obviously. :(
Are the springs the same or have you got odd ones?

MacGD016
08-07-04, 09:52 AM
They should be the same obviously. :(
Are the springs the same or have you got odd ones?
Interesting point, I will have to have a look when I have decorated the bedroom!!