Su carb Woes

[JamesR]

Having had some problems with my 11/4 Su's and getting them synched and the car starting up, I have now found that the slightest touch of the accelerator pedal causes huge revs, basically all the way round the dial. I have checked that the butterfly valves are near closed to the lowest idle speed possible, but they still accelerate far too much.
Does anyone have any ideas why this would be?
Plus I have no idea how to effectively use the vacuum advance. I can see the knob move in with an increase in idle speed but I don't know how much the knob should be wound in to start with??  

[GT6boy]

Hi James
This is a smut-tastic thread incidently. ;)
What engine/car is this on and is it a standard or modified affair? Also, has the car been running with the carbs in the past?
Fear not, i'm sure it is something that can be sorted quicky and easily.
Regards
  Bruce.

[Clive]

rather than vacuum advance do you mean the choke (ie knob iside the car. No jokes boys!)

[JamesR]

It's a spitfire mk3 and with kns.
There's nothing wrong with the choke as far as I know. The jets start down and when I push it in the jets go up and the idle speed lowers. They are both flush with the nuts.  :D
I guess the vaccuum advance is a separate question, but as I don't know how to set it I don't know how its affecting things.
What is perplexing me is how can this harsh acceleration occur with the butterflies almost shut.

[GT6boy]

If it has K+N air filters, people sometimes fit the heavier weight (yellow?) carb piston springs. What needles are fitted?
However, if you are observing the pistons and butterfly operations, you would have the air filters off, so being K+N's shouldn't make a noticeable difference in these circumstances.
The vacuum advance should be disconnected/blocked off when setting up the carbs.
Back to basics: are both pistons rising at the same time and rate when the revs rise dramatically?
Might you have an air leak in the inlet manifold(have you had this off?), so that even with butterflys nearly closed, it's sucking air in between the buuterfly and the engine?
As an aside, when you were synchronising the carbs, the interconnecting spindle should be lose, so each carb spindle can be individually set up for idle.
Bruce

[Spitfire1500]

Sounds like your idle is too high.

If you look down on your carburettors, there are two cross-head screws which control idle. Turn these anti-clockwise as far as you dare. Better to have your carburettors off than blow up your engine.

Theres a guide to tuning SU carburettors on this page:

http://www.mintylamb.co.uk/?page=sutune.htm

If you still get the same problem, it may be worth turning your mixture enrichment nuts anti-clockwise as far as you dare as well.

If the problem still remains, I would take the carburettors off and see if they are operating normally.

Hope this helps.

David

[Clive]

Check the butterflys are closing fully when at idle, and the pistons fall right to the bottom with a nice clunk when released(take out the damper from top of dashpot). The jets on those hs2 carbs need to be centered.
Oil in the dashpots, springs inside (you never know!)
Synchronise the carbs and set up as per the mintylamb istructions, all quite straightforward really.
And as above check for leaks around the manifold etc.

Vacuum advance is preset. Check it is working by sucking on the vacuum pipe, watch the dizzy toplate move. It should hold until you release the suck, and spring back to its start point.

[JamesR]

Thanks for the replies.
The idle is set as low as possible, I have had them off to check whether the butterflies are positioned correctly on the spindles with the screws off. (Apparently they are fixed...Shame)
The piston springs are black so I assume standard and the needles are BO. I did try to get some richer neddles from CC but they didn't have the fixed type and actually advised me not to change them.
I haven't had the inlet mainfold off so there shouldn't be any air leaks.
I can't guarantee that the pistons are rising at the same time because I have been too busy looking elsewhere when the acc is pressed... briefly!
The best thing for me to do would be to check the functioning of the pistons and adjust the jets, but what I'm not understanding then is how would increasing the amount of petrol decrease acceleration?

Btw. So what does the adjustment knob on the VA do then? It is a delco diz btw.

[GT6boy]

Ah-ha! I think you mean the ignition advance/retard knurled knob(oo-er) on the side of the distributor. This allows you make small amounts of adjustment to your ignition timing, with out having to slacken the pinch clamp base-plate on the distributor and turn the dizzy itself, to either advance or retard the ignition to obtain the correct timing.
It is not connected to the vacuum advance, but could be construed incorrectly as being part of that mechanism. Every day's a school day as they say!

[JamesR]

Timings out then... :P
Thanks. I'll bear that in mind!

[junkuser]

? pistons/needles sticking preventing full drop.

[JamesR]

I've adjusted the mixture per minty lamb, it wasn't quite right but I seriously need to use the colourtune on it on a non windy day!

As for the pistons they are moving up with acceleration, only a bit mind as I can't press the pedal/lever too far, and are returning on releasing the pedal. They move freely with the dampers out

Are there any non carb problems this can be related to?

[BarryH]

[i have now found that the slightest touch of the accelerator pedal causes huge revs, basically all the way round the dial. i have checked that the butterfly valves are near closed to the lowest idle speed possible, but they still accelerate far too much.]

Two things.

1) Have you had the butterflys out.  They are bevel edged and can be put in the wrong way leaving a gap and you will never get the idle down.  I can lend you my tee shirt for that one.

2) If you are getting the correct tickover until you touch the loud pedal is the cable or linkage sticking.  It's not unknown for frayed inner cables to catch on the outer and if you have had the linkage apart has it stiffened up and stopped returning properly, does it need spring loading?

Just a couple of thoughts as tuning and timing may be totally irrelevant and it is something more mechanical.

[JamesR]

I've checked the butterflies, they are correctly placed.
The cable isn't catching. Well its a mark III which has a lever and rod so that's not it.

Cards on the table, the idle is erratic at low revs, and is only steady around 2000rpm, but this was never a problem in terms of the accelerator...
The engine is now getting incredibly hot, but the temp guage doesn't go past halfway, but I think that's enough.  :o

As for air leaks I have checked everything as far as I can. As I said I haven't removed the inlet manifold, but in the interests of investigation I gave the nuts a turn with a spanner and they turned with a decent amount of pressure. I haven't got a torque wrench but they are supposed to be 14lbs/ft which doesn't sound a lot. Should they be turnable? It is relative? I've sprayed with wd40 whilst running and that doesn't seem to make any difference to the engine...

Also I've colourtuned the jets today and it hasn't helped. The timing is not set 100%. I have set it previously by screwdriver down the no1 cylinder and not moved the distributor. I have fitted a pertronix thingy so the timing isn't right for that. Could that be the problem?

I can see the obvious answer. Hunt down the air leak but it is no different at low idles from before I took the carbs apart and now.
I've got an MOT on monday so I'm getting kinda desperate, though the guy did say he'd pick it up Sunday (only so he knew it worked and didn't have to cancel the appointment, Monday). But he might be able to help...

[GT6boy]

Move to Bristol James- I 'd love to help! Can't help feeling that checking/sorting things to base settings has got to be the way forward: you've part achieved that so far with the carbs, but the ignition timing should be proved as well. Throttle spindles not worn are they, letting in air?

[JamesR]

Well I've tried sticking my finger over the outside part of the spindle and its made no difference. There is lateral movement but I'm not measuring anything that small.
It's also a point that the air leaks needle to be checked with the throttle open...

Someone well practiced in them would be great, everyone used to be an expert but aren't so sure now, though I've only just got them running properly. I guess I can get a proper diagnosis on Sunday but being quite busy at the moment I would like the time I have spent on it to count for Monday. Plus the rate its revving leads me to beleive that its something more than a little air leak.  

[JamesR]

Just a little update. It went for the service and mot. The guy said that he thought it was fine and was idleing at about 1000 and it wasn't accelerating too fast. He did change the brake pipes i did though so they should be ok.

Having driven it about a little in gear it seems to be ok. Now the next problem is, which happened when I first got it, is that in 1st and 2nd (having a bit of problem finding second, though it may just be my awareness of the gb having not driven it much)... It is juddering/bunnyhopping.
Is there any strict reason why a car should do this? I had a peak at the tssc board and there seem to be a number of posibilities...  ::) I have just been outside to check for sparks around the engine/diz and couldn't see any. So any ideas?

[mark spit]

Juddering when pulling away could be the clutch worn/contaminated. If it's with the clutch fully engaged and moving, ignition timing and spark strength and fuelling problems.

Mark

[JamesR]

Thanks, that's useful. I'm not sure off the top of my head which it is so I'll have to investigate further.

[JamesR]

I've had a short test run and it makes absolutely no difference what you do with the clutch, so that probably rules that out. I got someone else to drive it and he didn't think there was a problem in second...
I do need to change the plugs as a matter of routine, but there is nothing particularly wrong with them. As for the timing, going back to what I said before about setting having set it to TDC, having fiddled with the Advance timing knob, and set it just back of pinking that should be ok? Otherwise I'll have to get a timing light. Do I need one with an advance if I get one?

Plus can this actually be a result of a worn first gear. I can't thnk why but I'd like to rule that out as well. I'm trying to rule stuff out b4 spending money at the mo.

[rotoflex]

Get a timing light.  I had a sort of chugging on acceleration & it was the timing out.  As someone else mentioned, some non-original happycams really only idle at about 1000 rpm.  

Also when searching for leaks around the intake, a useful tool is that spray starting fluid.  Spray it around, & if it finds a spot with a leak, the engine speed will very noticeably change.

[JamesR]

Got the timing light and had a little play with it when the sun suddenly appeared this afternoon.
Not much to report except to say that i wasn't ready...

Anyone know the change in timing for using the pertronix/aldon ignitor? Plus I am wondering if the plug gaps should be bigger.

I take it the VA should be disconnected?
I getting a new head gasket and decoke set as well so I can fit everything knowing its correct.