Carburettors

[jongant]

MY series I 4.2 1965 FHC has a problem. It?s a problem that?s been with me for a fair while and I?ve had no luck in sorting it out. My car cannot idle without black and blue smoke pouring from it when driving off from stationary. Not while driving normally. Firstly, it is a little difficult to establish when the problem first reared its? head. I did have the head off for a refurb 10,000 miles ago and while we were there, the pistons were removed for inspection. My car is a daily driver at about 7,000 per year.

For the purposes of this report, the head has been checked since this work and found to be fine, IE nothing leaking past the valves. The pistons were re-ringed and the bores which were in tolerance ish, were lightly honed. I?m sure the bores could be better, but they?re fine. The blow by can be seen but it is not at all heavy.

The car runs very well, smooth fast and does not cough, splutter, vibrate or use copious amounts of fuel. It never fails to start on a very short press of the button, it holds 70 deg C steadily while cruising, it maintains 45psi (calibrated) at idle. In fact it?s fine...apart from after idling for a short while, perhaps at the lights, it starts off with a big puff of blue and black smoke.

I tried a lot of things to cure this and failed. All the things I tried I documented and measured and changed one element at a time. Changes were then set back to standard again and more research was carried out. What is obvious is that after 15 or so secs of idling, there is a pool of petrol in the base of the carb between the bridge and the butterfly........on all 3 carbs. Also, after a minute or so of idling the plugs are dripping wet and black. Clearly when the throttles are opened, the pool of petrol is sucked into the bore and burns black and blue.

Strangly, there is petrol evidence on the inside of the carb piston. IE the fuel must have made its way up the two holes in the piston base.

All my instinct tells me it is the carbs that are the problem.........but something else tells me to look further. Before anyone says try this or that, let me tell you what has been checked or renewed.

1. Were the float chambers flooding? No excess fuel ran out of the overflows and all chambers were checked. The floats have been tested at various heights with no improvement. Now back to 7/16.
2. The static height of the 3 pistons above the bridge is 12 thou. At idle with std springs this is about 40 thou.
3. I checked to see if using stonger springs which press the piston to the bridge at idle makes a difference. No improvement.
4. I wondered if the needle - jet ratio should be weakened? I tried both moving needles downwards in increments. Moving the needle down 30 thou and it still idles. Move it down 60 thou and it ceases to run at idle. In neither case does any movement make any difference to the pool of fuel or the black and blue smoke.
5. Half a turn on the richness screw is approx 14 thou up or down of the main jet. I have tried moving the jet up, (lean), in stages and this makes no effect on anything.
6. Whilst I had eliminated flooding as a possibility, I changed the pump for my SU spare and again no change.
7. The Jag manual says check the car is level. This could alter the fuel level in jet. Yes, it is level.
8. Was the valve timing affecting the pressure or suck in the manifold/carbs? The timing is spot on. That is according to the cut outs in the cams. TDC is absolutely correct. Does the engine start to draw air and fuel.......then stop dropping the fuel out of the air. Compression pressure is 170 psi which isn?t that high. But is that relevant?
9. What about ignition timing? Again it?s set at 10 deg BTDC on a 123 ignition. Difficult to suspect that as the engine runs really well at all RPM?s
10. The Jag book says consider vibration as a cause of flooding. No, the car runs really smoothly....even at idle.
11. Are the pistons in the carbs moving up and down smoothly? Yes they are.
12. Are the plugs a nice chocolate brown? On conducting a plug chop at 40 MPH and above, they are fine. Below that speed it is difficult to say.
13. Is the correct oil in the piston dampers? Yes, the official SU formula.
14. The main jets are new......and centralised. And set 5 x ? turns below level with the bridge to start with.
15. Various needles have been tried but yes, the standard UM?s work the best. Or at least no worse than all the others I have experimented with.
16. Is there play on the buttefly spindles? Only very, very slightly.
17. Have I run the fuel pump with the car not running to see if there?s any flow through the jet , or anywhere else? I have run the pump with the needle in and the needle out with the tops removed. This is not the cause of the pool of fuel. The fuel pool requires air to flow to replicate the problem.
18. Are the butterflies properly shut? Yes and I also increased the spring return pressure to be sure.
19. Are the volume screw chambers blocked in any way? All screws have been removed and blown through.
20. Are there any signs of cracks in the ally castings? None, that I can see.
21. Have you tried running the engine with the piston dampers removed? Yes and no difference.
22. Is the air filter clean? Yes, we have a new one...and have run it with no filter. Same problem.
23. Engine pistons are 9-1.
24. Car has been proven on the track to 140 mph with no problems.
25. After 5 mins of idling the car does become choked up.......the plugs are very black and wet.

I cannot think there is anything wrong with the carbs, only because I have tried just about everything and it makes the problem neither better.........nor in any way worse. Not least this has happened to all 3 carbs. If one carb/float chamber was to go faulty, it would go faulty on its? own.

So what else could cause this ?fuel pool??

Either the fuel is being sucked up the main jet or it is being pushed up.

Either the fuel is too much to go down the manifold into the engine or the engine doesn?t let it in.

Is the jet/needle CSA too great?

Is the engine blowing back down the manifold?

Is the ignition system incapable of burning the fuel.......but then how can it even try if the pool of fuel is in the carb body?

Why has this not happened with broadly the same equipment in the last 60,000 miles / 9 years?

So can anyone shed any light on all of this?

[christopher storey]

It sounds to me as though either

i.there is excessive fuel pressure - no more than about 2.5 lbs/sq.in ( about 0.2 bar) is needed - , or

ii. you have faulty needle valves in the float chamber lids . I do not think you mention having changed these, and they do wear quite rapidly

[Heuer]

Agree with Christopher - try new needles and a Filter King set to 2.5psi.

[jongant]

Thank you David and Christopher

Maybe I do need to install pressure regulation of the fuel. I did not know 2.5psi was reqd...so thats useful. I'm not sure why the pump could be pumping more now than before, when everything was ok.......(or was it ok). However, it is a cheap thing to add and I assume I can replace the current filter thats above the data plate. I shall report back

Jon

[Heuer]

Jon

Filter King installation details: http://etypeuk.com/forum/viewtopic.php?t=164

[jongant]

David thanks for the link to the filter king photos. What I have been able to establish as of 10 mins ago that the car std pump is currently running at 4 psi. I really hope this is the answer...........others seem to think so. I shall let you know.

Thx Jon

[christopher storey]

Jon : just one other thought - check that the overflow pipes at the top of the chambers are not blocked by corrosion, and also, check that the slotted ( vented) washers are a.fitted, and b. in the correct position nearest to the float chamber ( I hope that's correct!)

[PeterCrespin]

.., check that the slotted ( vented) washers are a.fitted, and b. in the correct position nearest to the float chamber ( I hope that's correct!)

Yup! :-)

[jongant]

Guys

Sadly my new pressure regulator has not cured my problems of the fuel pooling inside the carburettor. You will see the photo of the pressure regulator which I have mounted inside the aperture for the pump in order to leave the engine bay as std as possible. 2.5psi steady.

The second photo shows hopefully the fuel pooling in front of the bridge. I have therefore done some more checks to assist with the diagnosis.

All carbs suffer in exactly the same way.

Checks were done with a warm engine. All float needles were changed, albeit old ones look perfect. Float chamber vent pipes were checked and found to be open.

1) I have discovered that starting with bone dry carb innards, just cranking the car over for 8 secs (with fuel pressure at 2.5) is enough to flood to the extent shown.

2) Running the engine (on its own power) for 5 secs floods the carb innards the same.

3) Taking off the dash pot, leaving piston and needle in place and cranking for 8 secs (with fuel pressure at 2.5) does not flood the carb.

4) Doing 3 but with the 2 piston holes blocked does flood the carb.

5) Doing all the above with the float chamber centre bolt removed makes no difference. Done to ensure no blockages etc were to blame.

6) Doing all the above with the volume screw fully down and up makes no difference.

7) All the above checks were done with the piston spring in and out. No difference.

8) All the above checks were made without the needle. All the produced was a sea of Petrol in the carb.

9) Carbs are well insulated from the heat of the engine...........are they too well insulated?

10) Engine chop at 50mph / 2500rpm still shows wetness in the carb throat.

Is someone whose car does not appear to suffer from black and blue smoke when taking off from the lights, brave enough to idle their car for a few minutes, then lift off the dashpot to see how wet their carb is?

Maybe my condition is normal and I will just have to live with it, albeit it never used to do this. Clearly the engine depression is sucking fuel out of the jet........but the amount I get is immense in my book.

Any more ideas anyone?

[Heuer]

Jon

Contact David Lonsdale:
Classic Carburettors - David Lonsdale (SU carbs sale and restoration) T: 07786 912011 davidnlonsdale@aol.com

He is hugely knowledgeable, should be able to give you some expert guidance and is generally a really good guy.

[christopher storey]

Jon : perhaps your tests nos.3 and 4 set out above , unusual though they are , give a clue . If there is no flooding with the bell chamber off and the 2 air holes open, and yet if when the holes are blocked flooding occurs, it strongly suggests that this problem emanates from an obstruction to airflow ( akin to the time honoured choking method of putting hour palm over the carb air inlet! ) . Have you checked what happens if you run the engine with the air distribution manifold and the air filter disconnected . There are all kinds of maladies which can arise, from e.g. a rag getting sucked into the connection between filter housing and air manifold, to a squirrel having stored its nuts in the paper filter - and no I am not joking !!

[jongant]

Christopher

Point is definitely taken ref squirrels generally.......and while I have tried certain things with the air filter disconnected, I can't quite remember what and at what point. I shall just check again tomorrow to ensure I have not got something terribly wrong in the air inlet. And trust me, I shall never be happier if I find a sleeping rodent.

PS. I once got the fuel level warning light and the ignition (oil) light bulbs reversed. Caused me no end of consternation till some expert advice set me straight.

[jongant]

Thanks David.

FYI email address has changed to David Lonsdale <david@classiccarburetters.co.uk>

I've been through it at length with David and sad to say he is at a loss too. He gave me a few more checks to make but they all came up with the fact that everything should be normal. Not really sure what to attack next. Logic refuses to play a part.

Thanks though. Jon

[Heuer]

You need to look elsewhere, the carbs seem fine based on the tests you have done. Christopher has a point regarding air flow. If there is insufficient air getting to the carbs it would be the equivalent of running on choke as you are enriching the mixture hugely. Run the car without the air filter - in fact I would start by removing everything on the air side of the carbs. Take it for a run and see what happens.