fuel flow problems

Hi,  having had quite a few problems with hot running in the past few days, I am now fairly sure that the fuel supply is being starved by something. I have rebuilt the carbs, fitted new fuel pumps, blown the fuel pipes back into the tank, rebuilt the reserve / normal fuel tap, emptied the tank and washed out quite a lot of rubbish, etc..and still the car goes from running very nicely, to misfiring (2 carbs?), to only just running (1 carb?) to stalling altogether. The tank breather is ok, the ignition has mostly been renewed and checked with inline HT spark testers.  Is there a filter inside the fuel tank that I should find and check? (I have only managed to get one of the fuel feed pipes to flow compressed air properly so far) All ideas and suggestions welcomed - fuel vapourisation from incorrect timing or bad petrol are two obvious suggestions and I have ordered a fuel pressure meter to monitor the pipework between the pumps and the carbs ...  thanks  Dave Posted by DaveT 29/05/2012 19:51:16

Seems to me that you have all the classic symptoms of fuel vapourisation. Try car when ambient temperature is lower. Late evening perhaps. If problem occurs again when weather is hot park up in some shade, leave engine idling, with bonnet open for two or three minutes then go on your way again. If either of these tests has positive results then you have vapourisation due to high underbonnet temperatures. Fuel vapourisation is a problem again because moden petrol is so volitile. I have had this problem and carried out these tests on my 1925 Sunbeam this week. Good luck  Mike Wheadon Posted by Mike Wheadon 30/05/2012 09:11:58

Dave,  Are your fuel pumps on the manifold or drivers side of the car?  Is your exhaust manifold the early one piece integral inlet and exhaust unit, or the later separated?  After how long driving/how hot do the symptoms appear.  If you pour cold water on the fuel pumps and/or carbs does this help?  Do you have in-line fuel filters between the tank and the fuel pumps? Over the years I've had poor running and vapor lock on my SP25 and have gradually eliminated. Posted by johnlayzell 01/06/2012 01:45:51

Before you change too much stuff try a new ignition coil - they can give these symptoms too! Posted by Nickalvis 28/06/2012 18:15:59

Thanks for all the replies so far - Here is a quick update:  I have rebuilt most of the ignition system (including trying a new coil and resetting the timing which appeared to have slipped) I have rebuilt the carburettors I have rebuilt the fuel reserve switch and blown back the fuel line into the tank. On the reserve pipe, I was not happy with the result and went right back to the tank. There was a small blockage in the reserve pipe within the tank which I did not find until the tank was drained and the tank removed (but as I was not using reserve during all previous testing this is not relevant). The inside of the tank is quite rusty and despite cleaning it a few times there is still debris in the bottom of the tank which impedes the end of the reserve pick up pipe. I have now added 3/8 insulating spacers between the carb bodies and the inlet manifold (in 3 pieces and not combined with the exhaust manifolds) and remade the choke supply pipe and air silencer fixing to suit. I have made a new heat shield and fitted it so that it does not touch the exh manifolds (unlike the original) I have fitted 2 new fuel pumps (drivers side bulkhead) and added a T piece and fuel pressure gauge between the pumps and the carbs I have discarded all 5 gallons of the supermarket petrol (into the daily runabout) which I suspect was the main cause of the original problem  Later this week I should have the tank back in and with some new fuel will be ready to try it.  Lets hope somewhere in the above that I have found and fixed the problem without introducing any new ones!  Thanks for the advice so far.  Dave Posted by DaveT 02/07/2012 12:16:19

This is starting to evolve further. On the road, the car now struggles for power under load ( I live on a hill and the car is misfiring trying to get up that hill).  An inlet manifold vacuum measurement is indicating good vacuum at idle but this reduces with increasing rpm, whereas I believe the vacuum should increase. I am measuring the vacuum instead of the choke fuel supply pipe at the end of the balancing pipe. What does this mean? Partial blockage in the exhaust system seems to be the likely cause - so guess where I will be this weekend? - taking the exhaust to pieces!  BR Dave Posted by DaveT 20/07/2012 11:16:10

exhausts were fine but problems continue.... Posted by DaveT 23/08/2012 09:53:41

What a saga Dave! Lots of questions: Does this happen right from cold, or only when the car heats up a bit? If its OK when cold, is this only while on choke, or OK until the engine really heats up? What sort of air filter arrangement do you have? Have you tried it without it? Does it run rough, or just not as much power as usual? Was the initial problem a sudden occurrence?  Thanks Paul. Posted by pryantcc 23/08/2012 20:33:30

Have you tried a different fuel?  Do you always buy your petrol from the same garage? Posted by Angus D'Arcy-Drake 28/08/2012 12:53:42

Hi,  here is a quick update. The car starts every time without choke. The plugs look fine indicating mixture is ok. Dwell and timing have been set up correctly. An alternative double ended style SU pump appeared to make things much better but not for long!  I have tried a few different types of fuel but suspect that the main culprit is either a supply blockage or an air leak between the tank and the pumps. A new tank has now been ordered (old one is very rusty internally) and I am in the process of fitting new fuel lines. Filters will be included in both runs.  The current layout of fuel lines on my car is not original and needs to be changed but the original layout for a late SP25 is for the dual fuel lines to run along the outside of the chassis on the offside (twin electric SU pumps are on the offside of the bulkhead) ie. pipes will be quite close to the silencer / exhaust run. Should this be avoided? ie will it create a similar problem even when everything is new. Currently I do not accept that fuel vapourisation is to blame - I have a pressure gauge fitted between the pumps and the carbs and this would go off the scale in a vapourisation condition, whereas it drops to zero when the car starts to falter.  Does anyone have a few photos of the fuel line installation along the side of the chassis before fitting bodywork onto a late SP25 that might help me, please?  thanks  Dave Posted by DaveT 29/08/2012 12:02:25