fuel flow problems

Hi Dave, I'm absolutely no help with photos, but in my opinion, if you can possibly keep the fuel lines away from the exhaust, it's worth doing. It can only help you!  I'm too late with this one since you're already organising a new tank, but you say that the tank breather is fine, how did you check it? Have you tried running with the fuel cap open? Although, I don't see how the new fuel pump would have made a breather problem any better, even temporarily.  I really hope your new tank sorts it out! Good luck! Paul. Posted by pryantcc 30/08/2012 07:43:39

When engine is still cold or warm, i.e. before the problems start, have you switched from coil to magneto and from Main to Reserve tank to ensure no issues here?  Following this thread closely, and know you will get resolved.  Before I thoroughly cleaned and sealed my SP 25 tank, the fuel flow would frequently slow down.  When it's hot and faltering, are the SU fuel pumps chattering away or clicking slowly?  Apologies if some of this is redundant. Posted by johnlayzell 30/08/2012 19:54:53

Hi John,  my car (late SP25 SC) is coil only (no magneto option).  When the car falters, pumps are quiet. I have tried other pumps and checked pump operation.    Thanks for the suggestions Posted by DaveT 03/09/2012 09:13:47

Hello Dave, Have you disconnected the fuel feed pipe from the outlet of the pumps and before the carbs and connected a hose to direct fuel into a container? Check to see if the flow is continuous or stops.  Another possibility is that if the fuel level is low there may be a split in the pick up pipe in the tank which is covered when there is plenty of fuel but is uncovered as the level falls. This will then pull in air giving initially a weak mixture causing hot running , fuel vapourisation and eventually a complete stop.  Cheers  john Posted by speed20 03/09/2012 18:13:21

Hello John,  thnaks for your suggestions - I have tried an external tank and suspect this expeirment gave misleading results due to rust in the various filters in the system. I ordered a new tank to eliminate the rust issue AND to be certain any splits in the pick up pipes are also eliminated. RE tank breathing, I have run the car in the garage with the fuel filler cap completely open, also there is a breather pipe in the tank which is, in my opinion too large to have become blocked (it is fully uncovered at present and has not been squashed by the boot floor!)  thanks for your comments  Dave Posted by DaveT 06/09/2012 09:19:02

Dave - looking forward to the successful outcome of this longish saga.  Please keep us updated!  Rally on! Posted by johnlayzell 09/09/2012 15:44:56

Hi Dave, You never answered John Worrall's question... "Have you disconnected the fuel feed pipe from the outlet of the pumps and before the carbs and connected a hose to direct fuel into a container? Check to see if the flow is continuous or stops."  This seems very relevant, and would show if the pumps can continuously pump large amounts of fuel, and whether they are doing so on main and reserve.  Ian Hardaker Posted by IanHardaker 09/09/2012 19:46:05

Dave, Why would the fuel pressure go 'off the scale' with vaporisation?  If the pumps are only taking in vapour instead of liquid the pressure will fall as the pumps have nothing to work on.  If there is vaporisation between the pumps and the carbs, the vapour will escape through the carbs more easily than liquid, and the pressure will fall. Dudley Posted by Dudley Smith 10/09/2012 22:49:04

[quote="IanHardaker":1vrrzdm4]Hi Dave, You never answered John Worrall's question... "Have you disconnected the fuel feed pipe from the outlet of the pumps and before the carbs and connected a hose to direct fuel into a container? Check to see if the flow is continuous or stops."  This seems very relevant, and would show if the pumps can continuously pump large amounts of fuel, and whether they are doing so on main and reserve.  Ian Hardaker[/quote:1vrrzdm4] in response to this, I have tried this test numerous times with all three pump options and with each of the pumps isolated. The pressure reading usually builds up correctly with my thumb over the end of the open delivery pipe hinting at a float chamber problem or feed pipe leakage (or AED issue) - all of which I have tried to investigate (more than once!).  Re the next point about vapourisation. There are obviously two possibilities vapourisation before the pump and vapourisation after the pump. If the pump is unable to deliver fuel due to vapourisation (or pipe blockage or air leak) the pressure reading might drop (IMHO) - again I have been investigating this possibility for many weeks now. If the vapourisation occurs in the feed pipe to the float chambers I believe that the pressure will increase (it might bleed away but if the vapour pressure bleeds away (through the float chambers) the pumps should pump fuel and the pressure should read normal (IMHO).  This past weekend I have driven the car with a temporary but clean fuel tank and with new feed pipes. It ticks over very smoothly but now has a misfire under load which I have not yet pinpointed. It appears to be reliable but not right yet. I am happy with the ignition timing and the rpm related advance in the distributor appears to be working correctly. Also, none of the pistons in the carbs appears to be sticking although I concede that the jets and needles could be a bit worn. One other symptom is that the RHS exhaust seems to be runiing cooler than the LHS and the RHS seems to be emitting white smoke - a possible coolant leak into the combustion chamber or exhaust port perhaps. In the hot weather of last weekend it also seemed to be overheating more readily than I remember from last year when I took the car to France etc..  Investigations continue.... many thanks for the inputs so far.  cheers  Dave Posted by DaveT 11/09/2012 11:10:02

My question about letting the fuel flow into a bucket is more to do with volume than pressure. if the tank inlet or fuel pipe was blocked, the pump would make pressure with a small amount of fuel, but if it was restricted at the tank, the volume would soon be sucked out, and the output volume decreased. This may have been the case, and cured by the new temporary tank. I have had similar problems with hydraulic pumps sucking the inlet strainer flat due to blockage and the pressure being OK as long as the pump did not have to deliver any volume of oil. When volume was required (I.e. to move a cylinder), the pressure and volume dropped of dramatically.  It sounds like you're progressing, but with possibly bigger problems to come with coolant in the exhaust. Good luck! Posted by IanHardaker 12/09/2012 00:55:01