The LandRover is starting to run really well [and it's great to get the hard top off in the sun] but there is one intermittent niggle that I need some help with. Every now and then the engine starts to 'miss' and this is accompanied by a very noticeable loss of power / jerkiness. What I have found is that if I then stop, open the bonnet and prime the Fuel Pump using the large squeezy lever then some air bubbles come through the Glass Bowl and everything is just fine again - for another two or three trips.
All pipes appear to be tight and sound but air must be getting in somewhere. Fuel pump and carb have already been replaced by previous owner -Where should I start to look?
I'd look at the pickup in the tank - either a pinhole air leak (does the problem happen only at low fuel levels?) or the pickup pipe is partly blocked, possibly by loose debris in the tank that is sometimes sucked up against it. Also do not lose sight of the fact that it is just possible that the problem is not fuel, and the bubbles are a red herring - could be a loose connection in the ignition primary circuit, for example, or I have had similar problems due to insufficient points gap due to wear on the rubbing block.
I have installed a new fuel tank and the pickup looked fine when it was out though I didn't look too closely. It's simple enough to take it out again to confirm whether this could be the problem.
Both those certain sound possible explanations but I can consistently replicate the problem and the steps needed to rectify. Only that little lever on the fuel pump seems to do the trick (every time) which seems to eliminate all non-fuel explanations - or am I missing something?
It's possible that it's the wrong fuel pump if it's been changed recently. The mechanical type come with a number of different internal operating arm lengths, & it may not be making proper contact with the camshaft, which reduces the flow somewhat. I had one of these many moons ago on a different vehicle, & it took a while to work out what was going on. IIRC, the pump even came out of a sealed box with the right part number on it:-/ It may even be a spacer block missing / extra between the pump & the block, or a broken/ weak/ misplaced return spring on the operating arm.
If it wasn't a new factory type fuel pump, but just reconditioned (As I often used to) by changing the diaphgram & associated bits, it may be sucking air in past the edge of the diaphgram if it's been installed incorrectly. Either way, bubbles in the sight glass mean you've got an air leak between the fuel in the tank & the diaphgram. Could be a pinhole in the pick up pipe or at any of the joints, or even a corrosion pinhole in the pipework itself, which IIRC is steel, & probably as old as the vehicle.
It seems to be pumping *almost* enough fuel to cope with the engine's needs, but not quite enough. If it's petrol with a carb, bubbles in the fuel aren't necessarily a stopper, as unlike a diesel or injected engine, the bubbles just vent through the needle valve in the float chamber. The fact that you say it's a gradually worsening problem would indicate to me that the fuel level in the float chamber on the carb is gradually dropping, leading to a weaker & weaker mixture, until it finally stops, which is then rectified by using the hand primer to refill the float chamber.
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