09-11-2017, 09:45 AM
Thanks all for the input. I am getting a bit fed up with this. The symptoms all point to fuel weakness but...
To address the points raised:
The pistons rise and fall easily in the carbs.
The plugs seem a little black, but I put this down to the way the engine is runnning poorly. The actual ignition point on the plugs seem about the right colour.
Fuel level looks ok as to needle and jets. But to be honest, once those sorts of things are set on SUs, it's hard to see how they could change.
The carbs have been apart and are clean. All the filters (in the pump and the float chambers) are clean.
The floats are good.
The pump is good and delivers plenty of fuel.
All fuel lines run freely.
The engine ticks over perfectly, the fault only manifests itself on fast increase of revs.
I haven't checked the compressions but the engine had a complete rebuild over the winter with new liners and pistons and has done about 2,500 miles. I suppose I should do that..
Fuel is the last tankful I purchased in July but the engine ran perfectly about a week before the fault appeared. I could try fresh petrol.
To eliminate them, I have swapped the magneto and fitted new plugs.
It does spit back through the carbs when 'faulting' but that is the normal behaviour of a weak fuel issue...
The carbs are mounted on an ali inlet manifold which is in turn mounted on the block. I have remounted the manifold with a new gasket and some gasket silicone.
I have checked there is no internal failing of the inlet manifild allowing air to get in, but for completeness, I should perhaps try another one to eliminate that possibilty.
I will agree, it does feel like an air leak to me, but I buggered if I can find one..
The only thing that can resolve the fault is the test with the 2 screwdrivers down the top of the dash pots preventing the pistons rising too quickly. The engine revs away like a good'un when I do that. Yesterdays test was to fit 2, admittedly unidentified but matching springs to the pistons but they didn't make mcu difference. But as I say, the carbs shouldn't have springs as designed, so that would just be masking the fault rather than resolving it...
Thanks for the ideas. I will continue my efforts and provide an update...
To address the points raised:
The pistons rise and fall easily in the carbs.
The plugs seem a little black, but I put this down to the way the engine is runnning poorly. The actual ignition point on the plugs seem about the right colour.
Fuel level looks ok as to needle and jets. But to be honest, once those sorts of things are set on SUs, it's hard to see how they could change.
The carbs have been apart and are clean. All the filters (in the pump and the float chambers) are clean.
The floats are good.
The pump is good and delivers plenty of fuel.
All fuel lines run freely.
The engine ticks over perfectly, the fault only manifests itself on fast increase of revs.
I haven't checked the compressions but the engine had a complete rebuild over the winter with new liners and pistons and has done about 2,500 miles. I suppose I should do that..
Fuel is the last tankful I purchased in July but the engine ran perfectly about a week before the fault appeared. I could try fresh petrol.
To eliminate them, I have swapped the magneto and fitted new plugs.
It does spit back through the carbs when 'faulting' but that is the normal behaviour of a weak fuel issue...
The carbs are mounted on an ali inlet manifold which is in turn mounted on the block. I have remounted the manifold with a new gasket and some gasket silicone.
I have checked there is no internal failing of the inlet manifild allowing air to get in, but for completeness, I should perhaps try another one to eliminate that possibilty.
I will agree, it does feel like an air leak to me, but I buggered if I can find one..
The only thing that can resolve the fault is the test with the 2 screwdrivers down the top of the dash pots preventing the pistons rising too quickly. The engine revs away like a good'un when I do that. Yesterdays test was to fit 2, admittedly unidentified but matching springs to the pistons but they didn't make mcu difference. But as I say, the carbs shouldn't have springs as designed, so that would just be masking the fault rather than resolving it...
Thanks for the ideas. I will continue my efforts and provide an update...