08-11-2017, 04:28 PM
Ok, not Austin 7 related, but I know there are people here who are well versed in the ways of the SU carburettor and I'm hoping someone may be able to help.
As well as Sevens, I have a 1932 Riley Nine fitted with twin 1" OM SUs running GG needles. OMs do not require and never had a spring fitted in the dashpot to temper the behaviour of the piston. Over the winter, I built a new engine for it, after running in, had it set up on a rolling road and embarked on a 2000 mile tour of Scotland. Apart from a mag crying 'enough' in the damp and me changing it for the spare, I had trouble free trip. I've used the car a bit since I returned home at the end of June. The other evening, I went out in it and, all of a sudden, it has developed a fault whereby the engine wont rev. When changing up through the gears, the engine dies when accelerating in the new gear (with your foot hard on the throttle) and will also show the same symptoms when the throttle is blipped to change down.
I have had the inlet manifold off and replaced the gasket as there was a sign of a leak. No change. I have had the filters out of the carbs and the pump and cleaned them, I have checked the flow of fuel from the pump. I have also fitted new plugs and changed the mag as part of my trouble shooting. All to no avail.
A pal came the other day who used to be a motor mechanic and he did a test whereby he removed the caps from the dash pots, dropped a screwdriver down each one, applied downward pressure with his hand and asked me to blip the throttle. The fault immediately disappeared. So, problem is caused by the pistons rising too quickly, basically letting too much air in, weakening the mixture and the engine dies.
The questions for my learned friends are as follows:
What could have suddenly changed to cause the symptoms? and
What might the solution be? I have fitted a couple of lightweight springs today, that has made a small improvement, but not enough. But the carbs are not designed to have springs anyway, so am I just curing the symptom and masking the fault?
Your thoughts, no matter how random or obscure would be welcomed...
Hugh
As well as Sevens, I have a 1932 Riley Nine fitted with twin 1" OM SUs running GG needles. OMs do not require and never had a spring fitted in the dashpot to temper the behaviour of the piston. Over the winter, I built a new engine for it, after running in, had it set up on a rolling road and embarked on a 2000 mile tour of Scotland. Apart from a mag crying 'enough' in the damp and me changing it for the spare, I had trouble free trip. I've used the car a bit since I returned home at the end of June. The other evening, I went out in it and, all of a sudden, it has developed a fault whereby the engine wont rev. When changing up through the gears, the engine dies when accelerating in the new gear (with your foot hard on the throttle) and will also show the same symptoms when the throttle is blipped to change down.
I have had the inlet manifold off and replaced the gasket as there was a sign of a leak. No change. I have had the filters out of the carbs and the pump and cleaned them, I have checked the flow of fuel from the pump. I have also fitted new plugs and changed the mag as part of my trouble shooting. All to no avail.
A pal came the other day who used to be a motor mechanic and he did a test whereby he removed the caps from the dash pots, dropped a screwdriver down each one, applied downward pressure with his hand and asked me to blip the throttle. The fault immediately disappeared. So, problem is caused by the pistons rising too quickly, basically letting too much air in, weakening the mixture and the engine dies.
The questions for my learned friends are as follows:
What could have suddenly changed to cause the symptoms? and
What might the solution be? I have fitted a couple of lightweight springs today, that has made a small improvement, but not enough. But the carbs are not designed to have springs anyway, so am I just curing the symptom and masking the fault?
Your thoughts, no matter how random or obscure would be welcomed...
Hugh