Discussion
Hi . Further to my previous post, foot down but not floored , hesitation and not a lot of 'oomph'. Foot to the floor , a wee bit of hesitation and then 'begrudgingly' quite a bit more 'oomph'. It feels like it wants to go but cant, then all of a sudden it all falls into place and bingo off it goes.
I may be off track here but does the vacuum advance have a metered bleed? If I suck down the tube I'm getting resistance and I can see the dizzy plate moving but need to keep sucking to maintain position ie the diapragm is not completely sealed( should it be ) ? I've checked for leaks and changed the hose so the 'leak' is definately coming from the unit itself. I just dont want to go spending money on a new unit only to find that they are supposed to be like that !
I've also bought a new fuel filter which I'll be fitting tomorrow just in case its fuel related.
I noticed on a previous post 'Spend' mentioned the ignition amp in relation to a poor running question, is that a common fault and if so what are the normal symptoms?
Sorry to be asking so many questions, I'm trying to save money so's I can afford a new roof covering kit and a re-spray ,so any money I can save on diagnostics is a real bonus. Ta , Nick........
I may be off track here but does the vacuum advance have a metered bleed? If I suck down the tube I'm getting resistance and I can see the dizzy plate moving but need to keep sucking to maintain position ie the diapragm is not completely sealed( should it be ) ? I've checked for leaks and changed the hose so the 'leak' is definately coming from the unit itself. I just dont want to go spending money on a new unit only to find that they are supposed to be like that !
I've also bought a new fuel filter which I'll be fitting tomorrow just in case its fuel related.
I noticed on a previous post 'Spend' mentioned the ignition amp in relation to a poor running question, is that a common fault and if so what are the normal symptoms?
Sorry to be asking so many questions, I'm trying to save money so's I can afford a new roof covering kit and a re-spray ,so any money I can save on diagnostics is a real bonus. Ta , Nick........
The vac advance should have a 'slow return' valve in it.
The intended behaviour is that when suction is applied on the 'throttle body' end the vacuum advance only comes in very slowly. When suction is removed from the throttle body end, the vacuum advance is removed quickly. The purpose of this is to stop the vac advance from engaging transiently as you open the throttle from idle.
However, these valves are often understood and fitted back to front, and they can also fail and get blocked. So if in doubt just connect a plain hose between the dizzy and the throttle body.
The vac cannister should hold suction indefinitely, and if the hose is a good fit on the cannister and you suck on the hose to bring in the vac advance then block the hose, you should be able to feel the vacuum release if you unblock the hose a couple of minutes later. If it won't hold vacuum that implies the diaphragm in the cannister has split and the cannister needs to be replaced.
Unless there's something very wrong, none of this should affect the behaviour on load. So as a sanity check you can disconnect the hose to the dizzy, block off the spigot into the throttle body and leave the vac advance cannister on the dizzy open to fresh air, you will get no vac advance and none of this will have any effect. So if you still get erratic throttle response it's nothing to do with the vac advance.
A worn throttle pot would be my next guess. Sometimes you can prove there is a problem by measuring the resistance as the throttle moves, but unless you have a storage scope and logging equipment you can't really prove there *isn't* a problem if you see what I mean. So when the symptoms point to a possible throttle pot problem, your best bet is to replace it and see whether you still have the problem.
Note that the throttle pot has to be calibrated after fitting.
The intended behaviour is that when suction is applied on the 'throttle body' end the vacuum advance only comes in very slowly. When suction is removed from the throttle body end, the vacuum advance is removed quickly. The purpose of this is to stop the vac advance from engaging transiently as you open the throttle from idle.
However, these valves are often understood and fitted back to front, and they can also fail and get blocked. So if in doubt just connect a plain hose between the dizzy and the throttle body.
The vac cannister should hold suction indefinitely, and if the hose is a good fit on the cannister and you suck on the hose to bring in the vac advance then block the hose, you should be able to feel the vacuum release if you unblock the hose a couple of minutes later. If it won't hold vacuum that implies the diaphragm in the cannister has split and the cannister needs to be replaced.
Unless there's something very wrong, none of this should affect the behaviour on load. So as a sanity check you can disconnect the hose to the dizzy, block off the spigot into the throttle body and leave the vac advance cannister on the dizzy open to fresh air, you will get no vac advance and none of this will have any effect. So if you still get erratic throttle response it's nothing to do with the vac advance.
A worn throttle pot would be my next guess. Sometimes you can prove there is a problem by measuring the resistance as the throttle moves, but unless you have a storage scope and logging equipment you can't really prove there *isn't* a problem if you see what I mean. So when the symptoms point to a possible throttle pot problem, your best bet is to replace it and see whether you still have the problem.
Note that the throttle pot has to be calibrated after fitting.
Cheers for that V8s . Sounds like my diapragm is perforated 
(and I was waiting for the right one!!!)
However I'm sure that your connection between 'load' and vac advance is correct so it appears I have more than one problem , and the throttle pot is as good a place to start as any.
I can fit a new one but how , or who does the calibration?
Nick..........
(and I was waiting for the right one!!!) However I'm sure that your connection between 'load' and vac advance is correct so it appears I have more than one problem , and the throttle pot is as good a place to start as any.I can fit a new one but how , or who does the calibration?
Nick..........
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