Vac Advance & Shutny / Jerky Cruising
Discussion
So, I've had a very slight jerk while cursing at constant throttle.
After trying a few things, I think I have finally found what is causing this.
If I remove the vacuum advance pipe from the plenum, and block off the connector on the plenum, it runs silky smooth!
I then thought the vac advance diaphragm must be cracked/leaking. So I tried the suck-test - I can hear the diaphragm moving, and it will stick to my tongue. I can also feel/see the rotor arm moving very slightly as I suck on the vac pipe.
At the plenum end the connector isn't blocked, I checked that - can blow air through it.
The vac pipe itself isn't leaking, if I block one end with my thumb, I cannot suck air through.
Any ideas why disconnecting the pipe from the plenum & blocking up the plenum connector will smooth the rough running out, when all seems to be working?
Would this point to the timing being out?
If so, what would cause the timing to go out?
After trying a few things, I think I have finally found what is causing this.
If I remove the vacuum advance pipe from the plenum, and block off the connector on the plenum, it runs silky smooth!
I then thought the vac advance diaphragm must be cracked/leaking. So I tried the suck-test - I can hear the diaphragm moving, and it will stick to my tongue. I can also feel/see the rotor arm moving very slightly as I suck on the vac pipe.
At the plenum end the connector isn't blocked, I checked that - can blow air through it.
The vac pipe itself isn't leaking, if I block one end with my thumb, I cannot suck air through.
Any ideas why disconnecting the pipe from the plenum & blocking up the plenum connector will smooth the rough running out, when all seems to be working?
Would this point to the timing being out?
If so, what would cause the timing to go out?
SMB said:
RDJ said:
Inline plastic non reverse valve back to front?
This isn't standard fitment on a chimaera.Edited by RDJ on Sunday 17th August 17:44
ETA:
http://www.tvrshop.com/tvr-car-parts/engine/engine...
Edited by RDJ on Sunday 17th August 20:26
I have exacty the same issue since I had it serviced. he pointed out that the valve was the 'wrong way round' even though the 'dist' went to the dizzy etc, but when you blew in either side you can obviously tell which way its working. after that I have had this very light throttle judder at cruising. I have been told there is no reason to run the valve and to just remove it and have a single pipe
RDJ said:
Present on both of my 430s
ETA:
http://www.tvrshop.com/tvr-car-parts/engine/engine...
Hmmmn, not on my serpentine 400 owned for 19 years, perhaps was only on the 430 or pre serpentines, I see the op has a 2000 400 serpentine.ETA:
http://www.tvrshop.com/tvr-car-parts/engine/engine...
Edited by RDJ on Sunday 17th August 20:26
This seems to confirm my not original spec, but often added
http://www.pistonheads.com/gassing/topic.asp?h=0&a...
The original serpentine air line for the distributor was hard plastic so it would be impossible to fit this unless a new flexible pipe is added.
Edited by SMB on Sunday 17th August 22:05
Hi, exactly the same thing with mine (400 pre-serp Chim), seemed to be the wrong way around even though it was installed "dist" to dist and "carb" to the plenum. Looked in the Steve Heath bible and it says that they are problematic and a lot of people remove them with little or no side effects - so I did.
It has made a tremendous difference to be honest, it's not 100% cured the shunting but a hell of a lot better.
Jase.
It has made a tremendous difference to be honest, it's not 100% cured the shunting but a hell of a lot better.
Jase.
Yep, my car definitely doesn't have this attached.
Still no idea how removing the vac-advance from the plenum, and blocking the plenum connector stops this jerking at small-throttle inputs.
From what I understand, the vac-advance advances the timing under small throttle openings (i.e. high-vacuum).
When I suck on the vacuum pipe, I can hear the diaphragm move, and see the rotor arm move slightly (i.e. advancing timing?) so I would think this is working.
That would lead me to believe I'm not getting enough vacuum to enable this to happen?
Still no idea how removing the vac-advance from the plenum, and blocking the plenum connector stops this jerking at small-throttle inputs.
From what I understand, the vac-advance advances the timing under small throttle openings (i.e. high-vacuum).
When I suck on the vacuum pipe, I can hear the diaphragm move, and see the rotor arm move slightly (i.e. advancing timing?) so I would think this is working.
That would lead me to believe I'm not getting enough vacuum to enable this to happen?
Malcster said:
Yep, my car definitely doesn't have this attached.
Still no idea how removing the vac-advance from the plenum, and blocking the plenum connector stops this jerking at small-throttle inputs.
From what I understand, the vac-advance advances the timing under small throttle openings (i.e. high-vacuum).
When I suck on the vacuum pipe, I can hear the diaphragm move, and see the rotor arm move slightly (i.e. advancing timing?) so I would think this is working.
That would lead me to believe I'm not getting enough vacuum to enable this to happen?
maybe your dizzy is off whn it gets vacuum... try with another dizzy?Still no idea how removing the vac-advance from the plenum, and blocking the plenum connector stops this jerking at small-throttle inputs.
From what I understand, the vac-advance advances the timing under small throttle openings (i.e. high-vacuum).
When I suck on the vacuum pipe, I can hear the diaphragm move, and see the rotor arm move slightly (i.e. advancing timing?) so I would think this is working.
That would lead me to believe I'm not getting enough vacuum to enable this to happen?
You could try checking the integrity of the wires running from the pickup on the vac. Advance plate to the body of the dizzy.
Not unheard of for these to brake and loose contact when the vacuum advance pulls the plate around. A simple test would be to suck on the vac. Hose with the engine running and see if it stumbles.
Not unheard of for these to brake and loose contact when the vacuum advance pulls the plate around. A simple test would be to suck on the vac. Hose with the engine running and see if it stumbles.
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