I noticed when changing the speedo gear that the vaccum line has disconnected from the vacuum modulator on the trimatic. I suspect this happened when I changed the dissy over to electronic as I was moving the vac line out of the way. The car ran really well after the dissy conversion and shifted up/down with no worries at all. Seemed to have more grunt than previously. This was with the vac line disconnected.
Anyway, I reconnected the vac line to the modulator after I did the speedo gear and took the car for a spin around the block. It seems to be back to its old self.... sluggish, not down shifting properly and idling a bit rough.
I am going to disconnect the vac line from the modulator again as the car runs better this way but was just wondering what adverse effects this will cause, if any?
Vac line runs from the carb (edelbrock) to the modulator.
Cheers,
T
Vacuum lines. Carb to trimatic..
Started by
_Big T_
, Jan 03 2008 01:02 PM
16 replies to this topic
#2
rodomo
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Posted 03 January 2008 - 01:06 PM
It will suck un-filtered air straight into your manifold.
Re-connect and re-adjust your mixture.
Re-connect and re-adjust your mixture.
#3
_Big T_
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Posted 03 January 2008 - 01:16 PM
Oh, ok. I was about to ask how but I think my brain just had a bit of turbo lag. So the vacuum created at the manifold will suck the air up the line that is dangling under the car and possibly suck water, dirt etc as well. Makes sense. Blocking the line will have no benefit either I assume as both reconnecting or blocking are going to require a re-tune anyway.
Miss Big T is right when she says I should find out more before I embark on my hair brained ideas......
BTW Rodomo, that speedo gear has worked a treat
No longer am I puttering along at 120 kph !!
Miss Big T is right when she says I should find out more before I embark on my hair brained ideas......
BTW Rodomo, that speedo gear has worked a treat
No longer am I puttering along at 120 kph !!
#4
rodomo
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Posted 03 January 2008 - 01:25 PM
Actually, it would pay to blow the line out as it may have crap in it already.
#6
TerrA LX
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Posted 04 January 2008 - 03:48 PM
The port on the carb should be connected to the distributer if it has vac advance or block it off. The vacuum for the modulator should take off directly from the inlet manifold.Vac line runs from the carb (edelbrock) to the modulator.
Cheers,
T
Reconnect everything correctly and retune the carb etc before a test drive.
#7
_Big T_
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Posted 04 January 2008 - 06:01 PM
No vac advance on the dissy and the manifold vac is going to the brake booster i.e doesnt have any other connection. I think I asked a while ago about the vac port on the carb running to the vac modulator on the auto and was advised this is ok. From memory the carb set up instructions say I can run this config as well. I might have to re-visit old posts to make sure I got it right. Its all to bloody confusing somethimes 
#8
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Posted 04 January 2008 - 06:33 PM
You may well be right Tony, some carbs have a full manifold vacuum port at the BASE of the carby, so long as the modulator is getting FULL manifold vacuum it does not matter where it is hooked up.
Sorry for any confusion.
Sorry for any confusion.
#9
_Big T_
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Posted 05 January 2008 - 06:51 PM
No dramas mate. I went out and bought some fittings to make up a T piece for the manifold anyway. I couldnt find a manifold tree anywhere so I had to improvise. Basically I got a female T piece and then some 1/4 inch fittings for the manifold, brake booster and vac modulator. It tidies things up a bit and at least now I know I am getting full manifold vacuum without any air disturbance around the carb. Not sure if it will make a difference but hey, its worth a try. 
#10
_The Stig_
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Posted 06 January 2008 - 10:45 AM
Vacuum modulator lines were notorious for blocking up, I think (From memory) there is a restriction somewhere in the line, about 0.040", that gets blocked up and prevents the modulator from getting a vacuum signal. I had a 308 HZ once that this happened to, what it would do, if you were under a big load, eg going up a big steep hill, the box would start shifting back and forwards, 2nd-3rd, pretty quickly. Only happened on one particular hill, which was a long steep one, and it was worse when I towed a car trailer up there! I think from memory I sprayed carb cleaner through the line and compressed air and it worked a treat, ran better everywhere, not just on hills.
Was 20 years ago now, tho, so my memory is a tad hazy....
Some carbs have ported vacuum points and full manifold vacuum ports, also. I think from memory vac advance, EGR's, etc. go to ported, vac mod goes to full vacuum. Easiest way to tell is pull the hose off at idle and stick your finger over the tube. If it sucks at idle it's full manifold, if it doesn't it's ported.
BTW, with vac modulators, if they get a hole in the diaphragm they will suck tranny fluid out of the box and pump it into the engine. Symptoms will obviously be a heap of exhaust smoke and constantly having to top up the tranny. A lot of ppl have been caught out by this and rebuilt engine and/or tranny for no result! Just imagine spending 2 or 3 grand rebuilding everything just because of a pissy little hole in a peice of rubber
Was 20 years ago now, tho, so my memory is a tad hazy....
Some carbs have ported vacuum points and full manifold vacuum ports, also. I think from memory vac advance, EGR's, etc. go to ported, vac mod goes to full vacuum. Easiest way to tell is pull the hose off at idle and stick your finger over the tube. If it sucks at idle it's full manifold, if it doesn't it's ported.
BTW, with vac modulators, if they get a hole in the diaphragm they will suck tranny fluid out of the box and pump it into the engine. Symptoms will obviously be a heap of exhaust smoke and constantly having to top up the tranny. A lot of ppl have been caught out by this and rebuilt engine and/or tranny for no result! Just imagine spending 2 or 3 grand rebuilding everything just because of a pissy little hole in a peice of rubber
Edited by The Stig, 06 January 2008 - 10:47 AM.
#12
_Big T_
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Posted 08 January 2008 - 12:44 AM
That is some very handy stuff to know Stig. Thanks for that. I will blast a bit more air and carb cleaner thru the line on the weekend.
Just checked the edelbrock manual and it says the port I had connected up was manifold vacuum but for dissy. Didnt say anything about vac mod or the like. Carb has two vac ports, one right at the base (the one I had connected) and one a bit further up. Probably would have been fine to run the way I had it (just needs a tune to increase air mixture I've gathered), but I'd rather be on the safe side and run vac straight from the manifold. Hopefully I get some time this week to give it all a tweak.
Just checked the edelbrock manual and it says the port I had connected up was manifold vacuum but for dissy. Didnt say anything about vac mod or the like. Carb has two vac ports, one right at the base (the one I had connected) and one a bit further up. Probably would have been fine to run the way I had it (just needs a tune to increase air mixture I've gathered), but I'd rather be on the safe side and run vac straight from the manifold. Hopefully I get some time this week to give it all a tweak.
#13
ls2lxhatch
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Posted 08 January 2008 - 10:08 AM
The modulator is responsible for downshift on light throttle application. When the manifold pressure drops below the threshold due to engine load the modulator will open a valve that can cause the transmission to downshift. Once manifold pressure has increased above the threshold the modulator closes the valve and the transmission can upshift. The manifold pressure drop at 3000+ rpm is typically not below the threshold at full throttle.
The kickdown switch/cable is used to open a similar valve at full throttle regardless of engine load.
The kickdown switch/cable is used to open a similar valve at full throttle regardless of engine load.
#14
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Posted 08 January 2008 - 12:12 PM
So if i cruise along doing 200K @4000rpm and plant it it will kick back?The kickdown switch/cable is used to open a similar valve at full throttle regardless of engine load.
#15
_The Stig_
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Posted 18 January 2008 - 09:12 AM
I reckon governor pressure would be too high to allow it to kick back - Ls2lxhatch, is the kickdown electric on trimatics? I have a feeling it is, but like I said earlier, I haven't seen one in 20 years! lolSo if i cruise along doing 200K @4000rpm and plant it it will kick back?
The kickdown switch/cable is used to open a similar valve at full throttle regardless of engine load.
#16
makka
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Posted 18 January 2008 - 08:41 PM
yep, electric kickdown, I gotta fix my kickdown, it doesnt kick down from top when I boot it at 100kmh. doesnt kick back well from lower speeds either....I reckon governor pressure would be too high to allow it to kick back - Ls2lxhatch, is the kickdown electric on trimatics? I have a feeling it is, but like I said earlier, I haven't seen one in 20 years! lol
So if i cruise along doing 200K @4000rpm and plant it it will kick back?
The kickdown switch/cable is used to open a similar valve at full throttle regardless of engine load.
also, there should be a restrictor no larger than 0.060 in the Vac line, either the hole where the hose attaches to the carb/manifold or a restrictor in the line
Edited by makka, 18 January 2008 - 08:43 PM.
#17
ls2lxhatch
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Posted 18 January 2008 - 09:26 PM
There are other mechanisms that will prevent kickdown when it is not appropriate. That is why I said "that can cause the transmission to downshift.".So if i cruise along doing 200K @4000rpm and plant it it will kick back?
There is also a speed sensing valve on the output shaft that is closed by centrifugal force. It is the combination of the valve positions and their effect on the oil pressure which is rpm sensitive that determine if downshift or upshift is appropriate.
The speed sensing valve on my trimatic was sticky once which caused it to upshift later than it should.
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