Vacuum Advance Issue

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Jonymoto
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My tercel:: 1983, DLX 4wd wagon with 4.1 manual gearbox, tow bar). Manual steering. 313,000 kms. Tercel 4WD SR5, 1987, 500,000kms. Other car - 2003 Rav4, AWD, 2l manual, Peugeot 407 Diesel (93,000 miles) in Ireland for use when on holiday there.
Location: Melbourne, Australia

Vacuum Advance Issue

Post by Jonymoto »

Hi guys, I will appreciate it if you could help me with a timing/vacuum advance issue that I'm having.

I was checking the timing on my 1983 Tercel 4wd today.
With the vacuum advance tube to the distributor disconnected and plugged, the timing was at 7 degrees btdc. I, then, unplugged the vac tube with the engine idling, the timing or idling speed didn't changed. I plugged the tube to the distributor and again no change in timing. I found that there was no vacuum at the end of the vac tube to the distributor.
I disconnected the vacuum advance tube at the base of the carburettor. Again no change in timing or idling speed. There are 3 tubes connected to the base of the Aisin carburettor, on the right hand side. I disconnected all 3 one at a time whilst the car was idling. Again this didn't make any difference to the timing or idling speed. Shouldn't disconnecting these tubes caused the engine to lose vacuum? The car is running well in all gears but with no vacuum advance working. Would anybody have encountered this issue before?
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jimcrazy
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Re: Vacuum Advance Issue

Post by jimcrazy »

you need to put a vacuum gauge tester on it. when I did mine last spring went through the same thing, the line from the carb had no vacuum at idle but pulled about 10 on gauge when reved up quick. the dizzy I went through 5 different ones to get one that would pull a vacuum and hold it with the hand pump, the diaphragms in the advances were all shot.
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Mark
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Re: Vacuum Advance Issue

Post by Mark »

There are 2 sources of vacuum that can be used for distributor advance. Intake manifold (which gives vacuum at idle) and ported (near the middle of the carb. It only gives vacuum at part-throttle conditions, not at idle). The 3-hose distributor should have vacuum coming from both. My '81 Tercel only has one hose going to the distributor. It came from the ported advance so there was no advance at idle. This was for better exhaust emissions at idle. Your distributor diaphragm may be ruptured, but I'd also check to see if the hoses are routed correctly from the right source.
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Mark
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Re: Vacuum Advance Issue

Post by Mark »

The fact that your idle didn't get rough when you disconnected all the hoses and there was no vacuum from the end of any of them makes me think that your hoses are all coming from the ported vacuum.
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Jonymoto
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My tercel:: 1983, DLX 4wd wagon with 4.1 manual gearbox, tow bar). Manual steering. 313,000 kms. Tercel 4WD SR5, 1987, 500,000kms. Other car - 2003 Rav4, AWD, 2l manual, Peugeot 407 Diesel (93,000 miles) in Ireland for use when on holiday there.
Location: Melbourne, Australia

Re: Vacuum Advance Issue

Post by Jonymoto »

Thanks for the replies. My dizzy has two hoses. It's an electronic one that I got from a 4AC engine. I checked it by sucking on the vacuum tubes and the mechanism moved inside which would indicate that the diaphragms are ok. I have a spare dizzy. Without mounting it on the engine I connected the vacuum tubes to it whilst the car was idling. The advance mechanism didn't move when I connected the tubes. But I didn't try it whilst reving the engine.
Would it be best if I connect the dizzy directly to the intake manifold? The car seems to be running fine. I was just wondering why they say to check the timing whilst the dizzy is disconnected and the vacuum tube plugged if it makes no difference to the timing.
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dlb
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Re: Vacuum Advance Issue

Post by dlb »

Jonymoto wrote:Thanks for the replies. My dizzy has two hoses...I was just wondering why they say to check the timing whilst the dizzy is disconnected and the vacuum tube plugged if it makes no difference to the timing.
i don't know what emissions your aus tercels came with but if your dizzy has two vac advance ports, it's probably as follows:

- upper port connects to manifold vacuum (the ports right on the intake manifold, just in front of the carb) - advances timing by 8* at idle and highway cruising.
- lower port connects to ported vacuum (a port on the base of the carb) - advances the timing by 13* at highway cruising.

so if you unplugged the lower port, no, there would not be any vacuum at idle.

if you unplugged the upper port and there was no change in timing, either the upper advance diaphragm is ruptured, the hose is connected to the wrong port, or not connected to anything at all.

to check the diaphragms, suck on the hose going to the diaphragm in question. if you can suck air through the hose, it's ruptured and won't work. plug the hose for now until you can replace the vac advance unit.

to confirm the hoses are going to the right places, unplug each end and suck or blow through one end. feel with your finger for sucking or blowing on the other end.
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Mark
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Re: Vacuum Advance Issue

Post by Mark »

Yours is probably just connected to the ported vacuum port. If you follow the hose, it probably ends up at the carb (sort of near the idle screw). This port doesn't provide any vacuum at idle. That's normal. I think it's common on many cars since the 1970's to not have any distributor advance at idle since not having it improves emissions at idle. Like I mentioned above, my '81 had the distributor advance hose routed to the ported advance port at the carb. When I removed it, the timing didn't change, the idle stayed smooth and there was no vacuum from the hose. This is normal. I re-routed mine on my '81 to the manifold vacuum port (I think Toyota calls this component the gas filter) so there would be vacuum advance at idle. My reasoning was that for in-town driving I spend a lot of time at idle so I thought it might improve in-town fuel economy. So now it does have advance at idle, but I haven't really noticed much of a difference so far with the fuel economy.
You could try re-routing the hose temporarily to the manifold vacuum and see if the advance changes at idle.
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Mark
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Re: Vacuum Advance Issue

Post by Mark »

Even if you only had one hose going to the distributor coming from the ported advance so there was no advance at idle, it would still be a good idea to remove the hose when checking the timing since part of checking the timing would be to rev the engine and see if the centrifugal mechanical advance works.
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Mark
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Re: Vacuum Advance Issue

Post by Mark »

Jonymoto wrote: There are 3 tubes connected to the base of the Aisin carburettor, on the right hand side. I disconnected all 3 one at a time whilst the car was idling. Again this didn't make any difference to the timing or idling speed. Shouldn't disconnecting these tubes caused the engine to lose vacuum?
I don't think any of these tubes have a vacuum when the engine is at idle, only at part-throttle conditions.
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dlb
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Re: Vacuum Advance Issue

Post by dlb »

Mark wrote:
Jonymoto wrote: There are 3 tubes connected to the base of the Aisin carburettor, on the right hand side. I disconnected all 3 one at a time whilst the car was idling. Again this didn't make any difference to the timing or idling speed. Shouldn't disconnecting these tubes caused the engine to lose vacuum?
I don't think any of these tubes have a vacuum when the engine is at idle, only at part-throttle conditions.
on that note, it's important to remember that each port has vacuum only at certain times. just because they don't have vacuum at idle doesn't mean they're all the same. so make sure you connect the hoses just like the sticker on the underside of the hood (aka 'bonnet') says. don't mix any of them up.
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Jonymoto
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My tercel:: 1983, DLX 4wd wagon with 4.1 manual gearbox, tow bar). Manual steering. 313,000 kms. Tercel 4WD SR5, 1987, 500,000kms. Other car - 2003 Rav4, AWD, 2l manual, Peugeot 407 Diesel (93,000 miles) in Ireland for use when on holiday there.
Location: Melbourne, Australia

Re: Vacuum Advance Issue

Post by Jonymoto »

Initially the car came with a dizzy with a contact breaker. But I got fed up having to adjust the points gap/dwell angle and timing all the time. That dizzy only had 1 vacuum port which was connected to the carburettor (near the idle screw). I replaced it recently with an electronic dizzy from a 4AC with 2 vac ports, both are connected to the ported vacuum on the carburettor (same one that the original dizzy was connected to). So the emission sticker under the bonnet doesn't show where the second tube from the distributor should go as it's from a 4AC, not the original 3AC.
So should I connect the upper dizzy port to manifold vacuum (as mentioned by dlb) and the bottom dizzy port to the port on the carbie near the idle screw?
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dlb
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Re: Vacuum Advance Issue

Post by dlb »

Jonymoto wrote:So should I connect the upper dizzy port to manifold vacuum (as mentioned by dlb) and the bottom dizzy port to the port on the carbie near the idle screw?
yep, that sounds correct to me.
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Mark
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Re: Vacuum Advance Issue

Post by Mark »

Here's a random 4ac photo from the internet showing one hose going to the ported and one hose going to the gas filter (manifold):
86corolla4A_LCvacuumhose2.jpg
It looks like the hose closest to the distributor goes to manifold vacuum.
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Jonymoto
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My tercel:: 1983, DLX 4wd wagon with 4.1 manual gearbox, tow bar). Manual steering. 313,000 kms. Tercel 4WD SR5, 1987, 500,000kms. Other car - 2003 Rav4, AWD, 2l manual, Peugeot 407 Diesel (93,000 miles) in Ireland for use when on holiday there.
Location: Melbourne, Australia

Re: Vacuum Advance Issue

Post by Jonymoto »

Thanks dlb & Mark.

So the upper dizzy port also goes to K on the TVSV.

I will try that tonight.
When you lose, don't lose the lesson - The Dalai Lama
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Mark
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Re: Vacuum Advance Issue

Post by Mark »

I think port "K" is the TVSV's source of manifold vacuum. I think this is what sends vacuum to activate the Exhaust Gas Recirculation system at idle when the engine is warm. I don't think it has anything to do with the distributor. They just share the same vacuum source.
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