vac advance shunting ?!
Discussion
some time ago got myself a used chim. modded 4.6 engine fully mapped
of mark adams different cam injectors and things. when i bought the car
no vac hose from the plenum had been connected to the vac advance.
also no plug on the plenum. neither on both sides of the vac advance.
at this time had some shunting from 1200-2000 rpm. connected the vac
hose from plenum again to the vac advance but left the other side
of the vac unit ( atmosphere side ) fully open. car was more worse than
before and almost not driveable in slow traffic.
disconnected the vac hose again. sealed the plenum connection with a cap
and sealed the atmosphere side of the vac advance with a cap too.
the other side is still without cap. shunting has almost gone and revs
fall without starting to kangaroo below 2000 rpm. at 1200 it runs a bit rough
but maybe this is cam related.
is there any technical explanation why blocking the atmosphere side of the
vac unit and leaving the other side blank and disconnected can cause this cure ?
of course i know it must be magic
of mark adams different cam injectors and things. when i bought the car
no vac hose from the plenum had been connected to the vac advance.
also no plug on the plenum. neither on both sides of the vac advance.
at this time had some shunting from 1200-2000 rpm. connected the vac
hose from plenum again to the vac advance but left the other side
of the vac unit ( atmosphere side ) fully open. car was more worse than
before and almost not driveable in slow traffic.
disconnected the vac hose again. sealed the plenum connection with a cap
and sealed the atmosphere side of the vac advance with a cap too.
the other side is still without cap. shunting has almost gone and revs
fall without starting to kangaroo below 2000 rpm. at 1200 it runs a bit rough
but maybe this is cam related.
is there any technical explanation why blocking the atmosphere side of the
vac unit and leaving the other side blank and disconnected can cause this cure ?
of course i know it must be magic
There are some many small factors that lead up to shunting, that say altering the timing at low throttle openings may have some effect in your case- Mind you I would think that no vaccume advance connection would be the same as having the air port blocked on the other side of the diaphram anyway (?)
blitzracing said:
There are some many small factors that lead up to shunting, that say altering the timing at low throttle openings may have some effect in your case- Mind you I would think that no vaccume advance connection would be the same as having the air port blocked on the other side of the diaphram anyway (?)
will try that too. it was just a curious find with try and error as the engine is running quite smooth right now. before it was the well known shunting with kangarooing at lower revs. above 2000rpm no prob it just
pulls.
In town driving, at 1200rpm, driving technique can make shunting tolerable to nearly non existant on my Griff. Slight throttle on, throttle off, cruise, slight throttle on, throttle off, cruise etc, etc and so on.
Try and avoid driving with throttle on at a steady 1200. On open road and full throttle or at a steady 70mph in 5th no shunting.
Try and avoid driving with throttle on at a steady 1200. On open road and full throttle or at a steady 70mph in 5th no shunting.
Edited by EGB on Monday 24th December 13:32
Running vac advance will only make the lurching more pronounced 
my findings were that less advance in the lurch/low speed area masks the problem nicely (especially if you can add a little fuel too) 
Gaskson by doing what you are doing the vac advance as no say in the timing now hence more drivable at slow speeds 
my findings were that less advance in the lurch/low speed area masks the problem nicely (especially if you can add a little fuel too) Gaskson by doing what you are doing the vac advance as no say in the timing now hence more drivable at slow speeds Edited by Sardonicus on Tuesday 25th December 15:15
gacksen said:
some time ago got myself a used chim. modded 4.6 engine fully mapped
of mark adams different cam injectors and things. when i bought the car
no vac hose from the plenum had been connected to the vac advance.
also no plug on the plenum. neither on both sides of the vac advance.
at this time had some shunting from 1200-2000 rpm. connected the vac
hose from plenum again to the vac advance but left the other side
of the vac unit ( atmosphere side ) fully open. car was more worse than
before and almost not driveable in slow traffic.
disconnected the vac hose again. sealed the plenum connection with a cap
and sealed the atmosphere side of the vac advance with a cap too.
the other side is still without cap. shunting has almost gone and revs
fall without starting to kangaroo below 2000 rpm. at 1200 it runs a bit rough
but maybe this is cam related.
is there any technical explanation why blocking the atmosphere side of the
vac unit and leaving the other side blank and disconnected can cause this cure ?
of course i know it must be magic
Has your timing been set at 3000rpm to give more advance further up the rev range. If your connecting the vac pipe and it is set this way it will make the shunting worse.of mark adams different cam injectors and things. when i bought the car
no vac hose from the plenum had been connected to the vac advance.
also no plug on the plenum. neither on both sides of the vac advance.
at this time had some shunting from 1200-2000 rpm. connected the vac
hose from plenum again to the vac advance but left the other side
of the vac unit ( atmosphere side ) fully open. car was more worse than
before and almost not driveable in slow traffic.
disconnected the vac hose again. sealed the plenum connection with a cap
and sealed the atmosphere side of the vac advance with a cap too.
the other side is still without cap. shunting has almost gone and revs
fall without starting to kangaroo below 2000 rpm. at 1200 it runs a bit rough
but maybe this is cam related.
is there any technical explanation why blocking the atmosphere side of the
vac unit and leaving the other side blank and disconnected can cause this cure ?
of course i know it must be magic
Had mine setup this way ,gave me loads more power but low down driving anf fuel consumption was crap ..I also noticed a time lapse before it gained power ..
Timing may need retarding with the vac pipe connected
Hi a bit of a long shot but..
I had a simular fault when i bought my 400 2 months ago, terrible shunting and erratic idle, found the vac unit was u/s. Changed that and was a little better but not right as idle was still bad. Changed ecu temp sensor and plugs and again ran a little better and not as thirsty but shunting and poor idle. Then my fuel pump and ecu relays( metal ones in n/s footwell) both packed up completely and were in a right mess so resistances would have very poor, have bridged them and drove the car home from my wksp and it has never ran so well, no shunting, idle is spot on no hunting and stalling when coming to a stop.
So it may well be worth while at least looking. May be wrong but it cant hurt to check as one of the relays feeds tha lambda sensor heater, and i might be wrong again but if no heating then lambda will not work and your fueling will be wrong, the ecu will try to compensate and this may be the cause of the shunting etc
Hope this helps and a Merry christmas to all
Jamie
I had a simular fault when i bought my 400 2 months ago, terrible shunting and erratic idle, found the vac unit was u/s. Changed that and was a little better but not right as idle was still bad. Changed ecu temp sensor and plugs and again ran a little better and not as thirsty but shunting and poor idle. Then my fuel pump and ecu relays( metal ones in n/s footwell) both packed up completely and were in a right mess so resistances would have very poor, have bridged them and drove the car home from my wksp and it has never ran so well, no shunting, idle is spot on no hunting and stalling when coming to a stop.
So it may well be worth while at least looking. May be wrong but it cant hurt to check as one of the relays feeds tha lambda sensor heater, and i might be wrong again but if no heating then lambda will not work and your fueling will be wrong, the ecu will try to compensate and this may be the cause of the shunting etc
Hope this helps and a Merry christmas to all
Jamie
thx guys for the advice and whish you a all a happy new year.
had been driving for some days now and the shunting is only in higher gears
at around 1200-1300Umin. but not that much as before. blockes all the ports on the
vac advance with caps. fuel consumption seems ok. no change in that one.
only thing i have noticed is the fuel itself. if i change brand and octane
the car runs slightly different.
on my t350 i just drive it with aral ultimate 102 perfect for pops and bangs and it just pulls away. the chim does not like that. havent tried the shell yet but it seems to best running with 98 super plus.
currently looking into getting a new exhaust system and clives y piece
had been driving for some days now and the shunting is only in higher gears
at around 1200-1300Umin. but not that much as before. blockes all the ports on the
vac advance with caps. fuel consumption seems ok. no change in that one.
only thing i have noticed is the fuel itself. if i change brand and octane
the car runs slightly different.
on my t350 i just drive it with aral ultimate 102 perfect for pops and bangs and it just pulls away. the chim does not like that. havent tried the shell yet but it seems to best running with 98 super plus.
currently looking into getting a new exhaust system and clives y piece
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