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Low Vacuum

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Nik Nikolai Sklyadnev
Moscow, Russian Federation   RUS
Hello.
I checked my vacuum advance unit by means of a vacuum pump and a vacuum gage. It works correct. But when I checked a vacuum in a carburetor vacuum port (hole for vacuum advance tube) max vacuum was 7 in-Hg at 3000 rpm.
It’s too low vacuum for correct operation of the advance unit. It must be 14 in-Hg for full distributor vacuum advance.
Also I measured vacuum without air cleaner (I use paper type filter) and was very surprised to see 11 in-Hg vacuum but also too low.
The engine has a good compression in all cylinders and no visible leaks in carburetor and intake manifold gaskets (the engine works smoothly) and I have standard SU H4 carburetors.
What might have happened? Any ideas please.
Thank you,
Nick

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ghnl Avatar
ghnl Eric Russell
Mebane, North Carolina, USA   USA
1961 MG MGA "Calvin"
Where are you measuring the vacuum? The stock MGA distributor uses 'ported' vacuum - the vacuum take off is outside the throttle butterfly - vacuum is present only when the throttle is at least partly open. Thus you will not see the expected high vacuum at idle or coasting with the throttle closed.

If you are actually measuring intake manifold vacuum then low readings might indicate a mis-timed camshaft, valves out of adjustment or leaking valves.



Eric Russell ~ Mebane, NC
1961 MGA #61, 1981 Alfa Romeo GTV6, 1984 Alfa Romeo Spider, 1991 Honda ST1100



Edited 1 time(s). Last edit at 2017-09-13 07:47 PM by ghnl.

Nik Nikolai Sklyadnev
Moscow, Russian Federation   RUS
Eric, thanks for reply.
I measured «ported» vacuum in the throttle body and MAX 7 in-Hg vacuum was at 3000 rpm. It’s too low.

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oleanderjoe Avatar
oleanderjoe Silver Member Joseph Baba
Fresno, Ca, USA   USA
When the throttle is cracked open from idle to like FULL on, there is a ('VACUUM PULSE"winking smiley That pulls the advance plate. If you are holding the rev's at 3000 RPM, after the initial PULSE, around 17hg, it drops back down. You will NOT see a constant vacuum pull. at the Carb port.

Nik Nikolai Sklyadnev
Moscow, Russian Federation   RUS
Joseph,
Do you mean when cruising at constant 3000 rpm there is almost no vacuum advance?

oleanderjoe Avatar
oleanderjoe Silver Member Joseph Baba
Fresno, Ca, USA   USA
Nikolai: Not exactly. It is more complicated than that. If you hook up the Vac gauge to the PORTED PORT, (At the back of the carb) at idle there is no vac showing. Quickly crack the throttle open. There will be a pulse of vac to around 17hg and than back to 0 All is good, you have enough vac to work properly. At 3000 RPM there is vac, but not the full on 17hg 3000 RPM standing still is not the same as 3000 on a highway or going up hill. The advance helps control timing according to demand. it is not an ON-OFF Switch. Late model MGB used a vac retard to manifold vac. The retard unit held the timing at (Say 10 Degrees BTDC) when the throttle was cracked open, the instant drop in manifold vac allowed the vac retard to let go and the timing advanced. I believe it was year model 77-80 that have a 4th gear vac switch that let the vac bleed off at cruise speed in 4th gear to advance timing. I am not sure what Distributor you have, but almost all of the ones that have the knurled A-R knob can be very sloppy. I usually snug up that knob to eliminate any slop in the unit. Sometimes the vac unit cannot pull the plate, due to that slop so it looks like it is not working. I hope I did not muddy the water too much, Maybe some of the fellows here know of a link where you can read up on it a little. Don't have the right keyboard here so this is the best I can do. " DAS-VA-DON-YA" ?????? OK ?????

bobs77vet Avatar
bobs77vet bob K.
northern Va, Va, USA   USA
i am looking at this a little differently..... our distributor vacuum units operate at 7" to 14".....

https://www.britishvacuumunit.com/our-products.html
MGA 1955-62
​#423438 code 7 14 10...................meaning pulls 10* of advance between 7-14"


so if your distributor is only pulling advance at 14" i suspect the parts are worn, not flexible enough or are sticking and need to be cleaned. the vacuum pulse you are reading at the carb port is what the distributor needs to operate off of.

to increase vacuum overall try adjusting the valves.


what reading are you getting for manifold vacumm at idle?

and remember there is no vacuum signal at WOT and i am not surprised at 3000 rpm you have a lower vacuum reading since the throttle butterflys are really open wide. and at 3000 rpm you have base timing PLUS probably close to full mechanical advance and you want the vacuum advance to start backing off then so you dont create too much advance. does the vacuum unit pull any advance at 7"? at what vacuum level does it start to pull advance?



Edited 4 time(s). Last edit at 2017-09-14 10:50 AM by bobs77vet.

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B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, Minnesota, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Checking the vacuum advance function by revving the throttle in your driveway will not give you the same results as when driving the car. You will get higher than 11" of vacuum at cruise speed while driving.



jeff@advanceddistributors.com

Nik Nikolai Sklyadnev
Moscow, Russian Federation   RUS
Joseph,
Thank you for handy hints and tips and explanation.
(DOS-VI-DAN-IYA – is more correct (smiling smiley).
I made some additional real highway tests today.
The vacuum began to grow after 1500 rpm and reached maximum 7 in-Hg at 2500 rpm
(It was the same 7 in- Hg but at 3000 rpm during garage test).

Bob, thank you for message.
Some explanations.
I tested a vacuum advance unit. It works OK, it starts to pull advance at 7 in-Hg and has maximum advance at 15 in-Hg (almost correct).
Also I made road and «standing» test for measuring a maximum possible vacuum in carburetor Port.
Maximum that I could get was 7 in-Hg at 2500 rpm when driving and at 3000 rpm when standing.
The vacuum measured in Carb Port is «0» at idle, begin to grow after 1500 rpm and increasing up to 7 in-Hg at 2500 rpm (highway) or 3000 rpm ( when standing).
Vacuum reading at higher RPM is «0».
Thanks,
Nick



Edited 2 time(s). Last edit at 2017-09-14 12:43 PM by Nik.

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oleanderjoe Avatar
oleanderjoe Silver Member Joseph Baba
Fresno, Ca, USA   USA
Glad to help:

bobs77vet Avatar
bobs77vet bob K.
northern Va, Va, USA   USA
so one of several things.....


the engine vacuum is low due to poorly adjusted valves or something else......take an intake manifold reading. if this is low it wont get any better as you get further upstream.

and why dont you clean that carb port to make sure its fully open. use an O/A rig cleaning tip or something else really narrow like MIG wire or a Pin.

you dont have an aftermarket cam in by any chance do you?

Nik Nikolai Sklyadnev
Moscow, Russian Federation   RUS
Bob, thanks.
Yes, I have an aftermarket camshaft and I don’t know from which manufacturer.
Do you think it could be a problem?
And how do you suggest to measure a vacuum in intake manifold, to drill a hole?
Nick

bobs77vet Avatar
bobs77vet bob K.
northern Va, Va, USA   USA
In reply to # 3591897 by Nik Bob, thanks.
Yes, I have an aftermarket camshaft and I don’t know from which manufacturer.
Do you think it could be a problem?
And how do you suggest to measure a vacuum in intake manifold, to drill a hole?
Nick

do i think its a problem NO, do i think after market "hot" or "sport" cams develop low vacuum YES. do i think its effecting engine vacuum YES

on one of my corvettes the Comp Cams XE268 produced so little vacuum we had to swap out the vacuum unit on the distributor so it would start pulling vacuum at what the cam produced.

i have an 1800 intake and i just took out one of the plugs in the intake and put in a threaded nipple.

do you have any plugs you can remove on your intake? if not drilling a hole and tapping it is a good way to do it. once you know the vacuum level your engine produces with the cam you can look for a vacuum unit to match it or if none are available look to other distributor choices.

i am not surprised because of the valve overlap on "hot" cams that you are developing low vacuum. how "lopey" is your idle?

Gary E Avatar
Gary E Silver Member Gary Edwards
Kernersville, ,N.C., USA   USA
What are the symtoms of the engine performance. Does it run poorly, loose power.



Gary

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