i have recently rebuilt my a12 engine and wanted to set the carby mixture properely and was told to get a vaccum gauge so i do then the gauge needle dosnt stay steady it vibrated lots from 15 inch Hg to 17 dosnt stay steady and it says its late ignition timing on the gauge!

any ideas please help?

i haven't used one before but...

i'm wondering where you hooked it up? if it is on one of the inlet runners to the head the vacuum will pulse as it will only have full vacuum when that inlet valve is open. If it's on the base of the carb somewhere then it is hard to see why it would pulsate much

if it is pulsing i would say you would look at what is a reasonable maximum it is reaching and use that as your tuning point... if fact the idea as per workshop manual is to maximise vacuum so keep doing whatever improves it

as a side, how much did the gauge cost (might buy one too)

double post

hi i plugged the vaccum gauge in the connector in front of the manifold its tapped in the manifold dont no were else to put it i tried the dizzy advance but was barley any vacuum i have a pic the place is circled in red!

also the meter cost 22 dollars from ebay!

http://cgi.ebay.com.au/ws/eBayISAPI.d ... TRK:MEWNX:IT#ht_500wt_922

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You may have late ignition timing. What is your static timing (engine off)? It should be 5 to 10 degrees.

please explaint to me what static timing is i tried reading on wiki but didnt go through me!

the way i set my timming is turn on can and use timing light to advance to retard timing until set to what ever. i set it to 7 deg btdc like that.

how do you set it with engine off??

thanks!

That is the other way to set it. You can do it engine off OR engine on with a timing light. The latter is more accurate -- as long as the vacuum advance is disconnected when you checked it, it should be good.

Does the engine run well? If do I'd say your gauage is defective.

engine runs okay just a bit sluggish and slow pickup from standstill but is good once it picks up speed!

could this be some problem or just normal?

i made a video of the gauge while connected to show it but arnt able to upload it on this forum!

I have a professional workshop type vacuum gauge that my stepfather bough from Redex back i the 60's. It's nice & big & works well.
One of the things that came with it is an external clamp around the vacuum line & this is intended to cause a restriction in the line.

What this does is dampen out the wild fluctuations of the needle at idle, particularly in a four cylinder engine.
Some people use a vice grip to restrict the tubing [but not squash it, blocking the vacuum signal completely.
You should see the needle move smoothly when reving up, but not flicker wildly.

Good, now that this is done, disconnect the vacuum line at the distributor & plug it. A Philips screwrdiver of the right size is good for this.

Start the engine with a dwel angle meter connected to the distributor. Standard spec is from 49 to 55 degrees of dwell but I would set it to 49 degrees as the figure will move towards 55 degrees as the points wear over time.
Using just feeler gauges is OK, .... just & is really little more than a setting to get the engine started when installing new points.

Next, set the idle to factory spec's using the Tach that you previously connected.

Here's where the vacuum gauge comes into play. While watching the needle, gently screw the idle mix screw in or out. You are looking for an increase in the vacuum figure & with it will come an increase in idle rpm.

Get your best reading & adjust the idle sped to spec's.
Repeat the mixture adjustment.

Next, set the ignition to spec's at the nominated idle speed.
If revs increase then re-adjust the idle speed & check the timing again.
Once this is done, reset idle speed to spec if different from the speed needed to set the timing.

Re-connect the vacuum line to the distributor.
Now that these things are right, you can compare the reading on the vacuum gauge to any tech spec that you might have in the vacuum gauge's instructions.

With careful attention to detail, I can get 18-1/2" HG from my stock 1200 engine at 600 rpm at 2,500 ft elevation, where I live. Despite my engine being close to 40 years old, it is in near perfect mechanical condition.
You should get a slightly higher reading at sea level.

Oh yeah, all readings & adjustments should be done with the engine at operating temperature.