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Vacuum gauge reads 0 during cranking - why?


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I bought a new vacuum gauge and connected it to a port on the input manifold of my 1929 sedan - see photo.  The dial never moves off of zero during cranking.  I cannot get the car started, so I cannot say what the gauge would read during engine idle.  I verified that the port is not clogged and that the vacuum gauge does work by reading the vacuum pressure of my shop vac with it.  I tried this gauge on my car 3 times over 2 days and every time the gauge is pegged at 0 during cranking.

 

Any ideas as to why the gauge would not read anything during cranking?  Shouldn't I see at least a few inches of mercury vacuum when each cylinder is in the intake stroke?

 

After this I took off the engine valve cover and verified that all the valves open and close, at least per the cam and lifter action.  Next I did a (cold) compression check on each cylinder - cold because I cannot get the engine started.  I saw 50, 50, 40+, 55+, 45, and 45 psi on cylinders 1-6.  Finally I added oil to each cylinder and all readings but #1 went up by about 5 psi.  These readings are all 5-10 psi little lower than what I have measured several times in the past few years, but the ambient temperature now is at least 20 degrees colder than when I have taken readings before.  

 

 

Thank you

 

John

IMG_20201107_151457.jpg

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John,

 You will need a closed choke or the throttle closed to read on the gauge.   Use some starting fluid if you want to start the engine and it has not been started in a while.  If I let my Buick sit for a couple weeks, too much of the gas has evaporated.  The starting fluid saves a lot of cranking with the starter motor.   

my starting procedure is

I do the following rather quickly, as the starting fluid evaporates rather quickly.

- I open the choke.

- spray a shot into the carburetor - about 2 seconds worth.

- I get in the car and pull the choke closed. 

- I use the hand throttle on the steering wheel at 1/2 way and sometimes full.

- I push on the starter pedal.

Once it starts, I can lower the hand throttle and push the choke in some.

 

Note as well that your pot metal venturi in the carburetor may have grown.  This prevents the air valve from closing.  Another reason for low vacuum showing. 

I do not know how far your idle stop screw is turned in as this will effect the vacuum reading as well.   

 

Most all test and values on gauges are based on a warm engine.  See if you can get the motor started and then use the gauges.   

 

Let me know if you need the procedure for rebuilding the Marvel carburetor.  They work great if they are rebuilt correctly.

Hugh

Edited by Hubert_25-25 (see edit history)
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As ben says. Throttle closed, try pulling choke on full. I would think it should show a little, although I haven't tried it. You say it won't start. Have you got spark? if you crank it with the choke on does a bit of gas drip from carb?  As OD says, could have problems in a few areas,  but basic stuff first. 

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Even with the throttle closed there is still an idle circuit so there should be SOME movement of the gauge needle. I'm wondering if there is an intake manifold leak, how else can there be such good compression but zero vacuum? Engines can't compress air if you aren't bringing any in, and you can't bring any in without making some vacuum somewhere. If the manifold in leaking you can get a zero on the gauge.

 

I have had intake manifold leaks before, at the gasket. They are a pain. 

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OK, thanks for the suggestions.  I should mention that this is a continuing problem for me (not being able to start) - really going back to when I bought the car 6 years ago.  I finally bought the vacuum gauge because I spent this summer going through all the electrical reasons for not starting.  Also, I always feel like the plugs are too dry after a long unsuccessful try at starting when I look at the plugs to see if they are wet (note - I do get fuel dripping out of the carburetor/air cleaner at that point).  Also, the carburetor is newly rebuilt (1928 brass bowl carb with new venturi) and the heat riser is newly re-sleeved.  The car has a new/rebuilt mechanical fuel pump, and I am 100% positive that fuel gets to the carburetor.  For whatever reason, use of starting fluid has never worked for me on this car.

 

I took the vacuum measurement with the throttle wide open, so I can change to full closed if that is better.

 

I don't see cracks in the manifold, but I am thinking it must either be leaking at the gasket or there is some other major leak.  I can check the flanges for flatness.

 

Thank you all for helping out.

 

john

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John, 

    Are you getting a spark?  If you remove the coil wire from the distributor cap and hold the coil wire end near the block, and crank the engine, do you get a consistent spark?   Hugh

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Hugh,

Yes, I am getting good spark now.  I spent most of the summer getting that correct and adjusting the timing.

 

John

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You won't get any vacuum with the throttle open. just a breeze going through.  It should run, or at least fire at this point.  How are you setting the timing? the Buick book will baffle you.

Take the valve cover off and turn the engine over until #6 exhaust has just closed and the intake (second from back) is starting to open. The top dead center mark should be close in the timing mark window. Set it on Top dead center - it may say UDC ( upper).  with the lever down on the column to retard the spark the points should be almost opening and the rotor pointing to # 1 wire.  

 Fuel, spark,  and compression.  It should run

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This past summer I have adjusted and checked the timing several times and it is correct now.  I use the exact procedure described in the service manual and then use a self-powered timing light to verify that the timing is correct per the mark on the flywheel.  I have also used my oscilloscope to look at the disitributor pulses going into the coil during cranking and they all look evenly-spaced and no pulse is missing (ie, 3 pulses per 1 revolution of the engine).  As one last check, I place the clamp-on pickup coil from the timing light around the output wire from the ignition coil and see the same 3 pulses per engine rev on the output of the coil.

 

This Saturday I'll check vacuum again with the throttle closed and see what I get.

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If you still get no vacuum with the throttle closed, make a bead of silicone all around the intake manifold where it meets the head. Then check for vacuum. Not a permanent fix, may be messy, but you can clean it all up if you need a new gasket.

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On 11/11/2020 at 11:28 AM, Mark Shaw said:

You may also have one or more valves stuck open.....

 

I agree with Mark,

 

I might have missed it, but have you done a compression check on the engine?  If so, what are the readings by cylinder?

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ok, I just finished getting new vacuum readings with throttle fully closed (I set the idle adjustment to fully closed also) and with the battery just fully charged.  I get a peak reading of 3 or 3.5 inches mercury on the gauge.  It was about 35 degrees F ambient temp.

 

Is that normal, low, or high?

 

I also tried applying some mastic (playdough-like pliable sealant) around the intake manifold ports where the gasket is located, but I could not get access to two ports on the bottom sides (I couldn't get my fingers in to apply the mastic).  So I couldn't seal everything, but when I tried again to crank with some mastic applied I got the same vacuum reading ( 3 or 3.5 peak inches mercury).

 

One other question: What cranking speed do I need to achieve to get the engine to start if everything else is OK?  It seems like my starter motor is slow.  I measured the speed about 2 months ago and now I can't find my results, but I am thinking I saw 150-180 rpm.

 

Thanks

 

John

 

 

 

 

 

 

0

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Also, to possibly correct my earlier post/question:

 

One other question: What cranking speed do I need to achieve to get the engine to start if everything else is OK?  It seems like my starter motor is slow.  I measured the speed about 2 months ago and now I can't find my results, but I am thinking I saw 150-180 rpm.

 

I still have the same question, but I am thinking that I saw 300-360 cranking rpm earlier this year (forgot that there are 3 ignition pulses per engine rev, not 6.  I think there were 50 milliseconds between ignition pulses to a spark plug).

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Starter motor speed in RPM is nice, but these cars can be started by a hand crank, which has a starter speed of one. Not one revolution, not 1 RPM, just one. You pull on the hand crank, one cylinder compresses and fires. Boom.

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My Opinion - not having tried this, is that your vacuum is good. You wont see 17 inches until it is running.  Maybe a little weak but keep in mind this engine hasn't actually run for a good while. 

Checklist:  Spark at the spark plug. Not just the coil wire.

compression: it sounds not bad. 

Fuel:  Mainjet in carb open 3/4 turn, big knob flush with end of  lock spring

Fresh gas.  - should purr.

Other wild ideas: Exhaust not plugged

Valve timing out. (Compression would say not) but they have fiber timing gears that CAN slip on the hub.

Have you tried towing it to start it?  I've had to do that on worn motors. Once it has started then it usually will start on the starter.  If you haven't ever done that get experienced assistance. 

 

 

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Many, many years ago I learned the hard way to check for the color of the spark at the end of the  spark plug wire.  If blue, great, if yellow not enough spark to start engine  and install new coil.  It can be the cause and often overlooked test of an engine not starting.  

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Have you tried removing the spark plugs and giving each cylinder a short burst of starting fluid, then quickly put the plugs back in and try to start it?

That has always worked better for me on an updraft carb.

 

Good Luck

 

Bill

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7 hours ago, wmsue said:

Have you tried removing the spark plugs and giving each cylinder a short burst of starting fluid, then quickly put the plugs back in and try to start it?

That has always worked better for me on an updraft carb.

 

Good Luck

 

Bill

 

No need to remove the spark plugs.  You can just spray into the inlet for the carb.  Then crank the engine.  Works on my updraft cars/trucks.

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