VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 Vacuum Reading for a 216

In trying to pinpoint problems with tuning my 216, I find that the vacuum reading is 15 inches. Is that normal and would it operate the vacuum advance properly? I disconnected the wiper line and hooked to the intake for the reading. I've never measured vacuum before for any reason so I'd appreciate some expert input.
Thanks, Roger

That is low. Should be17-19.
Is the meedle steady?
Does the engine idle smooth?

I agree that 15 sure seems low, especially for a recently rebuilt engine. My '37 pulls a steady 18 1/2 at idle. I am not sure what level of refresh/rebuild has been done to the engine.

Some ideas to consider.
- Disconnect and plug the vacuum line to the distributor. There could be a leak in the diaphragm in the advance mechanism.
- Confirm that your vacuum gauge is reading correctly. Either put it on an engine that runs well or borrow a gauge. I have a mighty vac that I periodically connect to my vacuum gauge to compare the 2 readings.
- Check it again after you have really driven the car and seated the rings. And I mean drive it! If I remember correctly this engine has had almost no use since you restarted it after the rebuild.

Yesterday I did a leakdown test on all cylinders just to see if there were excessive leaks. Nearly had cardiac arrest. Three of the cylinders would not hold even 50% of the 50 pounds of pressure applied. The others all held over 90% of that 50 pounds. I was extremely careful to get the piston up top and both valves closed. I even tapped on the rockers with a rubber mallet to make sure the readings weren't skewed by sticky valves. I tapped on the gauge and finally gave up- knowing something was terribly wrong inside. Had bad dreams last night. Needless to say.

This morning I decided to do a compression test to confirm the leakdown. To my surprise, all cylinders show 120 pounds of compression. Don't think I'm not relieved. Can't explain it.

Still have the vacuum issue though. I bought another vacuum gauge to try. Hope the result is different. I'll let you know.
Regards, Roger

I woud drive it another 500 miles if it is running OK and idle is smooth.....and make sure the valves are not adjusted too tight.

The new vacuum gauge shows the same reading for vacuum- 15 inches. My biggest question is how can the leakdown test and vacuum test results be so disparate, not to mention between compression test and leakdown.
Makes no sense, especially since I made every effort to prove test equipment malfunction.
To locate the loss of vacuum I've tried everything I know to do. Should I replace the intake leak test propane with acetylene?
Thanks, Roger

I never had much luck chasing vacuum leaks with propane (and I used to do this sort of thing for a living).

Carb or Brake Cleaner works well as long as it is flammable. Some isn't today. I hear Berryman B12 still works. Since the exhaust is on the same side on a Chevy six, an exhaust leak could easily start a fire. Be very, very careful.

If the compression is good, and it is, I would roll with it. Leakdown is useful mainly for figuring out what is broke when something is. It is normally done after a compression test comes up bad. For instance, when the leakdown tester is running you listen to the carb (with the throttle open), the exhaust pipe, and the oil cap. The noise tells where the air is going.

There is always leakage through the rings, so there will be some noise here. The carb and exhaust will be real quiet. Don't be fooled by the sea shell effect. It can help to have a friend disconnect the tester while you are listening if you think you are getting fooled.

Burned exhaust valves will generally result in super-low compression, and show up in a compression test, triggering the need for a leakdown test. Burned intake valves are so rare they shouldn't even be considered until every other possibility has been tried.

Generally speaking bad valves make the engine run horrible and bad rings are barely noticed while driving.

Good compression suggests nothing wrong. Leakdown (through the rings) is always horrible until the engine breaks in. Some leakage is normal after break in. 10 percent is probably what a lot of guys shoot for. They probably wont get it. 5 percent or less is the province of high dollar race engines, honed to exact size on a power hone with torque plates and hot water running through the block, and every ring file-fit. Many cars rolling down the road are 35 percent or worse.

Maybe the vacuum gauge is out of calibration.

If you don't have any reason to suspect broken rings or pistons, run it!

Chev Nut's advice above is spot on, as usual.

Vacuum is 15 inches with a forty year old gauge and a new one. Exactly the same.
Everything in the motor is new. So worn parts are out. No sign of broken parts. I would like to drive it , but it idles very rough and is weak as a kitten. Wears me out trying to deal with all the problems. I've run about a tank of gas through it now. No changes.The vacuum advance is not moving the distributor any though it is free to advance by hand. Don't know how much vacuum it takes to operate the advance. I've plugged both vacuum outlets-no difference at idle or on the gauges. About the leakdown test, I could understand all the cylinders losing say 20%, but three losing 50% and the other three losing only around 15% or less. That defies explanation, especially with the compression test showing 120# on all six. Valve lash is good. Checked again for third time when I did the bleeddown test.
I Remain Puzzled, Roger

You are grasping at straws a bit until the engine is broken in fully as has been stated above. Your numbers will improve as the rings seat but you need to drive it under a good load and avoid idling it once warm.

Check the valve lash hot as Gene suggested. Worth trying a bit more ignition advance as that will liven it up. You can run a lot more advance than stock with today's gasoline.

If the vacuum is still low after you put a good additional 1000 miles of driving on it, you'll have to continue searching further for a vacuum leak. Might be worth eventually checking that your cam gear and crank gear are lined up properly, especially if it has a lumpity, lumpity idle like an engine with a really long duration cam...

How sure are you that the cam is ok? I would disable the ignition, (and maybe remove the plugs to not run the battery down), take the valve cover off, and have a friend crank the engine while you watch the valves to make sure they are all opening about the same. A bad one should stick out like a sore thumb.

How sure are you that the cam timing is right? Generally speaking on most old engines the "overlap" occurs right at TDC, one whole turn from where the cylinder fires. You can check this by cranking the engine slowly with a wrench, no spark plugs, clockwise only, or you could push the car backwards in reverse if you cant get a wrench on. Better with a wrench if you can.

EIther way turn the engine only clockwise.

What you are looking for is the intake opening just before TDC (UC) and the exhaust closing just after TDC (UC). You will be checking the OPPOSITE cylinder in the firing order. That means you will be checking #6 if #1 is firing. Feel the rockers and watch the timing mark. The manual will probably tell you exactly where these events should occur, but it is right around TDC. If it is wrong, and it will be a little wrong due to gear slop, you can then calculate (using 360 divided by the number of teeth on the crank gear) to figure out whether it is off a whole tooth.

As for the vacuum advance, are you sure it holds vacuum? The manual will most likely give specs to check it. I would be really surprised if it isn't all the way pulled in at 15 inches. Harbor freight has handheld vacuum pumps with a gauge on them cheap. You might have to improvise a little to get a good seal on the advance, since it probably connects with a hard line and a strange fitting. Or, you could just suck on it. It can't leak, at all. Vacuum is probably not supplied to it at all at idle, and comes in when you crack the throttle.

What year is this car?

The block and head are '50 216. Everything else is 1940. The cam being the problem was my first guess. Checked the gears. Dead on for the marks.

Would the excellent compression test (120#) not eliminate valve timing as a problem? It was exactly the same for 6 cylinders.

Good news that timing marks line up.
Check what Bloo suggests above just to see if the valvetrain looks fine while hand turning it over. With a variety of parts in there I also wonder if it's possible to combine the short lifters with the short pushrods? There are 2 versions - short lifter and long pushrod used up to 39 or 40 and a tall lifter with a shorter pushrod used in later years. Never compared overall lengths to see if it's even possible to mix/match. Just thinking out loud.

It would be impossible to use the wong lifter/push rod combination.
A check of the valve adjustment would be my first recomendation setting the straight intake valves a .008" and angled exhaust at .014" engine running at a slow idle AND HOT.

Hi Roger,

How many miles have you driven the cars with varying load? Running the engine at a high idle speed with no load will not seat rings and help the valves seal on the seats.

More than one of us has strongly recommended that you quit troubleshooting and start driving. Enjoy the car. All of your work is indicating there is nothing wrong other than a good break in is needed.

Post the results after you have 500 miles on the car.

Mike Deeter came over again today and brought his good carb. I replaced the advance ahead of his arrival because there was no sign of it working even though the vacuum reading did not vary whether it was hooked up or not. Has advance on the spark now. Put his carb on and it idled and run fairly well-still with a minor miss though. Checked the vacuum which has been at 15 inches no matter what and it was at 18 !/2 inches. In the green on the gauge. Checked all the plug wires with a tool he brought-all good. He took his toys and went back home so I installed my carb and restarted. Misses much worse with my carb and the vacuum is back to 15 inches. While he was here we checked the valves running and hot. All but one were good and it was an exhaust about .002 loose. The carb was the only variable introduced after pulling his carb off, so I'm convinced there's a problem there. The minor miss with his carb on could be a plug sporadically misfiring, doesn't have a rhythmic dead miss. The search will now center on the carb and plugs. I can't drive the car in it's condition for 2 miles much less 500, so that is out.
Still Bewildered, Roger

Well, at least you know which way to go now! Keep after it, you are almost there!

So it sounds like you have a 1950 block and head with 1940 manifolds, camshaft etc... right?
And it sounds like Mr. Deeter has narrowed it down to a carburetor/mixture problem. Wouldn't a low but steady vacuum reading suggest a mixture issue as well?
I reread the above and don't see it mentioned what carburetor you are running. Carter W-1? Have you looked to see which metering rod size you are running... and checked the setting of the metering rod with a T109-25 gauge? Float level?
Someone here smarter than me will be able to give the answer but... will it be the 1950 head? or the 1940 manifold that will determine which metering rod you use?
In other words, do you need a carburetor for a 1940 engine or a 1950 engine? Each requires a different set up as members like Gene have pointed out on other threads.

Stovbolt, thanks for commenting. Let me clarify the situation if I can. The block and head with their working parts are all 1950. Everything else on the car is 1940 including the manifolds. Carb is W1 (420S) I believe. As far as the mixture causing the low vacuum, I have no clue. That's way above my pay grade. But I do know the reading dropped 3 1/2 inches immediately when I switched back to my carb. So there's some indication. I'm guessing(only guessing) that too little gas flowing through the metering (rod) jet would cause more of a problem at higher engine speed. But I don't know much about the dynamics of that part. Perhaps it is a problem.. I just mailed the carb off to a man who knows them inside out. We'll see what he says. All work related to performance will be on hold for a while.

I do have something I can work on though. The road draft tube baffle plate is missing and can't be found at the machine shop. I need to find one. Gene says '37-'62 6 cylinders all work.
So I'm hoping someone has a junk block they could pirate from. I know it's a necessary part because even sitting still, oil is dribbling out onto the garage floor as it runs.
Regards, Roger

It will be interesting when your carb person installs the carb as received from you on an engine and runs it. I recommend that you ask him to do that first.

It would not surprise me if there is a small crack somewhere in the carb body.

Do you know what carb model Mike Deeter used for the test?

My carburetor is a 420.

At least you've made some progress and the problem is not internal to the engine. That's a relief.

It's possible for a carb to have a vacuum leak from a passage, the base or maybe even sloppy shafts or missing parts/plugs. It's good news overall. There are a couple guys on here that are experienced with many carbs and might have an immediate idea where the leaks is. If someone rebuilt this, you should be sending it back to them.

Yes, we've made some progress. The rebuilder is behind me on this problem 100%. So I'm just waiting now. My sincerest thanks to all who have helped me in the last year on this rebuild.
Roger

Bloo, I did a little test to see where the valves were opening and closing on #6 today. Setting the ball on the indicator for TDC on #1, made a mark on the rim of the distributor where the rotor center was pointing. Then turned the engine forward until the exhaust valve on #6 closed. Exhaust closed at approximately 28 degrees after TDC. Then I rotated the engine backward past TDC for #1 and checked to see when the intake for #6 starts to open. Intake starts opening at approximately 8.7 degrees Before TDC. All this measurement is using the rim of the distributor so I know it won't be an actual measurement, but it is proportional to the actual cam angles. Can I interpret anything from the difference here? I know for 100% sure that the marks on the crank gear and the camshaft gear nest perfectly.
Roger

Remember that you need to double the degrees you read on the distributor to get crankshaft degrees.

The readings for the intake valve might be off a little because you were turning the engine backwards.

A more accurate process would be to make your observations with the engine turning in the right direction. Now that you have marked the distributor you should be able to do that by starting with #6 firing (both valves closed) at TDC.

The fact that you got a more normal vacuum reading with Mike’s carb (thanks for clarifying the model number) is a strong indicator that the problem is with your carb. It could be as simple as adjustments or jet size.

Perhaps your carb person should try to duplicate the settings and jets on Mike’s carb.