What is the vacuum test telling me?

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Steve Jelf
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What is the vacuum test telling me?

Post by Steve Jelf » Fri Oct 18, 2019 1:19 pm

The first part is running at idle. The second part shows accelerating and returning to idle. Can somebody with vacuum test experience tell what it means?

https://youtu.be/glVtd05mFJc
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Re: What is the vacuum test telling me?

Post by jab35 » Fri Oct 18, 2019 1:57 pm

Steve: Thanks for sharing. Here's a link that might help, I know the Model T experts are poised to respond and I'm eager to hear their assessments.
https://dannysengineportal.com/how-to-do-a-vacuum-test/
Good luck, jb

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Re: What is the vacuum test telling me?

Post by cudaman » Fri Oct 18, 2019 2:16 pm

Looks pretty normal to me for a slow revving four cylinder engine. Is your 1915 still having starting difficulties?
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Re: What is the vacuum test telling me?

Post by Shane Lach » Fri Oct 18, 2019 2:17 pm

A fluttering needle typically leads towards a valve issue.

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Re: What is the vacuum test telling me?

Post by George Mills » Fri Oct 18, 2019 2:28 pm

hey Dauntless....

Before you lose me...which engine is this? I thought that you just rebuilt one here a little bit ago.

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Re: What is the vacuum test telling me?

Post by Ruxstel24 » Fri Oct 18, 2019 2:40 pm

I agree with Mark, looks pretty normal to me.
The fluttering is not very much, as slow as a T idles...IMHO

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Re: What is the vacuum test telling me?

Post by Steve Jelf » Fri Oct 18, 2019 4:00 pm

I thought that you just rebuilt one here a little bit ago.

That's right. This is the 1915 that just received a new crankshaft, pistons, and rings. Of course all the valves came out when the block went for new Babbitt, and I expect Mike checked them carefully when they went back in. That's why valve trouble would surprise me. My guesstimate is that this engine has less than 2500 miles on it. But the truth is that from the git-go it hasn't had the guts on hills that I was expecting from a fresh engine. I'm wondering if maybe there's a head gasket leak.
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Re: What is the vacuum test telling me?

Post by Charlie B in N.J. » Fri Oct 18, 2019 5:23 pm

You're a wee bit low. 17 to 21 is normal. That fluctuating needle bothers me. It's not much of a range of movement but that's usually an indication of mis-firing or sticking valves. Not buying the head gasket just yet either. I know you posted the dry comp readings before but I'd REALLY like to see a cold engine wet & dry test result. Cold to simulate an engine's condition in a cold start situation. You mentioned it re-starts easier when warm so lets eliminate that situation and see exactly what's up when cold.
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Re: What is the vacuum test telling me?

Post by Steve Jelf » Fri Oct 18, 2019 5:28 pm

I kept the dry compression numbers: 36, 40, 35, 38. Lower than I would expect on a fresh engine, but enough to start without it being a major project.
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Re: What is the vacuum test telling me?

Post by George Hand » Fri Oct 18, 2019 5:34 pm

Steve, A couple of other issues that may cause vacuum gauge fluctuation could be caused by excessive valve stem/guide clearance or a week valve spring. George

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Re: What is the vacuum test telling me?

Post by andrefordt » Fri Oct 18, 2019 5:57 pm

Hey Steve,

For me this is looking normal, knowing that this is a fresh rebuild engine. It needs a good run in.

At the start the needle is shaking fron 17 to 19 on idle, at the end it is shaking from 18 to 19, for me it does this because the model t engine is a low running engine at idle speed.

To be sure of the condition of your rings and valves make it run on a brisc idle and watch the needle. It should stay stable .Short one spark plug at the time and see what the needle does. It should get lower for all four cilinders the same value . If it is not stable or getting lower the same value at each cilinder you have a problem with the cilinder where the lowering value is the less. Shaking should say valves less lowering is compression, should be rings or wear, but it is a fresh rebuild engine and need a good run in.

Just my opinion.

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Re: What is the vacuum test telling me?

Post by George Mills » Fri Oct 18, 2019 7:36 pm

Steve,

I’ve been ‘noodlin’ This all day

My first thought was...not too bad

Then I remembered you had difficulty starts and not just “new resistance”...

Then I looked at your video about 10 more times...

Then before a conclusions you mentioned it was not any better at pulling a hill then before the rebuild...

What do the spark plugs look like? It’s sure pointing to a valve, but the same deductions would hold true if it only is a 3 cylinder car.

How’s your timer? If it’s one of those S things from about 8-9 years ago...they are trash right out of the box.

Make sure your timer rod isn’t shorting the timer when all the way down...

I’d also ask your gap on the plugs. I know you spend a lot of time nurturing your plugs. In lots of cases I find 030 works a whole lot better than 025...

All this as a double check...then I’d do as Andre suggest.

There is a p.s. here...you ARE sure your 15 does not have 3::1 rear? That could answer the no guts on a hill even with a fresh engine?


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Re: What is the vacuum test telling me?

Post by RustyRim » Fri Oct 18, 2019 8:04 pm

Broken or very weak valve spring


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Re: What is the vacuum test telling me?

Post by AZTerry » Sat Oct 19, 2019 12:20 am

What really stands out to me is your other posts that I believe may be related to this issue and the 2500 miles on the new engine.
Some replies imply valves, and I tend to agree it should be the first place you look.
The post prior to me writing this suggests it may be weak or broken valve springs.
I am going to suggest a related problem that I had.
My block was decked and new valve seats cut.
I pulled the stock head to put a Z head on and found the exhaust valves contaminated and on the way to being burnt.
The problem was the valve spring height.
I was running Chevy valves with Stock keepers.
For the past 13 plus years I have been running offset keepers to achieve the correct valve spring height.
No further valve problems.

I hope this helps
Terry

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Re: What is the vacuum test telling me?

Post by John Warren » Sat Oct 19, 2019 7:49 am

Hi Steve, You may have already done this, but you can check valve , ring and head gasket problems by doing a leak down test. Take out all of your plugs, pull your pistons top dead center on compression stroke, put your car in hi gear and block your tires. Now build up an air hose and fitting that will screw into your spark plug hole and use a regulated air pressure say about 60 psi. Gently introduce the air pressure, listen at the carburetor, ( open the throttle) listen to the exhaust and to the oil filler, and check other spark plug holes. Any leak should point to problems. This is a very non evasive test, and relative easy to do. This is how air planes are checked during an annual. Some one has to hold the propeller to keep the engine from turning. It is very easy to tell if you are on top dead center when you are the one holding the prop. If it is trying to turn, you over power it in the opposite direction until it is not trying to turn. Some one mentioned timer, spark plugs. My friend Danny was always having poor performance problems and was ready to take his engine out and overhaul it. It turned out to be a coil problem. He installed a set of freshly professionally tuned coils and wow all the problems went away. Idled better more power and starts great . It always ran and would miss if you grounded out the spark plugs (normal) just didn't perform.
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Re: What is the vacuum test telling me?

Post by Charlie B in N.J. » Sat Oct 19, 2019 8:35 am

Lordy John you mentioned a possibility there. Coils. I'm assuming Steve's are done but not sure. My '23 started dead cold with hand cranking for the first time since I owned it after replacing the caps and HCCTing.
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Re: What is the vacuum test telling me?

Post by Steve Jelf » Sat Oct 19, 2019 8:59 am

Yes, the coils were tested and adjusted at Greenfield Village in September. The plugs fire in the tester at 60 psi.
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Re: What is the vacuum test telling me?

Post by Lessumner » Sat Oct 19, 2019 10:46 am

Hi Steve, Have you completed the same tests on your other model T? Does your touring perform better than your new rebuild? Les

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Re: What is the vacuum test telling me?

Post by Steve Jelf » Sat Oct 19, 2019 11:28 am

Have you completed the same tests on your other model T?

No, the touring isn't performing at all. It's down with a busted low drum and its NH is on the runabout.
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Re: What is the vacuum test telling me?

Post by Tractordoc » Sat Oct 19, 2019 12:59 pm

I would suggest that you do a cylinder leak down test as John Warren suggested. We do them as a first step in engine diagnosis all the time at work really lets you know the condition of your engine before you start playing the guessing game. John explained the steps quite well. The one thing it might not show is a weak valve spring. Good luck finding the problem. George


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Re: What is the vacuum test telling me?

Post by otrcman » Sat Oct 19, 2019 8:53 pm

Tractordoc already seconded John Warren's suggestion of a cylinder pressure test, so I guess the best I can do is "third" it. But when doing a pressure leakage test it's a lot easier to listen at the intake, exhaust & breather by using a length of hose. Garden hose, vacuum hose, whatever. Just so it's long enough (about 24 inches, and 1/2 to 1" inside diameter) so you can put one end right at the hole in the engine and the other end close to your ear.

Trying to get your head down close to the carburetor inlet is for young guys with long necks. The hose method allows you to get a much louder signal which will make it easier to compare from one cylinder to the next.

Dick


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Re: What is the vacuum test telling me?

Post by Roverdriver » Sun Oct 20, 2019 1:56 am

Steve, John Warren's suggestion is a good one to locate a few different problems, so I endorse it. To make an adaptor to pump air into each cylinder, take a standard T spark plug, unscrew the upper section, remove the porcelain and replace that with a tyre valve. That works really well.
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Re: What is the vacuum test telling me?

Post by RustyRim » Thu Oct 24, 2019 7:10 pm

found these instructions in my 50 year old tester[image][/image]
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