Finding a vacuum leak

[39topless]

I am having a hard time starting my '39 stock V8 engine without priming the carbs first. Someone suggested looking for a vacuum leak. How do you find a vacuum leak? Do you spray some water onto the likely areas and look for bubbles? (like looking for a hole in a tubeless tire in the old days). Thanks,
Phil

[Jay in Mass]

I don't think that a vacuum leak would cause bubbles like looking for a hole in a tire which is blowing air out. Maybe you can spray something like choke cleaner around the suspected area. Let it get sucked in and look for changes in how it runs.
Someone will come up with a good idea.

[Ken/Alabama]

If your looking for a vacuum leak you can start the engine and let it idle and spray some carb cleaner or WD 40 around the base of the carb and other vac points. If you have a leak the engine will pick up. Another way to check for a leak is to place two or three fingers across the top of the carb. If you have a vac leak the engine will pick up. Hard starting sounds more like a leaky fitting or lid on the fuel pump to me.

[cmbrucew]

Originally Posted by Ken/Alabama

If your looking for a vacuum leak you can start the engine and let it idle and spray some carb cleaner or WD 40 around the base of the carb and other vac points. If you have a leak the engine will pick up. Another way to check for a leak is to place two or three fingers across the top of the carb. If you have a vac leak the engine will pick up. Hard starting sounds more like a leaky fitting or lid on the fuel pump to me.

Ken
I was told WD 40 does not use propane for a propellant now. Maybe an old can will have it.
Bruce

[COE Dan]

I'd recommend using a vacuum gauge. If you connect it to a vacuum source on the intake manifold, a normal reading at a steady idle is somewhere between 17-22 in. A vacuum leak will give you a low steady reading (somewhere around 5 in). If you see a different reading or fluctuating needle, there may be another issue. I'd recommend doing a Google search for "vacuum gauge reading chart" - you'll find a bunch that show you what different readings mean. I'd reproduce one here but I don't have one particular chart I'd recommend as correct so I'll let you do you own research to be certain.

[Willit Stop]

Sponsored Links (Register now to hide all advertisements)

Here's a link that shows how to interpret a vacuum gauge in different scenarios.Click on the little boxes and it shows how the gauge reacts to different problems.
http://www.secondchancegarage.com/public/186.cfm

[Jack E/NJ]

The propellant doesn't matter. The engine is running too lean from the vacuum leak. So the liquid from the WD40 or carb cleaner is supposed to temporarily block the excess air intake at the leak point. This should cause the engine to speed up. Same with cutting the carb air supply with your fingers as Ken suggests.

Jack E/NJ

[V8COOPMAN]

Originally Posted by Jack E/NJ

The propellant doesn't matter. The engine is running too lean from the vacuum leak. So the liquid from the WD40 or carb cleaner is supposed to temporarily block the excess air intake at the leak point. This should cause the engine to speed up. Same with cutting the carb air supply with your fingers as Ken suggests.

Jack E/NJ

The propellant DOES matter. In the case of propane, it adds a combustible fuel a fuel-starved LEAN mixture as it's sucked into the vacuum leak. DD

[39topless]

Wow! Great information guys. I will be a busy boy today. Don't know for sure yet ( I'll try the WD-40 trial ) if I have a vacuum leak, but I'm going to go buy a vacuum gauge anyway. Will post the results. Thanks again.
Phil

[V8COOPMAN]

Originally Posted by 39topless

Wow! Great information guys. I will be a busy boy today. Don't know for sure yet ( I'll try the WD-40 trial ) if I have a vacuum leak, but I'm going to go buy a vacuum gauge anyway. Will post the results. Thanks again.
Phil

Read the WD-40 can. The earlier ones with propane as a propellant used to say "FLAMMABLE". THAT was the stuff I used to run thru the potato gun.......VERY flammable. DD

[supereal]

I can post a vacuum gauge troubleshooting chart if you need one. You can also use an unlit propane torch to check for leaks. Put a hose over the end and move it over various connections until the engine speeds up. DO be careful.

[gearhead1952]

Starting fluid in a spray can works well also.

[scicala]

Originally Posted by gearhead1952

Starting fluid in a spray can works well also.

Especially for people with no fear of injury or death.

Sal

[V8COOPMAN]

Originally Posted by scicala

Especially for people with no fear of injury or death.

Sal

Boy I'm tellin' ya! Some scary ones in our midst. DD

[Mike B]

Maybe FH related, maybe not...if I suspect vacuum leak, it's because my idle speed is higher...

So...I spr y bottle water into suspect res, looking for decrease in idle speed s the water plugs the leak for bit.

Sorry, bd KB

[pooch]

I thought is you blocked the air flow at a leak, the engine would reduce revs...

[1935coupe]

OK I do these tests several times a week on todays cars, it is not hard but you may have to improvise on the equipment. I have a smoke machine, this handy tool will blow smoke out of a hose. Cover the air inlet of the carb and any other known large leak area (pcv system) connect it any vacuum port, then look for where the smoke comes out. I have heard some that use tobacco products know how to make smoke or buy a Halloween smoke machine and create an adaptor. Once you have the smoke machine set you can use it on a lot of things. It will send a low pressure smoke to find leaks on most anything.

[39topless]

Update. I purchased a vacuum gauge,(in the first parts store the counter guy had no idea what I was talking about), and the engine tested at a steady 15. 5 ". I guess a vacuum leak is not my problem.
Thanks for all the replies and suggestions

[V8COOPMAN]

This thread has gotten WAAAAAAAAY off base. You said it won't start if you don't prime carb first. Are you talking about after it sits for a week, or after eating a burger at the joint? When you have this "fuel" problem, are the carb bowls dry? Do you see fuel squirt when you move the throttle? IF the bowls are dry, the problem most-likely is NOT a vacuum leak. What are your REAL symptoms......the COMPLETE story? DD

[39topless]

Originally Posted by V8COOPMAN

This thread has gotten WAAAAAAAAY off base. You said it won't start if you don't prime carb first. Are you talking about after it sits for a week, or after eating a burger at the joint? When you have this "fuel" problem, are the carb bowls dry? Do you see fuel squirt when you move the throttle? IF the bowls are dry, the problem most-likely is NOT a vacuum leak. What are your REAL symptoms......the COMPLETE story? DD

Yo Coopmyster, the vacuum gauge reading came up because someone suggested my starting problem may be a symptom of a vacuum leak. I reached out in an earlier thread about the starting problem and how I had to prime the carbs to get it going. After sitting overnight, I had to prime the carbs (this is a 2 X 2 setup), to start the engine. The fuel system includes a new electric pump, filter and a regulator set at 2.5.
I do get gas in the throat of the carb when pumped but I have no idea if the bowls run dry overnight. I have the front carb setup with a choke but leave the back open. There seems to be a miss when running and it does backfire occasionally through the exhaust. I could live with these issues if I could get it to start easily.
Thank you for your insight and interest in the well being of my '39.
Phil