Finding a vacuum leak
 

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

Re: Finding a cacuum leak
 

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.

Re: Finding a cacuum leak
 

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.

Re: Finding a vacuum leak
 

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.

Re: Finding a vacuum leak
 

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

Re: Finding a cacuum leak
 

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

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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.

Re: Finding a vacuum leak
 

Starting fluid in a spray can works well also.

Re: Finding a vacuum leak
 

Especially for people with no fear of injury or death.

Sal

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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.

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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

Re: Finding a vacuum leak
 

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