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Vacuum Leaks

mylesw

Jedi Hopeful
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Hello

What is the best way to test for vacuum leaks on a BT7 (HD6 carbs)? I have a vacuum gauge somewhere and could connect but don't know where is best...

Thanks

Myles
 

Keoke

Great Pumpkin
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Hi
Check with your local automobile parts/supplies stockist he will have products that can be sprayed around on the carbs and manifold that are fairly safe to detect the source of a leak.
 

BOBBYR

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I've used window cleaner or water from a spray bottle for this - if there's a vacuum leak, the engine will stumble.
Hi Guy's on checking for vacuum leaks, I like to use a little starting fluid. If you put a portable tachometer on engine while using the fluid, you can usually find event the smallest of leaks by an increase in rpm . Remember your dizzy and brake booster also get their vacuum from the manifold and carbs . Hope this helps Bobby R
 

Keoke

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Hey Bob, I'm getting a mental picture of a guy checking with a lit torch. Ah, You guys are too much .
Bobby R

NO Bobby R :
Just make sure the engine is cold and the torch is unlit--:excitement:
 
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A can of WD-40 with the thin "straw" attached would also do very well and not be explosive.
 

aeronca65t

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I always use Micheal's method. WD-40 is nice and safe.
 
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Nutmeg--

You're right and so is propane but the concept is to spray it toward the carbs, not an open flame....
 
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Tom--

Sorry for the sarcasm....
 
OP
M

mylesw

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Used the starter fluid method and appear to have some leaks where the spacer blocks attach to the manifold. I've bought some new spacers and gaskets from Moss but the spacers appear to be made of plastic these days... Question is, should I use gasket sealant when attaching these to the car?

Also, took the opportunity to do a compression test on the cylinders and 4 of them are at 120 psi and one at 105 and another at 110. The reason I was looking for vacuum leaks is because I have been getting a backfire through the carbs. Could the low compression on the two cylinders cause this? Note that before doing the compression test I checked and set the valve clearances.

Thanks

Myles
 

NutmegCT

Great Pumpkin
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You'll get many good replies on the gasket sealant question, but my first suggestion would be to call Moss Motors and ask. If they say yes, then ask what sealant they recommend.

Another thing to check when you start the work - is the carb to manifold junction surface flat (not warped), to allow perfect mating of the surfaces? I had a major vacuum leak due to a warped (heat?) carb base; had to true up the surface by rubbing across sand paper on a flat steel plate. My brain has failed - can't remember the word for that process.

Tom
 

steveg

Yoda
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You'll get many good replies on the gasket sealant question, but my first suggestion would be to call Moss Motors and ask. If they say yes, then ask what sealant they recommend.

Another thing to check when you start the work - is the carb to manifold junction surface flat (not warped), to allow perfect mating of the surfaces? I had a major vacuum leak due to a warped (heat?) carb base; had to true up the surface by rubbing across sand paper on a flat steel plate. My brain has failed - can't remember the word for that process.

Tom

Lapping?
 

CLEAH

Jedi Warrior
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Decking or surfacing. I think lapping involves two pieces of material, such as a valve and a seat.
 
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