Vacuum gauge reading.

Bassman/NZ

Bassman/NZ

Well-known member
Finally got a vacuum gauge hooked up, and it's giving a steady reading of about 6 at idle. According to the interweb, this is the symptom of an intake leak. I had a new gasket on it, and all intake bolts were tight, same for the carb to manifold. I have taken the intake off, and the gasket looks fine, perhaps I need to face the surface of the manifold. Any comments on this?
 
What year intake? Are you sure all the vacuum ports are either used or plugged (including any on the carb)? I would think you would hear a leak large enough to only have a reading of 6. If you check the manifold and it is flat, use a new gasket with some spray on high tack on both sides and torque it down in steps from the center out. Also look at the underside of the manifold sealing surface for defects.
 
In the BIG picture .... Normal steady at btwn 17 and 21; a steady read below normal around 15 could indicate poor rings (retest by loading with oil); a steady read 13 to 16 could also indicate incorrect ignition timing; a steady read btwn say 8 and 14 could indicate incorrect valve timing; and a steady read below 5 indicates a leaky manifold or carb gasket. And if your vehicle has a heat riser ck that.

PS .... if possible confirm your readings by checking gauge connection and using another vac guage.
 
Y'know Marty, when I first read your other post, the first thing that came to my mind was vacuum leak. I didn't respond, because someone else suggested same thing. One thing that sticks in my mind from when I was a young fella, was reading once in a NZ Hot Rod Magazine about Rob Campbell trying to cure a poor running flathead. Tried everything, finally coated the inlet manifold gasket with grease, and, viola!! it ran sweet. Since that day, I have always done the same. Think about it, it allows the surfaces to slide over each other as they're torqued, any that oozes inside the engine will do no harm, and in the future, it allows one to easily remove and even reuse the gasket.
I know where you live; about 20 feet above sea level, you're running a stock cam, at idle you should be seeing 20-21 ins Hg on the gauge!
 
Update: I ran a flat file over the underneath surface of the intake manifold, and it was all over the place. I attacked it with great gusto, and had to actually take quite a bit off to get it flat. As well as not being flat, there were a lot of little gouges from the ports out to the edge. I kept flat-filing until it was smooth and close to flat, and I'll assemble it with a liberal coating of grease (thanks Brian). fingers crossed.
 
Another trick is to get something like "starter fluid" . . . and with the engine idling, spray some of it around the manifold - see if the RPM changes. Now don't spray too much and get the engine on fire!

It definitely sounded like you had a vacuum leak - 6 ins is a number that you should not see unless you opened up multiple carbs all at once . . .
 
Finally got a vacuum gauge hooked up
Click to expand...
How long have you had the problem? If the intake is as bad as your indicated it must have been that way from when it was installed on the engine? Sounds as though before you aquired it must have been mistreated / banged around pretty good before being installed. Are you speaking of the mating surfaces where they interface with the ports? If so a thin controlled amount of red RTV with copper on both sides of the gasket is a much more positive seal. several Mfgr's make the stuff. (http://www.zoro.com/loctite-rtv-sil...vjDicWOwdZiDelQUlpMwehoC2EPw_wcB&gclsrc=aw.ds )
It's for high temperature applications.
 
It may be better to check for a vac. leak around the carb, and intake area with a propane torch bottle with a hose on it to direct the flow of propane to the suspected area's. I use this method on Eng.'s that have paint, or other coatings that I do not want to damage. I also use Berrymans spray B12 Chemtool carb cleaner, but it will attack (eat) paint, and you have to be careful that you don't catch something on fire. be warned that using the chemtool, that if you use a lot it will run off the eng and puddle under the car and may catch fire. it has happened to me. this chemtool method seems to work the best, but it is the most hazardous and is the most destructive to paint. which ever way you choose just be aware of a fire hazard.
 
A couple vacuum leak tests we have used over the years.

1-Use propane with engine running and hold propane ( old torch etc) and apply around the intake, carbs etc . Engine will speed up usually when leak is found.
Note engine will slow down when carbs ar too rich.

2-Hook vacuum gauge to dipstick tube when running any vacuum ay all show intake leak internally under side of intake...
 
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