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Good way to check the health of the engine is by using a leak down test gauge. This test will show the true health of the engine and you can diagnose possible issues by listening to hear if the escaping air sound (hissing) can be heard from the oil fill (air coming past the rings) heard from the carb throat (intake valve not seating) or the air sound heard from an ear on the exhaust ( burnt exhaust valve or valve not seating) Most healthy type1 engines  can have up to 15% leakdown and still run very strong. Leak down over 20% and the engine will run hot and have issues pulling grades easily . This test is best done on a warm but not hot engine. High precision racing engines can approach zero leak down, which is amazing. This test should not be confused with a compression test. It is completely different and more accurate to tell the health of an engine. I am not sure why I decided to post this but I hope it may be of use to someone.

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Last edited by Jimmy V.

+2 on the manifold gaskets. 

Small, intermittent vacuum leaks are a way of life with built VW engines. You have two big, heavy carbs perched atop a long(ish) intake manifold which tapers, V-like, to bolt to the small(ish) dual intake ports with just two studs. Hot rodders alla time "port-match" the heads to the intake by taking material out of the heads, leaving only a teeny tiny Brazilian bikini's worth of material between the two now-big ports. 

Your gasket is setting on that, getting gently persuaded this way and that by the oscillating intake suckage on each side of it while simultaneously getting jiggled by the road bounces that are swaying those heavy carbs above.  

Add it all up and pretty soon, usually, something softer than aluminum gives way and just a leetle beet of extra air joins the race into one or another of your cylinders—or both, depending on the condition of the bikini's grundle bit.

And now you can't balance the carb, idle goes away and, yup, popping on decel. 

Worth a look I say.

Last edited by edsnova

Leaks don't HAVE to be a way of life. I've never had a manifold leak after I figured out what the problem was. Ed's info above is correct about the big weight on the top. Also, out of synch carbs cause a LOT of vibration, making it even worse. My problem was soft gaskets that squish out. Hint, don't buy the thick CB gaskets.

I got some gasket material from a machine shop. It's thin and stiff, about 1/16" or less. Loctite 517 on both sides and they've never leaked in 40,000 miles. I also put blue threadlocker on the nuts.

Good and tight linkage is just as important. And Synch! 

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