Attach a manifold vacuum gauge and you can determine
if you're on the right track about the vacuum leak. Use a long
enough hose to reach into the cockpit so you can see it while
driving. That's a serious vacuum leak for it to break that
much vacuum that you notice it at cruising speeds. Hm,
seems like a lot to get from parts expanding from warming up.
When you noticed that both carb pistons raised a little that
kind of tells me that the manifold vacuum is OK.
A distributor cap can do that sometimes, but every time I've
seen that it's been on cylinders adjacent in the firing order.
The vacuum gauge can also tell you if you've got a serious
problem with valve adjustments or operation. It can also
show if you are losing compression on those cylinders.
I guess that's what I'd do first (it's easy), especially since
you've already ruled out carburetion. Can test spark plugs
easily by just swapping front 2 with rear 2 (unlikely since
they are new).
The mechanics in the shop I had would have shot some
marine engine storage oil into the carb to see if it picked
up; that was a quick test they would do to check for that
symptom on a multiple carburetion engine, which is common
on large outboard motors. The improvement you saw by
blocking off the front carb is a clue. We probably should be
thinking in that direction, keeping in mind your carburetor
swap. Did the machine shop check the cylinder head for
cracks? There is an inspection process that can be used
on aluminum parts that is similar to magnaflux testing.
Carl Wade
Williamson, New York State USA (retired marine repair shop owner)
B395000858 LRX
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