<VV> '69 engine misfire?

[Dale Dewald]

At 21:46 12/08/2008 -0500, Michael Kovacs wrote:
>'69 110 PG 172,000 miles, never had heads off, doesn't smoke or use 
>excessive oil.? Starts instantly.
>
>?Problem is a slight back fire when cruising. This is a mild series of 
>pops through the carb(s) and a minor loss of power (to be expected). ?A? 
>stab of the gas will?usually bring a moment of hesitation than?normal 
>power. The carbs were rebuilt by me about 2,000 miles ago due to tired 
>accelerator pumps, they do?squirt very well now.
>
>?It has a protonics ignition installed years ago. It seems like a lean 
>mixture to me, but how? The manifolds are from an non smog engine and the 
>AIR was removed by PO 30 years ago. I've had this car since the mid 70's.
>
>?I can check almost anything, but with this 20 degree weather, I'd 
>rather?concentrate on the most obvious. I drive it 20 miles to work with 
>no other symptoms, but it seems much better when hot.

I think you are on to the probable cause being a lean mixture.  Try to 
check all of the likely sources of vacuum leaks; all hoses, balance tube, 
PG modulator & lines, etc.  Typically vacuum leaks become more noticeable 
when the engine is warmed up and the chokes close--this does not quite fit 
with your problem.

If the above yield no improvement, I would next want to check the condition 
of the valves.  It is possible that they are worn down enough, or that the 
guides have worn allowing them to wobble, so they no longer seal 
effectively. This would allow pressure pulses to back flow into the intake 
manifold (which screws up the carburetor flow) and perhaps cause the 
backfire condition you describe. Put a vacuum gauge on each cylinder bank 
at the choke pull-off port and check for flutter.  A compression check (or 
better a leak-down check) may show a couple low cylinders or obvious 
leaking through the valves.  The shop manual has more information on these 
tests.

Dale Dewald
Hancock, MI