Subject: Back firing
Darren wrote:
> Hey people,
> Just started getting a backfire through the exhaust. doesnt happen all the
time. Mostly when the the car is still on the cooler side. Seems to happen
around 2700-2900 rpm. Other particulars 69 2000, u20, n17 needles, no smog,
gary's dizzy, dean's header
Hi Darren,
In order for a post fire to occur, three things have to be present.
Excessive unburned gas in the exhaust, an ignition source and fresh
oxygen to combust with.
Unburned gas is normal during decel (carbed engines), but usually there
isn't enough oxygen left to allow a post fire combustion (even if sufficient
heat or other ignition source is present to ignite the mixture.)
The fact that this post firing occurs when the engine is relatively cool
tends to suggest that one or more exhaust valves are not seating correctly.
You may want to run a compression leakage test to see if air is leaking
past the exhaust valves (at TDC).
A compression check cannot check for such minute valve seat leakage.
Cylinders can produce excellent compression readings, yet fail the CLT test
completely.
This is because the CLT is conducted statically with the pistons
stationary (at TDC).
Even minute failures of the valves to seat can allow combustion flame to
enter the exhaust ports, lighting the poorly vaporized fuel of the cool or
cold engine.
If the exhaust valves are leaking, you can sometimes save yourself the
pain of a valve job by carefully tapping on the top of the rocker tips (just
above the valve retainers).
Be sure there is sufficient valve clearance (rocker to cam) prior to
tapping on the rockers to allow a full bounce of the valve spring to
hopefully dislodge the material caught between the valve seat and the valve
itself.
If simple carbon buildup is the problem, one can often times chip away
or dislodge this material using this "tapping" method.
Lastly, you might want to double check your cam timing. Late or
retarded cam timing (due to chain stretch) can allow for late closure of the
exhaust valves.
Kim Blough
Idaho Z Car
2913 Garrity Blvd.
Nampa, Idaho 83687-2629
1-208-466-0004
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