Backfire on a down grade

Russell Reay

Russell Reay

Well-known member
In my trip this afternoon one leg was 1 1/2 miles down a continuous 5-12% very crooked grade. With the spark advance at ~ max I kept the car in 4th (5 speed) to limit the braking, and experienced several backfires. Is there some tweaking of the advance that will eliminate, or minimize the backfiring?
 
Russell, In my car, while going down a grade, I can cause the engine to backfire out the exhaust by retarding the timing. Unburnt air/fuel is going out the exhaust because it is not getting burnt in the engine and the retarded timing ignites it.

Now that you have re-timed your car, how is it running? Is it still low on power?

The 30 degrees of backlash that you reported in your distributor worries me and may be related to your backfiring. Investigate where this is coming from and fix it.

A leaking exhaust valve can cause the unburnt air/fuel in the exhaust system to ignite when going down a grade.

You also reported that the point gap was different for different positions of the distributor cam. This indicates that there is wear in the distributor or that the cam is bad. This may be related to the backfiring as there will be different timing for different cylinders. Either fix this or get a different distributor without this problem.
 
Last edited:
If the muffler clamp gap is pointing directly at the carburetor's intake throat, some leaking exhaust will be sucked into the carb upon deceleration. It doesn't have to be an obvious massive exhaust leak. Just a little leakage in this area between the exhaust manifold and muffler pipe created by an imperfect seal will be sufficient, a very common problem with some cheaper mufflers or with pipe flanges that are not perfectly round. This leakage into the carb will cause a popping or even a backfire. Loosen the nuts on the clamp bolts and rotate the gap away from the carb's intake throat. Tighten the nuts and take a drive down a steep hill to see if this has cured the problem.
Marshall
 
It was backfiring thru the exhaust, not carb. After re-timing the car has as much power as ever. The need for a distributor rebuild seems clear. Johnb's link was helpful. I have never fiddled with the GAV, feeling that i would need a long level stretch of highway to sense any change. I normally run about 1/4 turn open--I will try some additional. I have not done a leak-down test, but compression is good and fairly uniform.
 
Recheck the timing. Set it with a lite or buzzer. By eye not so good.

Mine had to run full advance going down a long grade.
 
You must log in or register to reply here.
Back
Top