tjmckenzie
Junior Member
Don't know if this has anything to do with the backfire but the plugs were not real wet. Could not enough fuel cause backfire?
What am I missing? 1930 Model A
- Thread starter tjmckenzie
- Start date
nkaminar
Well-known member
TJ,
Does it backfire through the exhaust all the time or just when backing off the throttle or accelerating. If it backfires when backing off the throttle it could be a retarded ignition. If all the time or accelerating it is likely a stuck exhaust valve. A lean mixture will not help but it is probably not the primary cause. Use the mixture control knob to enrich the mixture to test the lean theory. It could also be that one or more spark plugs are not firing allowing un burnt fuel into the exhaust system where it explodes. Find a mechanic to help you.
Use the accessory hand crank that came with the car. It has the socket wrench on the other end to change the tires. Put the car in neutral and keep the ignition off. Insert the crank in the hole in the radiator and position it so that you are pulling up for a compression stroke. Pull up and note how hard it is to pull up. Reset the crank to pull up for the second compression stroke. Do this for all four cylinders and note if any seem to pull up really easy. If that is so than pull the head to see if you can find an exhaust valve that is sticking. There can sometimes be a bit of carbon under the valve that keeps it from sealing or there could be a section of the valve missing or it could be just sticking up from sitting. You may need a valve job. This is usually done by just lapping the valves and resetting the lash.
Does it backfire through the exhaust all the time or just when backing off the throttle or accelerating. If it backfires when backing off the throttle it could be a retarded ignition. If all the time or accelerating it is likely a stuck exhaust valve. A lean mixture will not help but it is probably not the primary cause. Use the mixture control knob to enrich the mixture to test the lean theory. It could also be that one or more spark plugs are not firing allowing un burnt fuel into the exhaust system where it explodes. Find a mechanic to help you.
Use the accessory hand crank that came with the car. It has the socket wrench on the other end to change the tires. Put the car in neutral and keep the ignition off. Insert the crank in the hole in the radiator and position it so that you are pulling up for a compression stroke. Pull up and note how hard it is to pull up. Reset the crank to pull up for the second compression stroke. Do this for all four cylinders and note if any seem to pull up really easy. If that is so than pull the head to see if you can find an exhaust valve that is sticking. There can sometimes be a bit of carbon under the valve that keeps it from sealing or there could be a section of the valve missing or it could be just sticking up from sitting. You may need a valve job. This is usually done by just lapping the valves and resetting the lash.
jerrytocci
Senior Member
Check the distributer cam for even rotation . Could be a stripped timing gear
jerrytocci
Senior Member
OHM out the ignition switch to dist. plate
Just a thought on timing: When you check/set your timing, the lever on the steering column must be in the up (fully retarded) position. Double check this when setting the timing, otherwise you will be WAY off. By the way, is the advance/retard lever connected to the distributor and working?
Tacoma Bob
Well-known member
MMMm I see that what little gas is in the sediment bowl looks very, very old. The gas should fill the bowl. You may want to remove the gas cap as the vent in the cap could be crudded up.
nkaminar
Well-known member
TJ, There is a Model A club near you and the members would be more than happy to help get your car sorted out. The chapter is the Delta Model A Club. Their web site is at http://deltamodelafords.homestead.com/. Check it out. In the Contact window are a couple phone number to call to get started.
How can this be? John
The dimple on the timing gear will indicate TDC on both the compression and the exhaust stroke. If you install the distributor shaft a half turn off and then time to spark at the top of the exhaust (thinking it's at the top of compression) it will be 180 degrees off.
180 degrees is pretty obvious. I once had to diagnose a car that turned out to have a timing gear installed one tooth out of sync with the crankshaft. A real pain to get to the timing cover, so just visually checking was the last resort. It ran, but poorly. That was interesting.
JayJay
J Franklin
Well-known member
JayJay;198162) If you install the distributor shaft a half turn off and then time to spark at the top of the exhaust (thinking it's at the top of compression) it will be 180 degrees off. JayJay[/QUOTE said:
sinbad339
Junior Member
Maybe I'm missing something, but because the crankshaft makes two revolutions for one four-stroke-cycle, the dimple will indeed indicate TDC on both compression and exhaust stroke. 180 degrees off isn't quite the correct term (360 degrees off?), but the basic issue is the same; yes?
J Franklin
Well-known member
The dimple is available on TDC on the compression stroke, it is half way on the exhaust stroke.
You are correct of course, thanks. I was thinking of other engines I've worked on that had the camshaft gears at the bottom of the distributor shaft and it's not all that difficult to get the distributor mis-timed to the camshaft.
And in hindsight - my whole premise above is flawed. i wasn't thinking, the dimple is on the timing gear connected to the camshaft, which only rotates once per two rotations of the crankshaft. So as J Franklin points out, there won't be any dimple at TDC of the exhaust stroke. Gotta go back to first principles! Doh!
So ignore anything I've said in this thread. I'd delete it all but too much discussion attached.
JayJay (hanging head in shame)
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