backfire 68 327

[greggome]

My car wont start and is backfiring through the carb when cranking. Was running the day before though not well ,hesitating and episodes of bucking /sputtering then running better. I have been chasing this problem for awhile but now its stuck in the garage and wont run. Fuel is being deliver and I have a spark. any ideas on possible cause and how to track this issue down?

 

[dougelam]

If the engine ran fine before and nothing was touched I guess that I would start by putting the number 1 piston at TDC (top dead center)
Then check to see if your rotor is pointing towards the number 1 spark plug wire
If not maybe a loose timing chain
If close just simply time for a tune up

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[Tom Bryant]

The timing chain was what came to mind first for me too. Has your engine ever been rebuilt? If not, it may have one of those nylon toothed cam gears. If that's the case I doubt there is much left of it and the chain might have even jumped a tooth or two.

While you have your distributor cap off when setting the #1 to TDC you can check for slop on the timing chain. Turn the crankshaft one direction until you see the distributor rotor move. Now turn the crank the opposite direction until you see the rotor move. How far did the crank move before the rotor moved? It should be nearly instant to just a couple degrees. Any significant movement before the rotor moves will indicate a loose timing chain.

Another thing that can cause these symptoms is a bad camshaft. You may have one or more flat lobes. While you are tuning your engine over by hand to set TDC and checking the timing chain, remove the valve covers so you can observe the rocker arm movement. All rockers should move about the same amount. If any don't seem to move as much as the others it is an indication of a worn cam lobe. You can measure this with a dial indicator but a significantly worn lobe will show up as a rocker arm that is barely moving. Real easy to see without any special tools.

Continue turning the engine until both valves on the drivers side front cylinder (#1) are closed and stop with the timing mark on "0" to be on TDC.

Tom

 

[greggome]

Thanks for the advice. The motor was rebuilt many years ago and has about 60K on it since.The crank and rotor move in unison. The rotor is pointing about 180 degrees off on the compression stroke of cylinder # 1 so it appears somethings screwed up.

A little history: The car was my brothers handed down to his son who had it up on block for about 20 years. 2 years ago I had the car shipped to me. I did all the basic procedures for preparing to start a motor that was stored for awhile. I just could not get the car to run. after weeks of trying I found the rotor pointing 180 degrees off of the #1 cylinder at TDC. I am still puzzled about how the rotor ever got that far off because I know the car was operating before it went into storage. I pulled the distributor and set the rotor to the proper position. I have run the car since but lately have been having intermittent hesitating, stalling, unstable idle speeds and hard cold starts which had been attributed to possible carb / fuel delivery problems. The weird thing is to see the rotor back in the same position it was in 2 years ago when the car wouldn't run pointing 180 degrees off. Dont know if there is any significance to these events but its very weird.

As a test If I were to re arrange the wires on the dist cap to match the new rotor position would I expect the car to run or would this cause damage?

The timing chain was what came to mind first for me too. Has your engine ever been rebuilt? If not, it may have one of those nylon toothed cam gears. If that's the case I doubt there is much left of it and the chain might have even jumped a tooth or two.

While you have your distributor cap off when setting the #1 to TDC you can check for slop on the timing chain. Turn the crankshaft one direction until you see the distributor rotor move. Now turn the crank the opposite direction until you see the rotor move. How far did the crank move before the rotor moved? It should be nearly instant to just a couple degrees. Any significant movement before the rotor moves will indicate a loose timing chain.

Another thing that can cause these symptoms is a bad camshaft. You may have one or more flat lobes. While you are tuning your engine over by hand to set TDC and checking the timing chain, remove the valve covers so you can observe the rocker arm movement. All rockers should move about the same amount. If any don't seem to move as much as the others it is an indication of a worn cam lobe. You can measure this with a dial indicator but a significantly worn lobe will show up as a rocker arm that is barely moving. Real easy to see without any special tools.

Continue turning the engine until both valves on the drivers side front cylinder (#1) are closed and stop with the timing mark on "0" to be on TDC.

Tom

 

[Vettehead Mikey]

As a test If I were to re arrange the wires on the dist cap to match the new rotor position would I expect the car to run or would this cause damage?

If the distributor rotor is actually off by 180* then it or the camshaft is way out of 'safe' range. Don't make things worse by bodging an ignition wire fix.

I'd double/triple check that #1 is on actually the compression stroke and not on the exhaust stroke. One can easily be mistaken for the other. When #1 cyl is on the exhaust stoke, #6 is on compression.

 

[greggome]

I have rechecked this to make sure by inserting a vacuum gauge /fuel pressure gauge plug into the # 1 spark plug hole cranked the engine and it blew out the plug at the same location about 180@ off, rotor pointing between #5 & 6 distributor cap wires. Is re positioning the distributor to TDC appropriate at this point?

If the distributor rotor is actually off by 180* then it or the camshaft is way out of 'safe' range. Don't make things worse by bodging an ignition wire fix.

I'd double/triple check that #1 is on actually the compression stroke and not on the exhaust stroke. One can easily be mistaken for the other. When #1 cyl is on the exhaust stoke, #6 is on compression.

 

[Vettehead Mikey]

How do you explain the distributor jumping exactly 180 degrees?

The test you performed is not a sure fire way of finding TDC compression stoke on a cylinder. The right way of doing it is pulling the valve cover and making sure that both valves are closed.

 

[greggome]

I have no idea how to explain the rotor pointing 180@ off. As I stated its about 180@ off between the 5 & 6 plug wires on the dist cap.That is why I have checked and rechecked the # 1 cylinder at least 5 times. When I plug my vacuum /fuel pressure gauge into the spark plug hole and it shows positive pressure as the rotor approaches the # 6 position on the dist cap and the gauge plug I pushed into the whole is forcefully blown out I would assume it is on the compression stroke. The only thing I have not done to verify this is pull the valve cover. I guess I will try that as a final confirmation.

How do you explain the distributor jumping exactly 180 degrees?

The test you performed is not a sure fire way of finding TDC compression stoke on a cylinder. The right way of doing it is pulling the valve cover and making sure that both valves are closed.

 

[Vettehead Mikey]

And you're sure that the wires in the cap at the 5 and 6 cap position are actually going to cyl. 5 and 6?

 

[Hib Halverson]

I've been reading this thread for a while.

Some reasons why an engine might spit-back through the carb

1) Spark timiing way off.
2) Cam timing off
3) Harmonic damper outer ring is loose and has rotated.
4) Wrong timing cover

Given the right timing cover, have you confirmed, using the "postive stop method," that the timing mark on the harmonic damper lines up with zero on the cover's timing index?

 

[greggome]

Yes they are. I have not moved any of the wires but doubled checked there position anyway.

And you're sure that the wires in the cap at the 5 and 6 cap position are actually going to cyl. 5 and 6?

 

[dougelam]

Silly question,
Did it ever run good?
Some history might help!

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[greggome]

Yes it was running fine up until about 2 months ago. The only issue I had was difficult cold starts. Once problems started the focus was on probable carburetor issues. That was worked on without success by a local shop that deals with older cars. At which point I swapped to another carb but issues continued. the last run with the car resulted in continued hesitation,bucking and rpms dropping to stall. Got the car home ,tried starting it the next day with no success. Forum communications lead me to the possibility that my timing was screwed up.

Silly question,
Did it ever run good?
Some history might help!

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[dougelam]

How about an overhead pic of the engine compartment without the air cleaner on?

And one of the carb that was removed

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