Jump to content

74 620 backfires loud out the tail pipe


sanyantho

Recommended Posts

Hi Ratsuns,

     My 74 620 backfires very loud out the tailpipe when I stop and turn off the ignition. Sounds like a shotgun and pops only once. Usually happen when I get out the car and closed my door. Very embarrassing because everybody ducks like they are getting shot at.Does anybody know what can cause of this? I have new weber, spark plugs, wire, cap and rotor, and also new points. I have a small exhaust leak.

Link to comment
  • Replies 17
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

 I have a small exhaust leak.

 

 

Which will soon become a big one.  Could be contributing- it's allowing air in which ignites the raw fuel pumped out when the engine winds to a stop with the ignition off.  

 

Other causes-  having incorrect timing or too rich an idle mixture.  Too fast of an idle as well, which makes the engine shutoff take longer.  

 

Or having no idle cut solenoid.

 

  • Like 1
Link to comment

I'm going with the exhaust leak igniting gas theory, or timing.

 

There shouldn't be any gas to ignite after the combustion process. How will a leak in the exhaust cause anything but noise. 

 

Timing for spark is 12 BTDC. The exhaust stroke begins 138 degrees after this with the beginning of the opening of the exhaust valve. Timing that far out wouldn't run.

 

 

To back fire out the exhaust there has to be abundant gas and air, and an ignition source.

 

The most common are... The interruption of the ignition spark. Un-burned gas and air (also in the correct ratio) is pushed into the exhaust pipe when the spark stops momentarily. When the spark returns it explodes. Any loose wire in the primary side of the ignition can cause this. This is the same as turning the ignition off and on.

 

Another is run on or dieseling. Ignition is turned off but the idle mixture continues to fire from the heat of compression just like a diesel. Most cylinders fire sporadically and incompletely and the engine shakes and kicks. Much of the fuel and air is un-burnt and ends up in the exhaust pipe where it can be set off by a cylinder that does fire. Lack of an idle cut solenoid, low octane gas, wrong spark plug heat range, carboned up combustion chamber from choke on or an over fast idle can all contribute to dieseling. 

  • Like 2
Link to comment

I adjusted the idle speed and the idle mixture. Also found that a little bit on gas is leaking from the Weber's fuel inlet and tighten it to stop the leak. So far so good, I will have to drive around a bit, because the backfires does not occur all the time. But once in a while. Going to take some time to know if it is fixed.

Link to comment

There shouldn't be any gas to ignite after the combustion process. How will a leak in the exhaust cause anything but noise. 

 

Timing for spark is 12 BTDC. The exhaust stroke begins 138 degrees after this with the beginning of the opening of the exhaust valve. Timing that far out wouldn't run.

 

 

To back fire out the exhaust there has to be abundant gas and air, and an ignition source.

 

The most common are... The interruption of the ignition spark. Un-burned gas and air (also in the correct ratio) is pushed into the exhaust pipe when the spark stops momentarily. When the spark returns it explodes. Any loose wire in the primary side of the ignition can cause this. This is the same as turning the ignition off and on.

 

Another is run on or dieseling. Ignition is turned off but the idle mixture continues to fire from the heat of compression just like a diesel. Most cylinders fire sporadically and incompletely and the engine shakes and kicks. Much of the fuel and air is un-burnt and ends up in the exhaust pipe where it can be set off by a cylinder that does fire. Lack of an idle cut solenoid, low octane gas, wrong spark plug heat range, carboned up combustion chamber from choke on or an over fast idle can all contribute to dieseling.

 

Contridict much?

Link to comment

There shouldn't be any gas to ignite after the combustion process. How will a leak in the exhaust cause anything but noise. 

 

Timing for spark is 12 BTDC. The exhaust stroke begins 138 degrees after this with the beginning of the opening of the exhaust valve. Timing that far out wouldn't run.

 

 

To back fire out the exhaust there has to be abundant gas and air, and an ignition source.

 

 

The key here is it happens AFTER he shuts it off.

 

When you shut the engine off, it doesn't stop immediately.  And the higher the idle, the longer it takes to windmill to a stop.  With no idle cut solenoid, there's still fuel going through the carb when the engine windmills to a stop because it flows any time there's sufficient air flow through the carb.  Since there's no spark, the fuel mix isn't ignited in the cylinder.  So it ends up in the tailpipe.  A hot tailpipe.  Normally, the system is closed, so there's no oxygen source except at the very tail end, and by then the residual exhaust has cooled off too much.  But a leak further forward, such as at the manifold, air can get let in, providing more oxygen.  Leaner fuel burns faster.  Mix that with already vaporized fuel from a somewhat rich idle mix, and BANG.

 

But it's entirely possible that a rich idle mix/too high idle alone can do it, and the exhaust leak is coincidental.

 

Now, if it was happening when running, that's another issue, usually a momentary loss of spark.  My Dad told me a couple tales from his youth of kids who would hot-dog it into the Mt Baker Tunnels on US 10 (now I-90) back when they were still round bores.  Midway through they'd switch off the ignition for a couple seconds, then switch it on, making a LOUD  "Bang" , which tended to attract the State Troopers.   I had that happen accidentally  on the freeway.  My tach feed wire fell off and was hanging by the clutch pedal arm, and occasionally touched the clutch pedal arm, grounding the coil (which kills the ignition) and when it swung off the ignition re-lit and blew my muffler to smithereeens.

  • Like 1
Link to comment

Years ago I was starting my neighbor's '64 Chev. It backfired and split the muffler under the diver's seat. I wouldn't worry about an exhaust leak right now.

 

Try this... usually it only takes a half dozen times to figure it out and get good at it....

 

Standard transmission.... place in 4th, hold brake on and let the clutch up part way to load the engine, just like you are going to drive off. Turn the key off. The engine will stop immediately without dieseling.

 

Automatic transmission.... shut off ignition in DRIVE then place in PARK.

  • Like 1
Link to comment

That's what I used to do with my mikuni side draft (since there's no gas shut off solenoid) but now I installed a fuel shut off valve to avoid having gas going in as the engine windmilled off, works great, now I don't have to worry about gas going into any cylinders while engine is off...it does create an issue to get it started since carb is empty, but it still fires up within 30 seconds, so not bad of a trade off...

  • Like 2
Link to comment
  • 2 weeks later...

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.