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Backfire Through The Exhaust

 
Cobbie
I love my Chevy Chevette!
Posts: 3
Joined: 05/09
Posted: 03/16/13 12:47 PM

I’m trying to trouble shoot a backfire through my exhaust. I adjusted the valves, reset total timing, tried another carb, replaced fuel nozzle the carb is a Holley 600 the motor is a 350 40+ comp 286hr 10 to 1. It only happens under a load at idle and above 5500 rpm. The only thing I have not tried is fuel pressure. Intake vacuum is 15. New msd ignition box, coil, distributor and wires.  

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pepsi1
Big Block power for the win
Posts: 940
Joined: 03/11
Posted: 03/16/13 04:32 PM

Cobbie
Your camshaft can be worn marginally and not effect the vacuum.
A few things come to mind though. You didn't mention if your
engine was roller cam or a flat tappet cam... Or the type of
distributor you have a small or large cap HEI. The small cap
HEI's can cause some problems as you stated..But we need to
know what what you have...

1.I would run a compression test.

2.Your valve springs could be worn out. You can do an on the head
valve spring pressure test. Moroso makes a few different type of
valve spring testers.

3.You can try running the engine without a valve cover on one side
and then go to the other side. But thats hard to see if the valves
are actuating the same amount.

4.Pull the spark plugs and see if there is any noticeable difference
looking at the spark plugs. IE broken porcelin, broken ground electrode
etc.

5.Timing chain may have stretched just enough to put the engine out
of time...

6.Don't forget to disarm the ignition, when doing a compression check

Bob  

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Cobbie
I love my Chevy Chevette!
Posts: 3
Joined: 05/09
Posted: 03/16/13 05:44 PM

Bob
The cam is a hyd/roller. Its a small cap with external coil.I did notice that one plug was a little rich while the others are a little lean. I just had the heads done with new valves and springs the cam has about a 100 miles on it. New double roller timing chain. Thank you for your advice.I will get the tools to do those checks.  

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waynep71222
My first time was in a Chevy
Posts: 218
Joined: 04/12
Posted: 03/16/13 06:14 PM

a few other things to check and a few questions first..

what type of ignition.. you said small cap.. remote coil...   points or electronic ignition..

do you have wire separators on the spark plug wires...

are the spark plug wires fully seated in the cap... and fully installed on the spark plugs without the boot being pulled back so far the terminal is exposed...

do you have a ignition coil output tester.. or can you position the coil wire removed from the cap.. not the coil.. about 7/16" for points type ignition and 3/4" for electronic ignition.. have somebody crank the engine.. you should have a nice bright blue snappy spark..

got any exhaust leaks??  what kind of exhaust.. factory manifolds... headers..  

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waynep71222
My first time was in a Chevy
Posts: 218
Joined: 04/12
Posted: 03/16/13 06:46 PM

you changed the pump discharge nozzle... can you verify that when you open the throttle.. you get an instant shot from both sides...



lets take a slight step to the side...
can you verify the ground wires are properly connected...
do you have a digital volt meter..  this is a quick test to verify the grounds... since electrons from from negative to positive..  grounds are IMPORTANT..

~~~~~~~ this chart is an image.. print it.. run it at the car..
Voltagedroptesting
~~~~~~~

battery neg to engine block,
battery neg to body
engine block to body.. usually at the firewall..
now.. just think of this... where did you hook up the ground wire for the MSD box.??? which one.. msd 6 or 7..

let me describe the spark path..    

the key sends power to the MSD BOX small red wire.. not directly to the coil..  

the MSD sends power to the positive coil via the orange wire..

the large red wire gets hooked to the battery positive...

the LARGE BLACK WIRE NEEDS to be hooked .. not just to the Negative battery post. but also TO THE ENGINE BLOCK or back of the cylinder heads..

the small black wire is hooked to the coil negative...


when the MSD is charging the primary side of the coil..  the small black wire is connected to ground thru a transistor..   this saturates the primary winding building a magnetic field inside the coil.

when the MSD decides to discharge the ignition coil.. the black wire is disconnected from ground.. the electrons stop flowing. this causes the magnetic field to collapse in the primary.. and since its properly designed.. this induces a high voltage spark in the secondary windings.. where it travels out the coil.. thru the cap, rotor, cap, spark plug wire, spark plug,, cylinder head block.. or where ever the big black MDS box wire is hooked up to ground the block/heads/battery negative terminal box..

this is why i like to recommend that you have a decent ground wire from the engine to the body.. usually right at the firewall..

if you only have the big black wire grounded to the battery terminal.. thats a LONG way for the negative side of the spark to go to make a complete circuit...  any loose or corroded connections.. will cause issues like you have...


the reason to also run my chart is to prove that the grounds are intact.. that the msd unit is getting full power all the time.. . full ground all the time...

there is one more thing to check...

the small red wire from the car to the MDS unit comes from the IGN 1 circuit..  depending on your year of chevy..  you could have a bad ignition switch internal contact.. you could also have an issue with the resistor bypass circuit coming up from the R terminal on the starter solenoid...

so a little more info on your car is needed.. along with testing..  

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pepsi1
Big Block power for the win
Posts: 940
Joined: 03/11
Posted: 03/17/13 03:30 PM

Cobbie
Those small diameter external coil distributors are okay. But they good bad quick. 100,000 miles is usually their limit. I would run through the ingition as you said you have a plug that doesn't look good.

If you need any schematics from MSD they have a great book with all the wiring diagrams.

While your at it I would do a compression test for the heck of it.

Also usually a small backfire at low speed (Idle) is usually a sign of an ignition problem. Sounds like a low grade fart or a foof...LOL... Good Luck.

Bob  

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Cobbie
I love my Chevy Chevette!
Posts: 3
Joined: 05/09
Posted: 03/17/13 03:35 PM

Thank You. I have a problem in the charging system some where. When the battery has a full charge the problem goes away.  

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waynep71222
My first time was in a Chevy
Posts: 218
Joined: 04/12
Posted: 03/17/13 09:53 PM

run the voltage drop test...

takes only a few minutes... will find your ground issues..

you can also use it on the positive side..

positive battery post to the alternator output terminal..

positive battery post to the starter top post..

positive post... to the RED wire that leads into the  firewall..  or by using a long jumper cable.. run it around or under the car from the positive battery post to the pavement under the open drivers door.. so you can measure from the end of the jumper cable to the various fuse connectors in the fuse box...  or on the wires to the ignition switch.. see if there is any voltage drop... you could have issues all over the place...


these again are engine running.. headlights on..


can you post a year make and model???  

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pepsi1
Big Block power for the win
Posts: 940
Joined: 03/11
Posted: 03/18/13 10:12 AM

You may have a diode that crapped out in your alternator, in the diode pack. Most of the GM alternators can be rebuilt with nothing more than hand tools.

Of course unless it had a major melt down....Get a service manual it will tell you how to test the bridge rectifier, and the diode packs. Then you can replace the bearings also. The early  GM alternators are cheap enough to just replace them. The later ones are more expensive.

Disconnect the battery from the source first.

If you rebuild it don't forget to "INDEX" it. So when you reinstall it, it will align with the brackets.

Even with low battery voltage the MSD ignition had enough to start the engine.

Bob  

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pepsi1
Big Block power for the win
Posts: 940
Joined: 03/11
Posted: 03/18/13 10:19 AM

Back in the day when aftermarket ignitions were starting to became known. MSD had a display at a speed shop in New Jersey (Van's) (Van Idersteins) it showed the spark jumping a inch (1") gap. It was really impressive for the time. It was 1982.
  The salesman showed it by burning a business card, between that (1") gap.

Bob  

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waynep71222
My first time was in a Chevy
Posts: 218
Joined: 04/12
Posted: 03/18/13 04:24 PM

there is another thing to monitor...

hook up a timing light to the coil wire...  rubber band the switch on... aim it where you can see the flashing light.. hang it under the wiper arm...  take it for a test drive... watch the flashing..

do the same test on each of the spark plug wires..   and if you don't have a tiny amount of dielectric tune up grease wiped inside the spark plug boots but not on the metal terminals.. the spark could be escaping down the spark plug insulator under the boot and shorting out to ground.. this could also cause a back fire in the exhaust system as one spark plug is going dead ..


if the timing light blinking flutters or goes dark for a split second.. you have found the point where the ignition stops firing.. the pistons continue pumping now fresh air and unburned fuel into the exhaust .

same effect that delivery van drivers have when they are driving .. turn off the key for a split second then turn it back on...  when they turn it back on.. it makes a huge bang from the exhaust system like you describe..

an intermittent ignition power supply.. could cause this...  or an intermittent power supply to the MSD box could cause this...  and the intermittent could be on Either side of the circuit.. positive or negative ..  hence the reason for my voltage drop test to check the grounds..

be vary careful with the Orange wire that feeds the positive side of the coil.. and the negative side of the coil...    some of the MSD boxes put out several hundred volts to aid the saturation speed of the primary windings..

the cars original power wire to the positive side of the coil is now connected to the small red wire into the MSD box.. that CAN be tested by clipping a 12 volt test light to it... sticking the probe end into a ground and taking a test drive..   see if the light flickers... this would narrow down the possible location of faults in the wiring..  

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pepsi1
Big Block power for the win
Posts: 940
Joined: 03/11
Posted: 03/18/13 09:49 PM

Wayne
+1 on the Dieletric Grease.
  I will be helping a friend completley rewire his 1966 Mustang 2+2. I really haven't done any major wiring in a few years. He's getting all the High End  MSD components. He has (2) different types of Dieletric Grease. One is a white paste, and the other is a clear. The clear is much lighter weight wise.
  Do they Rate them for a specific job. For example power supplies, versus ignition wires. If so what should I look for. Any help would be appreciated. Thanks. Wayne or anyone that can help clarify the differences....

Bob  

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waynep71222
My first time was in a Chevy
Posts: 218
Joined: 04/12
Posted: 03/18/13 10:29 PM

He has (2) different types of Dieletric Grease. One is a white paste, and the other is a clear. The clear is much lighter weight wise.
 Do they Rate them for a specific job. For example power supplies, versus ignition wires. If so what should I look for. Any help would be appreciated. Thanks. Wayne or anyone that can help clarify the differences....


i have used them interchangeably...

but i usually use the white silicone thermal grease under ignition modules.. and under transistors that need to be heat sinked... the white has titaninum dioxide in it along with silicon dissolved into the silicone jell..

the clear.. i have also used under ignition modules.. but usually not under power transistors..

i do wipe a TINY dab around the inside of spark plug boots..  but not on the metal conductor..

the clear is also used to seal HIGH VOLTAGE wiring to the back of TV picture tubes..


an important thing...    COP ignitions .. COIL ON PLUG ignitions...  if you FAIL to use a tiny dab to seal the boots to the coil towers.. and more to seal the boots to the spark plugs.. you will end up with a MISFIRE and usually burned coils...  this is especially important on motors like the quad4 that are in so many daily drivers cars..





for 12 volts electrical connectors.. i usually use white lithium grease.. NOT silicone..

there is a product at most parts stores in small packages.. its called bulb grease.. that i have thought about using on electrical contacts..

i also DO NOT use silicone dielectric grease as a LUBE..  why.. there is dissolved beach sand in it.. when it gets heated up .. the sand comes out of suspension and becomes crystals again. wedging what ever you have applied it to forever...    i had one brake caliper bracket that some tech used dielectric tune up grease on..  and oops..  it came back and was locked up so tight that the 20 ton shop press would not budge it..

there is a product that is NOT  DIELECTRIC or THERMAL called SILGLIDE... that is used for sliding brake components..  

i guess i should compile some info on the different materials used in various types of electrical connectors on cars..  and start a new thread..   perhaps later this week...  

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pepsi1
Big Block power for the win
Posts: 940
Joined: 03/11
Posted: 03/23/13 01:28 PM

Thanks for the input.
I was curious as to which one to use. I've never had a problem with any. We want this to not only look good, but we want it right the first time too. You know what I mean. The small details pay off.  

When I was working with power supplies that we set up by Frequencies, you want to talk about OLD Technology. But it never wore out. If your wondering what that was used on. It was one of the very first Cat Scanners.
  We had a piece of wood about the size of a Chop/Stick, and we set them up that way...We then got along piece of plastic, that we used...lol...  

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briando
I love my Chevy Chevette!
Posts: 12
Joined: 08/12
Posted: 04/11/13 10:09 AM

i know my chevygets that with low quality gas, happens alot after it rains too, i'm guessing both problems are from water in the tank.  

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