Knocking and back firing when gas panel is down

1941 - 1945, MB, GPW Technical questions and discussions, regarding anything related to the WWII jeep.
Peterdodge6volt
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Re: Knocking and back firing when gas panel is down

Post by Peterdodge6volt » Wed May 24, 2017 11:06 pm

Thanks
I have change the distributor and I have tryed 3 different new cap still same and no spark around the engine when testing in night.
I am so confused why I had to change coil 9 times and condenser 4 times at Corowa and still over heating and coil keeps steaming, I put resistor on the coil it last long 10 minutes until resistor died, I put the original distributor back on and still did nothing coil still over heating. Does any one thing it might been a blown gasket on the head or manifold.

What does it sound like if the gasket is blown.
Can the high voltage cause problems.


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Re: Knocking and back firing when gas panel is down

Post by Wolfman » Thu May 25, 2017 4:58 am

If you had a blown head gasket, you would not get the compression readings you did.
Have you changed the plug wires ??
Going to twelve volts would give a hotter spark.
If you have bad ( open ) plug wires and changed from 6 to 12 volts, the increased spark is going to go some place and electricity is "Lazy". It will take the easiest path to ground.
You said it started back when you went to 8 volts and got worse when you went to 12 volts.
That could be cross firing to another cylinder and if that cylinder happens to be on the intake stroke, with the intake valve open, you will have what you have. BANG out the carb.
To blow the carb off ?? It must be a pretty potent bang !!
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Re: Knocking and back firing when gas panel is down

Post by artificer » Thu May 25, 2017 10:51 am

John wrote:Where is this vehicle located?

There is absolutely no credible reason one would be changing an engine with these compression pressures:
1) is 120
2) is 119
3) is 120
4) is 125
One needs to diagnose what is causing the issues here!

What exactly is meant by 'knock'?
What happens when the spare engine is fitted, the accessories bolted on & the same problem is still there?
What does one do next when they pull the engine apart & find nothing wrong?

This is what I would have done a long time ago & would be doing next, before anything else.

5. Plug a vacuum gauge into an inlet manifold port & tell us [describe] your readings both @ idle & also steady @ about 800 RPM's.
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Re: Knocking and back firing when gas panel is down

Post by Chuck Lutz » Thu May 25, 2017 11:51 am

artificer thinks this is ignition related....I totally agree....if one cylinder is blackened and yet the compression is OK then it isn't a head gasket. Several of us have pointed out that the backfiring through the carb means an intake valve is not closed when the spark ignites the compression stroke.

Backfiring ONLY through the tail pipe is probably just a timing issue. That means to me that you go back to Square One and insure that the #1 plug wire is on #1 cylinder at TDC/
It isn't a fuel problem because a bad fuel pump usually STARVES an engine to death, not create backfiring so eliminate that. Your' carb isn't the problem for the same reason but be more specific about the engine "blowing the carb off"....you mean it cracked the carb flange or broke the studs in the manifold or what?

Since the distributor in the jeep turns counter-clockwise, once you get #1 piston at TDC/Compression stroke and align the #1 plug wire with the cap where the rotor is pointing you should have the firing order as posted on the head in a counter-clockwise direction.

However....it is NOT time to change the engine my friend....if you can't sort out what is wrong with this one I fear you will be equally likely to repeat what the problem is with the other one.

6 volt, 8 volt, 12 volt, changing distributors, changing coils, changing condensers....all in vain as the problem persists.

And yet the engine does not have a broken #3 intake valve spring and the valve train operates as it should when you use an engine hand crank to turn it over now that the head is off?

I know you are getting a lot of suggestions on what to do here but somewhere in all of them is the solution and when you have correctly sorted those suggestions out...you will have found the problem. Putting in a tired engine is giving up on what should not be this difficult to diagnose...

So...is it backfiring through the CARB or through the TAILPIPE?
Chuck Lutz

GPW 17963 4/24/42 Chester, PA. USA 20113473 (USA est./Tom W.)
Bantam T3-C 1947

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Re: Knocking and back firing when gas panel is down

Post by Peterdodge6volt » Thu May 25, 2017 12:31 pm

Thanks chuck
I have some the timing more than 4 times, I check the flywheel for ignition markings I read on the manual
And then check the distributor on number 1 and pistons first piston on number one both valves closed.

It broke my new carby off and the choke buttery is some where on the road at Corowa.
I have crank the jeep with the head off and checked all the valves and check the spring with the gauges and tools all fine and nothing broken on the springs

Thanks Wolfman

everything was running normal on 8 volt and it was getting pain in the butt to start it cranking very slow
And I have a 8volt charger and I also adjusted the regulator to 9volt.

I have change it to 12 volt and in two days that's where the jeep starts playing up at Corowa.



Thanks artificer

I will try and tested with the vacuum gauge on the manifold.

Thanks

Cheers Peter

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Re: Knocking and back firing when gas panel is down

Post by artificer » Thu May 25, 2017 12:37 pm

Where is this vehicle?
I'm prepared to come & work with you, if reasonably local.
And it is just possible someone else may offer if you are not in SE Queensland.
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Re: Knocking and back firing when gas panel is down

Post by Chuck Lutz » Thu May 25, 2017 1:08 pm

So....it backfires through the carb and not the tail pipe if it blew up your carburetor.
1) flywheel on correctly
2) distributor rotor/plug wire/#1 plug all at #1 Cylinder TDC/Compression stroke.
3) distributor is not loose

Since it is STILL a timing issue I can only surmise that the timing chain is stretched to the point that when under RPMs this is the cause of your problem.

Artificer...would you suspect this to be an issue on the short list of possibilities?
Chuck Lutz

GPW 17963 4/24/42 Chester, PA. USA 20113473 (USA est./Tom W.)
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Re: Knocking and back firing when gas panel is down

Post by artificer » Thu May 25, 2017 1:50 pm

Frankly with steaming coil, voltage changes, carburettors blowing up, changing everything many times etc. & some questions unanswered....I would be starting from scratch & spending 1/2 an hour eliminating what is not wrong, starting with ignition, timing & the manifold/s.

We are told the vehicle engine has good compression & idles OK? so I'm hooking up a vacuum gauge first to see if the reading points me anywhere.
And with this elusive problem it very likely will.

The weirdest thing on my '42 MB Jeep, in the 1960's was not dissimilar except that it took quite a few miles between bouts.
A head spark plug thread was cracked & when the vehicle got hot, the crack opened up internally. The crack's extremities in the combustion chamber got red hot & ignited the incoming fresh mixture.
This was not something one would normally be checking for & the crack closed up when cold [barely visible].
When the head was off the crack was only recognized because eventually the tips of the crack, that obviously glowed red when hottest, burned away & became rounded off.

Peter says he has had the head off many times & probably used different heads so a similar crack being his problem is remote although he did say something strange about number 3 some time ago.
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Re: Knocking and back firing when gas panel is down

Post by Joe Gopan » Thu May 25, 2017 2:09 pm

WARNING!!!

One should think twice before using a vacuum Gauge on a backfiring engine, seasoned mechanics would advise against this due to shrapnel from the disintegrating vacuum gauge.
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Re: Knocking and back firing when gas panel is down

Post by artificer » Thu May 25, 2017 2:19 pm

How about the Jeep King's suggestions on finding & solving this person's issue/s?
The engine is said to be idling OK! So this experienced, qualified mechanic is using his vacuum gauge sensibly!
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.

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Re: Knocking and back firing when gas panel is down

Post by rjbeamer » Thu May 25, 2017 2:39 pm

Joel, Joel, Joel. Where do you come up with these things? Surly you can't show me this "warning" in print from any publication or Manufacturer.
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Post by Ben Dover » Thu May 25, 2017 3:09 pm
WARNING!!!

One should think twice before using a vacuum Gauge on a backfiring engine, seasoned mechanics would advise against this due to shrapnel from the disintegrating vacuum gauge.

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Re: Knocking and back firing when gas panel is down

Post by Chuck Lutz » Thu May 25, 2017 3:35 pm

Hang on here....you have not convinced me for one second that this jeep is timed so that you have #1 firing on #1TDC/Compression stroke.
To begin with, looking at the flywheel is fine...but not if it is bolted on 180' from what it should be!!!...and the crank and flywheel turn TWICE during the sequence!!!

Take the dammnned valve cover off and have someone hand crank the engine over so you can watch all EIGHT valves move up and down.

Here is the sequence which requires turning the crankshaft TWO full turns to see. Now watch the valves and see what I am talking about!

So...as you can see, there is more than one time during the sequence between firings that BOTH valves are closed on #1 piston:
1) When at #1 TDC/Compression stroke.
2) As the intake closes and the exhaust opens at the bottom of that firing stroke.
3) As the exhaust closes and the intake opens at the top again.
4) As the intake closes at the bottom again.

FOUR times the two valves appear to be "closed".....which one are you on?
Chuck Lutz

GPW 17963 4/24/42 Chester, PA. USA 20113473 (USA est./Tom W.)
Bantam T3-C 1947

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Re: Knocking and back firing when gas panel is down

Post by artificer » Thu May 25, 2017 4:03 pm

Would timing set incorrectly @ 180* out, start, run or idle OK?

For info there is a 50/50 chance of a Jeep flywheel being on 180* out if not assembled in an observant manner....mark with Texta before removing & always make sure 1 is @ TDC compression & 4's valves are rocking when the flywheel timing marks are in the window space. Alternatively 4 @ TDCC when 1's valves are rocking.

If anyone's flywheel is on 180* out, correct timing is accomplished when 2 is @ TDC compression & 3's valves are rocking, when the timing marks are showing in the window. Alternatively 3 @ TDCC when 2's valves are rocking
John GIBBINS Member Institute of Automotive Mechanical Engineers [Ret], ASE Master Medium/Heavy Truck & Auto Technician USA -2002 Licensed Motor Mech NSW MVIC 49593 Current 2015
TO DIAGNOSE, TROUBLESHOOT OR FAULT FIND ANY AUTO SYSTEM....
Understand how system parts interact with one another. GOOD parts can then be established & the NOT GOOD problem/s part/s isolated for repair or replacement.

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Re: Knocking and back firing when gas panel is down

Post by Chuck Lutz » Thu May 25, 2017 4:33 pm

"Timing...is everything!"
Chuck Lutz

GPW 17963 4/24/42 Chester, PA. USA 20113473 (USA est./Tom W.)
Bantam T3-C 1947

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Re: Knocking and back firing when gas panel is down

Post by Peterdodge6volt » Thu May 25, 2017 5:18 pm

I am in Sydney Campbelltown which is to far from your place.

This was my first time took the head off 2 months ago and it still off.
I did had the valves cover off at Corowa and crank it they were fine also did the check it with the valves gauge tools
We also did run the engine to watch the valves and we used the timing lights. I had the timing light gun pointing on the valve to watch the flash and test with each time on the spark plug cable.
I will be going to farm today and will have a look for cracks and other things.

Thanks guys

I will keep it posted


Cheers Peter


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