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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Knocking and back firing when gas panel is down

Post by Peterdodge6volt » Wed May 24, 2017 12:46 am

Hi
I have a GPW 1943 jeep.
A few months ago I took my jeep to Corowa. On Monday everything was find and running normal.
Tuesday morning was on my way to Jindera she was fine, on late afternoon I was on my way back to Corowa, the jeep starting making a loud back firing every 1 second and coughing a lot and a knocking noise in the engine. I spent 5 days at Corowa fixing it and my friends were helping me fixing her.
We check the timing, valves, timing chains they where fine, I put new coil, spark plug, spark plug cables, new distributor, new condenser still doing the same thing. She runs normal if my food not on the gas pedal
If I put my foot on the pedal she cough, loud back firing, and knocking in the engine,
I check the cool and it was extremely hot and the condenser, I change it same thing, and it blew of the carby, the carby was brand new from milspares jeep.
I took the head off, and number 3 was very extremely black, their where no water in the pistons.
I crank to see if the valves are line up they were fine,
My friend was thinking that their might be a crack in the manifold engine block, I still haven't took the manifold off yet,

Any chance for help please
Thanks you


Cheers Peter


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Re: Knocking and back firing when gas panel is down

Post by artificer » Wed May 24, 2017 3:03 am

What is gas panel?
Accelerator?
Tomorrow or after 1st June....let's analyse this logically.
Solution will mostly likely be a very simple & money saving answer.
& without pulling things apart & taking a peek!
Avoid this inane peek option @ all cost!
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 Wolfman » Wed May 24, 2017 5:34 am

Broken intake valve spring on cylinder #3 ??
That is the cylinder that was black.
The valve operation can keep up at cranking speed but not when the engine is running.
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Re: Knocking and back firing when gas panel is down

Post by dinof » Wed May 24, 2017 5:56 am

Did you buy any gasoline during this trip you took? Almost sounds like bad gas/ water in fuel.
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Re: Knocking and back firing when gas panel is down

Post by dpcd67 » Wed May 24, 2017 6:44 am

He already removed the head to "take a peek", so the systematic diagnosis approach ship has sailed. Might have learned a lot from a compression test. Not now.
So, the Take a Peek adherents claim another victim.
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Re: Knocking and back firing when gas panel is down

Post by Joe Gopan » Wed May 24, 2017 6:45 am

Assume you meant "Gas Pedal" Sounds like a lean fuel mixture.
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Re: Knocking and back firing when gas panel is down

Post by Chuck Lutz » Wed May 24, 2017 11:08 am

Valve, valve spring....when the firing in a cylinder is "off", meaning not at TDC Compression stroke you get a backfire up through the carb. When all cylinders do that it points to the timing chain but since it is only #3 then the problem is isolated now that you have inspected #3 bore.

So...if the intake valve itself is not closing at TDC Compression stroke you will experience this problem.

I would suggest you inspect the intake valve spring to see it if it is broken or some damage has occurred to the valve or valve guide for #3 intake that is not allowing it to close on firing. With the head off, you can inspect the valve to see if there is any damage to it which would prevent a proper sealing.
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Re: Knocking and back firing when gas panel is down

Post by Peterdodge6volt » Wed May 24, 2017 1:33 pm

Thanks for the help guys
I have check all the compressions when it was playing up, the compression on
1) is 120
2) is 119
3) is 120
4) is 125
I check the valve and they are all sealing properly and their are no broken springs or crack. In valves
I took the oil lever dipstick out and smell a lot of fuel in it.
I am sorry i didn't mean to write gas pedal instead of accelerator pedal.
I am using unleaded 95 or 98 which is petrol
We have change the carby back to original because it blew off the new carby off. but playing up still there.
The only thing when we run the engine we try take I spark plug cable off each spark plug to see any difference
Number 1 stop the back firing but the rest sound like it's not drinking fuel or timing not right, so we took the cable off number one and run and drive it for 2 days but the knocking still there. Its knocking when I am going up hill but quite going down down hill or non hill road and the engine was going between 180F to 200F so we stop and we put it on the trailer and went home and we took the head off and valves but the valves were all fine and the timing were all line up in number 1. When we tryed to figure out while the engine running and back firing every 1 second the flame comes out 3 meters high.
It was blowing black and blue white smoke colour I check the choke but it was off so I tryed the choke did nothing as well.

I will see if I can work on it this weekend if I have time

I had so many people telling it wiring problem but I don't believe it's the wiring problem.


Thanks for the help will check again.


Cheers Peter

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Re: Knocking and back firing when gas panel is down

Post by Chuck Lutz » Wed May 24, 2017 2:20 pm

Hmmm...have you confirmed you have the firing order correct and the plug wires correctly run in the proper order to the proper cylinders?

Just a thought...
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 » Wed May 24, 2017 2:38 pm

Yes we heve check the firing order more then 5 times and the timings as well they we're all correct.

Cheers Peter
Last edited by Peterdodge6volt on Wed May 24, 2017 2:55 pm, edited 1 time in total.

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Re: Knocking and back firing when gas panel is down

Post by artificer » Wed May 24, 2017 2:48 pm

This is ignition related IMO.
Tell us what was done immediately prior to this fault developing?
Things that need checking:
1. As Chuck just indicated orientation of the plug leads & check again you are looking in the right DOR [direction of rotation]. Backfiring through the carb means a spark is occurring in one or more cylinders on the intake stroke i.e. the wrong time.
2. Cross firing or carbon arcing between leads [check in the dark] & or inside the distributor cap.
3. Spark plug heat range, if this backfiring only happens when things are hot.
4. What do the spark plug tips/electrodes look like
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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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 semberae » Wed May 24, 2017 3:40 pm

Worn timing chain. Sounds like timing chain jumped a notch.

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Re: Knocking and back firing when gas panel is down

Post by Peterdodge6volt » Wed May 24, 2017 6:37 pm

Thanks guys
Last time we check in the dark for sparks, when I start her up my father only saw 1 sparks came out of the distributor and when through the starter motor. I had her on 8volt before a few months ago with no problem and I change it to 12volt 1 week before Corowa and thing starts playing around.
I have that problem a few years ago and I change coil and condenser and everything when back to normal.
But this time it did nothing to her. I got the spark plug and spark plug cable from Joel. The distributor from Ian Thorburn,
I have tryed put back the original distributor did nothing.
I have check the timing chain it is fine because it was replace last year brand new one.
I did have a wiring catch on fire from battery to ignition and I replace a new wiring.

I father told me that I should change the engine and put the spare engine in before I do that I need to check compression in the spare engine. I will have a look tonight at my spare engine because I know it was fully rebuild and all complete ready to install.
I do have a another spare original GPW ford engine block and GPW head that originally came from the jeep with the match numbers it's Completely worn out badly so I don't know how Good it is so I will have a look at it this weekend.

I have to say that I agreed with my father that it's time to change the engine and fix gearbox and transfer case as it is leaking oil everywhere. I have never had a spare gearbox and transfer case for jeep. Only spare engine. Once I change the engine I will pull her apart and find out what's wrong with her. As I had her fix about 8 years ago as she had a crack in the engine valves block. So I will check it this week end.

Once again thanks you so much for all your help I appreciate it

I will keep posting when I find out.


Thanks

Cheers peter

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Re: Knocking and back firing when gas panel is down

Post by artificer » Wed May 24, 2017 8:22 pm

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.
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 conrod6 » Wed May 24, 2017 9:54 pm

How about a cracked distributor cap ? This can cause the firing of two cylinders at the same time with one cylinder in the wrong
position in the cycle . The engine seems to break down under load.

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