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Old 11-23-2019, 01:25 PM   #1
ottoschmidt
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Default backire through carburator on 1946 8 cylinder

Hello Men,
I am new member and need help. I had a competent machinist completely rebuild my 1946 ford truck 8 cylinder 1.5 ton from A to Z with all OEM parts . The only modification was putting a Pertronix Ignitor plate in it .I delivered the motor to the mechanic who installed it and he said it started right up and he put 13 gentle miles on it and it ran good .The next day when I went to pick it up he said it`s now backfiring out through carb intake and running like crap loss of power . He thinks valves are the issue and the machinist said no way . The machinist is saying that installing a Pertronix plate you must retime the distributor but not the primary timing but the secondary timing of the weights by unloosening a bolt and nut on the distributor and using a timing light . Mechanic and me don't understand how it can run fine day one and not the second day . He wants to take off intake manifold cover and look at valves . I say check timing first .It has the crab style distributor .It has a brand new Holley 94 from National Carburator . Any advice will help .
Regards
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Old 11-23-2019, 02:01 PM   #2
Fordestes
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Default Re: backire through carburator on 1946 8 cylinder

Check compression, or pressure up cylinders to verify the valves are sealing properly, it is nothing unusual for the valves to stick especially if the valves are stainless steel and the guide bores are not bronze lined or maybe the stem to guide clearance is too tight, I have found some of the off shore valve guide bores to be a bit too small i.d. , yes they can run fine then the assy. lube is washed off, then after sitting a short time start to stick . check for a faulty distributor cap or moisture collecting in the dist. cap causing spark scatter. , make sure the plug wires are not shorting (especially under load) across one another.
Just because the parts are new don't mean they are not defective. always add some marvel mystery oil to the fuel to keep the valve guides lubricated and avoid future problems with sticking valves .
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Old 11-23-2019, 03:30 PM   #3
tubman
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Default Re: backire through carburator on 1946 8 cylinder

If all else fails and the vehicle is 6 volts, take out the Pertronix and put some points back in. Good one day and bad the next? That's a 6 volt Pertronix.
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Old 11-23-2019, 05:44 PM   #4
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Default Re: backire through carburator on 1946 8 cylinder

Quote:
Originally Posted by tubman View Post
If all else fails and the vehicle is 6 volts, take out the Pertronix and put some points back in. Good one day and bad the next? That's a 6 volt Pertronix.
Good advice. Just go with stock. They are a very good distributor.
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Old 11-23-2019, 05:45 PM   #5
flatjack9
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Default Re: backire through carburator on 1946 8 cylinder

Dump the Pertronix. Also if the engine backfired, you might want to check the integrity of the power valve.
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Old 11-23-2019, 09:55 PM   #6
ottoschmidt
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Default Re: backire through carburator on 1946 8 cylinder

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it is a 12 volt system I forgot to say . I thought someone would say something about the timing/changing to Pertronix ??? But the valve lash will be checked im sure. But always go with the easiest way first ? Regards
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Old 11-24-2019, 12:05 AM   #7
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Default Re: backire through carburator on 1946 8 cylinder

I would. And pay heed to what Jack said about the power valve in the carburetor.
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Old 11-24-2019, 02:46 AM   #8
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Default Re: backire through carburator on 1946 8 cylinder

Take the plugs out and spin the engine while someone takes a look to see if all valves move as they should...
After that ignition or something in the carb making it run way lean would be next for me to check.
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Old 11-24-2019, 06:29 AM   #9
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Default Re: backire through carburator on 1946 8 cylinder

Check the distributor cap for crossfire trails/ cracks. If nothing, change the Pertonics system. And, yes, the PValve can easily become damaged from intake backfiring.
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Old 11-24-2019, 11:05 AM   #10
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Default Re: backire through carburator on 1946 8 cylinder

If the power-valve is blown, you'll usually have a VERY RICH condition - as it will act like it us under load. Your problem sure sounds like an ignition problem . . . though it can be a valve that is stuck open (or incorrectly adjusted).

If it was mine, the first thing I'd do is swap out the distributor to a stock points/condenser style - so you can rule that out being the issue.
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Old 11-24-2019, 08:10 PM   #11
flatjack9
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Default Re: backire through carburator on 1946 8 cylinder

I think your mechanic is blowing smoke up you know where.
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Old 11-25-2019, 03:45 PM   #12
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Default Re: backire through carburator on 1946 8 cylinder

Once the problem is resolved, if the idle is rough and stinky (fuel), then more than likely the power valve diaphragm was damaged by the backfires. At idle is where that issue shows up the worst.


Sal
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Old 11-25-2019, 06:24 PM   #13
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Default Re: backire through carburator on 1946 8 cylinder

Sometimes one thing can cause another. Back fire up through the venturis is generally caused by a lean condition in fuel delivery but as several folks mention, this backfiring can damage the power valve on a model 94. When they are that lean it usually indicates a plugged up fuel delivery line, jet, or port. I definitely won't idle worth a crap with a blown power valve but it will enrich the idle mixture.

If an ignition system is out of time then a back fire can happen if the plug fires while an intake valve is open but that can happen with a stuck valve too.
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Old 11-25-2019, 11:10 PM   #14
ottoschmidt
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Default Re: backire through carburator on 1946 8 cylinder

In my honest opinion... I don't think my mechanic knows what he is doing . He finally said he worked on a Ford 6 cylinder tractor motor once ..
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Old 11-25-2019, 11:24 PM   #15
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Default Re: backire through carburator on 1946 8 cylinder

Like I said.
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Old 11-28-2019, 07:06 PM   #16
ottoschmidt
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Default Re: backire through carburator on 1946 8 cylinder

Hello Men and Happy Thanksgiving,
And how happy I am .. My mechanic finally found my backfire through the carburetor problem . Before I bought this truck it was converted to a 12 volt . Here , there was a ceramic resistor mounted on the firewall with a wire running to the distributor because he said you cant run 12 volts to a 6 volt distributor . Since it had a 12 volt Pertronix replacing the points it needed the resistor removed . He simply didn't know the history of the change over . I appreciate all of your input .It will be all saved knowledge for future trouble shooting . It starts with a bump of the key . I have a question though , how many miles would be considered it to be broken in ? It runs nice but that damn one speed rear end will only let it go 40 to 43 MPH … Regards
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