Engine rough and backfires out exhaust when revved.

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GordonRead
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Joined: Wed Jan 12, 2022 1:47 pm

Engine rough and backfires out exhaust when revved.

Post by GordonRead » Wed Jan 12, 2022 2:34 pm

I am retired aircraft mechanic and have not ever worked on a Packard 8. My best friend owned this 51Packard convertible and I was helping him restore it to original when he suddenly died. Another friend and I have agreed to finish the restoration in honor of our friend. The engine was completely overhaul by a professional engine/machine shop in Portland, OR. It never ran smooth and backfired when revved. The engine shop said they assembled it but are not equipped to troubleshoot or run the engine. I checked compression on all cylinders, checked all plugs are firing, reset solid lifter valve lash hot (.008 intakes, .010 exhaust), had carb overhauled and adjusted on car, I checked distributor point gap, replaced plug wires and plugs, vacuum and centrifugal advances appear functioning, using fresh unleaded fuel. The engine seemed to run smooth when timed to 10-15 degrees after top center. I pulled timing cover and checked crank to cam timing. It appeared 1 tooth off. After resetting the timing chain it still runs rough and backfires. I am out of ideas. Is there someone in the Portland chapter who would be willing to come out to our shop and lend some advice. We are located south of Hillsboro, OR. My phone # is 503-260-4011.

Adam
Posts: 1141
Joined: Fri Sep 04, 2009 4:15 am
Location: lincolnshire, uk

Re: Engine rough and backfires out exhaust when revved.

Post by Adam » Thu Jan 13, 2022 5:29 am

Well done for honouring your late friend in that way. I am sure he would be very pleased.

However, that is a frustrating problem to diagnose with a long list of possible causes. I will offer a few suggestions here, but it may be any or none of them.

Firstly, if things smooth out at 10-15 degrees ATDC, that is simply because the ignition is so far retarded that it is producing very little power.

Because everything has been disturbed in the rebuild, it could be almost anything at fault now. But here are a couple of suggestions to check out, based upon my experience.

Check ignition leads follow the firing order; I expect you have done this already.
Check the distributor is built to run in the correct direction of rotation. Just in case it has been swapped from another car.
Check distributor cap and all HT components for tracking of sparks.
Check for short circuit in LT circuit (eg lead not well insulated inside the distributor).
Check quality of spark and condensor operation.
Check for an inlet manifold air leak (ie sucking air in past a gasket). (or vacuum wipers or automatic choke hot-tube or automatic clutch mechanism.).
check carburettor for correct float level and fuel supply, also for blockages (being rebuilt is no guarantee).
Check carb intake and exhaust system are free from blockages (eg no collapsed baffle in the exhaust).
Double check the valve timing (you said you altered it, can you check when a valve commences to open relative to the crank position (eg with a degree wheel and DTI)) to ensure that the timing marks are not incorrect. Compare with the correct valve timing for the engine.
Check all valves open and close fully ( you said you had done a compression test already), so should be ok.

Hope some part of this inspires you to find the cause.
"Do not underestimate the English cousin.....they are not as stupid as they look!" - Signor Altabani in The Italian Job.

GordonRead
Posts: 5
Joined: Wed Jan 12, 2022 1:47 pm

Re: Engine rough and backfires out exhaust when revved.

Post by GordonRead » Thu Jan 13, 2022 8:44 am

I am inspired so I will go back through your list, item by item. There has to be something I have overlooked. Thank you very much for your response.

Adam
Posts: 1141
Joined: Fri Sep 04, 2009 4:15 am
Location: lincolnshire, uk

Re: Engine rough and backfires out exhaust when revved.

Post by Adam » Fri Jan 14, 2022 7:20 am

Please report back with what you find. There maybe further ideas based on what you find this time around.
"Do not underestimate the English cousin.....they are not as stupid as they look!" - Signor Altabani in The Italian Job.

GordonRead
Posts: 5
Joined: Wed Jan 12, 2022 1:47 pm

Re: Engine rough and backfires out exhaust when revved.

Post by GordonRead » Thu Jan 20, 2022 5:46 pm

Many thanks to Adam who inspired us to go back through his list of probable causes looking for something I missed. I found the oil pump timing in error and when retimed, the distributor points opened at the correct place, ie 6 degrees BTC. Sounds so good when it runs smoothly.

Adam
Posts: 1141
Joined: Fri Sep 04, 2009 4:15 am
Location: lincolnshire, uk

Re: Engine rough and backfires out exhaust when revved.

Post by Adam » Fri Jan 21, 2022 3:00 am

Brilliant news!! It is so gratifying when a problem is sorted out. And it turned out to be something not on my list! :D

Please keep us updated with your exploits with the car.

Adam..
"Do not underestimate the English cousin.....they are not as stupid as they look!" - Signor Altabani in The Italian Job.

GordonRead
Posts: 5
Joined: Wed Jan 12, 2022 1:47 pm

Re: Engine rough and backfires out exhaust when revved.

Post by GordonRead » Fri Jan 21, 2022 6:51 am

Will do Adam. Thank you again for your assistance and encouragement.

GordonRead
Posts: 5
Joined: Wed Jan 12, 2022 1:47 pm

Re: Engine rough and backfires out exhaust when revved.

Post by GordonRead » Tue Mar 01, 2022 6:08 pm

To Adam: I disassembled the carburetor and reinspected all passages and jets. Then I followed the service manual procedures for setting the accelerator pump and then the dual metering pins. Amazing, the engine runs smooth, accelerates, and does not backfire. Hallelujah! Thanks again for your help mate.

Adam
Posts: 1141
Joined: Fri Sep 04, 2009 4:15 am
Location: lincolnshire, uk

Re: Engine rough and backfires out exhaust when revved.

Post by Adam » Fri Mar 04, 2022 2:34 am

Fantastic !! So pleased that you have got it running well. The weather should be picking up soon so you can hit the road. Perhaps you could post a photo or two on here?

Adam..
"Do not underestimate the English cousin.....they are not as stupid as they look!" - Signor Altabani in The Italian Job.

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