413 Backfires

[RKC]

1963 Imperial 413 engine with 25,546 original miles. I replaced the points, rotor, cap, condenser, plugs, ignition wires, installed a carburetor kit, installed a new intake manifold, new water pump and timed the engine. The Y pipe is on the exhaust but the muffler and tail pipe are gone and will be replaced as soon as I can solve the backfire problem. The main reason for all of this work was to detail the engine, which ran fine prior to doing this work. The car starts fine, idles fine, but when I increase the engine while in park, it will occasionally backfire and it runs a little ruff. No blue, black or white smoke from the exhaust. Any suggestions will be most appreciated. Thanks!

 

[Big_John]

Double check firing order.

 

[PurpleBeeper]

Snap call? vacuum leak on intake/carb/vacuum hose when you changed the intake. Maybe look for a single fouled plug, a loose plug wire (boot backed off of plug), burnt plug wire leaning up against the exhaust.

 

[RKC]

Double check firing order.

Firing order ok. On idle, no miss, runs smoothly.

 

[Pete Kaczmarski]

My '59 Imperial did that while driving home from a car show several years ago and blew up my mufflers. It turns out to be the condenser which was defective. I replaced it and have not had a problem since.

 

[Trace 300 Hurst]

Double check firing order.

Let's hope this is your problem. Inadvertently switching 5 and 7--either at the plugs or at the cap--is a FAMOUSLY EASY tune up error with the Mopar 1-8-4-3-6-5-7-2 firing order. The engine will run a little rough but "almost okay" at idle and no load with these wires switched But because number 7 is getting sparked 90* too early while that intake valve is still somewhat open, you will get a POP!

 

[Big_John]

Let's hope this is your problem. Inadvertently switching 5 and 7--either at the plugs or at the cap--is a FAMOUSLY EASY tune up error with the Mopar 1-8-4-3-6-5-7-2 firing order. The engine will run a little rough but "almost okay" at idle and no load with these wires switched But because number 7 is getting sparked 90* too early while that intake valve is still somewhat open, you will get a POP!

My Dad showed me something years ago about #5 and #7. If the two plug wires ran next to each other and parallel for any length, you could get a crossfire between the two cylinders. The solution was to cross the wires at right angles. I think that hasn't been an issue with newer materials, but it was a little gem on info that he passed on.

 

[Big_John]

Firing order ok. On idle, no miss, runs smoothly.

The thing to do next is start pulling plug wires, one at a time, to see what cylinder is your problem. If the car runs worse, that's not the cylinder. If it doesn't change, that's the offending cylinder.

Another check that's real easy is to open the hood and run the engine in the dark to see if you have any "leakage" that will show up as a light show. I'd also recheck the timing. The outside ring on the balancer can be known to slip and then the timing marks aren't correct. Try advancing or retarding the timing a bit and see if there's any improvement.

Did this car run OK before doing these repairs?

 

[65Port]

From experience (too much, actually), don't trust new parts. But, assuming they check out and your timing is on, recheck your points and throw a vac gauge on it to see if you now have a leak after intake removal and reinstall. Good luck

 

[RKC]

The thing to do next is start pulling plug wires, one at a time, to see what cylinder is your problem. If the car runs worse, that's not the cylinder. If it doesn't change, that's the offending cylinder.

Another check that's real easy is to open the hood and run the engine in the dark to see if you have any "leakage" that will show up as a light show. I'd also recheck the timing. The outside ring on the balancer can be known to slip and then the timing marks aren't correct. Try advancing or retarding the timing a bit and see if there's any improvement.

Did this car run OK before doing these repairs?

Yes, the car ran fine before the repairs. Reason for the repairs is that I wanted to detail the engine bay, which is now completed. As a result, I have a great looking engine bay in a car that backfires!!! My Dad always said, "leave well enough alone".

 

[66 Monaco 500 365]

Usually a backfire is from going lean, ether the float level is low, or the accelerator pump isn't working properly or mis-adjuisted, if it happens when the throttle is opened quickly. Since it is idling smooth, I would guess it doesn't have a vacuum leak
My $.02

 

[Badvert65]

Using an infrared temp gun, compare the temps of the exhaust manifolds at each exhaust port. If one is different than the rest, that cylinder is likely misfiring. You can also use a squirt gun and squirt water at each port, a 'normal' firing cylinder will immediately vaporize, a dead cylinder will not. Works on cast iron manifolds but is usually best done from a cool down for more distinct results. This will help identify a misfiring cylinder, not WHY it is misfiring, but it will give you a place to start. V-8 engines are inherently smooth so one cylinder being dead or randomly firing can be difficult to notice at lower RPMs.

 

[65sporty]

Do you still have the old condenser? Like Pete mentioned, he had a condenser problem. If you still have the old one it would be easy to swap out and see if your backfire goes away. New doesn't always mean it's good

 

[RKC]

Do you still have the old condenser? Like Pete mentioned, he had a condenser problem. If you still have the old one it would be easy to swap out and see if your backfire goes away. New doesn't always mean it's good

A very good idea, but I no longer have the old condenser.

 

[mr gtx]

check your points adjustment gap. did you use a dwell meter ?

 

[Blusmbl]

Maybe recheck initial and total timing as well. It can't hurt.

 

[RKC]

check your points adjustment gap. did you use a dwell meter ?

I have not, but will now, based on your suggestion. Thanks!

 

[65sporty]

The condenser is cheap, I would buy another and swap it out

 

[Dads300]

Back when I was still running vehicles with points 20-25 years ago, I would always keep my working used condenser for the glove box, and re-use the old condenser hold down clamp. At that time, I found that the new clamps were too tight and squeezing the condenser causing failure.

This once saved me out in the woods in the middle of nowhere. My 71 w200 just quit running while driving up a two track. After some quick basic diagnostics, I pulled the old condenser from the glove box, installed it, and she fired right up. The known good spare saved my bacon that day.
Travis..