engine and drivetrain Backfire on highway after several miles at 4-5k

[eurotrash]

Search just turned up Tii threads, and one good one that fit my explanation to a T, but no one responded to it. 

 

 

This began on the way home from Vintage, and I just powered through to get home. The car is a 76 with flat tops, 284 cam, no name header with a leaky collector, and side drafts. Remanned dizzy with points ignition with a blue coil (no resistor wire), no vacuum advance connected.. 

 

At first ts was a misfire/bog when I would toe in for more speed at cruising. Sounded like someone rolling their Rs for a few seconds. - then a random back fire began, out the tail pipe. If I gave it a good bit of pedal, it would power through, but then return when it leveled out.

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On a gas stop, I filed the points, cleaned the dizzy posts and button, and gave her the slightest bit more advance.  This cleared it up for a while, but it came back.  I guess it points to the ...errr... points.    

 

But then today, on the way to work, it ran great on the secondary roads.  But after 5 or so miles at 65-75 mph on the interstate, just the backfire started to creep in.  It doesn't effect power or anything, its just annoying. 

 

I am going to pull the plugs and inspect, to see whats going on inside the motor.  But I wanted to get any suggestions from the crew, for when I get home to nose around. 

 

thanks all!

[wegweiser]

Hard to say without "feeling it" myself…. my last side draft car had a lean miss at 3/4 throttle cruising. Sounds like yours is more dramatic though (?) What's your current jetting? 40s or 45s?

[percy]

7 hours ago, eurotrash said:

Search just turned up Tii threads, and one good one that fit my explanation to a T, but no one responded to it. 

 

 

This began on the way home from Vintage, and I just powered through to get home. The car is a 76 with flat tops, 284 cam, no name header with a leaky collector, and side drafts. Remanned dizzy with points ignition with a blue coil (no resistor wire), no vacuum advance connected.. 

 

At first ts was a misfire/bog when I would toe in for more speed at cruising. Sounded like someone rolling their Rs for a few seconds. - then a random back fire began, out the tail pipe. If I gave it a good bit of pedal, it would power through, but then return when it leveled out.

.

On a gas stop, I filed the points, cleaned the dizzy posts and button, and gave her the slightest bit more advance.  This cleared it up for a while, but it came back.  I guess it points to the ...errr... points.    

 

But then today, on the way to work, it ran great on the secondary roads.  But after 5 or so miles at 65-75 mph on the interstate, just the backfire started to creep in.  It doesn't effect power or anything, its just annoying. 

 

I am going to pull the plugs and inspect, to see whats going on inside the motor.  But I wanted to get any suggestions from the crew, for when I get home to nose around. 

 

thanks all!

 

 

Backfire or misfire?  Backfire is more typically a description of premature gas explosion in the intake manifold - backwards.  To be fair it may also describe a premature or unintentional gas explosion out of the exhaust.  I see where you say "out the tail pipe."  You say "no loss of power" and "no name header with a leaky collector."  This makes me wonder if that leaky collector isn't affecting your mixture, especially at certain rpm.  Are you sure about not losing power?  Unless it is a simple exhaust leak, every misfire may be a loss of power, except you don't feel it.

 

It is a long shot, but, header exhaust design including runner and collector length, diameter and position can have a desired scavenging effect, so that exhaust is effectively funneled or "scavenged" out of the combustion chambers on the exhaust stroke.  You mention side drafts and I would wager that you tuned-jetted those fuel spitters to take advantage of the header.   If you upset the balance of exhaust flow by an inconvenient exhaust leak, the scavenging effect is incomplete and one or more cylinders, depending upon the location of the leak, may be impacted.  In short, some of the exhaust reenters the cylinder and this changes the air-fuel mixture, enough to get an occasional lean misfire.   Pull the plugs next time you notice the misfire and look for indications of one or two cylinders "running" lean.

 

Of course, a slight high speed misfire can also be caused by difficult-to-detect vacuum leaks, improper jetting, ignition timing that is too far advanced or retarded, improperly gapped plugs or the wrong heat range, or even plugs that are not properly torqued.  In the old days when dinosaurs ruled the earth and MikeS was not old enough to legally purchase beer, I might also suggest examining your plug wire routing to check for inductive cross-firing.  Having read some of the other posts regarding spark plugs, they can suffer from cracked insulators and burnt electrodes; however, if that were the case, I submit the problem would be far more noticeable when accelerating.  This was not something you described.  There is something to be said about an overheated exhaust - glowing red under a sustained load.  But that is generally a racing situation where an engine with a big cam is pushed to its lean limits.  Maybe that's the way your engine is set up and the way you drive it, yet if that is the case, an exhaust leak close to the cylinder head with exposure to ambient air may still upset engine performance and cause popping and an occasional misfire.

 

 Finally, maybe your exhaust leak is only becoming very obvious to you at higher speeds where it cannot contain the exhaust?  Run an uncorked header and see if the popping and flaming isn't far more pronounced.  

 

 

 

 

 

 

 

 

 

 

 

 

 

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Nomenclature?

 

From wiki:

 

 

Quote

 

A back-fire or backfire is combustion or an explosion produced by a running internal combustion engine that occurs in the air intake or exhaust system rather than inside the combustion chamber. Unburned fuel or hydrocarbons that are ignited in the exhaust system can produce loud sounds even if flames are not present at the tailpipe. A visible flame may momentarily shoot out of the exhaust pipe where the exhaust system is shortened. Fire may also travel into the air intake piping. Either condition may cause a loud popping noise, together with possible loss of power and forward motion. A back-fire is a separate phenomenon from the fire produced by Top Fuel dragsters.

 

 

A Backfire does not occur in the exhaust, that is called an After-Fire. The term Backfire is when unburned fuel moves back into the intake, and combusts, whereas an After-Fire combusts unburned fuel in the exhaust side of the combustion cycle. A likely cause of this is from running rich, which in certain cases could be from the combustion not achieving high enough temperatures to correctly burn the fuel. Meaning a foul sparkplug, coil, or plug wire could be the cause. In most cases, after-fires are bad due to the combustion, or fire within the muffler or exhaust system. This will overtime cause damage to the mufflers and piping of the exhaust - eventually leading to an exhaust leak, and a burned out muffler/catalytic converter. Also their will be performance lost due to a weaker combustion.

Also, an explosion in the inlet manifold, carburetor/throttle body, or air cleaner of an internal combustion engine can occur when the intake valves are not shut prior to fuel combustion.^[1]

 

The term derives from parallel experiences with early unreliable firearms or ammunition, in which the explosive force was directed out at the breech instead of the muzzle.^{[citation needed]} From this came the use of the word "backfire" as a verb to indicate something that produces an unintended, unexpected, and undesired result.

 

Quote

In both cases (combustion occurring before and after the combustion chamber), the result is a sharp pop, which is colloquially referred to as a "backfire". However, for troubleshooting, engine mechanics more strictly define an ignition of fuel within the engine exhaust system as an "afterfire", while a "backfire" is this same process taking place in the induction system.

 

Quote

Exhaust system backfires occur in engines that have an emission system malfunction, like an air injection system diverter valve problem, an exhaust leak, or when the catalytic converter has been removed. In some high-performance vehicles, when a driver shifts up and lets off the accelerator, the engine has a moment of running rich. This causes an incomplete burn which causes the fumes to explode in the exhaust system along with an audible pop or bang sound.

 

   

Edited May 26, 2016 by percy

[roadhog0]

Spark plugs maybe? I've had spark plugs do random things and fire at the wrong time (delayed) causing one hell of a hard diagnosis. I've had them fail and do this overnight. They will appear to work, but like I said, timing is delayed. So your plugs might be doing this when warmed up. Other than that, maybe you have a jet/passage that got a little clogged and is now causing a lean running condition where fuel doesn't ignite in the cylinder but does int he exhaust. Those are just some random ideas I've got.

[burndog]

This Backfire is a result of unburned fuel / vapor accumulating in your header, the heat from the header is causing it to ignite. Sounds like a plug, wire or even rotor / cap issue.

[TobyB]

I like the plug wire theory... I've had plug wires that didn't like the prolonged heat, would lose contact, and miss occasionally...

 

Fine around town, not so good after 15+ minutes on the highway.

 

But that's anecdotal...

 

t

 

[eurotrash]

Thanks all.  I will be pulling the plugs today. Had to put a trailer hitch on the family van last night and totally forgot about the car..  

 

The drive to vintage certainly worsened the exhaust leak in the collector.  It was louder yesterday than it was originally.  I may have to swap out the unknown, rusty header for the crusty Stahl.  I just need to weld on a new flange for the mid-pipe connection.

 

I will report back with plug findings as soon as I can.  

 

The car ran fine on the way home, but I was in much slower traffic.  Its weird.   

[Gthielen]

Dirt in your fuel/carbs?

George

[downhillwolf]

This is common problem with using on carb filters. Due to bad flow gasoline does not spread in to mixture but some bigger drops remain that does not burn. While in exhaust there is enough heat and due to leak a bit more oxigen and here we go with the flames out of exhaust pipe

This is my theory.

[TobyB]

Yeah, the exhaust leak's certainly not helping things...

t

 

[eurotrash]

Well. Those look pretty damn good. Best maybe I have ever seen in my own car... Ever.

 

all are in order 1-4. 

 

Discuss. 

[downhillwolf]

Put down thise filters and try again

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[eurotrash]

The thing is, I had this kit on another, freshly built motor and drove 24 hours to Arkansas and back without seeing the bog/misfire issue. So that tells me the air cleaners aren't necessarily the cause. 

 

I'm leaning toward ignition for the high RPM issue, and the added oxygen for the high RPM, high heat "after-fire"...

 

now, where to start.   First I need an event that's really far away, for testing.  !?

[eurotrash]

...but then, if it were ignition, how would one explain that when I gas it, the bogging goes away...  ?  hmmmm

[downhillwolf]

Hmm did you do compression test? Also what kind of carbs are you using? Maybe resonance frequency makes floats in carb unstable and make overflow of the gasoline chamber inside carb? Please record backfires... We than can determine what does that... Also did you run AFR?

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