back firing through tailpipe

[RalphG]

Originally Posted by GB SISSON

My 59ab backfires through the tailpipe, but only on deceleration. There are some fairly long grades here, so I need to address it. Engine is fairly worn, carb rebuilt, but I may not have switched the power valve, which I don't really understand. Recent Bubba crab. Other than the backfire, a good runner and easy starter.

I've had the exhaust explosion on deceleration (but not at idle) happen on an IH V8 and it always turned out to be the points. Either cleaning them up or replacing would fix mine. Might be the same on a flathead.

[GB SISSON]

Points new and set up by Bubba. Forgot to mention a mopar type oil fill cap with hose plumbed into the air cleaner to help with blow-by. Packed the oil breather cap with a stainless kitchen scrubber pad.

[rotorwrench]

An internal fuel delivery seep can blow unburned fuel into the exhaust on deceleration where it will eventually be lit off. Leaky or sticky exhaust valves will cause the same thing.

Harley Davidson motorcycles bee bop & pop due to the ignition design on the older high compression bikes. The coil fires twice during the 4 cycles of operation so they light off unburned fuel on deceleration normally.

[GB SISSON]

Originally Posted by rotorwrench

An internal fuel delivery seep can blow unburned fuel into the exhaust on deceleration where it will eventually be lit off. Leaky or sticky exhaust valves will cause the same thing.

Harley Davidson motorcycles bee bop & pop due to the ignition design on the older high compression bikes. The coil fires twice during the 4 cycles of operation so they light off unburned fuel on deceleration normally.

Vacuum about 18, but real steady, so I lean towards the fuel delivery seep. Is fuel delivery more about the needle and seat or the power valve or what? What should I address on carb? I think the kit had a caged disc instead of traditional needle and seat.

[40 Deluxe]

Originally Posted by GB SISSON

My 59ab backfires through the tailpipe, but only on deceleration. There are some fairly long grades here, so I need to address it. Engine is fairly worn, carb rebuilt, but I may not have switched the power valve, which I don't really understand. Recent Bubba crab. Other than the backfire, a good runner and easy starter.

Where's that hidden teenager in you? Remember when backfiring was cool? Relive your youth! Don't mess with it!!

[Gary Tosel]

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A friend of mine had a 40 Ford and a 36 Ford that both would backfire through the exhaust. The 36 Ford would run okay for awhile than backfire loudly and then shutoff. The 40 ford would backfire for quite awhile.


In both cases we changed the condenser and the problem went away. Just our experience.

[Bulligen]

I think the Marvel Oil will help the situation, good luck with it!

[rotorwrench]

Most times it's power valve related but other little leaks can happen. Some modern valves don't seat correctly in the housing.

A failing condenser has little or no capacity so it will cause a bit of delay in spark since it can't boost that energy through very well. The thing about a failing condenser is that it will soon fail altogether so it will become obvious soon enough.

[GB SISSON]

Since the only auto parts store we have on the island is a napa, I should stop in tomorrow and get a condenser, maybe a spare too. Because I'm lazy, what is the part # so I don't have to do a search. Thanks!

[jms50f5coe]

I have installed a Marvel Mystery Inverse oiler device that has worked very well to prevent sticky valves. Uses about a pint of MMO per thousand miles. And looks cool under the hood.

[rotorwrench]

The NAPA Echlin catalog calls out the FA54 for the 59 type distributor/coil set up. I've gotten in the habit of purchasing several at a time just to have spares around or for troubleshooting.

[Flathead Fever]

Originally Posted by melisahe

My 8rt is back firing through the tailpipes at low rpm. as rpm increases to 800 it stops. What causes this? The timing is set right and I double checked the firing order. Thanks Mike

If it has a sticking valve below 800 rpm while idling I think you would here the lifter clattering away. It should also show up as a fluctuating needle on vacuum gauge.

Is it consistent, does it always do this, hot or cold? Does it go away if you give it a little choke or add some propane to the carb?

Its hard to tell what "back firing" means? Are you talking about an ignition misfire sound in the exhaust like its dropping a cylinder or an actual popping noise? I hear engines all the time with definite misfires. I point them out to the owners who have no idea its happening. They look at me like I'm nuts Sometimes on a V8 its not that noticeable to some people.

Does it run okay below 800 rpm other than the exhaust noise? Is the sound happening while just idling in the garage. A slight burned valve problem at idle should make it run rough and then it will get better with higher rpm. An ignition problem (other than timing adjustment) will usually get worse as rpm increases the demand on the system.

Can you pull the plug wires one at a time while it is running with your "best" insulated pliers and see if they each drop the rpm the same amount or have any effect on the noise in the exhaust. Never do this on a electronic ignition unless you ground the disconnected spark plug wire first before starting the engine. If the voltage cannot jump from the plug wire to ground it will find an alternative path to ground by backtracking through your electronic ignition module or computer. Only do this test on old fashioned points systems. It should help you narrow the problem down to one-cylinder and then do a compression test. While the piston is on TDC put compressed air in to the cylinder and see if it comes out the , oil breather, exhaust or intake.

There are so many problems this could be. Its so important to hear the sound in person before making an educated guess. Then your guess can be verified with actual testing, ignition scope, timing light, dwell meter, vacuum gauge, compression gauge or air fuel ratio checks. Your at a huge disadvantage without these tools but you can still figure it out with out them.

[billfish]

had a similar situation with backfiring through exhaust when the car was backing down- was advised to advance ignition- that solved the problem for me.

[GB SISSON]

Idles well, with no sign of sticking valves. My favorite run mode of a flathead is that super steady perfect idle. This one is pretty good, and I don't hear a miss. I'm pretty fussy about a miss and have been known to chase them down, sometimes going to great lengths.
The lowest compression is #5 at 75 lbs, highest is #4 at 95 lbs. Squirt of oil and they go up as much as 15 lbs. I am currently collecting the parts I need for a total rebuild on one of my spare blocks, using a 4" crank. Might not happen til next summer. The backfire is sometimes like a genuine gun shot on a long grade, but often not as loud as that, but yes, a real backfire. I don't have a dwell meter or an exhaust gas tester, but I bet the exhaust gas tester would be pegged on 'stinky'. At idle the exhaust is very strong smelling. I have messed endlessly with the idle mixture screws, and get best idle when they are about a half turn out, both sides. Since I have three 59 series flatheads currently on the road I think I'll put a kit in another 94 carb and try that out. Some of the kits I've used have 2 types of power valve. I will know later when I get down to the shop. Any advise on power valve installation welcome. Thanks.

[rotorwrench]

CharlieNY has a fix for ill fitting power valves. The rest would be insuring that the gasket surfaces for the lower housing is still in good shape. If you turn the idle screws near all the way in and it still idles, it points toward an internal leak. Holley always had problems with power valve blow outs.

[Init1]

It might be a little to rich. Does the exhaust smell rich at idle?
Slightly too much ignition advance will also cause backfiring.
The two things together will cause issues.

[Flathead Fever]

Your compression reading should be within 10% of each other and it looks like the rings are getting a little tired but I'm not sure that would cause your backfire.

You say the back firing is on a "long run", is that while still under acceleration or when you back-off the gas after this long run?

The reason I ask is on smog cars from the 1960s to 1980s the air injection pump injected air into the exhaust manifolds to keep the burning process going. That burned up any unused fuel before it went out the pipe. There was a diverter valve installed with the pump that would switch the air injection to the air cleaner when you suddenly backed off the gas pedal. The reason for diverting the air was when the throttle plate closes at high rpms it creates momentary high vacuum which sucks out way too much fuel out of the carb. The engine cannot burn it all and it goes out into the exhaust. If the air pump was still pumping into the exhaust system that extra oxygen along with that extra fuel causes a violent explosion. It will blow a muffler apart. I replaced them all the time at work. The blast would open them up from end to end, right down the crimped seam. Our Ford F-600s and Chevy C-50s were the worst. They were so overloaded you had to keep your foot buried in them everywhere you went and if that diverter valve did not do its job when you backed off the gas pedal it would scare the crap out of you. its like a shotgun blast going off inside the car.

You will see old muffler ads where they say "blowout proof". The glasspack mufflers that have the crimped ends can blow the ends right out. The later glasspacks where the tubes narrow at the ends don't have any crimped ends to blowout. When you were a kid somebody probably talked you into turning the key off while you were driving and then turning it back on again. It makes one hell of an explosion. Its because the engine was still sucking air and fuel into it but there was no ignition to ignite it so it just went out into the exhaust. The second that key was turned back on those cylinders started firing again and they ignited off that mixture in the pipe.

So what if your engine were running rich and you had an exhaust leak the same thing might happen. When you back off the gas pedal extra fuel is pulled into the engine. We think of the exhaust leak as air being pushed out of the exhaust but there are negative and positive pulses. The negative pulses will pull outside oxygen back into the pipe which combined with the un-burned fuel could cause your backfiring. Sometimes a backfiring condition can be repaired with a simple exhaust leak fix.

Dual exhaust can be a neat tool. It lets you know if you have a problem effecting just one cylinder and which side of the engine that cylinder is on. Is the sound in just one pipe or is it in both?

Normally there is very little oxygen left in the exhaust after combustion. You have hydrocarbons (gasoline), oxygen and nitrogen going into the engine. During the combustion process the hydrocarbon molecules get split apart and combine with the oxygen and the nitrogen. You end up with H2O (water), CO which is carbon monoxide, CO2 carbon dioxide and NOX (the brown haze you see in the sky). The molecules link up differently depending on how rich or lean the mixture is along with the temperatures and pressures in the cylinders. If the air fuel ratio is correct there should be very low amounts of hydrocarbons and oxygen left coming out of the engine to backfire in the exhaust.

Now if you had a cylinder misfire the unburned hydrocarbons and oxygen could backfire in the exhaust. To rich a mixture along with an exhaust leak could do it. You have too think why is there excess fuel and oxygen in the exhaust pipe at the same time as the backfire? How is getting onto the pipe? If it were too lean there would be excess oxygen but no leftover hydrocarbons to burn. Unless it was so lean the mixture never ignited in the cylinder and it went out into the exhaust. Now you have oxygen and fuel in the exhaust. But normally a lean engine will backfire through the carb so that's probably not the problem. What if its so rich the mixture does not get ignited. Now you have lots fuel and oxygen in the exhaust and its going to get ignited by the flames coming from the other cylinders and you get the big boom-boom.

[melisahe]

Problem solved with MMO and running regularly. Tks for the replies I learned a lot!

[Flathead Fever]

Originally Posted by melisahe

Problem solved with MMO and running regularly. Tks for the replies I learned a lot!

That was an easy and cheap fix! I wonder if a different weight engine oil would help? Maybe thinner oil, so it could splash around easier inside there? What oil are you using? My 2012 pickup uses a synthetic 0W-20W. Its about as thin as water.

[Tinker]

Originally Posted by melisahe

Problem solved with MMO and running regularly. Tks for the replies I learned a lot!

Nothing makes a engine fail like letting it sit.