back firing through tailpipe

melisahe

melisahe

Senior Member
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
 
weemark said:
could be lean
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Typically if it was running lean it would pop out the carb (looking for gas).

Now if it was running rich it might pop out the exhaust (igniting the left over gas in the exhaust).

Mike, What do your plugs look like?
 
You can, while its running, drizzle MMO down the carb till it smokes out. Let sit a few minutes then put some in the gas and drive a good distance (5-10 miles), repeat if its getting better. If it doesn't get better something else is going on.

I've done this on my truck that has a new rebuilt motor with low miles. Usually in the spring after its been sitting for a few months, I MMO it and it frees up the valves everytime.

My condition is a bit different though. Idle is fine, underload or at higher rpms it will buck and pop.

Worth a try as its fairly simple procedure, wont hurt anything, and cheap to do.



.
 
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Tinker-Plugs are clean, light gray and slightly brown. I'll try some MMO and see what happens. How much per gallon of MMO?
 
I think there are directions on the bottle. Few ounces pre tank, should be good. I usually just pour in a bit less then a 1/4 of the qt bottle on a fill up. But I don't use it on every fill up.

Others here probably have a proper mix formula that they use. I'm also one of those cooks that goes "a dash of this, and a splash of that!". :)

I would drizzle some down the carburetor till you get some smoke and smell that MMO burning. Then drive it with some in the tank. But the drizzle will kick start the oiling.
 
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Sticky valves as mentioned, burned valves, or a leaky carburetor can all cause after fire in the exhaust. Sticky or burned valves will generally show up on a compression or leak down test. A leaky carb body will drain the float bowl and be hard to start sometimes. It can run rough at idle plus possible after fire if too rich.
 
have a steady reading of 19 0n vacuum gage. I don't have another carb to check against. but sounds like a leaking carb. I have to crank it awhile on first startup of the day. who does carbs on the barn? Mike
 
Four (4) fluid ounces per 10 gallons of fuel. It's not a critical measurement though, "a few glugs" per tank is probably close enough. I usually measure it to stretch the supply out though. It does work; infrequently driven engines are subject to fuel souring or varnishing in the carburetor, stuck rings and lifters, valves etc.
 
melisahe said:
have a steady reading of 19 0n vacuum gage. I don't have another carb to check against. but sounds like a leaking carb. I have to crank it awhile on first startup of the day. who does carbs on the barn? Mike
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Try Charlie ny. He will set you straight.
716 662 9159
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Charlie Schwendler
Charlie ny
 
I had some back firing like yours it wasn't, all the time.
I think its the 2 middle exhaust together firing left over gas or miss fire.
 
Tinker
Once you get them Unstuck, use a couple of caps of MMO in every fill up of gas from here on out!!!
 
melisahe said:
have a steady reading of 19 0n vacuum gage. I don't have another carb to check against. but sounds like a leaking carb. I have to crank it awhile on first startup of the day. who does carbs on the barn? Mike
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Vacuum on a idle car might be inconclusive. Since your issue is not at idle as much as under-load. I have added a full time vacuum gauge to the dash just because.
 
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.
 
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