Topic: exhaust backfire help

[toyodaaddict]

catalytic converter was shot so i hallowed it out.  now every once in a while ill get a fairly loud single backfire. the truck has a stock 22R, stock exhaust man,2.25 inch pipe and a flowmaster and the hallow cat. so what do you guys think is most likesly causing the backfire? lack of back pressure,bad gasket,hole in exhaust??? the truck never backfired before i hallowed out the cat

[MiniSimp]

Hollowed out cat is the cause.

[toyodaaddict [OP]]

Hollowed out cat is the cause.

can you explain how the hallowed out cat is causing this,i dont understand?

[toe]

A carb'd engine will occasionally have unburnt fuel in the exhaust (during decleration, where EFI would just cut off the injectors).

The catalyst would burn the fuel up.    With no cat,  the unburnt fuel/air mix hits a hot spot and explodes......

[toyodaaddict [OP]]

A carb'd engine will occasionally have unburnt fuel in the exhaust (during decleration, where EFI would just cut off the injectors).

The catalyst would burn the fuel up.    With no cat,  the unburnt fuel/air mix hits a hot spot and explodes......   

ok this makes sence but how come my 80 toy doesnt backfire it has no cat and i dont think it ever did?

[Snowtoy]

When does the backfire occur, i.e., on acceleration, holding constant, or when letting off the gas?

[toyodaaddict [OP]]

When does the backfire occur, i.e., on acceleration, holding constant, or when letting off the gas?

its a single backfire on deceleration

[BLACKDOG]

its a single backfire on deceleration

mine did that when I had an exhaust leak (where the cat was bolted in)  when I welded up the pipes, the backfire went away.

ok this makes sence but how come my 80 toy doesnt backfire it has no cat and i dont think it ever did?

First, I have little to no experience with carburated toys.  That being said, I would venture to guess that it was never designed to have a cat in it, so the carb settings are a little different.  When catalytic converters are installed in vehicles, contrary to what most people think, the entire vehicle's exhaust system gets a little tweak.  This little tweak could easily account for why your truck with a hollowed out cat backfires, and your '80, with no cat (never had one) doesn't.

[87hillbillyoter]

funny, i was just thinking about posting a thread on this today.

anyways, mine does that too, except i have a cat and i dont think the PO hollowed it out, havent checked yet. but when i let off the gas, there are about 3 or 4 quiet backfires, mostly when the engine is cold. why is this?

edit: sorry to hijack your thread

[pumkin toy]

my rig has a cat and it backfires on decel and when I shift fast at high RPM's  Thats probly because my blow off valve doesnt recirculate into the air intake though

[toe]

funny, i was just thinking about posting a thread on this today.

anyways, mine does that too, except i have a cat and i dont think the PO hollowed it out, havent checked yet. but when i let off the gas, there are about 3 or 4 quiet backfires, mostly when the engine is cold. why is this?

edit: sorry to hijack your thread

When cold, the engine is running rich; so there's unburnt fuel..

Exhaust leak makes sense, as the leak is usually both ways.       The unburnt fuel gets some oxygen from the leak to combust with.....

[87hillbillyoter]

When cold, the engine is running rich; so there's unburnt fuel..

ok thanks for your help man.

[toe]

And without air leaks, the exhaust has cooled (and there's no hot spots) by the time it exits the exhaust system; so no backfiring.........

[89_yota]

Engine running a little lean causes that aswell. I'm assuming your truck still has the air pump attached.. right? Well if it is... that's a huge cause. It pumps fresh air into the exhaust to meet the emmisions and now with the hollowed out cat, you actually hear that AFTERFIRE. It's technically not called a backfire... that's only if it blows flame out of the intake. My 89 yota did that aswell... untill I took off the air pump.