Hi All,

I am looking for opinions about the shop diagnosis I was given today.

Symptom: occasional "popping" type backfire.

The setup: 300 I6, YFA carb, no smog pump (long gone), no EGR (long gone), glass pack single exhaust. 6000 miles on crate rebuilt motor.

Diagnosis: With good compression in all cylinders (185 psi), Vacum at idle around 19 (#?), and timing set the unspent fuel in the exhaust is igniting in the relatively small glass pack chamber. This is because it has just the right mix of pressure, oxygen and fuel. With more air (smog pump reinstalled) or less unspent fuel (EGR reinstalled) the backfire shoul be eliminated.

Any opinions would be appreciated. Thanks in advance!!!

 

You did not say when this is happing at idle or decal? At idle you will get some popping once in a wile as you will have some fuel that is not burnt all the time.


Also will happen on decal. You are on the rich side (high speed side) of the carb and then go to the idle side (lean side) so will also have un-burnt fuel in the exh.
Also an exh leak can be pulling in air on decal can be a cause of the popping.


If you feel it is un-burnt fuel try leaning out the carb on what ever system (side) the popping is happing and see if that does anything.


Not knowing how bad or loud it is I may not even worry about it.
BTW I had a Toyota FJ40 with the factory I6 and 3>2>1 header & turbo muffler and on decal it would back fire. To the point a cop thought I was shooting at him when he was pulling me over for no trailer lights. Not too fun of a stop till he knew I did not shoot at him.
Dave ----

 

Get rid of the glass pack and it should go away. A six with a single glass pack doesn't sound that great anyway.

 

Fuzz - It happens at idle with a frequency of less than 5 seconds. Not steady. It does not happen that I notice on deceleration. When it happens its a quiet pop, not like your "gunfire" ...... WOW!!!

Dave - I will consider the glass pack removal, thanks.

I'm definately interested to see if anyone agrees with the shop assessment as the next thing they will be doing is quoting $ amounts to me.

Thanks!!

 

At idle I would not worry too much about it as even my 02 Dodge FI V8 360 pops a little at idle.


Now look at where the carb is placed on your motor and how far to either end the fuel/air mix has to go. If the air flow is low the fuel can fall out of the air and cause issues. That and some cly to run rich and others to run lean and shows up as a pop.


As for spending money to stop the popping well again if not bad I would do nothing.
Air pumps of this age lock up and what is a core charge if you don’t have one to hand back in? EGR valve also uses temp control items and if you have them need to be tested. If you don’t have them where can you get them and if used will they work and for how long?


What does the shop say will happen if you don’t stop the popping? What did motors do before the air pump and EGR?
Don’t need them in my book.
Dave ----

 

Before the A.I.R. pump and EGR there were no emission requirements for ultra lean mixtures and retarded cam timing.

The shop is right about the *source* of the afterburner.

 

Ok this is happening at idle. I am almost 100% sure the EGR does not work at idle and when it does the motor runs like crap. So no need for EGR if happening at idle.


I do not know how the AIR pump works does it work at idle and if so does it work fully or just a little then opens up more as the RPM goes up?
If it also does not work at idle then again no need for it.


My .02
Dave ----

 

The AIR pumps had varied regulation of when and where air was injected into the exhaust stream. But in general, the systems were designed to minimize injection of the air into the exhaust during idle, or at least, during extended idle. That is because the fuel/air mixture at idle is relatively rich, and injecting air into a rich exhaust (higher load of hydrocarbons) had a greater tendency to overheat the exhaust components (e.g. catalytic converter).

If your AIR (Thermactor) system has a "dump valve", you can put your hand over the dump valve ports during idle and feel if the air is being dumped out into the atmosphere. You will feel air pressure if it is. Then if you can reach both this valve and the throttle, you can increase engine rpms and see if the "dumping" of air goes away.

 

Sorry, I see you do not have a Thermactor system installed. So you can't test it. I still say that in general a functioning Thermactor system injects more air into the exhaust stream at high RPM than it does at idle.

 

Thank you Glendor for the lesson on the air pump. So if it works a little at idle then it would help to get rid of the popping at idle.


I was just trying to point out to the OP that putting the air pump and egr systems back in place may or may not fix the popping.
So air pump maybe, egr no.
Dave ----

 

I didn't pick up on it being a slight pop at idle. It's probably more what I would call a "putt" through the exhaust correct? That's just a slight miss-fire from one of the cylinders. I don't think the glass pack would cause that, and I doubt the other stuff being gone would cause it either. A small vacuum leak, or a minor adjustment on the carb mixture screws may get rid of it. Sometimes it's very difficult to get rid of though.

I know it's annoying to listen to it, but it will run good for a long time just like it is.

P.S. Run it at night and see if you see any blue sparks from the ignition wires when it does it. Some people also spray water around on the wires to see if it aggravates it.

 

Before I did anything that costs money I'd read this thread and consider buying one of those carbs. It is a pre-emissions carb and the engine won't be running rich so no popping. https://www.ford-trucks.com/forums/1...-9l-300-a.html

 

I agree with Dave and Gary - reinstalling a Thermactor system rigged to work with some kind of modified (no-CAT I assume) glass-packed exhaust is NOT the answer to overly-rich running problems. You would realize it halfway through the attempt, if even that far.

 

David is enjoying very smooth running and excellent MPG with that carb, as outlined in the thread, and I highly recommend that combo for others. But, don't expect to get quite as good of MPG as he gets unless you drive like he does - again described in the thread. (He describes it as driving like a grandpa, but I'm a grandpa and I don't often drive that way.)

 

A very conservative southern grandpa!