Suddenly, it's backfiring! Ideas?

[LaGrasta]

I checked the vacume advance yesterday to make sure it held... it did perfect. Some of these suggestions are ruled out by the fact the car starts and idles perfect. It only pops under a load and even then, not always.
My next move is to check the plugs visually, then a compression test.

 

[Frankenstang]

Well I'd take this thought for what it's worth (which might not be much), I've always heard a carb backfire referred to as a lean pop versus an exhaust backfire being referred to as too rich.

EDIT: Maybe lean like a leaky/malfunctioning accelorator pump...not sure what style carb you have.

What are the chances you got some bad gas, or what was the last thing/service/repair done to the car before the backfiring started?

I noticed petronix in your sig, and have heard of them doing some flakey things, but I'm always working under the assumption that ignition either works or it doesn't especially for you guys with solid state setups (while there is always possibility of misadjustment, timing, etc.)...really just spit balling.

 

[cobraguy]

I've had this scenario a couple of times one was a broken valve spring, one time was a flat cam lobe on the exhaust side of that cylinder. Both times started and idled good and compression tests showed good.

 

[LaGrasta]

cobraguy, if it's a flat cam lobe, what's next? Rather, what did you do?

 

[cobraguy]

It was in a Dodge pickup with 240k on it. I sold him a longblock. If you have the valve cover off use a ruler to check the general valve movement while cranking( ignition disabled) a flat lobe will move the valve noticeably less than the rest. To check for broken valve springs just try twisting the spring with your fingers the one that twists easy is broken. Do a compression test just for giggles and peace of mind. Good luck Anthony, Rich.

 

[cobraguy]

Here's why I'm thinking cam. Under idle conditions the fuel load has plenty of time to burn then exhaust even if the exhaust valve dosen't open all the way. Pour in more gas with less time and that exhaust valve become a big problem and only exhausts half of the combustion gasses. When the intake opens that exhaust gas ingites everything in the intake manifold giving a backfire that will pop the choke closed in some cases. If the rest of the motor is fairly fresh I'd get a cam and lifters, if its got alot of miles rebuild it.

 

[Anonymous]

I still go with the cam and possibly lifters. I do agree with cobraguy as well as do check the valve springs and make sure that you only have 3 to 6 psi on the fuel pressure going to the carb. Do you have a elect fuel pump? If so most have a pressure rating of 19 psi and thats way too much for one of these engines. Let us all know what happens. Tom

 

[LaGrasta]

I spent some time with the car this past weekend. With the valve cover off I ran the car. It was beautiful. Ran fantastic, the valves working together in harmony and the oil spewing over each one just as Henry intended, couldn't find a single thing wrong. Next is a compression test...

 

[sasktrini]

LaGrasta, it may not be real obvious to check the deflection in the timing chain without removing the cover, as trying to turn the crank counter-clockwise will loosen the bolt. Turning the crank the normal way, you may not notice any deflection, as the chain will be tight on the side that's applying the pulling force to the cam sprocket. When I pulled the cover off mine, the top section of chain that was not under tension was real floppy.

I recently replaced my timing chain set... it was quite surprising to see the difference between the old and new... the new had very little deflection! As a result, I would guess my chain was stretched almost an inch (net, coupled with sprocket wear), and so my camshaft timing was extremely retarded in relation to the crankshaft.

My timing chain set may have been 40 years old. How old is yours?

Good luck!

 

[LaGrasta]

Although I've no idea of when, my engine has been rebuilt.

Slowly but surely I'll rule out each item until I find the problem. Removing the timing chain cover will be one of the last things I'd like to try.

 

[Stubby]

I spent some time with the car this past weekend. With the valve cover off I ran the car. It was beautiful. Ran fantastic, the valves working together in harmony and the oil spewing over each one just as Henry intended, couldn't find a single thing wrong. Next is a compression test...

If all of the valves were working correctly, look at the ignition very closely.
Check the cap for problems. Maybe carbon tracking. I might have an extra DSII dizy that I could send you to rule out the Pertronix and dizy alltogether.

 

[Asa]

if his signature is up to date, he's got the 170ci I6, and as far as i can remember, the DSII dizzy will not fit inside the 170's block

 

[LaGrasta]

thanks for the offer, but yes, i have the 170 (Pertronix). I really inspected the distributor; looks good. No bent shaft, new cap; everything looks tight.

 

[Anonymous]

You have no idea when the engine was rebuilt! So with that and the length of time you have had this and now it is backfiring! How many miles are on this engine since you have had it? If it is a lot then added to the don't know miles and last OH I would still think that one of the lobes on the cam is getting rounded, even slightly would cause the backfiring at added power. My last one that did that idled fine and the engine run very good but as soon as you started to add power it started to backfire back through the carb. I know it is a bitch to really bad to have to pull the cam but I am afraid thats what you will have to do. This is my assumption. Good luck.

 

[LaGrasta]

you may be right 2211. I alone put over 30k on this engine.

I plan to do some more troubleshooting over the weekend. Hopefully I can pin-point the problem once and for all.

 

[Anonymous]

I didn't mean to rush into doing the cam, what I was implying was to finish checking out the rest of the things first and then and only then tear it down. The cam could be ok but I doubt it with it doing what you are saying. Have you checked your fuel? Pop the top on the carb and look in the bowl for water. It is very easy to get water or condensation in your tank and carb. Good luck and I hope you don't have to tear it down. Champion2211

 

[69.5Mav]

The simplest way to tell if the cam chain is stretched is to watch the distributer rotor while turning the crank CW and CCW. The angle between the two points where the rotor is moving will give you an idea about how much slack there is in the chain.

You can remove the spark plugs to make it easier turn the crank.

Do this before doing anything else, it is easy and will tell you a lot about the cam chain condition.

 

[Anonymous]

All the information you can find to get the job done the easiest way please do it. It is up to you first and foremost. Champion2211

 

[Williboy]

Personally I would triple check the fuel and ignition before getting into the mechanical. We've all seen faulty or premature "new" spark plugs, wires, cap and rotors, ignition coils, fuel pumps, etc...I'd throw in a new set of points for a few buck just to eliminate the Pertronixs.

I think the intermittent is the clue. A mechanical problem would most likely be more consistent. But never say never...

My intermittent rod knock was a broken motor mount causing my oil pan to bounce slightly on my belly bar.

Good luck

 

[Mercury Mike]

Anthony, I work at a shop in Laguna Hills, why don't you bring it down and I'll pull the timing cover with you sometime next week, or let me drive the car down and do it, I can pick it up after work one of these days, and I'll go through it and report back to you. My shop is right off of El Toro, which is real close to your house. Let me know what you think, in case you don't have my number I'll PM you. Call me tonight!