66 Mustang with exhaust backfiring issues

bridgemute

bridgemute

Active member
Hello, new member here. Hoping you guys can help me. I have a 66 mustang with a 200 six and I took it out for a ride and I may have pushed it a little harder than I have in the past and it started backfiring in the exhaust.

I have a few mods. 2 barrel 2100 autolight carb. Split exhaust with Clifford performance header. Racing cam.

Things I have checked and the issue still persists:

Fuel filter replaced
Fuel flow verified
Different carb installed
Changed points, condenser, spark plugs
New distributor cap and rotor
Checked spark plug wire resistance
Replaced torn exhaust header gasket
Watched valve operation while engine was cranked
No lifter rods appeared bent.

Any other ideas? Maybe a valve knocked out of adjustment? Maybe a slightly bent lifter rod or lifter damaged?
 
:unsure: Possibly you have a lifter that pumped up or a rocker that tightened up if you have an adjustable type. You might also do a compression test to determine the condition of your engine. good luck :nod:
 
Forgot to mention a few key items:

Compression test showed 120 psi on all cylinders (pressure might be low, but it is consistent).
Spark plug 3 and 4 had some black unburnt fuel. No. 3 has more than 4. All other spark plugs had a tint of brown like they are burning cleanly.
 
what other changes were done before/after the above (6?)?
It sounds like U have another carb you've been swapping out.
Was it running fine B 4 that?
What's the SCV/LOM system doing w/the different carbs?
At what point in this process did it begin running poorly?
Had it been running or sitting B4 the backfires?
How often does it back fire & under what conditions.

I like ur idea abt the fouled plugs being related. Isn't a back fire due to unburned fuel in the cyl firing off at the "wrong time"?
A '66 may have SCV/LOM compatability probs, timing relates here to "wrong time firing".

Sorry, just more - Qs but need as much info as possible...
 
It sat for one year and I was driving it a bit this spring. Seemed to run fine, but it tended to stall out occasionally while cold. It wasn't until May that I took it out for a spin, pushed it hard, and then it started backfiring. It sounds like it is the passenger side exhaust pipe. Backfires heavily on WOT on acceleration but it doesn't backfire when lightly feathering the throttle until I decelerate from light throttle. It will pop occasionally while idling.
 
:unsure: well how is the condition of the distributor? Check the shaft side play for excessive movement by pushing it towards the engine and pulling it back towards you. There shouldn't be much play. Also check the condition of the distributor lead wire old ones can be britial and wire broken inside the insulation, also check that the point plate still has a good clean ground to the distributor body too. Good luck :nod:
 
Hi, X2 the advice. Maybe the points closed. Check point gap .025 on the high spot of the lobe. You also need a small dab of grease on the rubbing block. Dwell angle is a better check, 40 degrees. Dwell changing on a running engine shows a worn distributor shaft. Make sure you have the correct spark plugs, Motorcraft BSF82C. I wouldn't use any fancy platinum plugs in the old engines. I would maybe file the old points with an emery board and put the old points and condenser back in. Make sure your coil is not wired backwards. + is battery ignition side and - is distributor side. If you have a tach hooked up to the - side take it off for now. Double check the firing order. Can you check voltage to the + side of the coil? It should be about 6 volts with the key on. Good luck
 
I replaced the points and set to 0.025" on the high spot of the lobe. I did not check the ground wire in the distributor. I can definitely do that today.

This car has run fantastic for years. It was restored completely in 2004. It had an automatic and we switched over to 3 speed manual in 2007/2008.

I am using Autolight 46 spark plugs. These aren't platinum as far as I can tell. I had some NGK's in it when the issue started. Changing the plugs made no difference. Could the coil be weak? No tach in place. Wiring has not been changed since the car was originally restored. Movement of distributor shaft is very minimal. It feels solid.

I'm hoping I don't have to pull the head off and inspect the cam and lifters..

It appears the general consensus here is that it is more likely an ignition system issue vs. a valve not seating all the way?
 
You can test your coils primary and secondary by using a Volt / Ohm meter if you have one. Or You also can test your coil output by pulling the coil wire and watching how far the spark jumps when running and check its color should be a nice brite blue spark and jump about 3/8 inch or more. Be carful if you don't have a set of insulated plug pliers. You can also test the same way at each plug wire each one should drop the engine RPM about the same if you find one that doesn't then check that plug or switch it into another cylinder and see if it acts the same then you may have a bad plug. Good luck :nod:
 
Hi, yes I would strongly suspect ignition. X2 on Bubba. The ground wire is a braided bare wire that should attach to the points hold down screw with a metal loop. It is easy to miss it but it is in there. Look at the wire from the distributor to the points. It could be brittle or broken. Some people contort this wire to keep it away from the rotor. Check were it passes through the grommet. Good luck
 
Ok. I replaced the coil. Still the same. Checked the 12v wire to the distributor points for continuity, all good. Checked the braided ground wire in the distributor for continuity, all good. Not sure where to go from here. Maybe the vacuum advance mechanism isn't actuating fully? It does move when I pull vacuum on the line.

Couple other details that may be helpful. We put a comp cams cam shaft in this engine. P/N 65-236-4. And we put valve springs from a 302 in to help close the valves with the increased lift. But this car has run with this since the intial build in 2004.

Maybe I just need to put an HEI system in like DSII.
 
Y mix the 2 different systems? (ford/che**y)
May B we diagnos the prob 1st B4 obscuring it w/changes.
I like the idea of figuring it out, then replacing what needs to be.
Do U need it as a DD?

I'm still back @ the above:
"...It sounds like U have another carb you've been swapping out.
Was it running fine B 4 that swap?
What's the SCV/LOM system doing w/the different carbs?
At what point in this process did it begin running poorly?…"

I have beliefs it's w/the timing too...
 
I assumed HEI just another way to designate electronic ignition. I would put in a Duraspark II if anything. No Chevy parts here. It was running fine with the first carb, ran fine for years until I pushed the engine too hard. I tried a spare carb and it exhibited the same issues. I also rebuilt the 1st carb and cleaned and soaked it. Same symptoms. What is the SCV/LOM system? Both 2100 carbs are doing the same exact thing.
 
The vaccum advance unit should pull the point plate freely and hold it in full advanced position when you apply a constant vacuum source to it and then hold it. You could try installing two new plugs in cylinders 3 and 4 to replace those that are fowled. After that you need to pull the valve cover and check each pair of rockers when that cylinder is at TDC on the compression stroke (ignistion). Look for any of the valves,that isn't closing fully compared to the others could be cyclinders number 3 and 4 since those plugs aren't as clean. Yes a DuraSpark II would be a great upgrade to gain some performance with a hotter spark but I don't think the distributor is the cause of your backfiring right now. One other thing you could try is to loosen up the rocker arm Pedistal bolts remove the rocker assembly and put a straight edge across the top of all the valves to check for any major differences. Good luck :nod:
 
1) I'm just a rank beginner . After coming here (20 yrs into vehicle ownership) I learned abt timing a ford 6 pre '68/post '68. There is a 'feedback' system between the carb & distrib. which changed on that piviot yr. The Classic Inlines site listed on our ("144 - 250 'Sm Blk' Six Perf") forum's Index page with a star taught me abt this system. As backfire seems to B abt timing I thought I'd mention that information for you to look at in doin your trouble shooting. That & the "ford 6 performance handbook" many of use use are invaluble sources of info for you. If U mix carbs from non-SCV yrs you'll have timing issues (that may contribute to backfire I assume).

2) I deferr to bubba - he may be offering assistance w/a seperate issue? a colapsed lifter?

Thanks for your sharing of these issues. I learn from these adventures & it's certain I need more of that !
Let's keep going till reaching success...
 
I will inspect and measure the valves thoroughly and let you guys know, focusing on the No.3 and 4 valves

A friend recommended taking a mag-base dial indicator and hand cranking the engine measuring lift and comparing numbers down the valve train there. That would tell me if I have a collapsed lifter or bent rod.

If that is the case, maybe I need to install a tachometer so I know exactly how hard the engine is running. I got a little carried away :D

Thanks,
Chris
 
bridgemute":1ex6z8cy said:
I will inspect and measure the valves thoroughly and let you guys know, focusing on the No.3 and 4 valves

A friend recommended taking a mag-base dial indicator and hand cranking the engine measuring lift and comparing numbers down the valve train there. That would tell me if I have a collapsed lifter or bent rod.

If that is the case, maybe I need to install a tachometer so I know exactly how hard the engine is running. I got a little carried away :D

Thanks,
Chris
Click to expand...

Your friends recommendation is a very good one. You can also check for bent push rods by rolling them on a flat and true surface looking for if they have any wobble. I always also try to keep the push rods in the same order they were installed when reinstalling them. Yes a tachometer with or without a shift light is a real good tool to have. Good luck :nod:
 
:unsure: Something else to check out.Do you have any leaks in the exhaust system?No matter how small,they can let fresh air into the system and that will also lead to backfiring or popping in the exhaust system if the engine is running even a little bit rich.
Good luck.Have fun.Be safe.
Leo
 
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