Backfiring

[Her69fastback 3]

The car backfired when we turn it off. I pulled it up to top dead center 0 in the harmonic balancer. The rotor points to the #1 plug on the distributor cap . What should I set the timing at?The pic shows the duration & lift of the cam. I bought the car with the motor rebuilt. It ran great. I put a ederlbrock intake & a Holly 600 on it. It starts & runs good just backfires when you shut it off. 

[Brian Conway 264]

From the 69 Shop Manual 6 degrees.  Most people run at 10 degrees.  I run at 12.  Brian

[Her69fastback 3]

Thanks I’ll set that tomorrow. Will that make it backfire? 

[Brian Conway 264]

Backfiring is often a timing issue.  If your running at 0 that is probably the problem.  6 degrees is the conservative stock setting and will most likely put an end to the backfiring.  For a better performing motor set a 10.  Try the new setting and let us know how it turns out ?  Brian

[Her69fastback 3]

It’s still backfiring. Set timing at 12 ,10 & around 6 it’s still backfiring. 

[Her69fastback 3]

I just noticed that I put shorty valve covers on the car. It had tall ones on it when I got it. Would that make a difference?

[69ShelbyGT350H 61]

I just had an issue where I lost compression on a cylinder. Doing a compression check and #4 is dead. Pull the valve cover to check the motion, see if anything is wrong while I crank it and suddenly it has compression. The same thing happened to #5. Looked under the Valve cover and where the PCV and Oil Filler/vent are, are metal baffles. I used cork valve cover gaskets and after running the engine I tightened everything down again. Valve cover baffles are now rubbing on #4 and #5 rockers. I'm in the process of replacing the single layer Felpro cork gaskets with double layer, and longer valve cover bolts.

Fel-Pro 1645 Valve Cover Gasket Set

[Her69fastback 3]

Thanks. I’ll check the compression. I still have the original valve covers I’ll put them on tomorrow 

[69ShelbyGT350H 61]

[Brian Conway 264]

 Still backfiring ?  You set your timing to  ' BTC ' Before Top Dead center at 6, 10 and 12 with no effect.  The vacuum line from the vacuum advance to the carb was disconnected at the carb. and the hose pinched off right ?  So probably start checking the parts of the vacuum advance system ?  Your going to need a gauge to check the vac. at the carb the intake manifold and probably the brake booster.  With the engine off; Remove the vac line from the carb, remove the dist. cap, suck on the vac. line/hose.   Does the vac advance arm move ? I don't think the cam spec's are of any help here and I don't see where the valve covers have any impact at all.  I would suggest you check your vacuum at the intake manifold with a gauge.  Should be at least 14 or better.  Maybe some of the other guys can jump in ?  Brian 

[1969_Mach1 333]

Very odd issue.  I don't think he is chasing a base ignition timing issue or valve sealing issue.  For that cam, 10 to 12 degrees BTDC base ignition timing should be at least a good starting point, and possibly fairly correct.

Does it backfire instantly when the ignition key is turned to the off position?

What type of ignition system does it have, stock points, Pertronix, MSD, etc.?

Make certain your firing order is correct.  A 351W and later model 5.0 is 1-3-7-2-6-5-4-8.  A 289 and early 302 is 1-5-4-2-6-3-7-8.  Since cams physically interchange the cam specs usually indicate the firing order for it.

[Her69fastback 3]

No it not . I’m back to work. I’ll work on it Friday. Thanks for all the help 

[1969_Mach1 333]

I think you are experiencing a very small amount of "engine run on", or the slang tern "dieseling".  The engine tries to keep running after the ignition is shut off due to high temperatures or hot spots in one or more combustion chamber that keeps igniting the fuel.  Sometimes it will "run on" a very small amount, spit out of the carb, and maybe rotate the crankshaft backwards a small amount.  If that is the case, the first thing to try is a higher octane fuel.  Make certain the primary throttle plates are correctly positioned at idle.  Sometimes people arbitrarily turn the idle speed screw to set the speed on a Holley carb and get the primary throttle plates open far too much at idle.  Also, try to cool the operating temps of the motor.

Since the motor is fairly new, is it running hotter than normal?  What temp thermostat is installed?  Are the correct heat range spark plugs installed?

[Her69fastback 3]

The coil is a PerTronix Flame-Thrower lll. The ignition is a PerTronix Ignitot lll solid state system.  They come on a the car when I bought it. 

[69ShelbyGT350H 61]

Pertronix Ignitor III with a FlameThrower coil is a good combination. Never leave your ignition ON with out the engine running with these, even though there have been no reports with the Ignitor III having a problem, the Ignitor and Ignitor II have been known to fail. briefly, turn your ignition on and check the voltage from the pos side of the coil to the ground of the battery. You should get just about battery voltage. If you are getting more than 2 volts less than the battery, you have a resistor in line that needs to be removed/bypassed. All Ignitor units need the full 12V from the battery to work properly. Check the voltage and turn the ignition key switch OFF. 

The Ignitor III Rev-Limiter is preset at 5500 RPM’s. The Rev-Limiter can be user set to a minimum 4000 and a maximum 9000 RPM’s or turned off. All external resistors must be removed to achieve optimum performance from the Ignitor III ignition system.

http://www.pertronix.com/docs/instruction-sheets/71181.pdf

[1969_Mach1 333]

Is it truly backfiring through the carb after the ignition is turned off or is it engine run on?  I've never used a Pertronix ignition.  A lot of people like them and some have had odd issues with them.

[69ShelbyGT350H 61]

On 7/30/2018 at 6:35 PM, 1969_Mach1 said:

Very odd issue.  I don't think he is chasing a base ignition timing issue or valve sealing issue.  For that cam, 10 to 12 degrees BTDC base ignition timing should be at least a good starting point, and possibly fairly correct.

Does it backfire instantly when the ignition key is turned to the off position?

What type of ignition system does it have, stock points, Pertronix, MSD, etc.?

Make certain your firing order is correct.  A 351W and later model 5.0 is 1-3-7-2-6-5-4-8.  A 289 and early 302 is 1-5-4-2-6-3-7-8.  Since cams physically interchange the cam specs usually indicate the firing order for it.

 

Her69fastback, I might have missed which engine you have, 351, 302, 289, etc. If its a 351w, check the valve opening and closing's and see if it might have a 302 cam and firing order in it. When I bought my Comp Cam 280H, they did not make one for the 351w, so I had to use the 302 version and change my firing order to the 302 order.

[Her69fastback 3]

I’m going to pull the plugs Friday to see what they are looking like. Running  stock thermostat I think it’s 195. Running stock mother craft plugs. 

[1969_Mach1 333]

Maybe unrelated to the issue, but a 180 degree thermostat is more suitable.  I and some others use a 160 degree thermostat.  The high flow style, like Stewart brand, or Mr. Gasket work better.  These are actually just like the old Robertshaw brand thermostats.  In some parts of the US you can still find Robertshaw thermostats but I haven't seen them on the West Coast. 

[Her69fastback 3]

The motor is a 302. The only things I have done to it is intake Holley 600 & headers

[Her69fastback 3]

Pulled the plugs. They all look good. I only run 91 octane in the car. The fuel is only around 2 weeks old

The compression is

#1 120

 2 120

 3 118

 4 118

 5 125

 6 123

 7 123

 8 123

 

[barnett468 418]

does your vacuum advance work?

is it connected to manifold or ported vac?

did you check the timing with it connected or disconnected?

what rpm does the engine idle at.

did you check your float/gas level in the carburetor. it should be around 1/16" below the inspection holes with the engine idling or immediately after turning the engine off.

 

[Her69fastback 3]

The vacuum is connected to the carburetor. I check the timing with the vacuum unplugged. I don’t have a tach for the car. 

[barnett468 418]

2 minutes ago, Her69fastback said:

The vacuum is connected to the carburetor. I check the timing with the vacuum unplugged. I don’t have a tach for the car. 

is there any vacuum at the distributor hose port at idle?

did you recheck the timing after you reconnected the distributor hose?

does it still backfire at all the timing settings you mentioned with the distributor vacuum hose disconnected?

what about the carb float level?

does the car sound like it is idling higher than a stock one?

[Her69fastback 3]

Started it. The driver side headers are glowing red & the passenger side cylinder 1& 2 are doing the same thing. I put a set of headers from eBay on there. They are small tube.