Sudden back firing issue when starting when engine warm

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Jan 13, 2012
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Washington
My Car
1971 Mach1, 351 Cleveland, Ram Air (not factory), C6 Automatic, AM/8 Track, Bright Red.
It was a beautiful day so took the Mustang out for a drive. No issues starting cold but twice to day had issues starting it after it was warm. Both times car didn't start first cranking attempt but upon second try started right up but back fired...then ran great after starting. Engine was warm but not hot and didn't back fire or have any issues when starting it early when cold. Suggestions?

 
Could use a little more info. Did you change anything before this started?
Nope, that's the confusing part, not sure what brought it on but I haven't really been driving it much due to the weather. The last thing I changed were the plugs but I have about a 500 miles on them with no issues. It runs fine after starting. The fact that it doesn't attempt to start until the second cranking then starts right up (with a back fire) is baffling.

 
If it backfired through the carb, it would either be timing or a fuel issue. Possible blown power valve if it's a Holley wih the older style valve, without the backfire protection.

If it was out the exhaust, then that means raw fuel got past the cylinder. If you did nothing different, then I'd say you have a fuel issue.

If it cranks right over and doesn't role slow when you crank it, kind of a similar crank to when a starter solenoid is going out, then it's probably not timing.

Maybe you got some bad fuel and it just didn't lie it. I'd change the fuel filter and add some Chevron super unleaded and maybe some Redline SI-I Injector Cleaner if you can get your hands on a bottle. If you do all that, and it does it again, I'd say you illiminated fuel.

 
Through the carb, or out the exhaust?
Through the exhaust, drivers side both times.



.

If it cranks right over and doesn't role slow when you crank it, kind of a similar crank to when a starter solenoid is going out, then it's probably not timing.

Maybe you got some bad fuel and it just didn't lie it. I'd change the fuel filter and add some Chevron super unleaded and maybe some Redline SI-I Injector Cleaner if you can get your hands on a bottle. If you do all that, and it does it again, I'd say you illiminated fuel.
It cranks rapidly but it's as if it's not firing, when I crack the second time it fires right up with the backfire out the exhaust then runs OK. I've tried a couple more times since my initial post and I still have the issue with it not starting the first time I crank but starts on the second attempt, no backfire this time.

Timing or running to lean? Lots of leads for me to check out.

 
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Out the back is definetly fuel. Timing would be back through the carb.

Sounds like maybe the float is stuck, or maybe something dirty got in your carb and is clogging a port.

What kind of carb is it?

 
Out the back is definetly fuel. Timing would be back through the carb.

Sounds like maybe the float is stuck, or maybe something dirty got in your carb and is clogging a port.

What kind of carb is it?
Edlebrock thunder series, less than 6 months old.

 
Perhaps you have an exhaust valve that isn't sealing or perhaps is sticking somewhere prior to sealing

The fact that it is happening on the same side all the time would lead me to believe it is a valve issue and not a carb issue or timing issue. However, the starting issue is curious.

I might suggest looking into the carb prior to starting and see if the throttle plates are wet.

Disclaimer...take the stogie out of your mouth prior to putting face above carb.

 
The plot thickens...my experience would say you got some bad fuel and your carb ain't liking it. Does it backfire everytime(the second start like you described) or just every now and then?

What kind of fuel pump, electric or manual? If electric, then you got a few things to check. The regulator would be first on my list. if you don't have one, you need one.

If your running a manual pump, back to the carb.

Sound's like it's dumping too much fuel in the first crank and flooding it. Then when you crank it over the second time, it sparks, starts and ignites the unburnt fuel sitting in the exhaust port. BOOOM!

If your float is stuck, this would cause that scenario. You sure that it's only coming out on side??

You could try this. Before cranking, put your foot to the floor so you open the primary butterlfy's all the way.

Hold your foot there and see if it fires the first time you crank it. If it does, you may want to start it like this for a while, put some good gas in it and some additive to clean the ports, change the filter and put some miles on it. If it cleans up, your ok.

If it keeps backfiring, I'd stop driving it and pull the carb off, gets some Berrymans and clean out the port ands valleys, then blow it out with some compressed air.

That would be my approach anyway. Hope you get it licked , good luck Jim!

 
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Don't mean to pile on here, but is your choke working properly? Easy enough to check cold and hot.

 
It was warm yesterday.. Is your edelbrock still having peculation problems?
Yes I thought about that, yesterday was the warmest day of the year for us at 77, I thought I had the peculation issues whipped when I put the heat insulator gasket on it because at that time it stopped the starting issues. But who knows since it does start with no issues when cold.



Don't mean to pile on here, but is your choke working properly? Easy enough to check cold and hot.
I'll check today.



You could try this. Before cranking, put your foot to the floor so you open the primary butterlfy's all the way.

Hold your foot there and see if it fires the first time you crank it. If it does, you may want to start it like this for a while, put some good gas in it and some additive to clean the ports, change the filter and put some miles on it. If it cleans up, your ok.

If it keeps backfiring, I'd stop driving it and pull the carb off, gets some Berrymans and clean out the port ands valleys, then blow it out with some compressed air.

That would be my approach anyway. Hope you get it licked , good luck Jim!
Thanks I'll give that method a try, concerning if it's only back firing out of one side, at least that's what it sounds like and I noticed a puff of smoke from the drivers side exhaust last time if back fired.



Perhaps you have an exhaust valve that isn't sealing or perhaps is sticking somewhere prior to sealing

The fact that it is happening on the same side all the time would lead me to believe it is a valve issue and not a carb issue or timing issue. However, the starting issue is curious.

I might suggest looking into the carb prior to starting and see if the throttle plates are wet.

Disclaimer...take the stogie out of your mouth prior to putting face above carb.
The engine only has about 1,700 miles on a complete rebuild, of course that doesn't mean it isn't a a valve, had to take it back to the builder for two prior issues as a result of intake and then head gasket leaks weeks after the rebuild. The builder quickly repaired both but my confidence level isn't high.

 
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It was warm yesterday.. Is your edelbrock still having peculation problems?
Yes I thought about that, yesterday was the warmest day of the year for us at 77, I thought I had the peculation issues whipped when I put the heat insulator gasket on it because at that time it stopped the starting issues. But who knows since it does start with no issues when cold.



Don't mean to pile on here, but is your choke working properly? Easy enough to check cold and hot.
I'll check today.



You could try this. Before cranking, put your foot to the floor so you open the primary butterlfy's all the way.

Hold your foot there and see if it fires the first time you crank it. If it does, you may want to start it like this for a while, put some good gas in it and some additive to clean the ports, change the filter and put some miles on it. If it cleans up, your ok.

If it keeps backfiring, I'd stop driving it and pull the carb off, gets some Berrymans and clean out the port ands valleys, then blow it out with some compressed air.

That would be my approach anyway. Hope you get it licked , good luck Jim!
Thanks I'll give that method a try, concerning if it's only back firing out of one side, at least that's what it sounds like and I noticed a puff of smoke from the drivers side exhaust last time if back fired.



Perhaps you have an exhaust valve that isn't sealing or perhaps is sticking somewhere prior to sealing

The fact that it is happening on the same side all the time would lead me to believe it is a valve issue and not a carb issue or timing issue. However, the starting issue is curious.

I might suggest looking into the carb prior to starting and see if the throttle plates are wet.

Disclaimer...take the stogie out of your mouth prior to putting face above carb.
The engine only has about 1,700 miles on a complete rebuild, of course that doesn't mean it isn't a a valve, had to take it back to the builder for two prior issues as a result of intake and then head gasket leaks weeks after the rebuild. The builder quickly repaired both but my confidence level isn't high.
From recent experience back fire thru one side of your exhaust tells me you are having an issue with valve train on that side of the engine. I am guessing that you have torque down rocker arms. Remove valve covers and check all rocker arm nuts for proper torque while engine is warm-wear gloves,check at idle speed less mess and put cardboard between valve springs and cylinder head to prevent oily mess! you will probably find 1-2 valves out of adjustment. Good Luck1
 
Perhaps you have an exhaust valve that isn't sealing or perhaps is sticking somewhere prior to sealing

The fact that it is happening on the same side all the time would lead me to believe it is a valve issue and not a carb issue or timing issue. However, the starting issue is curious.

I might suggest looking into the carb prior to starting and see if the throttle plates are wet.

Disclaimer...take the stogie out of your mouth prior to putting face above carb.
1 erratic cylinder will typically not effect the engine from starting. Pull a plug wire and try it without grounding the wire.

My favorite troubleshooting method:

1. Connect your timing light to each plug wire to verify fire

2. Pull plugs to see how they look

3. Do a compression test before re-inserting plugs

These 3 simple tests will tell you volumes!!

 
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1 erratic cylinder will typically not effect the engine from starting.

Can't deny that....but you have to admit, backfire consistently out one side is strange.

2. Pull plugs to see how they look

3. Do a compression test before re-inserting plugs
These are probably good tips to do on any engine rebuild after 1000 miles.

I was even thinking that some of these cameras with eyes on the end of a shaft might be a good idea to look down at your piston when you have the spark plug out. Not sure if they can look up at the valves.

Anyone try this?
 
Lots of good tips to help me narrow this down, I appreciate it guys. Looks like it's going to be a busy this weekend trouble shooting. I did start it a couple of time yesterday, starts right up when cold no issues, after it's warm same thing, have to crank it over several times then on second try starts right up, no back fires this time though. Took it for a short drive ran like a bat our of hell, no issues, to busy at work to do more with it until this weekend.

- Starts fine cold every time

- Runs great once started

- Warm engine requires cranking several times doesn't start, then starts right up on second cranking attempt

- At times back fires out of drivers side exhaust upon warm start up.

I have a long list of recommendations that'll I go down the list doing this weekend to see if I can resolve it without having to leave it with the strangers at a repair shop.

 
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