High RPM Backfire when fully warmed

[TheGrimSpeed]

Hello, I have been having this issue on my magnum engine! When it’s cold it will rev up to 6k-6500 RPM with no issues. But when she’s warmed up really good she will backfire around 5500RPMS and that’s about it. I have a stock Mopar electronic ignition system, a rpm air gap, a Holley 3310 750 with 70 primaries and 76 secondary’s. I have 35 degrees total no VCAN attached. Camshaft is 224@.50 regrind.

I’m thinking when the car warms up my carburetor mixture leans out and is causing this issue at the top end. Does my jet combo sound okay?

Or my ignition system can’t hang?

What do you guys think!

 

[remytherat]

@junkyardhero and I both had this problem on two separate occasions with overheating coils, both edelbrock.

 

[SRT_DSTRHOLC]

Well a backfire can happen because a few different things. I would recommend checking the spark plugs first and getting an idea of what the overall tune looks like. You have to also take into account how you drive and how you have been driving the car. You could have some carbon build up temporarily causing an issue. You haven't even mentioned if this is under load or if you are in the car in park operating the throttle with your foot or what.


Could you provide a little more information?

 

[RustyRatRod]

A small exhaust leak can produce this result.

 

[SRT_DSTRHOLC]

A small exhaust leak can produce this result.

in more ways than one lol

 

[TheGrimSpeed]

@junkyardhero and I both had this problem on two separate occasions with overheating coils, both edelbrock.

I actually have a MSD blaster round coil, with a msd ballast resistor supposedly helps it get a better spark. What brand coil did you switch to?

 

[TheGrimSpeed]

Well a backfire can happen because a few different things. I would recommend checking the spark plugs first and getting an idea of what the overall tune looks like. You have to also take into account how you drive and how you have been driving the car. You could have some carbon build up temporarily causing an issue. You haven't even mentioned if this is under load or if you are in the car in park operating the throttle with your foot or what.


Could you provide a little more information?

It would be under load with me in the car. I’ll pull spark lugs and report back.

 

[TheGrimSpeed]

@junkyardhero and I both had this problem on two separate occasions with overheating coils, both edelbrock.

Also, would taking a voltage reading from the coil cold and when it’s hot help me know if it’s v overheating?

 

[TheGrimSpeed]

A small exhaust leak can produce this result.

Would the exhaust leak cause a backfire and rev limit me to 5500rpms? When she’s cold it’s fine only when fully warmed up.

 

[RustyRatRod]

Would the exhaust leak cause a backfire and rev limit me to 5500rpms? When she’s cold it’s fine only when fully warmed up.

Certainly. When it's cold, the mixture is overly rich. As it warms up, the mixture tends to lean out. Add in more air from an exhaust leak and it's possible. That's why old log trucks with neglected leaking exhaust pop and backfire a lot.

 

[TT5.9mag]

76s in the secondary are small for a 750 carb on 360 inches. Go up 4 (80) sizes in the back and retest.

 

[TT5.9mag]

Certainly. When it's cold, the mixture is overly rich. As it warms up, the mixture tends to lean out. Add in more air from an exhaust leak and it's possible. That's why old log trucks with neglected leaking exhaust pop and backfire a lot.

Exhaust leak shouldn’t limit the rpm. Fuel or ignition will.

 

[TT5.9mag]

@TheGrimSpeed how is the reluctor gap in your distributor?

 

[RustyRatRod]

Exhaust leak shouldn’t limit the rpm. Fuel or ignition will.

I somehow missed the "limit" part. In that case, something is causing it to lean out at higher RPM it sounds like.

 

[TT5.9mag]

also post the list number on your Holley 3310 please.

 

[TheGrimSpeed]

The reluctor was set to .008 with a brass feeler gauge
Here’s the part number Holley 3310-4 1952

 

[TheGrimSpeed]

Plugs I know I’m running rich at start up I have no choke set up, there’s some oil from a valve cover leak I had

 

[TheGrimSpeed]

76s in the secondary are small for a 750 carb on 360 inches. Go up 4 (80) sizes in the back and retest.

You think I should stick with the 70s in the front? Just go up in the rear?

 

[mbaird]

Plugs I know I’m running rich at start up I have no choke set up, there’s some oil from a valve cover leak I had

View attachment 1716455289

You need more timing . The band is almost at the end of the strap .

 

[mbaird]

And it looks lean .

 

[TheGrimSpeed]

You need more timing . The band is almost at the end of the strap .

Should I add more initial?

 

[TT5.9mag]

You think I should stick with the 70s in the front? Just go up in the rear?

Leave the 70s in front for now. Do you have a power valve in the front?

Plugs I know I’m running rich at start up I have no choke set up, there’s some oil from a valve cover leak I had

View attachment 1716455289

That plug looks better than most plugs I see. Total timing is close, maybe a couple short, that specific plug is a heat range (maybe 2) too cold. To judge fuel you need to cut one open. Light tan on the porcelain is nice.

 

[TheGrimSpeed]

The mechanical is at 20 and initial I just checked is at 14 for correction I am at 34 degrees total

 

[TheGrimSpeed]

Leave the 70s in front for now. Do you have a power valve in the front?


That plug looks better than most plugs I see. Total timing is close, maybe a couple short, that specific plug is a heat range (maybe 2) too cold. To judge fuel you need to cut one open. Light tan on the porcelain is nice.

Okay might have to check that out tomorrow and cut one open. Any specific plug part number you recommend?

 

[TT5.9mag]

The reluctor was set to .008 with a brass feeler gauge
Here’s the part number Holley 3310-4 1952

That carb should have a metering plate (134-21) on the secondary side with a .081 hole. Did you put the metering block kit in it with jets?