TheGrimSpeed
Well-Known Member
Yeah I have a 10.5 power valve up front, the rear I installed a metering block so I can screw in jets
High RPM Backfire when fully warmed
- Thread starter TheGrimSpeed
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TT5.9mag
Two atmospheres are better than one
Leave it there for now. It’s safe for a magnum chamber. And that’s definitely not causing a backfire.
TT5.9mag
Two atmospheres are better than one
Ok gotcha. You’re close, but definitely stick 80s in the back and try it
SRT_DSTRHOLC
"There is never enough horsepower...."
The timing is too much, should be in the bend, start there. IF the timing is at 34 then you might have a fuel delivery issue causing the lean issue not a timing issue, but the plug says the timing isn't proper
TT5.9mag
Two atmospheres are better than one
Huh? Not true at all. Timing mark in the bend is light for max power/efficiency
TheGrimSpeed
Well-Known Member
If I remember it was around 22 or 21 The 10.5 was the best choice.
TheGrimSpeed
Well-Known Member
So that carb stock would have at least came with a 81 secondary jet size?
SRT_DSTRHOLC
"There is never enough horsepower...."
how is it not true and then good for one setup? do you know what his build is? how would it be not true if we don't know the setup? We are looking for examples that lead to his issue. For a max power and efficent engine, the timing could be better...
I will add that timing is NOT the main issue right now, but it could be better. The issue looks like fuel, that plug looks like it could be practically new or not idling much.
TT5.9mag
Two atmospheres are better than one
Holley lists that sec plate with an .081 fuel orifice. That’s not jet size. A Holley 80 jet has a .089 orifice. In most 4150 style carbs, to keep fuel flow consistent front and back with a power valve in the front, you want “about” a 9-10 jet spread. That 3310 has a 71 main jet stock, so it’s been leaned a little on the primary. An 80 in the back would be a good starting point.
TT5.9mag
Two atmospheres are better than one
I’ve built a lot of magnums. That’s how. I actually currently have almost the exact same engine as the OP in my D100. On top of that I’ve tuned everything from 100hp NA 4 cylinders in roundy round cars to 1500hp turbo LS drag and drive stuff. I have many hours tuning and looking at plugs on engine dynos. This ain’t my first rodeo.
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TheGrimSpeed
Well-Known Member
The carb pretty much was rebuilt by me, floats adjusted added a quick change secondary pod with the yellow spring. Fuel pump is stock mechanical when we changed the fuel line due to rust we went with the bigger 3/8 line.
When the engine warms up, the mixture gets richer, not leaner. When cold, two things happen:
- raw fuel can drop out of suspension, deposit on the walls of the intake, less fuel getting into the chambers, lean mixture. This is precisely why a choke is needed, to richen the mixture & provide a combustible mixture for a cold engine.
- once the engine is heat soaked [ warmed up ], the incoming air gets heated by the surrounding metal & richens the mixture.
This problem sounds electrical, like running out of spark. I would try this. Warm up the engine. Run a jumper wire from the coil +ve terminal to bat +ve terminal & test drive. Drive just long enough to test for the problem, because the coil will get hot.
- raw fuel can drop out of suspension, deposit on the walls of the intake, less fuel getting into the chambers, lean mixture. This is precisely why a choke is needed, to richen the mixture & provide a combustible mixture for a cold engine.
- once the engine is heat soaked [ warmed up ], the incoming air gets heated by the surrounding metal & richens the mixture.
This problem sounds electrical, like running out of spark. I would try this. Warm up the engine. Run a jumper wire from the coil +ve terminal to bat +ve terminal & test drive. Drive just long enough to test for the problem, because the coil will get hot.
SRT_DSTRHOLC
"There is never enough horsepower...."
That explains what you have done, but doesn't answer my questions. Thanks for saying that I was wrong but that I was right because my information was for a "efficient engine". I think it might have helped to clarify what you were exactly responding to, but that's okay. I'll concede to the more experienced guy with the time put in looking at plugs.
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SRT_DSTRHOLC
"There is never enough horsepower...."
OP, is the backfire happening when you're holding the rpm steady or are you modulating it at that or around that rpm? What plugs are in it?
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For God's sake.
MOPAROFFICIAL
If it has tits, it's a liabilititty.
What valve springs....?
TheGrimSpeed
Well-Known Member
I believe we used P4876062 with hughes retainers and locks
Correction valve springs used were ERV901
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mygasser
Well-Known Member
i was waiting for this, my first thought was valve bounce/float. similar sound to the untrained ear but a completely different cause.
neil.
furrystump
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Hyd cam? It could be collapsing or pumping up a lifter at higher rpm. More likely collapsing a lifter. Hot oil, lifter bleeds oil faster. Chased that issue for months myself. Went through fuel system/ignition and valve springs. Finally said screw if and went solid flat tappet. Now it will pull until it throws the rods out.
Sounds like your valves are not closing when hot and everything is expanded. Could be lifter pump, Lighten up on the lifter preload.
TheGrimSpeed
Well-Known Member
Hmm, but if they weren’t closing wouldn’t I get a rough and different idle when warmed up? Like I would see a big change in idle vacuum from cold engine to a fully warmed engine? The motor gets a steady vacuum reading when warm and also pulls more vacuum, when it’s hot. No misfires or anything.
TheGrimSpeed
Well-Known Member
@MOPAROFFICIAL let’s say I have the wrong valve springs and I am getting valve float. Can you point me into the right direction for springs pressure rating?
Reground specs are
218/224@.050 .478 on both intake and exhaust installed 4 degrees advanced on a 106
TheGrimSpeed
Well-Known Member
Thanks for all the info, I will start with the simplest things first, I will swap my ignition coil, and ecu box double check reluctor gap, jet up my secondaries.
And if the problem still presents it’s self when fully warmed up then I’ll pull the heads and swap valve springs.
And if the problem still presents it’s self when fully warmed up then I’ll pull the heads and swap valve springs.
TheGrimSpeed
Well-Known Member
Okay to confirm the valve springs I used, along with the Hughes locks and retainers kit these are part number
ERV901
ERV901
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