Misfire @ High RPM. Valve float or...???

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340Duster247

340Duster247

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Ok guys I have a new problem for ya. My new 408 stroker comes apart around 63-6500rpm. It's almost like hitting the rev limiter but more nasty.
The weird thing is it almost seems to come and go.
Before my engine was a 340 with the same top end and camshaft and I never had this issue.
I'm hoping it's not valve float!

Here's my set up
Scat stroker around 12:1
Lunati 60405 cam hyd flat tappit
Eddy rpm heads. Looked over by engine builder but not re worked
Holley 750 dp
Msd billet dist
Msd digital 6 plus

Timing set 34 total.
94 octane gas. Most I can find here in calgary
 
I would say valve float but.......strokers usually develop max torque and HP at 5-5500. Really no reason to rev one that high with a hydraulic cam. tmm
 
If the springs / heads / other valve gear are " run tested " in your previous engine, it doesn't sound like float

How 'bout did anyone check the new build for piston to valve clearance? (Not familiar with the cam)

Changed rocker ratio?
 
did you have it dyno-ed?

it might be worth seeing where your optimal shift point is
and if it turns out that is well below where this racket starts I would not even worry about it
 
I'm sure all the clearances were checked again ....but I'll ask to be sure.
 
Not dynoed yet unfortunately. I hear what your saying. I'm gonna post camshaft details
 
I forgot to mention. I'm running a Holley black fuel pump and when this misfire happens the car definitely lays over
 
sounds like , might be running out of fuel?? no volume!! ever time the pump?? see how long takes to pump a gallon of fuel? what size lines you have? filter.? other engine was smaller, right? you're trying to run a bigger engine with same fuel system from smaller engine. it can't feed the extra cubes!! you have to check every thing. fuel delivery is first place to start at. this is from mopar race.
 
The springs are not questionable are they?
I didn't see any mention of what they are besides maybe came with the heads?
 
I agree always use recommended springs and retainers. Any time you reach max horse power like 5500-5800 it time to grab another gear not to keep reving it just because it will.
 
perfacar said:
sounds like , might be running out of fuel?? no volume!! ever time the pump?? see how long takes to pump a gallon of fuel? what size lines you have? filter.? other engine was smaller, right? you're trying to run a bigger engine with same fuel system from smaller engine. it can't feed the extra cubes!! you have to check every thing. fuel delivery is first place to start at. this is from mopar race.
Click to expand...

X2

Correct springs?

Hydraulic lifters get unstable in that RPM range.
 
possibilities: (as said fuel volume) check psi at the carb inlet with a gauge under load at the offending RPM (mount/plumb a gauge & take it back for a spin. Rotor phasing (if that distance is too great the available voltage cannot keep up with the required voltage to jump that gap distance
 
Do'nt know what ignition you have but i had a couple of coils that where going bad the first indication was a miss at high rpms
 
diymirage said:
did you have it dyno-ed?

it might be worth seeing where your optimal shift point is
and if it turns out that is well below where this racket starts I would not even worry about it
Click to expand...

tekslk said:
I agree always use recommended springs and retainers. Any time you reach max horse power like 5500-5800 it time to grab another gear not to keep reving it just because it will.
Click to expand...

Many smart-phones have an accellerometer chip in them,, so if you download a G-Meter app.. you can find your shift points by taping your cel/g-meter beside your tach,, and note when you start to loose power,, a coupla hundred rpm below that is shift point,, or you could go buy a g-meter...

A stoked 340 may not rev like a stock 340...
 
This may sound stupid...but here goes.....

Load test your battery. You might have a battery with a weak cell or two...or three. At high RPM, might not be enough current to properly fire the plugs.
 
Did you up your jets? 408 will eat more than a 340.... lean on top perhaps. A bigger pump (engine) can pull more air through the carb because it is compressible but the fuel is not.
 
Are the float levels set right? Too low might cause the issue, do you have jet extensions in the secondaries?

There are many cheap and easy things t check......
 
Wow! Lots of good replies here.
Ok I'm using the springs that came with the eddy heads because I believe they can handle up to 600 lift.

I'm gonna look into the fuel pressure thing too. Like I said I run a Holley black with -8 to the regulator. And a return. I'm using a stock tank without a sump and honestly feel the pump is working harder then in should..but anyways I have a fuel pressure gauge under the hood but it sucks cause it's liquid filled and reads wrong when hot.
I would like to install a fuel pressure gauge up under the cowl so I can read it when I'm at WOT.
As far as battery power I'm using a optima red top.

I have a innovate air fuel gauge and I actually jetted down the car a bit because I felt I was running rich. That being said I'm still in the process of tuning at wot.
 
d55dave said:
Are the float levels set right? Too low might cause the issue, do you have jet extensions in the secondaries?

There are many cheap and easy things t check......
Click to expand...

Yes float levels are good but I'll double check again
 
I would think you need double springs with that cam, not the ones that came with eddy heads.
 
perfacar said:
sounds like , might be running out of fuel?? no volume!! ever time the pump?? see how long takes to pump a gallon of fuel? what size lines you have? filter.? other engine was smaller, right? you're trying to run a bigger engine with same fuel system from smaller engine. it can't feed the extra cubes!! you have to check every thing. fuel delivery is first place to start at. this is from mopar race.
Click to expand...

This !
 
Our Edelbrock heads came with double springs. But they are not spectacular in terms of spring rate; just the typical upper 200 to lower 300 pound open pressure for the 500-525 lift range.

What rockers are being used? Just thinking about valvetrain weight.... But a lot of other good possibilities have been mentioned
 
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