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

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Thanks a lot guys. Weather has been poor here te last couple days so nothing new as far as testing.
I was gonna re do the heads during the build but ran tight on money. Possible this winter. So unfortunately there stock style rockers. I'm gonna try and figure the fuel first since that's easiest and Maybee back off the timing. I don't notice any pre ignition tho at 34. But it does hard start when hot. I have a start retard for the ease.
 
Put a good fuel pressure gauge on it that can be seen from the drivers seat and watch it when it happens! No sense in doing anything else until this is done!
 
340Duster247 said:
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.
Click to expand...

Lift is one thing, spring pressures are another. You may want to revisit that.

Use only the valve springs that will give the correct spring pressure with the valve both on the seat and at maximum lift.

I would check with Edelbrock and see what the spring rates are.

Fuel could be the issue also, check that first, cheaper and easier.
 
Once you find out what the springs are rated for then you can compare those rates to what the cam manufacturer recommends.

The last thing you want is piston to valve contact, it's ugly.

I'm going through this at the very moment, lol. Been on the phone with the cam guy and with another guy that's fixing me up with the parts.
 
Edelbrock advertises the lift on their RPM Performer heads as good up to around .575" lift, not .600". You can find all the spring specs on their site. But being able to accommodate that amount of lift does not mean things are going to be always under control up that lift.

But checking the fuel pressure as others have pointed out is the best starting point. Just be careful with a fuel pressure gauge plumbed in so that you can read it when driving.
 
My vote is weak valve springs.

I recently had to
pull the stroker , due to a felpro head gasket and a torque converter problem.

Decided to put a street 340 in with the spare 4200 converter> temporary
until i get the other converter back
stock spring> Edelbrock rpm heads

it's a new built engine, so thought it would be a good time to get some runs on it
made 3 passes, took it home> popping and miss firing above 5700

thought it was a ignition problem.

cosmetic head gaskets arrived> tues night
so swapped engines

using same carb, ignition, etc
ran last night>fri
ran good, no popping etc
1.47 60ft
6.81

017_zpsydwfib1x.jpg


when I built the street engine
I knew the springs were only 120 psi, on seat
and thought they would be ok
due to using a flat tappet cam
and not needing a lot of pressure, on the street cruiser

but looks like i will be increasing that pressure just a little
( single spring/damper)
 
My new 408 stroker comes apart around 63-6500rpm
Click to expand...

My question is does it do this in every gear or only at the top of High Gear?

If it only at the top of High Gear then its most likely ign or fuel. I found mine in my ign system but not were you might even look. Cheap/bad spark plug Wires. do your self a favor, and get your volt/oms meter out and compare reading. cheap and simple.

Fuel pressure gauge were you can read it, can tell you if fuel is the road you need to take.
 
Just exploring possibilities....so FWIW here. This topic may coincide with something I have been looking at in regards to the Lunati Voodoo cams versus others. If you look at the Lunati Voodoo cams specs versus, say, the equivalent Crane flat tappet hydraulic, you'll see that that Lunati Voodoo's (and the Comp XE's too, I think) have a steeper opening ramp. It is hard to make exact comparisons, (because Lunati measures advertised duration at .006" lift and Crane measures these particular profiles' duration at .004"), but there is indeed a steeper opening ramp. This puts more dynamic force on the hydraulic lifters and can result in the lifters partially collapsing at higher revs. This suddenly lowers lift and duration when it happens.

You can do searches for this and find plenty of experiences described. The steeper rate cam lobes can be accommodated with different lifters; not all lifters are created equal. I read one where it was determined that the check ball was gaining so much inertia that it floating off of the seat.....

Did you change anything in the valvetrain or oiling system when going from the 340 to the stroker? The RPM range ratings given by any manufacturer assume certain valvetrain parameters. Is the oil pressure any lower at elevated RPM's with the stroker? Different oil?
 
nm9stheham said:
Just exploring possibilities....so FWIW here. This topic may coincide with something I have been looking at in regards to the Lunati Voodoo cams versus others. If you look at the Lunati Voodoo cams specs versus, say, the equivalent Crane flat tappet hydraulic, you'll see that that Lunati Voodoo's (and the Comp XE's too, I think) have a steeper opening ramp. It is hard to make exact comparisons, (because Lunati measures advertised duration at .006" lift and Crane measures these particular profiles' duration at .004"), but there is indeed a steeper opening ramp. This puts more dynamic force on the hydraulic lifters and can result in the lifters partially collapsing at higher revs. This suddenly lowers lift and duration when it happens.

You can do searches for this and find plenty of experiences described. The steeper rate cam lobes can be accommodated with different lifters; not all lifters are created equal. I read one where it was determined that the check ball was gaining so much inertia that it floating off of the seat.....

Did you change anything in the valvetrain or oiling system when going from the 340 to the stroker? The RPM range ratings given by any manufacturer assume certain valvetrain parameters. Is the oil pressure any lower at elevated RPM's with the stroker? Different oil?
Click to expand...

Before my engine was a 340 with the same top end and camshaft and I never had this issue.
Click to expand...

this^^^^^^^ is why i don't think it is the cam, or valve springs.
 
Re reading your first post.........

You have a 12 esh to 1 compression(My scat crank, icon piston and 62cc heads put me in the 12.7 range) running on 94 octane and you live at about 3400 esh foot elevation right?

ARE YOU SURE YOUR NOT DETONATING?????? Pull a plug and make sure! and or add a bunch of octane additive and see if it goes away.....................
 
Great stuff guys and your right cudafever mine is scat crank and hypo flats that came from scat, with 62cc (12.1:1) I've checked the plugs and have seen no evidence of denoation, but could be a possibility tho. Unfortunetly 94 is the highest here unless I run race fuel. I'll check plugs again......and try adding oct. Booster.

As far as this happening threw the gears. This is from 1st to 2nd. Haven't had too much play time to be hitting 2nd 3rd at Hightower rpm.

The weird thing is this problem seems to come and go..when I first noticed it we thought it was because my temp was below 180 and I was getting on the car a little to early (temp wise). Around 185ish I never had any misfire problems.....last week tho on my way home from a cruise the car was heated up plenty, and the misfire cam back....((no changes to the engine) I still haven't had the car out since i started this thread and will try to get some updates to everyone soon
I appritate all the help!!!!
 
eddy springs are good to .570 ish lift....one thing to check is the installed spring height....eddy says the springs should be at 1.800 installed height for the correct pressure...

last set of stock eddy heads with stock springs i check were around 1.86,,,,,
 
Cudafever said:
Re reading your first post.........

You have a 12 esh to 1 compression(My scat crank, icon piston and 62cc heads put me in the 12.7 range) running on 94 octane and you live at about 3400 esh foot elevation right?

ARE YOU SURE YOUR NOT DETONATING?????? Pull a plug and make sure! and or add a bunch of octane additive and see if it goes away.....................
Click to expand...

Just curious what plugs you were running. I'm running auto lite 3923 and feel they may be a little hot still for my set up
 
nm9stheham said:
Just exploring possibilities....so FWIW here. This topic may coincide with something I have been looking at in regards to the Lunati Voodoo cams versus others. If you look at the Lunati Voodoo cams specs versus, say, the equivalent Crane flat tappet hydraulic, you'll see that that Lunati Voodoo's (and the Comp XE's too, I think) have a steeper opening ramp. It is hard to make exact comparisons, (because Lunati measures advertised duration at .006" lift and Crane measures these particular profiles' duration at .004"), but there is indeed a steeper opening ramp. This puts more dynamic force on the hydraulic lifters and can result in the lifters partially collapsing at higher revs. This suddenly lowers lift and duration when it happens.

You can do searches for this and find plenty of experiences described. The steeper rate cam lobes can be accommodated with different lifters; not all lifters are created equal. I read one where it was determined that the check ball was gaining so much inertia that it floating off of the seat.....

Did you change anything in the valvetrain or oiling system when going from the 340 to the stroker? The RPM range ratings given by any manufacturer assume certain valvetrain parameters. Is the oil pressure any lower at elevated RPM's with the stroker? Different oil?
Click to expand...

Thanks. Unfortunetly I'm not the most knowledgeable when it comes to camshaft/valvetrain extreme specs. I know the basics about duration and lift ect. But not so much about valve spring pressures and heights and so on. That what I got my engine builder to keep everything in line. And they highly reputable in my city.

Nothing on the top end changed. Oil pressure is high threw out rpm.
 
Look a little on the rich side. Hard to see any detonation. May have seen a couple specs on some plugs ditches back can't be 100% sure it's detonation
 

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