Hemi predator Valve float

6.5hp Shinebox

6.5hp Shinebox

Member
#1
#1
Heres my build......

Arc rod
Arc flywheel "32 degree advance"
22mm round slide mikuni
Black mamba cam
Fuel pump
Staged header
2 disc 6 spring Bully clutch


Valve float is the issue, the bike goes fast dont get me wrong but i reach valve float fast as well

*My questions are*

#1 can i put heavier springs in?
#2 will heavier springs make anything Bind up?
#3 will i need diffrent rocker arms & Newer pushrods
#4 whats a bigger cam than the black mamba

Any advice would be grearly appreciated
 
minibikin'

minibikin'

Sho-nuff
#2
#2
I believe Tim recommends 26# springs with the Black Mamba? If you're running stock springs don't even start that thing.
You shouldn't have coil bond with that cam but always check
I like to upgrade the stock pushrods when spinning them at high rpms but you don't have to with 26#.
Why do you need a bigger cam? If the head is untouched, more cam won't help it. I'm not an expert on the hemis, but I believe they become limited after .275" without some work by someone who knows that they're doing.


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6.5hp Shinebox

6.5hp Shinebox

Member
#3
#3
minibikin' said:
I believe Tim recommends 26# springs with the Black Mamba? If you're running stock springs don't even start that thing.
You shouldn't have coil bond with that cam but always check
I like to upgrade the stock pushrods when spinning them at high rpms but you don't have to with 26#.
Why do you need a bigger cam? If the head is untouched, more cam won't help it. I'm not an expert on the hemis, but I believe they become limited after .275" without some work by someone who knows that they're doing.


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Click to expand...

Engines already built and running long time ago, im looking to get more out of this motor..
Im currently running 26lb springs with the mamba which is a good cam..

With the 26lb springs i have valve float its very annoying

If i cant run heavier springs with the black mamba What cams Can i run? & what Head work would i need done?
 
minibikin'

minibikin'

Sho-nuff
#4
#4
So, what rpm are you getting float? How old are the springs? I'm not saying you can't run heavier springs, but your setup is a pretty basic and proven combo unless something is installed wrong or tired out.


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6.5hp Shinebox

6.5hp Shinebox

Member
#5
#5
minibikin' said:
So, what rpm are you getting float? How old are the springs? I'm not saying you can't run heavier springs, but your setup is a pretty basic and proven combo unless something is installed wrong or tired out.


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Click to expand...
Floating at high rpms...like normal

I just want advice What needs to be done to the head
What cam & springs

& will i need to upgrade my valve train if i do so
 
DeadPixel

DeadPixel

Member
#7
#7
Are you sure your getting valve float? Looked up the cam profile and it says it goes to 8K. I cant answer your 4 questions as good as some of the more experienced people on here so I will leave that to them but I do have some advice. I know your probably dead set of upgrading your valve train but is the above your whole build? What piston are you running? You did not mention a piston. Is the rod stock length? I ask because if your running stock compression than you would get more HP from upping your compression than a new cam. Also like mentioned above some head porting and maybe a larger carb are what I would do for more HP. Again though if your running stock compression with that cam I think your largest gains in HP will be from more compression. If you do bump compression and your running regular pump gas you might need to raise the octane or back off your timing a little after your done.
 
6.5hp Shinebox

6.5hp Shinebox

Member
#8
#8
DeadPixel said:
Are you sure your getting valve float? Looked up the cam profile and it says it goes to 8K. I cant answer your 4 questions as good as some of the more experienced people on here so I will leave that to them but I do have some advice. I know your probably dead set of upgrading your valve train but is the above your whole build? What piston are you running? You did not mention a piston. Is the rod stock length? I ask because if your running stock compression than you would get more HP from upping your compression than a new cam. Also like mentioned above some head porting and maybe a larger carb are what I would do for more HP. Again though if your running stock compression with that cam I think your largest gains in HP will be from more compression. If you do bump compression and your running regular pump gas you might need to raise the octane or back off your timing a little after your done.
Click to expand...

Stock Piston-But my piston sits flush as the wisco flattop piston would...stock length Rod.. & yes im sure its valve float cause it happens when im going about 40 ?

& to up my compression i would need to
*port & polish my head*?
*upgrade my stock rod*? Im not sure wheres thats gunna get me tho if my piston sits flush

Id like to start with that before upgrading to a bigger carb

Also i still want to be able to use my pull starter

1 more thing, valve float ......if i throw 28 lb springs or even 32lb springs in with an upgraded valve train chrome molly pushrods the works, that would take care of my issue no?
 
Last edited:
DeadPixel

DeadPixel

Member
#9
#9
The reason I suggest a higher compression piston is that high duration cams lower cylinder pressure and if your compression is only around 7.5:1 like the 196 clone motors are than your compression is way to low for your cam. Your static compression is the calculated compression but your real compression when you turn the motor over is lower and called your dynamic compression. Your going to want your dynamic compression closer to around 8:1 to be on the safe side. You can probably get away with 8.5:1 but 8:1 is safe. The motor in my truck for example if some one asked what the compression was I would tell them about 9.4:1 but the dynamic is about 7.8:1. Longer duration cams usually close the valves a lot later than stock cams and bleed off more pressure and require a compression increase to see the full potential. Low compression will kill power.

Yes the stiffer springs will allow you to spin the motor higher before valve float but I notice the springs recommend stronger rocker arms and probably new pushrod's. I don't know much about the predator but believe your piston is a dished piston if it is stock. If you still have the stock dished piston I would put a flat top in. I know that on the 196 motor my 160 head with the 18cc chamber with a flat top would yield about 10.3:1 which would be perfect for your cam. I might be wrong but I think just the flat top piston would put me at about 9:0.1 or close. You could have the head milled but check piston to spark plug clearance after this. Might have to use indexing washers to move the plug back a little of you mill too far.
 
minibikin'

minibikin'

Sho-nuff
#10
#10
The hemi uses a flat top from the factory.
I'm very doubtful that piston is at zero deck with a stock length connecting rod. Haven't seen one yet. More like .020-.030" below deck.
Valves float at 40mph? That tells us nothing. No tachometer? Then you have no idea what the engine is spinning.
You haven't said how old this combo is. If you've been riding WOT for the last year, get new springs.
Dead pixel is correct. You can run that engine with 10:1 static compression and 91 pump fuel to get more out of it.
Changing head gasket thickness and or milling the head will raise the static compression ratio.
I HIGHLY suggest reading up on the hemi head and it's limits before going to a larger cam. Talk to someone who does the heads such as Barry Young or the like.


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