I got a problem when i rev my engine past 6000RPM it start to make kind of intake backfire or missfire in all gear...The A/R meter still reading ok.Valve float with CC 986 spring on 0.509in and 0.512ex of lift bether not...



I got a 355 10.3:1 cc - vortec heads with comp cams 986-16 double spring & shave valve guide-Manley pro flo valves- comp cams XE 268 hyd. flat tappet cam-comp cams full roller 1.6 self align rocker-RPM air gap intake-holley 4150HP 650cfm with 71&73 jets (plugs and A/F guage read ok)-MSD 6AL ignition box set at 7000RPM (and brand new MSD wires-3/8 fuel line -edelbrock victor fuel pump-holley regulator pn:12-803-timing 29deg at 3500Rpm

What could it be

 

The stock HEI ignition can't fire the plugs at that high of an RPM. You should really upgrade it if you are going to be spinning that fast.

 

yes upgrade the ignition for sure ...and be carefull that you arent floating any valves if your running it up to 6000rpm all the time like that

 

The only stock HEI parts is the ditributor and the pickup coil...The coil is a ACCEL-the stock 4 pin module replace by a MSD 6AL.

 

damn, you are actually using the aftermarket weights?

 

Yes they came from a MOROSO kit.

 

What valve springs are you using?

 

Comp cams 986-16
Seat Load: 132# @ 1.750''
Open Load: 280# @ 1.250''
Coil Bind: 1.150''

 

Any idea?