question about comp ratio and pinging (big block officials needed)

[k30gambi]

Alright, i have less than two tanks of fuel on the k30 with my new 468 i assembled. It has hyperuetectic pistons that give a 9.5 compression ratio, 049 casting heads with 2.19/1.88 valves, a crane energizer 226/226 degrees duration @.050 lift, .533/.533 lift and i think 108 degrees lobe seperation, an old GM squarebore holley highrise squarebore intake and an edelbrock 750 performer carburetor.
At half throttle or a little more (basically not being into the secondaries) it runs great, it feels like it has as much power as the old mild 454 that was in it. But more foot feed and it almost feels as if two or three spark plugs quit and runs really rough or out of gas, or too much gas. I tried richening the carb up and leaning it way down with little change. I've gone the max in both directions. Timing makes no difference either and yes there is spark all the time.
Before i got it running i filled the tank all the way with ethanol on our farm (89-90octane), I ran that down halfway and filled with non ethanol premium 93 octane and maybe there is a very slight difference but basically no difference. Do I have too high of a compression ratio? I never really heard anything knock or ping before (besides a diesel) so I am at a loss. I was thinking the carb needs to have more cfm but i am too cheap to shell out money on a new carb just to see if that covers the problem. I want to fix this soon because i am missing truck pulls because of this /forums/images/graemlins/1zhelp.gif /forums/images/graemlins/ears.gif

 

[Sandman]

750 CFM should be enough. It wouldnt cut out like that, you would just loose top end if you didnt have enough CFM. 9.5 compression should be OK also.

Does it make a difference what RPM you are at when it does this?

What kind of ignition are you running? How is your vacuum advance hooked up? What kind of oil pressure are you running? Solid lifters?

I am suspecting ignition problem or lifters that are pumping up under high oil pressure (if it happens at a higher rpm).

 

[k30gambi]

It's a hyd flat tappet cam and the oil pressure is at 60 anywhere above idle and it dips to maybe 40 at idle or when in gear but its the same type oil pump and pressure as the old motor (i switched some spring in it for high volume only). The ignition is just the stock hei and the vaccuum advance was hooked to the carb then i put it directly from the intake manifold with no difference. I think before I do anymore pissin around I am going to fix the tank switcher which quit the day i got it running and put some race gas into the currently empty right tank and see what happens. thanks /forums/images/graemlins/1zhelp.gif

 

[k30gambi]

oh and it does this sputter when you initially step on the gas then cleans out at maybe 1200 then if you run through the gears at zero to maybe 1/2 to 2/3 throttle it will run fine but if you get into the secondaries (floor it) it will stumble but its a constant stumble, not a sputter then run. I also do not get that backfire through the carb like Im runnin lean which led me to believe i didnt have enough carb cfm.

 

[Sandman]

Throw a timing light on it and rev the motor and see what the advance does. If you hook to the manifold, you will have full advance at an idle and it will go less as you open the throttle. You need to be hooked to the right port on the carb (no vacuum at an idle, lots when you open it up).

Maybe the secondaries are too rich.

 

[Leadfoot]

I can't help you, but if you figure it out, let me know. I am having the exact same problem with mine and am missing truck pulls too.......

I swapped out fuel pumps (went from mechanical to electric) and bypassed the dual tank selector valve to see if it would help as it felt like I was running out of fuel but it did not. Floor it and it would put you in your seat, then as the RPM's increased and stayed there it would cut out until you backed out of the throttle and come back. Floor it again and it would start to build RPM's and then cut out again until you let off the gas. If I keep it floored, it will rev then cut out, then rev, then cut out, rev, cut out, etc... If I find out what is going on with mine, I will let you know.

FYI, I'm running a 355 SBC with cast 2.02 angle plug heads, Cranes Fireball II cam (not sure of the specs, will have to look those up), Keith Black hyperutechnics, HEI (completely rebuilt as I thought that was part of the problem), and an edelbrock Q-jet carb. Compression is a hair over 9:1.

 

[DieselBurb]

Maybe a vacuum leak somewhere? /forums/images/graemlins/dunno.gif

I've got a 56 Pontiac that cuts out at full throttle until I back out just a hair. The carb is old and I'm pretty sure that it has an internal vacuum leak.

 

[Leadfoot]

I "thought" I checked for leaks in all the vacuum lines, the carb base, and the manifold base (by the heads), but I will check again just to make sure I did not miss anything...

The original poster way want to check that as well.

 

[k30gambi]

Which vaccuum port is the advance supposed to hook to on the carb? On the old motor it was hooked to the right side if you're looking at the front of the carb and it ran fine. This is the one, while the engine is running, has a vaccuum at idle. I had the left one, which has no suck at idle, plugged. I switched the two and the idle went way down but when i drove it seemed to stumble differently almost like the lean conditions explained in the carb manual. Since the carb is leaner than when it was on the old motor, I'll try richening it up and see if I'm on the right track. we'll see

 

[83ZZ502_Jimmy]

Usually a vacuum leak will not cause problems under full throttle application.. There is little to no vacuum under full throttle. Throttle plates are open.

John

 

[Leadfoot]

Did you try fixing the selector valve or running off of the other tank yet?

Also, what happens if you floor it with no load? If it still cuts out at a specific RPM it is most likely ignition related, if it runs fine except under load, it is most likely a carb or fuel delivery problem.