sn93 small block chevy help

[Buck-Strickland]

A newer carb, with tight throttle shafts, should be fine in the 4-6psi range.

[415ss69]

well i got it all together i bought new valve covers that slope down on the ends work great
once i got it all together just for kicks i pulled the timing slapped on the carb hat made no adjustments to the carb and took it for a spin it pulls hard to 3000 rpm hen chokes hard and falls on its face any suggestions? I know your supposed to increase the secondary jets by 25%? is that my only prob? im running the supplied fuel pump my carb is a 650 mighty demon i put the special floats in it (forgot the name)is my carb too small i also have a 850 speed demon would that fit my application better? thanks guys.....
mike..
ill post new pics in the mourning

[pavement pounde]

Hard to tell without a wideband.
Do you see black smoke from the exhaust ?
If not it looks like a lean mixture.

[Buck-Strickland]

Does it work fine when you floor the gas, then at about 3k rpm starts to cough? It could be leaning out, after the accelerator pump shot finishes.

Could be fuel pressure not rising with boost. You hooked the reference hose for the fuel pump up to the bonnet or blower right? You don't want that hose hook up below the throttle blades. Did you verify the pressure rises with boost? Easy to do when idling, just blow really hard into the hose to the fuel pump and it should flood the engine as the pressure overcomes the floats.

I don't suggest running the motor without jetting up a little first to be on the safe side.

Pavement Pounder is 100% right, get an AFR gauge, especially for a carbureted system.

Also, how much timing did you pull? What's your total and when is it all in by now? Do you know the current timing curve?

Did you drop a heat range or two on the spark plugs? Also, did you re-gap them to 0.030-0.040, could be blowing the spark out. You have RPM heads right, change the RC12YC plugs to RC9YC (Champion part# E344) or RC7YC (2 heat ranges colder; Champion # E340).

Lot of stuff to review, but you should be careful, baby steps.

650 carb is fine, it's not a restriction. I'm running a Holley 650DP same size car, and I run high 11's at Sears Point.

[415ss69]

well i installed my 850 mighty demon and its running like a beast! it seems like it runs better when i lean into the gas instead of stabbing it any suggestions i took a look at the plugs and they look good
im going to switch to some colder plugs tomorrow sorry the pics are crappy also i dont have any type of pressure relif like a b.o.v am i hurting the blower?

[Buck-Strickland]

How much boost are you seeing under full throttle?

[415ss69]

no boost gauge yet rejetted the 650 feels a little better

[Buck-Strickland]

Might be a good idea to rig up/ or borrow a fuel pressure gauge too. Just to make sure pressure is not dropping in the upper RPM. Rule it out. I've seen the referenced fuel pumps work fine to a point, then start dropping pressure and leaning out.

Did you re-gap the plugs when you checked them?

[415ss69]

you were correct i rigged up a fuel pressure gauge and i start dropping pressure at around 4000rpm
are there any similar mechanical type pumps with christmas coming around im on a budget! does the pressure need to rise with the boost or can i just get a stronger pump and regulate it at 7lbs ???

[Buck-Strickland]

The fuel pressure must be at least 4psi higher than the boost. Example 5psi boost plus 4psi fuel pressure = 9psi fuel pressure minimum. If the fuel pressure is not greater than the boost fuel will not flow into the bowls past the needle and seats.

That would cause it to nose over in the upper RPM. Mine did the same.

I swapped out for a mechanical race pump http://racepumps.com/DragRacingIndex.html , but there are some good electric pumps and boost referenced regulators out there, Aeromotive etc. The flow rate at higher pressure is what’s important. Most pumps claim huge flow numbers, but the ratings are at 0psi. I have 12-14psi under boost with 7lbs of boost. Due a quick google search for blow through carbureted fuel systems, most will be over kill, but should give you some options.