[mustangdug]

I need help figuring out which plug to use. Currently im using NGK R5671a-8 gapped at 32. The enigne is 30 over, 8-1 comp mild blower cam, sorry dont have the specs. BT Alum heads w/ chvy exhust ports no porting, fresh 6-71 street blower underdriven making 5-7 psi. Carbs are Holley 660's w/ no PV 74 primary(had 76's). Plug are soot fouled now. Mallory unilite and flame thrower coil, no MSD box. It pops when cold or under light load in the pipe and surges going down the freeway. Fuel milage is about 5-6(terrible). Any sugestions would greatly apreciated!

 

[res0rli9]

nice hot rod you have there

 

[DaveMcLain]

Probably what is happening is that the plugs are getting black and the engine is getting loaded up running around at part throttle without a power valve to lean the mixture. What will work best on a blower setup is a boost referenced power valve where you have a line that runs from under the blower to the carburetors' power valve chamber. What this does is give you the power mixture whenever the manifold vacuum drops and you're under boost and it keeps the power valve from closing at high rpm when the blower can pull so hard on the carburetors that it can pull enough depression under them that it'll close the power valve.

Drop the plug gap down to .025 or .030 with a clean plug and see if that helps too. It's very easy for a supercharged engine to build enough cylinder pressure that the plugs will not fire under load.

 

[mustangdug]

Drop the plug gap down to .025 or .030 with a clean plug and see if that helps too. It's very easy for a supercharged engine to build enough cylinder pressure that the plugs will not fire under load.

That was my next move on the plugs. Im gonna call Holley and ask them about running the 660's. I guess if I have to I'll get some new carbs.

 

[Frank Merkl]

on 660 center squiters the idle curcuit needes to be leaned out with a piece of .015" wire
Frank

 

[mustangdug]

on 660 center squiters the idle curcuit needes to be leaned out with a piece of .015" wire
Frank

Could you elaborate a bit?

I called holley and the guy said i have 2 conditions. rich at idle and lean at cruise. New plugs might help but if not then need to change to a blower card and he recommended the 700.

 

[DaveMcLain]

Take a piece of wire that's about .015. Usually if you take a piece of stranded electrical wire, strip it and use one strand it'll be about .013-.015 diameter. Take a piece of the wire and place it in the idle feed restrictions in the metering block, bend it so that it'll stay in there when the carburetor is reassembled. This will lean the idle and transition portion of the carburetor.

I think I would probably use a couple of 750 double pumpers and do a boost referenced power valve. You could also jet them up a few sizes as a place to start with the blower.