So i'm back to trying to diagnose why this expensive Sean Murphy built Stage 3 Holley 750 VS won't run right for me.

It has 74/83 jetting, fancy boosters, a billit secondary metering plate... and who knows what else that the "stage 3" model got... I've got it running on my GMPP 454HO crate- but it runs like crap- super super rich at idle. It does a lot of power up top - but idles like garbage, and off idle is garbage.

Just to note, I also have a Holley 770 Street avenger that runs great out of the box- just not quite as much power as the other carb. No idle issues, no driveability issues. Its Jetted 72/75.

I spent so damn much money on the sean murphey carb i refuse to give up on it...

One of the mods they did to it was to drill out the throttle butterflies.

My motor, while it does have 425hp, it is super low compression, 8.5.1.
Could the butterflies being drilled be causing it to run so rich at idle, and the idle & off idle problems? The 770 SA does not have them drilled, and definitely doesn't seem to need them.

I made several calls to SMI last fall trying to diagnose the problems and his responce each time was that it was something wrong with my setup, timing, electical, etc. I finnaly gave up and bought the 770 SA, and it has run great all winter- that tells me its something in his carb- not with the way my car is setup.

What do you guys think?

For kicks i'm now starting to peice it out. I'm going to put the fancy secondary metering block(and fatter jets) on my 770SA to see if that wakes up its top end to match the other carbs... Should be interesting.

The drilled butterflies would not cause the carb to run rich IMHO. They are to allow more air in the carb at idle so you can close your primary throttle blades more so your transition slots are not open too far (they should look like little squares). If the transition slots are open too far that can cause stumbles and hesitations just off idle and at cruise as well. Usually, drilling the throttle blades is done for motors with big cams and low vacuum. A rich condition would either be not enough air or too much fuel. I think your problem is somewhere else.

can you fill us in on the jet sizes ,powervalve sizes,? ANd are the floats set properly?

That 454 HO comes with such a mild camshaft that no butterfly bleeds are reqd to maintain proper throttle angle at curb idle.750 cfm over 454 CI is a no no IMHO,if peak Hp is your goal.The old school 780 was configured for 396 CI.Stab an 850 DP on your stick car or the VC version on your auto trans if you are heavy with a stock stall converter.

jets and powervalves don't affect the idle unless the PV is leaking fuel.

tried a different PV? for diagnostic purposes you could put a PV plug in the front, see if that cleans up the idle. it won't drive like that though.

can you kill the engine with the idle mix screws?

on the front, are the outer air bleeds larger than the inner?

How much of the transfer slot is exposed?

How responsive are the idle mixture screws?

What is your base timing? Is the vac advance hooked to manifold vacuum?

Ben, what kinda vacuum are you pulling? I was getting about 14-15" at around 700 rpms, so you should get close to that. If your not, somethings way off. I'm wondering if he hogged out some internal circuits to help compensate for stout cams. The 454HO doesn't need a pig-rich carb, quite the opposite actually (as you found out). Are your mixture screws responsive?

BTW, put 84's in the rear block of that 770 and that carb will pull as strong as does that SMI jewel.

I once had a "trick" carb from some high price carb "guru". For my 402 I had to restrict the idle fuel orifice with a strand of bent wire. It seems hokey, but it works. I had a picture of the procedure in one of my carb or performance books somewhere.

Dont get too worried about having the holes in the throttle blades(unless they are way too big). If you do turn out to have too large of holes then you can simply use a butane or propane torch and some solder to reduce their diameter. (remove the blades first, of course).

Ben,

Call Sean at SMI and let him know the problems you're having.
I've delt with him, along with my brother and he is the kind of guy that will make it right. My brother was having some minor problems with a q-jet that he modified. He ended up sending it back and taking care of the problem, no charge.