C3...Holley carb question

[bruce11495]

I'm having some problems with my Holley running rich, don't they all!! What I wanted to know was; Does the power valve only come into affect on the cruise mode? If the carb has 2 power valves, wouldn't it be more efficent if the valves were staggered,ie primary 65 secondary 55, instead of them both being the same number/
Thanks in advance.

[Craig S.]

Re: C3...Holley carb question

Bruce - you are mixing up power valves and jets. The jets are sized and reflect the hole size to control the fuel flow to the primary and secondary venturis on the carburetor. The power valve is used for enrichment (if I recall correctly, if not, someone will correct me) on the primary side only. Holleys are known for blown power valves due to backfiring in the carbueretor (usually due to a timing problem in the engine). This can cause excessive richness in the primary circuit of the carburetor and may or may not be your problem. Usually jet changes should only be done when changing other engine characteristics such as camshaft profiles, pistons, heads, etc etc....and are NOT necessary for other reasons...Craig

[bruce11495]

Re: C3...Holley carb question

The carb has "blow out" protection in the power valve area.This kit is available from Holley and the aftermarket. The PVs were not blown out when I checked them. This Holley has PVs in the prim and sec. areas. Its an original GM LT1 carb. So if the PV "is used for enrichment on the primary side only", why the PV on the sec. side?

[Clem Z.]

Re: C3...Holley carb question

same reason. post the "LIST" number on the choke housing and i will post what internals that carb should have. some carbs have 1 PV and some have 2 depends on the application.

[Craig S.]

Re: C3...Holley carb question

Thx Clem for the clarification, I hadn't seen one with the 2nd PV on the secondary side, live and learn...Craig

[John H.]

Re: C3...Holley carb question

Craig -

My (stock/original) '69 Z/28 has the original Holley List #4053, and it has both primary and secondary power valves; very few production Holleys had two PV's, but this is one of them.

[Craig S.]

Re: C3...Holley carb question- thx John! *NM*

[Jeremy D.]

Re: C3...Holley carb question

1970 LT-1 4555 list Holleys have both primary and secondary power valves in the metering blocks, #125-85. I asked Holley basically the same question as you did, and I never got a straight answer....seems these carbs are kinda rare and not many people have tweaked them.
I dropped 2 sizes on my primary jets (from #70 to #68)and my car ran a lot stronger off idle. I've also been told that with a 780 carb on a 350, you can lean them out pretty far without any real danger, as long as everything else is OK. Make sure the gaskets between the metering blocks and your float bowls are PERFECT, and that there are donut gaskets around the sight plugs.

[bruce11495]

Re: C3...Holley carb question

I had the carb OH and recoloured a couple of years back and it ran good. I had just rebuilt the engine and was checking plugs and they were showing a rich condition. I took the carb apart and checked the primary jets and found them to be 76s when they should have been 72s. I'll check the secondaries today. I'm betting he put the jets in the wrong end.
As far as leaning it out, I found you can drop down to the low 60s in the primary end with no noticeable difference. The carbs ran rich from the factory in all those years, don't ask me why. Maybe just a little "preventative maintenance" for warranty on pistons
Thanks.

[Clem Z.]

carbs are jetted for "sea level" and

you can reduce the "area" of a jet 2% for every 1500 ft you are above sea level.