Holley rebuild questions

[Gary S.]

In my search for a cause/cure for my slightly rough idle on my 72 LT1, I pulled the carb to see if all was in order. I do have a few questions for the carb gurus. There is evidence of a prior rebuild with new blue gaskets throughout.

1. the primary metering block has a 3.5 power valve. What is the logic in doing that since my search of the archives indicates that a 6.5 may be better? Should the secondary block have a 6.5? Mine has a 5.5.
2. is there a home workshop way to check warpage in the metering blocks or the carb base?
3. I have been to the Holley website looking but where do you find out the size of the original needles and jets?
4. is a vacuum gauge the best method for setting mixture? I have always screwed the needles in until lightly seated, backed them out two full turns and then adjusted the idle using a vacuum gauge for highest vacuum. Should both needles be about the same distance out/in?

Thanks,
Gary

[Clem Z.]

Re: Holley rebuild questions

6239A holley correct? pri jets 68, sec jets 73, pri PV 65, sec pv 85 you need a short straight edge to check the metering bodies,i cut down a 6" machinist scale

[Gary S.]

Re: Holley rebuild questions

Clem, thank you for the numbers. It appears that both PV's were changed but the jets are correct. Where does one find this information? Holley resources?

Can you, or anyone else, elaborate on the theory behind changing PV? How does that help performance or cruising?

Thanks again,
Gary

[Gary S.]

Re: Holley rebuild questions

I just pulled the primary and secondary jets for a numbers check. Primary are 67 and secondary are 73. In Holley terms, does a 67 flow more or less than a 68? What am I losing and/or gaining by keeping this 67 primary in?

Is there a good source to read this information? I have been visiting the Holley site and, while it has a wealth of information on newer carbs, it certainly lacks for data on older carbs as well as logic for changing factory settings.
Gary

[Clem Z.]

Re: Holley rebuild questions

the power valves are used to determine at what engine vacuum setting the fuel curve richen up or leans out. i have used certain PV to lean out the fuel curve at the top end of the RPM range to make more power but that is a racing "trick". there are also "2 stage" PV that are use to increase fuel milage but you never want to use them in performance applications like a corvette. the higher the jet # the richer the jet. a lot of the early 70s carbs were jetted lean to help with emissions and sometimes jetting up richer and leaning down the PVR will help performance. holley sells a book,"holley illustrated parts and specs" that lists all the carb specs.

[Gary S.]

found Holley specs online

I did a Google search and found the MorTec web site lists Holleys, jet sizes, and power valves. Great site for other engine related stuff too.
Gary

[Craig S.]

Re: Holley rebuild questions

Gary - a 67 flows less than a 68, but not by much. The lower number is a smaller hole, if I recall correctly, it may mean 0.067" and 0.068" holes, and the numbering is just realative to the hole size...Craig

[Clem Z.]

some jets

have the same diameter hole but the length of the hole is different for a different flow rate

[Craig S.]

Re: some jets

Now that makes sense too...longer path is more resistance....thx for the tidbit Clem...Craig

[Clem Z.]

i am full of tidbits but mrs clem calls

it something else.