dgheinen

Long story (kinda) short, my crate engine I got from year one is has a nice dual plane intake that is very similar in appearance and mold to a Edelbrock Performer Air Gap. The only real difference is the lack of branding, and the receptacles for only a square bore carb. I purchased and installed an adapter plate, but it raises the air cleaner so high that I had to use a 1" filter to be able to fit the quadrajet, and I'm unsure if that will dramatically reduce air flow. I have a recently rebuilt (not yet run) 4778-5 Holley carb, 650 CFM with manual choke (which I plan to convert to electric) but the primaries are jetted with 74s and the secondaries are jetted with 78s. From the extremely limited information that I have about tuning these carbs, if the secondary metering block does not have a power valve (mine only has a p valve in the primary metering block), the secondary jets should be roughly 10 higher than the primaries. As it stands, I feel like 74s might be kinda rich for a 405 hp 420 ft/lb motor. What jets should I start with in the primaries? Thanks in advance!

Damon

Jebbysan

Need to know more about engine and what kind of vacuum it pulls at idle.....
1" filter is going to choke it immensely....especially with a full choke horn.....
When the lid gets close to the bowl vent...that can be troublesome too....
6 to 8 sizes front to rear is the rule......and it depends on whether it has a standard or hi flo power valve in it.


Jebby

MelWff

somebody has already made jet changes since that 4778-5 left the factory. Holley web site shows it came with 69 primary and 78 secondary. I would also install a 6.5 power valve front and rear.

Jebbysan

Mel....does it show a secondary power valve on this model number?
If so....the jetting should be square.

Jebby

MelWff

unfortunately the way Holley builds the columns on the list it only says "Power Valve" and does not distinguish between front and rear unless there is a note which for this carb there is none

http://documents.holley.com/techlibr...al_listing.pdf

Jebbysan

Most likely it does not....usually Marine Holley's are the only ones with two.

Jebby

d555

This above as MelWff states.

There is no secondary power valve in this 2 circuit Idle adjust
700 CFM Holley. Fixed idle/high speed bleeds. 28/31 squirters
I'd start 69/76 though as these carbs are somewhat fat to start with.
Make sure to square the idle transfer slots before install. Turn carb over small slot or allen head stop screw on secondary side. Idle screw on primary side....and yes start with a 6.5 power valve....and change jets in 2 size increments. Holley did not use close tolerance jets in these carbs.

Dave

dgheinen

I appreciate it guys, this is new territory for me. I've been doing tons of reading, but that only gets me so far, especially when most of what I'm reading doesn't directly relate to the engine that I've got. The engine is a Year One, Part # CT350PC1, 9.7:1 CR, 405 hp 420 ft/lbs, produces 12hg @ 800 rpm, red line at 5800, etc. I'm not sure exactly what type of information you guys might need for your suggestions. I think I'm gonna get some 69s or 70s, and maybe keep the 78s that are currently in the secondaries. That way I'm within the 8 size difference between primaries and secondaries. It's been a while since I checked but I do believe the power valve is a 6.5 I'll pull it apart again today and check. Thanks guys!

Jebbysan

A good start...

Jebby

cagotzmann

Since you have no way of measuring AFR you are stuck with trial and error. The old school way would be looking at gas mileage and the color of your spark plugs.

Let the engine warm up and take it for a short drive. Then remove a few of the spark plugs to check the color. Black plugs = rich white plugs = lean at the extreme's. Shoot for light brown color deposits.

To properly tune primary vs secondary you would need to read AFR (air fuel ratio).

Steady state driving light load to tune primary jets sizes, WOT to tune secondary jet sizes.

Quick acceleration changes to tune squirters / accelerator pump cams.

The other thing you can do with a vacuum gauge is tune the idle screws.

Disconnect your vacuum advance. Plug in a vacuum gauge to full vacuum port and turn idle screws equally unit you get MAX Vacuum. Record the number of turns to close. each side should be with-in 1/4 turn. You can also record the number of turns before you start and compare the before and after.

bluedawg

Replace the secondary metering block with the actual jetted metering block and buy a jet kit. By like 3 sets of plugs or an air fuel meter or take it to the strip(my favorite) either jet for tan plugs, 12.8 in second gear under wfo conditions or the best mph. Clean plugs indicates slightly lean, slightly tan plugs indicates optimum to slightly Ritch. 12.8 to 12.6 is were I found the most power when I dyno’d the L48 on the chassis dyno. Ultimately the mile per hour on the strip is the most fun and real world will get you to what your engine wants, first play with the timing, then run the jets until the mph starts to back off, run the timing again and the the jets again. If your using the drag strip make two to four jet sizes the change until the mph backs off, the narrow it in a size at a whack.