carb tuning question

[evilokc]

my truck is running pretty well but I have some questions about tuning the mixture on my holley. to start I know my carb needs larger main jets. I used a vacuum gauge to set the mixture. i plugged into the constant manifold and set the screws (all4) out 1&1/2 turns then adjusted then while watching the vacuum gauge. once it was at the highest reading i took it out and it went lean on me.

1) the highest reading is the correct mixture?

2) if the main jets are to small will i be able to get the mixture right?

3) when you change the jets do you need to reset the mixture?

[geezer#99]

What 4 corner idle carb do you have?
Mixscrews are for idle setting.
At elevation you need to lean your jets, not richen them.

Can you screw any of the mix screws in all the way without the motor quitting?

[mr48chev]

I'd say we first need to know the model number of the carb you are dealing with.

Is this an original out of the box carb or is it an older carb that possibly someone mixed and matched parts on?

Yes the highest vacuum reading you can get is normally what you want that and a steady vacuum reading. Normally secondaries don't have an idle mixture system in them. In the past I have mixed and matched Holley parts to build a carb that didn't exist But I don't know if a primary metering block can be put on the secondary side.

[Black_Sheep]

Some Holley carbs have reverse idle mixture screws, the “list” number off the choke air horn will identify what carb you have. Unless you are running a big cam, the carb jetting should be close right out of the box.

I like to set for peak vacuum then lean it out until it just begins to lose rpms.Reset the curb idle and tweak it again.

[Black_Sheep]

What makes you think it goes lean?

[evilokc]

its a new model quickfuel hotrod series carb. I believe the part number is hr-580-vs. they have 4 mixture screws. one on each corner. it also has a traditional singe idle speed screw. the truck surges and struggles at constant throttle which is a sign that the jets are off. i did the vacuum test to see what size power valve i needed and then pulled the metering block to see what was in it. the factory jets are 66 but it has 64s in it now. holley tech said the carb is good size for the motor but since its on the small side needs bigger jets. they suggested i try 72s. the power valve IS the size they suggested. i guess what I'm asking and I'm sorry if this was answered and went over my head. will the known problem of small jets be more apparent once the mixture is leaned out?

[evilokc]

Originally Posted by geezer#99

What 4 corner idle carb do you have?
Mixscrews are for idle setting.
At elevation you need to lean your jets, not richen them.

Can you screw any of the mix screws in all the way without the motor quitting?

I'm not at elevation anymore. I'm at sea level. when i was adjusting the idle mixture screws all it was trying to die on me when i started screwing them in

[geezer#99]

So what’s the rest of your motor’s specs?
The struggling and surging isn’t always the carb.
Most of the time it’s timing.
What’s your timing curve like?

[Black_Sheep]

The idle circuit supplies fuel as the carb transitions to the main jets, it can effect the air/fuel ratio as engine rpms increase

[mr48chev]

I pulled up the instruction sheet and it has a pretty good section on setting the idle mixture screws. http://documents.holley.com/hr_slaye...ons_rev._1.pdf

There are a lot of tuning points on that carb that may affect the off idle transition including the choice of air bleeds. It seems to be infinitely adjustable in the right hands.

Still you are back to basic carburetor theory and how each circuit of the carb works at what time under what conditions. Meaning is the going lean condition at an off idle point of mild acceleration, is it under hard acceleration or is it at higher road speeds?

[evilokc]

I believe I have the initial issue solved. I swapped jets and its running great. put about 60 miles on it in the last 2 days and its pretty happy now. next question. I found some trash in the main jets when I changed them. its possible that had always been there but I want to add a post pump smaller filter. every website seems to say 10 or 20 micron. the 10 micron guys says that 20 is too large and the 20 micron guys say the 10 wont flo well enough. what say you guys? 10 or 20 micron between pump and carb?