Inliners International Forums General Discussion Hi Performance Jet size for Holley 390 on chevy 250

I know it's going to take some playing with to get just right but I can't even find anything to give me an idea of what till get me close for jet size and power valves on my Holley 390 carb. It's a 1971 Chevy 250 with a Holley 390 on a Clifford intake, headers and full exhaust engine itself is still stock. I would reall appreciate something that will just get me in the park or has worked for other guys.

Thanks

Start with stock jetting for a list 8007 390 holley.list 8007 390 (F)51,(R)plate 134-34 6.5 4160 Hope that is what you have.
Change the power valve to a lower #, like a 4.5.

Yea I would have the check if that is what on it. I have it running and idling really well and does extremely well if you come onto the gas slow but anything sudden and it stumbles and doesn't feel like you're getting full power on the acceleration at all. Just cruising it sounds and seems to be running well and idling well though.

I hope it is not a list 6299.

No it is what you said I changed the jets to 45's on primaries. And then the power valve tried 4.5 and couldn't even get the car to run. Switched up to try an 8.5 over the 6.5 and going to try that tomorrow. It was 9:45 on a Thursday night didn't want to drive my neighbors too nuts.

The power valve doesn't affect starting, idle, or full throttle regardless of which one it is.

That's what I thought it was supposed to be. I think the change in idle is from the new jets. Still can't for the life of me figure out the issue with the 4.5? Has me really confused like I couldn't even get it to run like at all and idling was out of the question.

The only issue that needs fixing really is removing the stumble when you step on the gas. So will the 8.5 help with that over the 6.5 or give me the opposite effect I am after

The power valve number is not a size or diameter, and has no effect on how much fuel is added when the valve is open.
It's the opening vacuum point: a "65" (6.5) is closed at vacuum levels above 6.5 Hg", and open at all levels below it. When you put your foot down slowly, a 85 opens earlier than a 65, etc. The problem is that if you guess wrong, it's open all the time and there goes your gas mileage. Using too low a number means no extra fuel is added until later in the throttle position. With a really small carburetor and low PV number it will never open.
How much fuel is added is determined by the power valve channel restriction located in the body casting. It's a drilled hole, not a jet or an adjustment.

IIRC Clifford suggested 48 jets, yours may just be too lean.

Changing main jets has very little effect on idle mixture. Changing the power valve has none.

On my son's 240 ci inliner we found that the stock 390 jetting was just about perfect. But the accellerator pump circuit was too lean. We had an off-idle stumble. We changed the pump shooter from (I think) #25 to a #37 and it made a world of difference. You may want to change the pump cam to a larger one too. It somewhat depends on the rest of the drivetrain - stick or auto, hi stall converter or stock, etc. Good luck.

X2 with Frenchtown

There is one way to see if the metering block has any issue is to turn the air fuel mixture screws all the way in IF!! the motor does stall From either side There is a blockage.When turned all the way in the motor should die.IF it does not then there is a issue with the metering block as well.

Thanks yes I am beginning to agree I haven't ever messed to much with carbs but it was about perfect before, so probably change it back, maybe go in the middle on jets at a 48 like recomended it seemed a little rich. Other then that off idle stumble it us pretty good though.I will try I new pump shooter and let you know how it comes out.

I run a Clifford intake and Holley 6390, jets 52 all 4 corners.
Not sure if the jetting is perfect but that carb has won several carb shootouts so I havent fiddled with jets yet.
With headers it might like even more.

Mine has a Powerglide 3.08 but launches like a big coil spring with that Holley 390, no bog at all.
Try 52 jets in there (at least) and 10-12 initial timing, see if that wakes it up.

There could actually be a couple of more sinister reasons for your issues:

- warped main body casting
or
-pourous main body

I know I have posted these pics before, but with your problems you need to know.



The two carbs were fly cut. One has a pourous casting. It also did not clean up after a .005 cut. Worse yet the lowest spot is the accellerator pump circuit transfer boss in the middle. Just sayun'...

I was hopeing you would have added Those to this topic.Just to give this guy a little more insite.

Thanks for all the help guys, pulled the accelerator pump, one concerning thing is the manual I had read said it was a Phillips screw yet mine was secured with a flat head? Maybe just nothing thought I would mention it anyways I am going to go in and order the suggested #37. The car really runs well with the 51 primary jets and once I get this down can experiment with the power valves (seems to like the 85?) anyways it had a #28 pump in it and honestly other then that hard stumble when you stomp on the gas there aren't really any issues.

Is the intake heated?
What is your timing set at?

Intake is heated, timing is at about 6 advanced right now

I'd kick the initial up from 6 to 10 but of course really the whole curve matters.


And, at part throttle, make sure the vac advance works, otherwise it will feel soggy there.

Well guys.... The grim reaper may have come for the nova tonight on Halloween. Went and picked up a #35 engine was getting on it's last leg but starting pretty well (it has always not wanted to turn with the started when cold and the starter will just click) well started right up and flooded or something terrible wouldn't run at all. Not sure put the old #28 in and now it won't turn at all.....