Hi everyone,
Recently I got back to working on the tuning on my carburetor (Holley 390) and found that it was running really rich. I dropped the jet size by 4 (from a 52 to a 48, and a 47 may even be alright).

I originally didn't think it too rich was because of some of the lean hesitations I was having whenever I'd put leaner jets in. But it turns out its been running really rich this whole time and the rich condition was just covering up other issues I was having.

So, now that it's leaned out, it's running really, really well (better than it ever has). Pulls hard, accelerates great, cruises nice, etc. Except for a sharp lean hesitation I'm having as the engine vacuum transitions between 10 - 12hg vacuum. Seems to be the transition from the main circuit to the power valve circuit. The fuel mixture is great all the way through the power band until it hits this one spot when it goes way lean.

I've tried a 6.5, 8.5, 9.5, and a 10.5hg powervalve (they don't make any bigger than that), and it didn't get rid of it.

I contacted Holley about the issue and they said that if richening the mixture back up fixed the problem but caused the engine to go rich, that I should put in the larger jets, and then compensate the extra fuel with a hotter, more potent spark plugs to give it a more complete burn.

Another suggestion I had elsewhere was to start messing with the Power Valve Circuit Restrictions in the body of the carburetor, that they may not be big enough. This was backed up by the fact that the Offenhauser DP installation guide for a 300 (which recommends using a Holley 390) suggests drilling the Power Valve Circuit open to 0.040.


Thoughts on this issue? If I can nail this lean transition, I think I may be, finally, done with perfecting my tuning. I'm out of ideas though, besides the ones mentioned above.

Thanks!

 

:cough: edelbrock :cough:


LOL.

Seriously, are you sure the lean spot is from the power valve, and not from too light of spring in the secondard diaphram pod?

 

Yeah, yeah..... And I have thought about the Edelbrock. But this is the one I currently own, and it's also the closest I've been to actually getting it to run great.


Currently, the secondaries are disconnected and wired shut. This allows me to focus on just the primaries before tackling the whole thing. I've read about it on a couple Holley tuning write-ups. The idea being, get it running as a 2bbl first, then open it up to a 4bbl.

 

Did you get your problem figured out? I think I would try going with a 6.5 power valve and then bump up the main jets 1 or 2 sizes and see what happens.

 

I was starting to, and then did a cam swap that I ran into issues with.

Actually, it turned out to be the accelerator pump squirter. It was weak and wasn't squirting as much as it should have been. A larger one seemed to help quite a bit.

 

What size squirter did you end up with? I ended up with a 37...but I still get a slight hesitation/stumble squeezing the throttle from low engine speeds WHEN the engine/manifold is not completely warmed up. When I use the block heater I can drive off within a few minutes of start-up and have virtually no problem.

 

Right now, with the 48 jets, I upped it to a size 35 squirter. It started to give it a slight bog off the line, so I'm thinking of trying the pink cam which delays the squirt into later into the acceleration. Works for me, since that's where my issue was.
Once it's up and running again, I'll see if this works.


I've heard about block heaters. I may need to invest in one.

 

Up here in Canada they gave them 2 block heaters. I'll never be without one.

 

Nebraska winters provided me my education in block heaters...won't be without them either.

 

Have you looked at the 50cc accell pump that holley makes? Made a difference on my 2 bl

Ron