View unanswered posts | View active topics It is currently Thu Aug 24, 2017 6:22 am

Reply to topic  [ 5 posts ] 
 Holley Idle Air Bleed Size 
Author Message

Joined: Tue Feb 18, 2014 1:48 pm
Posts: 55
Wanted to ask a question about Holley idle air bleeds. My car is a 67 Corvette, stock 327/300hp engine with it's stock 3810 Holley (585 CFM), this carburetor is generic to the 300 & 350hp engine and they have two completely different sets of valve timing. I have always thought it to be a little rich on my 300hp engine so I decided to lean it some.

I tapped the IFR 6-32 and reduced the size from .031 to .028 and the engine runs very nice, the mixture screws are now out approx 1 1/8 turns from the approx. 5/8 turns before. I then drilled and tapped the IAB's 8-32 and changed the size from .070 to .076 in a attempt to further lean it. The engine motors up fine but if you crowd the throttle like going around a corner in forth gear it stutters badly then picks up. Is the transition circuit just not responsive enough now with the larger IAB.

My question is about the idle air bleeds, I have read I should leave them at .070 or even make them smaller because of a smaller IFR. After the IAB change the mixture screws are still approx 1 1/8 turns out.

I really enjoy this stuff and I am starting to understand better, I was very careful adding the adjustable bleeds, everything done correctly, so going back is no problem but I want to learn...

Thanks for all the responses..

Mon Aug 10, 2015 11:23 am
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
Your reasoning is correct. Sounds to me you need to take an assortment of air bleeds and go cruising. Use whatever size idle air bleed makes it run well. If this is a situation where it recovers from the miss if throttle is held steady after the crowding, then you might study the accelerator pump cam and lever to see if you can get the discharge to have more duration. Sometimes the angle of the lever portion above the cam can be 'adjusted' to accomplish more pump stroke to provide a longer discharge. A smaller pump nozzle which will lengthen the discharge time is OK if it doesn't result in a sag at other throttle conditions.

You say "going around a corner", have you fiddled with the float level to see if it has any influence? How hard are you "going around a corner", gradual bend in the road or drifting? Does it act up if it isn't in a corner?

Mon Aug 10, 2015 12:42 pm

Joined: Tue Feb 18, 2014 1:48 pm
Posts: 55
Thanks for the reply tuner, going around the corner like grandma and it has the same stutter going straight at low speed when the throttle is opened about a third. I think it's going lean at the same time the vacuum advance is retarding so I think more initial timing will help. The distributor is set up stock with that lazy curve, 6* initial 15* vacuum advance connected to full manifold vacuum.

I am just not sure if I am in the ballpark with this IAB, is it uncommon to reduce the IFR and enlarge the IAB. If I motor up normal the engine runs fine and there does not seem to be any surge at 2000 or 2500RPM. I did adjust the accelerator pump to where there is just zero clearance on the pump lever at idle, this way the over ride spring has good tension and it does not raise above when the throttle is pushed hard, pump squirter size is .025 (stock size).

Mon Aug 10, 2015 2:14 pm
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
How far is the secondary open on the idle adjustment screw in the base below the vacuum pod?

I recently had an experience with a "670 Avenger" 600 Holley which I cloned to be essentially the 3810 like yours. The 3810 is essentially a '64 327 350-365 HP R-2818 Chevy, carb but with a secondary jet plate instead of metering block and removable jets. The engine is a 350 Chevy with a cam very much like a 350 HP hyd., Edelbrock Performer intake and headers.

The particular base plate on this carb has some production slop and the T-slots are too long combined with the throttle shaft hole is a little high so the slot is exposed about .060" when the throttle is closed tight in the bore. This makes it not need so much mixture screw opening as it would if the slot were less exposed below the butterfly at idle.

The guy I did it for installed it and when he adjusted the idle mixture he noticed it only wanted about 1/4 turn on the mixture screws so he called me and asked about it. I suggested he open the secondary some so the primary could be closed some and see how that worked. Well, it idled fine but developed a part-throttle stumble that sounds like you describe yours. If you were at a steady speed about 30-45 MPH and crowded the throttle just a little it had a hiccup before it accelerated. I could bore you with the details of the several days we tinkered with pump cams, squirters, idle jets, idle air bleeds, metering blocks, float level, etc, etc, and no joy. Well, no carburetor can do this to me (LOL), so it was game on. No change of primary metering could completely eliminate this flat spot, even making it stupid rich, no joy.

To cut to the chase, I finally tried opening the secondary a little further and then it had a reeeeely big flaaaaat spot. Changing the secondary idle speed adjustment was the first thing that had a significant effect one way or the other and since opening it some made it worse, closing it some seemed appropriate. Well, that made it much better and closing it all the way, but for a tiny preload so the butterflies don't stick in the bore, fixed it, flat spot gone, it finally acted normal so we cruised around with a vacuum gauge and an MTX-L and tuned it, changed MJ, IJ and IAB until it was close to the tuneup it had off the bench, proving once again it's better to be lucky than good.

So ...... try closing the secondary some.

Mon Aug 10, 2015 8:55 pm

Joined: Tue Feb 18, 2014 1:48 pm
Posts: 55
I spent quite a bit of time on this carburetor, the secondary throttle blades are closed to where they just don't stick like maybe 1/8 turn on the stop screw. This 300hp engine idles at 600 with the primary idle speed screw turned in approx 1/2 turn so the factory PCV is providing much of the idle air. The engine makes approx 19" vacuum at that speed, there are no vacuum leaks and the emulsion screws will kill the engine turned in and make it very rich turned out.

FWIW at idle speed after the choke opens the engine will lean surge a little until the cast manifold heats up hot. It runs very nice, the primary throttle blades have a taper in them to just expose the bottom of the transfer slot at this 1/2 turn on the idle speed screw so it's the opposite of the issue you describe above.

Do you think I should go smaller on the IAB and see if the stutter goes away then maybe try one size smaller on the IFR from .028 (current) to .026 to see how it responds.

Tue Aug 11, 2015 6:06 am
Display posts from previous:  Sort by  
Reply to topic   [ 5 posts ] 

Who is online

Users browsing this forum: No registered users and 1 guest

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group
Designed by ST Software for PTF.