In depth Holley tuning help needed 2

[MWPC]

I usually hang around on the structural forums but I hope no one minds me asking for help here. I am looking for a good book on Holley tuning. I have a kit car with a fairly radical SBC engine (250+ degrees duration at 0.050 lift). It runs fine at the track and so so on the street but I think I can do better if I understood exactly how the air bleeds work.

I have already progressively drilled larger and larger holes in the butterflies to get only 0.040" of the transfer slots exposed. It has very low vacuum at idle so I opened up the idle circuits a little. This made the off idle transfer slots rich so I opened up the idle air bleeds a little and drilled and tapped the transfer slot port for a restrictor.

Now idle is fine, off idle is fine, and a fast cruise where the primaries are working is fine. The problem seems to be that I get a lean condition (I use an O2 sensor and a digital VOM) just before the mains start flowing. Do the main air bleeds help control where the primary circuits start to flow?

Can anyone point me to a good book or paper on the subject? I know I should just go to EFI and I have done that on another vehicle but I want to keep this one "old school". Thanks for the help.

 

[motorsportsdesign]

The concept is simple:
1. Raising the float causes the fuel to come through the booster with less throttle.

2. Reducing the air bleed size causes the fuel to come through the booster with less throttle.



Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[MWPC]

Thanks Jonathan. Do the air bleeds effect the A/F ratio on the primaries? It seemed like I could not get the idle and off idle rich enough when I drilled out the idle air bleeds, but I think that circuit works more on straight vacuum than air flow signal.

 

[MWPC]

P.S. Any good books, articles, or links?

 

[MikeHalloran]

If you can find 'em, pre- smog shop manuals provided helpful descriptions and illustrations of how carburetors are supposed to work.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

At least go here and get the official instruction manual for your carburetor:

http://www.holley.com/TechService/Instructions.asp

Mike Halloran
Pembroke Pines, FL, USA

 

[Peter7307]

MWPC, Try these books:
Any decent bookshop should be able to get them for you when you quote the ISBN. Costs are about $30 / 40 each.

MODIFYING & TUNING HOLLEY CARBURETORS S.A DESIGN
ISBN: 1884089321

HOW TO BUILD & POWER TUNE HOLLEY CARBURETORS HAMMILL VELOCE
ISBN: 1903706610

HOLLEY REBUILDING & MODIFYING
LIGHTHOUSE PHILATELICS
ISBN: 093147227X

HOLLEY CARBURETORS REVISED EDITION EMANUEL LIGHTHOUSE PHILATELICS
ISBN: 1884089283

Cheers , Pete.

 

[Peter7307]

Forgot to mention this site:

http://www.popularhotrodding.com/enginemasters/

which deals in engines where too much horsepower is barely enough!
and these books by David Vizard:


HOW TO REBUILD YOUR SMALL BLOCK CHEVY HP1029 REVIS VIZARD HP BOOKS
ISBN: 1557880298

HOW TO BUILD & MOD.CHEV.SMALL BLOCK V8 CAMSHAFTS VIZARD MBI AP TITLES
ISBN: 0879385952

HOW TO BUILD & MOD.CHEV.SMALL-BLOCK V-8#116406AP VIZARD MBI AP TITLES
ISBN: 0879385472

BUILDING SMALL BLOCK CHEVY ON A BUDGET VIZARD
ISBN: 1884089348

Good luck , Pete.

 

[patprimmer]

I use two books

The SA Design book mentioned above and a Holley catalogue.

For American V8 engines and a single Holley 4 barrel carby, I find that most times when working on tricked up carbies, that going back to the original settings in the Holley catalogue improves performance.

It is normally only necessary to adjust main jets slightly to suit the manifold and cam timing. In extreme cases where the manifold plenum design is bad, and the manifold floor cold you might need more pump shot.


Make sure the carby is correctly sized, make sure the pump squirts at the slightest touch of the throttle, make sure there are NO AIR LEAKS, make sure everything is set to std specs and in good order.

I ran my boat with a 350 CI SBC with basically the LT1 specs, with Pat Primmer ported 292 castings a 250 deg at 0.050 lift and .500 total lift at valve roller cam for years with various carby, manifold and exhaust setups, for ski racing then social skiing.

Originally it had over the deck tube exhausts and a tunnel ram and twin list (from memory) 4224 Holley 660 centre squirter's, as they came, straight from the box except for slightly richer main jets. It Idled at 800 rpm and made power to 7000 rpm.

Then I went to a single 650 double pumper on a Victor Junior. Once again as it came except slightly richer mains.

I then went to cast aluminium water cooled log type exhaust with under the floor pipes and a Holley list 3310#3 780 cfm vacuum secondary carby. Same story, as it came, except slightly richer mains.

I go to a lot of trouble to keep the carby, especially the floor of the float chambers protected from heat soak.

Boats like it a bit richer than cars because the under bonnet temp is quite cool and they mostly operate at sea level.

The real trick is to only change things if you have a problem and only change things when you understand how that change will fix the problem.

As I remember there are 4 air bleeds per metering block.

Of these, 2 supply air to the main circuit emulsion tube wells and 2 supply air to the idle circuit.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MWPC]

Thanks Mike, Pete, and Pat. I will go home and see if my Holley book is the SA Design book. If not I'll order it. The one that I have is older and covers all Holleys. It just touches on performance tuning. That is drilling the holes in the butterflies to get them closed until only 0.040" of the transfer slot is exposed.

As I mentioned above, it seems to run fine everywhere except one lean spot well below where the cam should be operating. The basic problem is: I just had to see if I could make this old 302 SBC with 12+:1 compression run on the street with pump gas. The long duration cam took care of the high compression problems now I am trying to cure the low manifold vacuum problems by modifying a street type carb. Oh the fun of hotrodding.

 

[Peter7307]

Just an extra thought on the low vacuum...you may want to think about using an alternator fitted with a vacuum pump. These are often fitted to diesel vehicles to provide a source of vcacuum to operate the usual brake servo units, ventilation units and so on.
It may be a more simple route than trying to compromise the carb set up.

Cheers , Pete.

 

[bobqzzi04]

There is an article in popular hotrodding

http://popularhotrodding.com/tech/0511phr_carb/

Which talks about the problem of low carb signal and the various types of boosters availble to help. Might be worth a read

 

[MWPC]

Thanks bobqzzi04. I glanced at the article. It looks good. I will read it in depth a little later. I decided to put some of the above books on my Christmas wish list.
Ken