Holley Air/Fuel curve nonliniarity

[Jerry G.]

Just ran my road race 327 on the dyno. ( AKA Black-death resurrected)I use a Holley that came over the counter back in the 60's. It was an old L-88 dual feed with a single power valve on the primary. The problem I'm having is that it a little rich between 3000 and 4500 RPM and goes lean between 4500 and 6000 and then goes rich again up to 7500. I played with the jets and the spread but it's still there. I'm not a carb guy so I don't know how to shape the curve to put a little rich in the middle. Any experts out there that know how these things work? Jerry

[Timothy B.]

Re: Holley Air/Fuel curve nonliniarity

Jerry,

If the carburetor does NOT have a secondary power valve, the secondary jets are approx 12 sizes larger than primary.

Do you think the primary power valve could be opening normal in the 3 to 4500RPM range and closing because of high vacuum (high RPM) causing lean condition. You may want to consider a higher # power valve so it stays open when vacuum increases with RPM. All trial and error but just do one thing at a time.

WOT air fuel ratio is determined by jet size area and power valve channel restriction area (primary) and jet size (secondary). Timing of power enrichment (power valve opening point) is determined by vacuum opening point of P/V.

[Duke W.]

Re: Holley Air/Fuel curve nonliniarity

That's typical. You can never get a carb to provide a constant A/F ratio. There's a lot of wave dynamics going on and they effect both fuel metering and distribution.

As long as the WOT A/F is in the range of 13.5-12.5:1 you are okay. Error on the side of a little too rich at max revs if it goes below 13.5 anywhere else in the normal rev range.

Duke

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

Jerry,

If the carburetor does NOT have a secondary power valve, the secondary jets are approx 12 sizes larger than primary.

Do you think the primary power valve could be opening normal in the 3 to 4500RPM range and closing because of high vacuum (high RPM) causing lean condition. You may want to consider a higher # power valve so it stays open when vacuum increases with RPM. All trial and error but just do one thing at a time.

WOT air fuel ratio is determined by jet size area and power valve channel restriction area (primary) and jet size (secondary). Timing of power enrichment (power valve opening point) is determined by vacuum opening point of P/V.

That's helpful. My primaries are 71 and my secondaries are 81. using your rule of thumb I should go bigger on the secondaries. 83? This would put me richer than I am now at the top end. My power valve is a 6.5. With a racing cam and setup I haven't paid a lot of attention to the power valve since it's usually a WOT condition. Not a lot of vacuum at idle. Maybe I should be using the power valve to better tune the carb. Your thoughts? Jerry

[Timothy B.]

Re: Holley Air/Fuel curve nonliniarity

Jerry,

I would try three jet sizes on the secondary and see if that helps.

Consider that if the vacuum rises above 6.5 at high RPM that power valve will close and cause the WOT air fuel ratio to lean. I realize you don't want power enrichment (power valve open) at idle so you have to be careful here.

Check idle vacuum and select a P/V with opening point 1 to 2" lower. I am no tuning expert but you may be able to run a higher # power valve. Just do one thing at a time because you can go back if you mess up. What's the idle vacuum??

[Edward C.]

Re: Holley Air/Fuel curve nonliniarity

Terry I would be interested in what numbers you are getting when you get it right and what mods you did to the engine. Thanks Ed

[Michael H.]

Re: Holley Air/Fuel curve nonliniarity

Just ran my road race 327 on the dyno. ( AKA Black-death resurrected)I use a Holley that came over the counter back in the 60's.

Jerry..... is that the GM #3965736 over the counter Holley out of the HD book?

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

I'm not sure how to tell. The choke riser was machined off. (Old racer trick that was done before modern race carbs were available.) I'm happy to check whatever you'd like. Just let me know. Jerry

[Michael H.]

Re: Holley Air/Fuel curve nonliniarity

I'm not sure how to tell. The choke riser was machined off. (Old racer trick that was done before modern race carbs were available.) I'm happy to check whatever you'd like. Just let me know. Jerry

If you can give us the metering body numbers, and if they're original, we may be able to ID the unit.

Just curious.....

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

The secondary metering block has the numbers 6501 with a 3 below it. The primary has 8543 with a 3 below it. The main throttle body has GR 3752 B on one edge of the rim and on the other edge 6 09 . If we have some real carb geeks out there we may be able to figure this out. Jerry

[Michael H.]

Re: Holley Air/Fuel curve nonliniarity

The secondary metering block has the numbers 6501 with a 3 below it. The primary has 8543 with a 3 below it. The main throttle body has GR 3752 B on one edge of the rim and on the other edge 6 09 . If we have some real carb geeks out there we may be able to figure this out. Jerry

Ok, thanks Jerry. I'll see what I can find.

[Clem Z.]

Re: Holley Air/Fuel curve nonliniarity

Just ran my road race 327 on the dyno. ( AKA Black-death resurrected)I use a Holley that came over the counter back in the 60's. It was an old L-88 dual feed with a single power valve on the primary. The problem I'm having is that it a little rich between 3000 and 4500 RPM and goes lean between 4500 and 6000 and then goes rich again up to 7500. I played with the jets and the spread but it's still there. I'm not a carb guy so I don't know how to shape the curve to put a little rich in the middle. Any experts out there that know how these things work? Jerry

those L-88 carbs had modified boosters to allow for the removal of the manifold divider so it may not be the best carb for your application.

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

Hi Clem, my dual plain manifold divider has been modified as shown in the Chevy Power service manual. Were they modified for additional boost? What I wish this carb had are adjustable air bleeds. Jerry

[Jerry G.]

Re: Holley Air/Fuel curve nonliniarity

The secondary metering block has the numbers 6501 with a 3 below it. The primary has 8543 with a 3 below it. The main throttle body has GR 3752 B on one edge of the rim and on the other edge 6 09 . If we have some real carb geeks out there we may be able to figure this out. Jerry

An update. I pulled the carb apart and found I have a power valve that is stamped 7 5 E 5. Any wizards know why someone would put in such a high vacumm opening on a pure race motor?

[Dick W.]

Re: Holley Air/Fuel curve nonliniarity

I think that carb might just flow a litte more CFM (830) that the engine can use on a road course. It would probably be OK on an oval course. I believe that somewhere between 750 and 780 CFM is where you should be.