Chuck Harmon

I have a factory spec 427 L88. Chevy's Power Book recommends #80 and #76 primaries, and#76 and #78 secondaries as a start for racing. I'm changed to #77 and #73 jets on my primaries, #75 and #77 on the secondaries. It helped reduce the eye burn at idle and still pull hard on acceleration. Drivability was still not great, nor was throttle response crisp below 2500. When I changed the primary jets to #72 and #68, drivability greatly improved, but it is now too lean for power. My vacuum is about 5.5 - 6.0 hg at 1200rpm so I am using a standard 35 power valve.

My question is: Will using a high flow power valve (part #125-135) allow me to use leaner jets for cruising and still have a rich condition for power. Is there a table that shows what this higher flow valve flows relative to jet sizes? i.e. Standard PV with #77 jets = High Flow PV with #70 jets

Should I use high flow pv's on primary and secondaries with similar jet changes or just the primaries?

If more details on my car's configuration is needed, please click on my website below for details.

Your help is greatly appreciated!

Chuck

clem zahrobsky

the I.F.R neds to be reduced in area,try .015 wire in the I.F.R will lean out the idle and make sure you not have the I.T.S open to far. if you do not have a L-88 carb with the fuel distribution tabs on the boost venturies you are way off on your jetting. do you have the intake with the cut out divider? also what is the list # on your carb?

mountainmotor

First thing I saw was you have a Non stock Holley 950 and you are trying to tune to the original specs.Post the stock jets that came with the carb and I can help from there but you are going to have to remain consistant with the fuel you run with that compression.For optimum performance you will need 38 degrees of total timing for that motor and as far as that eye burning thing goes it comes with the territory of an engine like that.Leaning the primaries and richening the secondaries to make up for it does not promote even distribution with that type manifold.Will also help the motor to detonate.
Edited to add are you certain the cam is L88 spec?Post you,re cam card please


[Modified by mountainmotor, 9:21 AM 12/15/2001]

gkull

I looked for, but couldn't my Holley power book to verify what your doing. They actually tell you to run different jets side to side?

950 cfm is a big carb. The jets shout not have any effect on idle A/F ratio. It's called the idle cuircut for a reason. The holley book says things like installing a .005 wire to restrict idle fuel flow and drill holes in the primay throttle blades to work as air bleads to lean out the idle. I drilled .125 holes and it brought the idle vacuum up when I dialed in the four corner adjustments. The easiest way to tell if a carb is way to rich at idle is too pull a vacuum line. If your idle speeds up and smooths out. It's time to drill the blades.

Your low vacuum might be bouncing even a 3.5 power open at idle. Then it's low number might be making it ineffective once you have the rpm up. I have driven with my vacuum gauge taped to the window when I was trying to figure out the best power valve. I have not used a high flow PV.

This is my opinion. I would run both sides with the same jets. That way your not getting fuel dist. problems between cylinders. All out race cars run all the same jets because they are only interested in 3/4 - WOT A/F ratio

In yours and my case being street driven. I would put 71 or 72's in the primary Your 72 and 68 average about 70. Then in the rear go with 81 or 82's. That will give you a lean crisp cruise and plenty rich at WOT

Chuck Harmon

Thank you very much for your replies.

1) The engine is over the counter L88 short block with L88 cam
2) The carb does not have the distribution tabs that came with the original L88 carb. I have a used 4296 that I bought from Chevyland in Texas many years ago. It doesn't have the tabs either. I'm sure someone ground them off or changed the booster venturis long ago.
3) Chevy Power Books suggest the stagger jetting that I have done for manifolds without center divider as mine is (#3933198) but for whatever reason, I have never seen one word in their guides suggesting use of tabbbed booster venturis (which I know came with the L88's)
I am also using a 3/4" open spacer under the carb, a 1" was too close a fit.
4) the 950 Pro-Shop Holley is a smoothed 850 with choke tower removed. It has 4 corner idle screw adjustments. Carb came with #78's all 'round. I never ran it that way due to the need of stagger jetting for the L88 manifold. But I don't believe that I am that far off with the jetting I had been running.
5) I am running 38 degrees total advance. Car runs best using lightest silver spring bringing all advance in by 2000 rpm. Mechanical advance only.
6) All IFR screws set at about 3/4 turn. They very much affect idle speed and quality.

Yes, I know that I will have to live with some burned eyes with the setup I've got, but I am not content to simply give up regarding a sharper state of tune either. The factory 454 LS6 ran #70's in primaries and #80's in secondaries. This is what prompted me to consider running a similar combination for the primaries in the L88. For part throttle use, the #72,#68 really works well. But as you guys pointed out, I am not comfortable using such a big difference between primaries and secondaries with this manifold. That is why I asked about the single stage HIGH FLOW power valves Holley has. The are supposed to flow more fuel when open than the standard power valves rated at the same vacuum. Have you used or are you familiar with this power valve? It would seem that I can have a smooth running part throttle jet setting and still have the rich mixture needed for full throttle with them.

The guy who did my valves, Hendron, is/was supposed to be an authority on Chevy's. He has since moved to South Carolina I think. Anyway, he stated that the mixture tabs were absolutely unnecessary. Chlem, you raise very relevant concerns about them and I believe your credentials are most likely stronger than Hendron's

For playing, the car is an absolute joy above 3,000 rpm. But when I drive around town or on the freeway it is nice to have a sharper throttle for those little blips when double clutching into parking lots and not washing my cylinder walls or burning my eyes and every body else’s at stop lights.

I look forward to your further esteemed and greatly appreciated replies!

Chuck





[Modified by Chuck Harmon, 9:37 AM 12/15/2001]

redvetracr

Chuck,
FORGET the books and start fresh from the beginning. You may also want to get your carb "freshened" by a carb expert and lucky you have one in (or near) your backyard. T.O.E. is in Suisun CA. They did a 830 annular discharge in the early 80`s and it is still in use today (after a freshen up of course) I think you want Tony sr. or Tony jr. ...redvetracr

74vetteman

IMO the best way to tune a carb is to use a Lambda (O2) sensor. Have a single wire sensor installed within 4 inches from your head or use a heated sensor in your exhaust collector. Using a voltmeter, you can determine what air fuel ratio you have at any given time.

Throttle response depends alot on what accel pump cams and squirters are used. Again this can be observed on your volt meter.

I would install a power valve plug in your metering blocks before setting your idle so fluctuations do not cause the PV's to open. Also set your main jets with the plugs installed. This will set your cruising air fuel ratio. Be sure to use light throttle when setting this as you wouldn't want your engine to run too lean while adjusting this!

Lastly set your power valves. If the power valve doen not supply enough fuel at WOT then you will have to drill the power valve channel restrictors so it does flow enough fuel. Using a pin drill you can open them up in .003 thou stages till you get the right readings on your volt meter.

For cruising you want a 14.7:1 Air/fuel ratio (idle circuit, transition circuit, main jets) which is around 400 millivolts.

At WOT (power valve circuit, accel pump circuit) you want a 12:1 air fuel ratio which makes the best power. This around 800 millivolts.

I've used this method to tune the dbl pumper in my vette with excellent results.

As a side note it is better to err on the rich side for two reasons. It will prevent going lean and damaging engine components, and if you look at a hp-air/fuel ratio chart, power drops off quicker when you go to the lean side versus the rich side.

I hope I've made sense here.

mountainmotor

Remove the spacer.It creates a half butt open plenum manifold by doing this spacer mod.Trust me on this.Your throttle response will come back after doing this.Put the squared up 78's back in it then recheck vacumm then post the results.Then along with how to jet the carb for max power and drivability I can tell you what power valves to run in front and rear.They are normally referred to as wide window power valves.After all these steps are taken there is a screw that can be adjusted under the carb to open the secondary plate just a tad to give it a little more air at idle to reduce the richness.

clem zahrobsky

the power valve has nothing to do with idle richness unless the butterflys are open to expose the idle transfer slot too much. i would run #65 power valves to prevent part throttle lean out. the idle air fuel ratio is controled by the idle fuel restrictions and the idle air bleeds. the screws only control the amount of fuel not the air fuel ratio. you can reduce the jetting but i would increase the area of the power valve restrictions the amount you reduce the jetting to get you back to the orignal richness at full throttle. if you do not have the distribution tabs in the carb those corners should be 1-2 steps richer than the one with the tabs.

mountainmotor

"the power valve has nothing to do with idle richness"

I did not refer to the power valve having anything to to with the idle being rich.Just trying to help the guy fix his car over the net one step at a time without typing a volume of information