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

 

I don't consider myself an expert, but if tou have a vacuum gauge (appears that you do) drive the car normally, idle, cruise/part throttle and look at the vacuum. My holley books state that the power valve should be one that opens at .5 to 1 inch of vacuum below part throttle. Example: If you see the lowest level or 5.0, the valve should be 4.0 or 4.5 (numbers 145 or 140). This is for primary side. secondary should be 35.

However the book states that if duration of a cam shaft is a lot as it appears yours is, you may find that you have to raise idle to increase vacuum to a level to keep the power valve closed at idle, or the worst case is to jet rich but plug the power valve

I'm confused by you jet numbers though. My book seems to have #80 jets for secondaries, but primaries are either #70 or #80. But this all depends on the cam.

Note: I bought a holley from a swap meet, that was too rich at idle and chased the problem for a few weeks. If was wrong metering block gaskets! Seems that on some double pumpers the gaskets are different depending on the accelerator pump transfer method. Some use a brass tube with o-rings between the metering block and main housing while others have a flat machined surface with just a hollow dowel to line up the transfer port. If you use a gasket for the transfer tube that is brass with o-rings on the other kind, the carb leaks internally causing it to run rich.

 

Chuck, not to be a smart aleck here, but is there ANY way to make a factory spec L88 streetable? :crazy: Between the compression ratio, cam, and carb you really don't have a car suited for the road do you? I would think any alteration of the original Holley jetting would compromise what limited street-ability your car may possess!

Gary


[Modified by GaryS, 7:03 AM 12/15/2001]

 

Hi Chuck...
I am by no means an expert, however I can offer "what worked for me".
Several years ago I installed a Holley 750DP. Out of the box it came with
75 Primary & 76 Secondary jets, and 85 Power valve.

You mentioned "eye burning", well yes, big time. This combination worked
very well at the strip with open headers. Not so good on the street.

I tried the lean jet route, and like you only succeeded in making it run like crap.

I'll skip all the trial and error stuff and get to the point.
Here are the changes that kept the power and made it very streetable.

750DP have a very rich idle circuit/feed. In the front metering block, there
are two brass idle feed passages. Since these were large, they provided
too much fuel to flow at idle and low rpm. (Too much for a LT1 engine).
I cut these down by about 30%. How ? By inserting a gold plated wire into
each hole. (Long enough to bottom out and be flush with the hole).
This was only temporary for testing. They have been in there for 25 years.

On the top side of the carb, I enlarged the idle air feed holes. The little holes
just in front of the choke.

Final jetting was 73 P. and 77S. with a 65 Power valve.
No more sore eyes and great throttle response.

All these changes were made for a 750DP on a small block. You may need to do something entirely different. It's just what I needed to do.

I guess to sum it up, if you have poor low end performance/ mixture issues
then it is not likely the jets.

You need to be aware of at least five things when tuning.
- Idle mixture adjustments.
- Idle feed circuit (affects idle and low speed)
- Acc. pump.
- Power Valve.
- jets.

I hope this is of some help.
Barry

 

Thank you very much for your replies. I ran this post over in "engine mods" too, but I posted it here also because until last night I had not even been aware that the engine mod section existed. I don't want to lose any of the high octane brain power in this section if others may not have been aware of it either. Yet by my not being aware of the engine mod section surely makes a strong case for my being in the unleaded 87 octane category.:bb

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 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

 

Chuck,

The Power Valve does not determine the amount of enrichment. The valve only opens/closes at a set vacuum level. The actual amount is determined by 2 small passages behind the PV, called the PVCR (Power Valve Channel Restriction). The reason for the 'high capacity' PV is that on some carbs the PVCR's are so large the regular PV can't keep up.

Just installing a high flow PV in your carb with no other changes will not change anything.

What you need to do is lean the idle circuit like the previous poster suggested. Note that the idle circuit also doubles as the low-speed circuit, so leaning it will also affect low speed cruise.

Next, you are correct in that the power enrichment circuit is insufficient on Holley DP's. They are meant to be strip only. For street use you want more power enrihment, which will let you run smaller jets with the same WOT mixture.

To achieve this you will have to enlarge the PVCR's. Get a set of precision drill bits, and insert bits to find the current size. Drill them both out equal amounts. Drill them out in small increments. A few drill sizes is all that is required.

You can also further stagger the jets between primary & secondary. At WOT the engine gets the average of all 4 jets, so you can decrease primary jets for driveability and increase secondary jets for WOT.

 

Zwede,

I think you hit the bulls-eye! Many thanks!

Each jet change is worth about 4.5% fuel rate difference, right? If the preferred jetting is 7 sizes less, should I increase the area of each power valve restriction hole by ~31.5%

Chuck





[Modified by Chuck Harmon, 12:33 PM 12/15/2001]

 

All excellent advice. My instinct would be to run lean enough primary jets to have good throttle response without a lean-hesitation, and then richen up your secondary jets to where your spark plugs tell you that you are running stoichiometrically at WOT. You may have to play with squirter sizes and accelerator cam sizes to help avoid the lean-hesitation. If you go too far, this won't help you either. But they'll help you tune it in if you're close.

How about the "carb expert", Lars. Any experience with race engines that you could apply here?

 

you have to calculate the area of the jet and the power valve restriction.