750 DP Still Running Rich - Tuning Tips?

Langadorf · 02-03-2003, 08:28 PM

I've been trying to tune my Holley 750 DP for the past few days and am having problems keeping it from running really rich. I installed an O2 sensor and an air/fuel gauge and have been using that in tuning the carb. I'm only pulling around 8" of vacuum, so I can't use a vacuum gauge to help tune the carb. When I first installed the carb I jetted it a little rich to be on the safe side. I drove it this way for a couple months and then decided to port the heads. When I pulled the heads off they were really sooty so I know I was running REALLY rich. I reset the carb to stock jetting which is 70p/80s. I also put .015" wires into the idle circuit to try and lean out the idle a little. I was able to get it to idle OK (still a little rich), but at cruise it would still read off the far end of the rich scale. I then put in #68 jets on the primaries and it still runs rich at cruise. I know my old 650 vacuum carb came w/ #67 jets stock. Might I have to go even leaner to get the cruise mixture right or am I missing something else? My engine specs are in my sig. What jets are people w/ similar configurations w/ 750 DPs running? Thanks for any ideas!

Langadorf · 02-03-2003, 08:41 PM

I just pulled a spark plug to check it's coloring. Here it is:

This actually looks OK to me. After installing the #68 jets I idled the car for about 10 minutes and drove it around the neighborhood for about 20 min. This is mostly at idle/cruise and 3 WOT runs. Do I need to drive it longer to get a better reading or is this OK?

Taijutsu · 02-03-2003, 09:07 PM

The first thing I would try is a 180 thermostat! I don't know if that will cure your problem, but it will help! Please keep us posted on what finally works.

CFI-EFI · 02-03-2003, 09:22 PM

One backfire through the intake can blow the power valve. Have you checked, it? That would be the FIRST place I'd go. Good luck, and...

RACE ON!!!

Langadorf · 02-03-2003, 10:38 PM

Don't all new carbs come w/ power valve blowback protection? The motor did backfire through the intake when I was fooling w/ timing when I reassembled the motor. What exactly would I be checking on the power valve?

L79vette · 02-03-2003, 11:32 PM

If it responds to idle mixture screws, then the power valve is OK. You can also often see fuel dripping through the boosters at idle if the PV is blown.

Since it is a new carb, it should indeed have PV protection.

I would bet the problem is that you have so little vacuum you are engaging the power valve at cruise. Most Holleys come stock with a 6.5 power valve. That means the power valve is open, enriching the mixture, whenever vacuum is 6.5 inches or less. If you are only pulling 8 inches at idle, you should go to a 5.5 power valve, maybe even less.

What transmission? If you have an automatic and have 8" vacuum in neutral, you probably have a lot less in drive and that could open the power valve when idling in drive.

What cam are you running? 8" vacuum ain't much. Either you're swinging a very big stick or you have some kind of valve train problem.

Langadorf · 02-04-2003, 12:01 AM

I'm actually running a 3.5" power valve. First off, I double checked my vacuum at idle in gear (automatic TH-350) - I had forgotten that I had fixed a vacuum leak. I read 10" of vacuum at a 950ish RPM idle.

My cam is a Comp Cams 07-306-8. It's a hydrualic roller running 1.6RR with a duration of 230/244 at .050" and 290/307 at .006" on a 112 lobe separation and lift of .544/.576". From what I've read this vacuum is typical with this cam and an auto tranny.

To further test my cruise mixture, I popped in two new plugs. All I had were 8 new platinum and 1 new copper plug, so I popped in one of each on cylinders #3 and #4. Here is a picture of the new plugs:

Then I drove for 20 minutes at an average of 3000 RPM and popped the plugs out. Here is a picture of the used plugs:

These look basically white to mean which makes me think I'm running lean. Now at this RPM, my air/fuel gauge goes completely off the lean scale. I also noticed a few pings from the engine when slowly accelerating from a stop. The strange thing is that around 2000 RPM the mixture on the A/F gauge goes back to the rich level. Is it possible that at that lower RPM I'm getting back into the idle mixture area? Also, might my power valve be too low now that I'm running around 10" of vacuum?

[Modified by Langadorf, 10:03 PM 2/3/2003]

L79vette · 02-04-2003, 12:58 AM

Power valve number too low will cause a flat spot in power at part-throttle acceleration, until you open the throttle enough to open the power valve (due to low enough vacuum). Cruise mixture should be unaffected on level ground. It does sound like you might be seeing too-lean an acceleration mixture. Since you're running 10" vacuum now, I would go to a 6.5 power valve to avoid a lean flat spot.

If the gauge says lean at cruise, then you do need to jet back up.

In fact, with a double pumper you need to run rich (not pig rich) at cruise. They are designed to run basically in power enrichment mode all the time. If you lean out until cruise mixture is near stoichiometric (middle of the gauge), you will get acceleration flat spots because the power valve channels are too small for this kind of setup (designed to add only a little power enrichment because the main jets are set to the rich side already). They can be drilled out, though. The double pumper is designed this way to provide the absolute best throttle response and power under all conditions, and gas mileage be da*ned.

Check out this article:
http://www.bob2000.com/carb.htm

The link is for a step-by-step Holley tuning method using an oxygen sensor. Also explains double pumper operation and why they will always run to the rich side of the gauge. That doesn't mean the plugs will read rich, though. "Correct" plug color is associated with best-power mixture, which is richer than stoich.

Langadorf · 02-04-2003, 02:00 AM

Well I put back in the #70 jets and installed a 6.5" power valve. I popped in two white looking plugs and drove for about 15 minutes at 2500-3000 RPM. I pulled the plugs and this is what they look like:

While driving, my A/F gauge said I was running about a 13:1 mixture. The plug from cylinder #4 looks lean still but plug #3 looks about right. Also, it still seems rich around 2000 RPM so I guess my transfer circuit is what is running really rich. Are these different plug colors OK or might I need to split the jetting? I'm running a dual-plane intake and the tan plug is on short runner side while the white plug is on the long runner side. Thanks for any ideas.

[Modified by Langadorf, 12:12 AM 2/4/2003]

HeaderDesign.com · 02-04-2003, 02:12 AM

You need to read the plugs at the deepest part of the porcelain. It colors first. The power valve channels will give you about 4 jet sizes on your race calibrated double pumper. Street carbs give you 6 or 7 jet sizes. Each jet is like 4.5% different in flow.

If you are certain that the power valve is not leaking (because they will blow even with the blow-out protection), then you have an idle/transfer fuel mixture problem. Light cruise is dominated by the transfer slot fuel flow. You should not need the wires in the idle feed restrictions if your engine responds to the screws.

You should remove your carburetor and check the primary and secondary throttle positions. If either the primary or secondary throttle plates uncovers too much of the transfer slots, you will run into rich/lean idle tuning problems. I set the primary throttle plates so that 0.040" of the transfer slots is showing, then set the idle RPM with the secondaries. That will give you a good starting point. The primary transfer slots should show no more than 0.060" at the most. The secondary transfer slots really shouldn't show at all. Good luck.

Langadorf · 02-04-2003, 02:19 AM

I seem to have the idle mixture set in the ballpark so I don't think I'm exposing the transfer slots at idle.

It's just at low cruise speeds w/ hardly any throttle (around 2000 RPM) where I'm seeing it run really rich now.

HeaderDesign.com · 02-04-2003, 02:26 AM

The primary transfer slots will always be exposed at idle to some extent, that really helps the response. I've run into front/rear mixture distribution problems because the rear throttle plates were too open or too closed. Fine tuning the idle fuel screws won't help the distribution. Does the engine load up at idle?

Langadorf · 02-04-2003, 02:40 AM

I guess that distribution comment brings me to another question. On a four corner idle, should all the screws be turned out roughly the same amount? Right now, I have the rear screws out about 3/4 turns and the front out about 1 1/4 turns. Should I make these all the same? I'm not sure if I know what "loading up at idle" means - can you explain it? Thanks.

L79vette · 02-04-2003, 03:03 AM

Mixture screws should all be the same.

Loading up at idle means running too rich at idle and fouling the plugs with excess, unburned fuel.

An alleged professional had my carb set with the idle mixture screws as follows:

driver side: front 1/2, back 3/4
passenger: 1 1/2 turns on both screws

The rear float was also way low.

I had weird behavior in which coming off the throttle resulted in the engine going to 1000RPM (normal idle speed with my 235@0.050 cam in the 327), and then slowly dropping to 850RPM. The drop was due to some cylinders loading up with fuel due to their overly rich idle mixture setting. With my dual plane intake, I could actually trace out which cylinders were connected to the rich idle circuits and found the plugs covered with fuel.