Langadorf

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

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?

MotorHead

Looks not too bad. The way i check them is to install new plugs or just one if you're cheap like me then drive it around for about 1/2 an hour with no idling at all, then check. :D


[Modified by MotorHead, 6:52 PM 2/3/2003]

Vetterodder

Keep in mind that your idle circuits and cruise circuits are two separate things. With only 8'' of idle vacuum the fuel probably isn't atomizing well which results in less than complete burning. If you'd pulled a plug after idling but before driving they probably would have been much darker. Running on the cruise circuit for 20 minutes would have lightened them but they would probably have lightened even more with more time at cruise. It's even possible that your jetting could be too lean but the plug's color still looks ok because you're seeing it combined with a rich idle. To get an accurate plug reading for your cruise jetting you should use new plugs, installed after the engine is warm, and drive it immediately. Plan your route so that the engine won't have to idle any more than necessary until you're ready to pull the plugs for a reading. That will give you a much better indication of your primary jetting than trying to guess how much of the coloring is from idle mixture and how nuch is from cruise mixture.

Langadorf

Are you saying I just might be always stuck with a rich idle?

I'm gonna go pop in a few new plugs and go for a spin real quick. I'll post pics when I get back.

Barry's70LT1

I have had the same Holley 750 DP on mine for about 30 years now.
Here are some one liners that may help.....

- I found that jets 73P 77S is the best combo for my engine/driving.
- I also run a wire in the idle circuit. (Can't remember what size)
- IMPORTANT, I had to go with a #55 power valve. Any higher made it rich.
- Idle mixture screws have a fairly course thread. A 1/4 turn makes a huge difference. When adjusting the mixture, turn slowly, allow the engine to catch up with the change.

Hope this helps.
Barry

Langadorf

First off, I double checked my vacuum at idle - I forgot that I had fixed a vacuum leak. I read 10" of vacuum at a 950ish RPM idle. Next 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. 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, 10:02 PM 2/3/2003]

Barry's70LT1

Here are the basics of the carb functions. (As I understand it)

At Idle, the carb operates on the mixture from the idle screw settings.

Just off idle, the transfer slot, in the throttle base is exposed, and the carb operates mostly thru the transfer slot. Mixture controlled mostly by the idle feed restriction. (Where the restrictor wire was placed)

Additional throttle will then cause the fuel to flow from the main jets. The idle and transfer slot have little effect when the throttle plates are 30+% open.

The real trick is to have all these circuits balanced, so when you enter/leave each circuit, it will not adversely effect each other.

One of the biggest mistakes in carb tuning is to change jet sizes to correct a low speed mixture problem. The reason is that the transfer slot and idle mixture are still involved to a great extent.

To clarify the power valve operation.........................
My 750 came with a 85 power valve. This means if the vacuum drops BELOW 8.5 in. then it opens, pouring gas into the engine.
In other words, it needs 8.5+ in of vacuum to stay closed.
The power valve should not open until WOT or under reasonably hard accelleration.

On my engine, a slight opening of the throttle plates caused the vacuum to drop below 8.5 in. I immediately went to a rich condition.

ANOTHER POINT that I just remembered.
I had to enlarge the idle AIR feed restrictor on the top of the carb. These are the two little brass plugs in the choke area. (The outer ones I think)

In fact, I removed the brass plugs completely, allowing more air to enter the idle feed/transfer slot circuit.

I would go back to your original jet sizes and see what results you get. Then try to determine what circuit is still causing a problem.

Note: On mine, around 2000 rpm I am mostly running on the transfer slot circuit, not the main jets.

Barry

Langadorf

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, 12:11 AM 2/4/2003]

Langadorf

ttt

Barry's70LT1

Check the size of the Air Bleeds on top. If they are too small, then the transfer circuit could run rich. As I mentioned before, I removed the air bleeds completely. I'm not saying to do this to yours.
There are several versions of the 750DP, and I'm not sure if all air bleeds are the same size. Mine is a 30 year old 750DP R4779-0.
On the old original LT1 Holley carb, the idle bleeds are very large.
My 750DP came with holes not much larger than a pin hole.

Your pictures are not showing up.
Typically you don't need to split the jetting on a small block. I've never taken the tuning to that point, however some experimenting might net you some good results.

I would fix the rich/low speed problem first.
Basic rule-------- Fix the obvious first, and sometimes other problems get fixed as well or at least change.

I still have the original LT1 dual plane intake. I don't think you will get a perfect color on all plugs. I only tried to get a good average over all plugs.
Light tan to dark tan.



[Modified by Barry's70LT1, 2:30 PM 2/4/2003]

Langadorf

The pictures should be working again.

These air bleeds - do they need to be drilled out or do these brass caps easily just come out?

Barry's70LT1

My first experiment was drilling to make the hole larger.
With additional tuning, I ended up pulling them out completely.
I can't exactly remember how I popped them out. That was almost 30 years ago, but I don't believe it was very difficult.

I also had to do a little adjustment on the rear throttle position.
The 'stop' for the rear throttle was keeping it open just a bit too much allowing some air to pass. This resulted in having to richen the idle to compensate.
This upset the tuning process a bit.
Just so there is no confusion, here is a picture as to where the air bleeds are located.

OatBoy

i say... dump the DP and use a normal 750 or street avenger 770 :)

Langadorf

Ah, what's the fun in that? :lol:

I'll have to look into the air bleeds tomorrow. I'll let you know what I find.