Well this week we changed the power valve which seemed to be leaking, from the 5.5 size to a 3.5 size. No change in performance, did not stumble but did sound wet or rich stil at idle. I went to the Frankenmuth car show today which is about 30 miles away and mostly highway speeds, checked the plugs when I got back and they were all black! I just put new ones in last night before my drive too? Do I need to go smaller on the jet size from 78 to maybe 74 or 75? I have not checked the new power valve yet to see if its defective but it should not be opening at highway speeds with light throtle..... :confused:

 

Better than running poor, no money. No include your approx. Hp level, timing, head cc, valve sizes, header tube size, etc., etc., etc., etc. and things like that so people can nail it down unless you are just providing a play by play.

 

Actually I am running poor after all of the time spent and money on attempting to make the 850DP work!!!! Motor is a .060 over 454 about 450 to 475 hp, crane hydraulic cam with 296\306 duration and 553 and 571 lift, Keith Black pistons, 9.2-1 compression, merlin rect port heads, 310cc intake runners 118cc combustion chambers, valves 2.25 and 1.80, header tubes, 1 3\4, vacuum advance msd system with msd 6al. Holley has 78 jets up front and 80 in back.........

 

On a performance Holley DP, 750 and above, you usually need to run restrictors in the idle feed circuit. (Restricts fuel flow on the low speed circuit, for street driving)

You also may need to increase the size of the idle air bleeds. (Increase air flow to idle circuit)

Jets are usually the least of the problems when tuning for street driving.

Barry

 

Is it possible to just adjust the four idle screws in so less fuel dumps? I know that the Holley book should be a must read for me!

 

A Holley carb has three circuits that it uses to deliver fuel.

IDLE - Basically for idling control. Does little for mixtures while driving.

TRANSFER SLOT - Used for off idle and light throttle requirements. Eg. crusing at lower rpm's. It provides the fuel for the transition between idle and running on the main jets.
Probably the most important part of the carb for low end street driving.

MAIN JETS - Used at higher rpm and WOT operation.

Of course the power valve and acc. pump shot figure into this as well.

On a performance Holley the idle/transfer slot idle feed circuit is very rich. This ensures enough fuel is supplied to the engine before the flow of the main jets are used.

If you only idled the engine then went to wide open throttle, (Drag racing), then this discussion would be done.
However, there are some of us that want it all. A carb that produces maximum power at WOT and expect the carb to run on the street and not burn gas like flushing a toilet.

This is where the tuning to your engine/driving comes in.
This usually falls in the area of the idle feed circuits.
As I mentioned in my other post, the idle feed circuit is almost always too rich for street driving.

Without knowing how your engine is setup, this may not need to be done, and may not be your problem.
It's difficult giving "long distance" advice.
Many people on this forum have a "it worked for me" advice. They are correct. It did work for them.
You will still need to determine what will work for you.

I usually do one of two things to control this. Sometimes both.
- Idle feed restriction.
- Increase idle air bleeds.

The idle feed restriction is a fairly common requirement for street driven, large Holley applications.

NOTE.... Many people try to fix a rich running engine by jet changes. This is just basically wrong. WOT mixtures is about the only way to determine what jet size you need.

Less than WOT mixtures (every day street driving) needs to have "other things" like the feed circuit & power valve tuned.

If you want to experiment with this circuit, I would restrict the flow by about 40 - 50% and see if this makes any difference.
You do this by adding a wire about 3/4 " long into the idle feed circuit on each side of the carb.
See picture for placement.
The diameter of the wire should be about half of the idle feed hole size.
Note.... Since you have 4 idle screws, you may need to restrict all four idle feeds.

Then re adjust the idle and go for a cruise. See if the throttle response is ok and there are no flat spots in the low end driving.

Also, the air bleeds may need to be enlarged to let more air into the low speed circuit.

This will require some "tinkering" on your part to get everything just right for your engine and driving.

The thing to remember is the Holley can be made to work well on the street.
You just need to understand how it works and tune to your application.
Barry

Idle feed restriction (Click on picture to enlarge)


Air bleed



Transfer Slot



[Modified by Barry's70LT1, 12:54 PM 9/7/2003]

 

Barry, Thanks much for the info, I have printed it out and will read in detail, I have to admit its a little frustrating when you don't know the correct way to solve a problem!

 

I don't agree. The idle circuit works at all speeds. The motor at light cruising has vacuum so the idle circuit is dumping fuel. The transfer slots start dumping fuel and they keep right on dumping . If the idle circuit is right jetting will be rich.
Lean out the idle circuit by drilling .100-.125 holes in the throttle plates and add a .015 wire to the idle circuits.
I run 2 850 4 corner idle double pumpers and stock #80 jets and don't run rich. |My plugs are white and I like it like that.
The mustang started to run rich and found a plugged PCV valve.

 

What should the wire be made of? Steel, brass and how do you install. Do you just remove the front bowl and stick the wire in?

 

Everyone has their own version, but here's a little more.

I usually start getting it to idle clean first. That often does mean drilling the throttle plates if your cam requires a pretty high idle speed. Often the idle transfer slots are getting uncovered at idle due to large throttle plate opening. The holes allow you to close blades a little.

The idle circuit does control idle and initial off idle and very low speed cruising. It feeds the engine until airflow increases to the point to get fuel flow through the main metering (jets). It also continues to contribute as engine speed increases, but has of course less effect. But at low speeds it's very important. In cases where you have a stronger vacuum signal to the circuits than Holley anticipated for a race deal, you can get too much fuel flow. Your cam duration looks fairly large, but on advertised numbers it's pretty tough to tell. I think you posted it once, but what vacuum does it hold at idle? You may need to do as Norval says, and restrict idle circuits. Usually, IF your idle screws are responsive and you can make it try to die as you close them, they are OK. But anyway, the wire trick is usually done by pulling a strand of copper wire (or any kind) out of a piece of electrical wire. Different gauge wires have different thickness strands, so it's pretty easy to get what you need. Make the restrictor about 1/4"-3/16" long and put a small bend on the end. Insert it into the two referenced holes with the bent end back towards you. Lay the gasket in place and carefully reinstall it all so wires don't fall out.

I later found that if you use some lead shotgun shell pellets and tap them into any of these orifices, you can redrill them to whatever you want and if you go too big and need to change them just dig it out and install another pellet.

But since you are talking about a highway driving issue, I believe you need to work on the main jets. Every combo is different, but somewhere around 1800-2000 rpm main fuel enrichment is beginning through jets. I put whatever jets in it I have to to get it to cruise nice and clean. Often that is a long way from stock, but then I work with the PVCR's (Power Valve Channel Restrictions) to work out WOT mixture. My gut feel is that somewhere around 70-74's will be needed on the front side. Don't worry about rear right now. Just lean it 'till it cruises well with no surging etc.

Then work on WOT stuff. Often it's easier to just kick up rear jets one or two sizes. Arguably that will richen rear cylinders, but it's not too bad. The right way is to increase PVCR's slightly until WOT is correct. But this takes extreme care to get correct. You have to make sure you shut it off clean and immediately pull a plug on the side of the road to make sure it's right.

I work out PV opening setting after I get main jets cruising right. I juggle them until I get a smooth transition as I slightly open the throttle without really activating accelerator pump. You want it to pick up with no lag under light throttle acceleration..say in the 30-40mph range....real easy increase. Generally if you have 8-9" of idle vacuum (in gear with an auto) you will need a 4.5-5.5 PV. Higher vacuum will allow a 6.5 or even a 8.5" one. My 540 holds 6-7" at idle and it is happy with a 3.5" PV.

Then when you're all done, work on accelerator pump tuning to get instant WOT response. But you have to get everything else right first.

Be very careful playing with air bleeds. The idle ones aren't as critical, but it's easy to lean one at high rpm too much with only a few .000's change. Usually they are for FINE tuning a combo. I think you're a long way from that stage right now, but can get there easily with a few hours work.

Hope all this helps,


JIM

 

Jim, My cam has 236 and 246 duration at .050, and I am running 10 inches of vacuum at idle. I have 78 jets up front and 80 in back with a 3.5 power valve. Is there a way to adjust the accelerator pump, I know mine gives a big shot of gas when you hit it because if you put the car in neutral it takes a while for it to drop to idle speed again..

 

This may be a stupid question...or I may have missed some of the replies...but have you checked your needle/seat assembly? Whenever my car runs rich it always seems to be a little spec of dirt caught in between the needle and seat. I pull out the little mechanism from the top of the carb using a 5/8" wrench and flat blade screwdriver, blow the whole thing out with compressed air and reinstall. Screw down as far as necessary so that the level of fuel is at the bottom of the sight plug hole. This sets your float level and once properly set....lock it down. If the car WAS running properly and then suddenly started running rich, I'll bet this is your problem.

 

Thanks for the idea, no I have not checked the float level or the seats, looks like a project for tonight!

 

You have to get it to idle right before anything else. It has to return to idle easily. If not it will keep acting crazy and will make you chase ghosts.

Make sure linkage isn't binding, or throttle shafts worn out real bad or bent. it can happen with misadjusted linkage. Get it to idle down well first.

Check your float levels. Everything else depends on that being a steady constant. Adjust them with engine running where fuel is just at bottom of holes for starters. Go slowly, because they are slow to react. Better to start out real low and slowly adjust them up one "flat" of the nut at a time to the proper level. If you go to far, "wing" it a couple of times to get fuel out of bowls and sneak back up again.

My gut feel is still that you will need to drop primary jets down a few numbers and then most likely go back up on PV with that 10" of vacuum. A 6.5 would probably work fine. You're probably so rich now on jetting that the lower opening PV is helping by not adding additional fuel too early.

Accelerator pump adjustment is done by adjusting lever screw/locknut to remove all play in linkage. Again, start out loose with slop and "loosen" locknut to make it longer to remove slop. *Just* barely remove slop...don't add preload. When done make sure that when you fully open the carb, the pump is not bottomed out. Do that by trying to depress pump arm slightly when it it at WOT. it should move a little-.010-.030" maybe.

A good healthy pump shot is OK. Once you get these other things ironed out, we can work on tuning the pumps. We need to get mixtures right first.

I saw your post on the maxjets. They are a "nice to have" but not necessary for what you're doing.


JIM