Jonathan Dyer

iTrader: (5)

Hi, I'm looking for some Holley tuning advice.
The setup is a fresh zz383 crate from GM mated to a T56 with a 3:73 10 bolt at the rear. Hedman headers feed a single 3" pipe, no catalytic. The car is a daily driver.

I haven't gotten as far as jetting yet. I'm just trying to get a reasonable idle. I've never been able to make any of my holleys run anything but excessively rich at idle so I picked up a carb with changable air bleeds and restrictions.

The idle mixture screws are turned in as lean as I can make them without killing the engine. Too much transfer slot on the primary side with secondary blades closed versus the correct amount of transfer slot and the secondaries cracked open versus primary blades closed and little more of the secondaries open all yield similar results.

I swapped out the original air bleeds (67) for much larger ones (75 or 77, I forget). I usually blame the power valve first but I have a drawer full of power valves that appear to be fine (no visible tears in the diaphragm that I can see) and changing it has never made any difference in the past. I have yet to try downsizing the idle feed restrictions (I don't have any on hand) but on previous carbs when I tried reducing idle flow with pieces of wire the effect was not huge.

Timing is currently at 16* and the vacuum is disconnected. The distributer itself is an MSD streetfire with an adjustable vac advance that I intend to use once I get a little further along.

I don't appear to have any vacuum leaks. The idle is not erratic, just rich. What am I missing? and what info have I not supplied to help answer?

On another note; in the sticky Sofa refers to a 6.5 power valve as stupid. As vacuum drops when you hit the pedal a 10.5 would open sooner than a 6.5 right?

Thanks,
Jon

JSD MOD 2/5/09

Jonathan Dyer

iTrader: (5)

OK some more info. I neglected to mention that I'm running a dual plane intake. Cam specs are as follows: 222°/230° @ .050", .509"/.528" lift, ground on 113 centers. That's a heap more duration and lift than any engine I've ever owned but from numbers I've seen in other posts it's still pretty mild. Unfortunately, I'm way out of my depth when it comes to cam shaft theory. But I think that with an LSA of 113 the intake and exhaust events are far enough from one another that these numbers do not represent a great deal of overlap (respondants showing my ignorance are welcome). Actually, come to think of it, this is not relevant to my current problem given that I have over 15" of vacuum at 1000 RPM. That should be more than adequate for a good idle right?

The original idle air bleeds were 71 (not 67 as originally posted) and I upped them to 75. Thinking that the carb is perhaps alot more sensitive to air bleed step sizes than jet step sizes I decided that perhaps all I had done was to decrease the signal to the idle discharge port sufficiently that I was now overly enriching the mixture to compensate. I stepped back to 72 air bleeds. This allowed me to lean the mixture screws very slightly. Idle quality improved somewhat. Still stinks though.

If I was to step down futher, bleeding away less idle signal strength, until I could close the primary throttle plates all together; would that just be an even trade? My thinking here is that with only the mixture screws and the idle feed restrictions in play control might be more precise?

Unfortunately, I wasn't able to get a fresh power valve until the end of the day. So I'll have to wait to rule that out.

Sorry for the long-winded post,
Jon

thewolf

iTrader: (5)

Yes. The rating should be as high as possible, but low enough to keep the valve closed at idle and normal cruise vacuum levels.

Jonathan Dyer

iTrader: (5)

In terms of determining vacuum at cruise, does the engine need to be under load? Would I need to rig up the vac gauge in the cockpit of the car?
Thanks,
Jon

thewolf

iTrader: (5)

Yes and yes. For determining the best power valve, you'll want to monitor your manifold vacuum when cruising and during 'normal' acceleration, i.e. light throttle around town. You probably don't want the power valve opening unless you really nail the throttle. Rigging a vacuum gauge in the car while you're driving is a good way to judge that.

Jonathan Dyer

iTrader: (5)

Thanks for the reply. It might be a day or two before I can get into it though. I seem to have a half dozen issues all at once. The car won't run properly at all now. It's shuddering like it's missing on a few cylinders but I can't find an ignition porblem. I pulled the plugs and some were a bit wet but after reinstalling there was no change.
Hopefully I'll get a bit of time to look into it tomorrow.

Jon

Jonathan Dyer

iTrader: (5)

OK, this is good (or stupid). The car appeared to be running on 5 or 6 cylinders. After pulling the carb off I noticed alot of fuel on the intake to carb gasket. I ran an extension hose for the fuel line so I could fill the fuel bowls off the car. I held the carb while a friend jumped the fuel pump relay. I watched as fuel bowls filled and then as the primary side just drained out through the bottom.

The gas appeared to be coming from either a screw that secures the base plate to the main body or the little vacuum hole right next to it that supplies the power valve. Wild thoughs of a cracked or improperly cast main body went through my mind.

After removing the metering block though, I found that I had somehow installed two gaskets on the power valve. They had both cracked when I tightened it down and gas was literally draining out past the power valve.

With that sorted I wound up back with 71 air bleeds on the primary side with just a touch of the transfer slot exposed and 73 air bleeds on secondary side with the secondary throttle blades opened up some. This is by far the cleanest idle I've had on this car to date.

Next I disconnected the secondary throttle linkage so i could drive the car on the just the primaries. There was a pretty good stumble off idle and a hesitation at cruise. Moved up from 64 primary jets to 66 which made the hesitation worse. Going to 68 made it a bit better. I'll have to try 70s next week (so much for fuel economy).

An accelerator pump cam change has helped with the stumble a bit but I need to experiment some more. I'm confused though over the position 1 and 2 on the pump cam bracket. I've read that position two makes the same cam slightly more agressive. Certainly rotating the position of the plastic lobe on the shaft would change things alot. Regardless of which hole the screw is in, the two hols on the cam and the two holes on the bracket line up. So how does this affect anything?

Thanks,
Jon

thewolf

iTrader: (5)

The two holes on the cam should not line up exactly with the two holes on the throttle shaft bracket. If you look very closely, they are a few degrees off, i.e. you should not be able to put two screws in the pump cam at the same time. When you switch from position 1 to 2, the cam will shift ever so slightly clockwise (advanced). More radical changes will probably require a different color cam.

Also make sure your accelerator pump linkage is not loose or binding.

Jonathan Dyer

iTrader: (5)

Yeah, looking closely is not my strong suit at the moment. I need new glasses but spent all my money on my swap! lol. But I had the carb off again today and I see what you mean now.

I moved up to 70 main jets and the hesitation/ surge at cruise is gone. I also switched to a blue pump cam lobe and the off idle stumble is all but gone as well. I say all but gone because while clutching between gears I don't have to feather the gas or let the clutch slip excessively. However, as the revs drop while in gear to say 1700 - 1500 RPM the car starts to buck. At first I thought that I just didn't have enough RPM to keep the transmission happy but it feels more like pulses from the engine as the carb moves back off main metering and into the idle transfer slot. Not sure if richening the mixture here might help.

I must confess that up to now I have not been adjusting the idle mixture screws for best vacuum but rather least smell! Can I assume that if the jets are adequate for 3000 RPM that they must be sufficient for 1700 RPM as well? Like the jets become restrictive as vacumm drops of a bit? Just grasping at straws here.
Jon

thewolf

iTrader: (5)

Did you ever look into sizing your power valve correctly?

How is your idle currently? How do you have it set? This article may help.

Jonathan Dyer

iTrader: (5)

Thanks for taking the time to post that link. That's how I started off but after a while I decided to set the curb idle screw to how the transfer slot is supposed to look and try to make everything else work around that. It's interesting that the article doesn't bring up timing and a how that affects things.

I haven't been able to do much with or even drive the car in the last few days but I was having a problem with it dieseling after shutting it down. Assuming that that was an indication of too much timing I dialed it back a bit (8-10*). That didn't help and it began to smell a little richer at idle. I ordered colder plugs and asked one of my guys at work to put them in but that didn't help either.

I had an oportunity to drive the car briefly today so I fired it up and advanced the timing and jumped in. I didn't use the timing light so I have no idea where I'm at now but the idle is better, leaner and no more running on after turning of the ignition.

I have been keeping base timing on the conservative side because over 20* or so my last engine was hard to start. So I'm thinking that once I finally get a chance to spend some time on this I'll wind up with the vac. advance plugged into full manifold vacuum. I think that this would allow for a relatively little advance while cranking and the vacuum canister would ramp up timing once the engine catches. Hmm..that might necessitate a recurving of the distributer though...

Anyway, that's step 6 I'm still on step one. I'll try to do my homework before coming back for more advice. Thanks again for the link.
Jon

Jonathan Dyer

iTrader: (5)

Haven't managed to get too far recently. I drove around for a few days with a vacuum gauge piped in below the throttle blades and it never dipped below 10" at any time. So I figured a 95 or a 10 power valve would be a good choice. An 85 was all that was available locally so I took that and ordered a 95. Particularly with the 95 the car smelled richer at cruise so I've stepped back the 65 for the moment.

The dieseling problem was solvable two ways. One was to close out the secondaries so that fuel was only coming through the idle well. This is OK but necessitated enriching the mixture a bit and is still too rich for me.

The second was to open both sets of blades so that the exposed portions of the transfer slots looked like a little square. With 72 air bleeds in front and 73 on the back the idle was great. But the car would run on if base timing was set at less than 28*-30*. The poor starter sounded as though it would give up the ghost at any minute and I was into pre-ignition in no time.

To further complicate things, my "fast burn" heads don't seem to like very much timing at all. I used to run 50* total on my old smogger heads but this engine pings with 40* all in @ 3000 rpm.

If I wanted to make this tune work I'd be looking into a start retard and limiting mechanical travel to like 7* above base timing (28* at idle + 7* = 35* by 3000 rpm). Comparing other posts I've read about setting up timing for street driving this all sounds ludicrous.

Any more thoughts?
Jon

Jonathan Dyer

iTrader: (5)

OK, so I took the car in for a dyno session today. Sure enough, the motor responded best with 28* total advance (354 HP to the wheels). Any more advance and power dropped off across the board. I'll have to repeat the setup that yielded best idle (28* initial) to prove that I'm not making it up in my head. If so, I'd be looking at locking out my advance all together...for a street car????

At the moment I have the base timing at 8*. The distributer adds 20* by 2800 rpm. But no combination of air bleeds, mixture settings etc. etc. yields anything but a stinky idle.

I'm so frustrated I'm just about ready to abandon this carb and put my Edelbrock on for a try.
Jon