DucatiDon

As promised, here is a new thread to document the tuning process of my new hand built (by me) 383.

As a quick recap, its specs are: 388ci, 3.75 stroke. 6" rods. 10.8 CR, AFR 195 heads, Howards hyd roller cam .56/.56 237*, Z28/LT1 dual plane intake, Holley 4160 750 vac secondary. 2.5" rams horns 2.5" full undercar exhaust Off road mufflers.
3,73 gears, 27" tire, and 3.26 1st gear with the RS500.

Timing set at 12* at idle. Full mech comes in around 2800 at 35
It has 15" vacuum at 750 rpm.
Ive tried up to 5* in both retard and advance with little change.
Mech advance appears to come in early, like 700 or so.

RPM Advance Pre-final:
400 11*
800 15*
1000 20*
1500 27*
2000 30*
2500 32*
3000 34*

Carb has 76 primary jets, a 6.5 PV and a 31 squirter and a metering plate secondary (72).
Idle screws are about 1.5 turns out at the highest vacuum reading.

With the vac can hooked up, (Full manifold) it ran fine at idle and in neutral, but in gear it stumbled and trailer hitched until about 2500 rpm. No backfire.

I stuck a toothpick in a primary air bleed, and the engine slowed down.
When I pulled the PCV hose off, the car died.
When I pulled the Vac advance line off, nothing really happens. Yet it has 15" vacuum.
When I rev the throttle, vacuum drops to about 9" before increasing again.

Vac advance remains disconnected. Car still has bog/stumble trailer hitch/miss until about 2k. It seems to run smooth above 2k and revs smooth, even with the secondaries opening up, until (Ive been letting off around) 4500.

Im ok if it is a dog on power from idle to 2k, but I do want it run smooth and be an embarrassing ride.

I noted the Vacuum and the PV number....if my vacuum never drops to 6.5", perhaps the PV is never getting a chance to kick in and help?

Hard fuel lines make jet changes a PITA, so Id like to try everything else first if possible.

Please share your thoughts or follow along as I Science the hell out of this.

GTOguy

Engine should have 18-21" vacuum at idle. Should drop to zero when you gun it, or close to zero, and shoot past 21" when you release the throttle after goosing it. Vacuum advance can be manifold or ported....manifold will give you vacuum to the advance at idle. I prefer this method over ported. Different engines like different things. if hooked to manifold, and you pull the line and get no RPM drop, you have a restricted line or a blown diaphragm in the vac advance. Without a scope to look at or a dyno, you are going to need to make sure the electrical, ignition, and fuel systems are up to snuff. You are either a bit lean on transition, or have an ignition issue. My very well worn 383 with a bit of blowby but with 461 heads and 195PSI compression will pull from a stop in third gear like nothing.....and produces huge torque below 2500 rpm.....that's what makes a 383 so much fun....no need to rev it to really pin you to the seat. You need access to scope or a dyno, or an O2 sensor system. Good luck.

karkrafter

I am no tuning expert...but I have learned one very important thing, street cars HAVE to have vacuum advance hooked up. These things need like 50/52 degrees of timing at part throttle. I blueprinted my distributor last month and my car is like a different car. So much more power off idle...there are may threads right here on the importance of proper set up of vac adv. Good luck.

Westlotorn

He has a large cam which reduces the normal idle vacuum. With his cam 15 inches is good. Duration is 237, the factory duration that gives 18-21 vac at idle is close to 200.
It makes a difference. Larger cams will have even less vacuum. The L79 327 cam only has around 224 duration and I think a good running L79 will have 15-16 inches at idle.
Based on the posted advance numbers for Don's engine it looks to me like stronger advance springs could slow the advance and help with your bucking off idle and he could hook the vacuum back up.

Vacuum advance will help any engine run smoother in very light load situations, cruising steady speed for example but vacuum advance is not in play while accelerating.
That is why the mechanical has to be dialed in first and second you fine tune by adding in the vacuum advance.

DucatiDon

Ill experiment with springs tonight... I have an assortment.

My 327 had only 12" at idle.

GTOguy

I totally missed that about the cam....and I agree with West. Try slowing down the mechanical a hair. And will it surge off-idle with the car sitting in the bay and you hand-working the throttle? If so, you can spray a little carb cleaner down the carb and see if it picks up.....that will tell you if it's lean or not.

cardo0

First of congrats on getting your dream build together and running. I have enjoyed following your build Don. Looks great and i imagine a lot of C7 owners are jealous.

Well as for the tune id say that cam is a fairly large cam even for a 383". Something large cam motors suffer is they dont breath well at off idle as the vac can doesnt react fast enough when the load is added and the engine stalls. Too much adv.
As much as im a fan of manifold vac i would think your motor is having a bad experience with manifold vac once in gear and load added. U could probably see the same if u where to turn on the A/C. Too much adv with vac adv added to the intial adv for the engines new load.

So what im saying u may want to consider trying a ported vac source on the carb. The technique is to tune it to idle well on a ported source with initial timing adv low enough to allow putting it in gear or the starting the A/C. Then the vac can doesnt operate and add adv unless throttle blades start to open increasing RPM and vac is high/load is low. Hope this makes sense.

To find a ported vac source on the carb u have to look for slots above the throttle blades when closed. Many times u can't push a fine wire through a ported vac port to/through the carb vac fitting (like u can w/unported source) but i use WD spray through the application tube with success.

Good luck hope this helps more than it confuses.

DucatiDon

Stiffer advance spring made no change. I'm beginning to believe it's a lean condition.
Under light throttle, accelerating, it pops like a miss until about 3k or so... Seems ok with the secondaries in. Lean? Is it possible the vacuum isn't dropping enough to activate the power valve?

Runs fine in the shop....issue only appears on the road.

update: interesting data point: I have a pcv restrictor fitting for the factory Holley, obviously this carb doesn't accept the fitting, but I just threw it in line with the pcv valve for smirks....guess what? Huge improvement. Too much vacuum (leak) with unrestricted PCV?

I drove it, it's much better. Noticed the idle dropped slightly, so I hooked up the vac advance ....drove it....huge improvement. Still a slight miss .......

And...it appears my primary float level is high. So gonna adjust that next.
Stay tuned.

DucatiDon

Dare I even say it.........


I got er licked!!


I believe that I can attribute the problem to two issues:

1) Primary float level too high. (Although I can't explain what this would cause.....)
2) vacuum leak through the PCV circuit, causing a lean condition. The restrictor fitting lessens the bleed. The vacuum is now 14" at idle, at 750rpm.

It's totally drive able now. Enough to worry about fine tuning after I fix all the leaks. It looks like either the rear pan is also leaking, or it's the rear main seal.... Won't know till I take the pan off, but won't do that until I've got good use of the break in oil.
Radiator hose will be here next week, and I'll get that leak fixed.

The wife is razzing me about the spots all over the driveway.... <sigh>.

Will keep posted what else I find. And what adjustments I make along the way.

65air_coupe

Glad to hear you got it running better but I still think your vacuum is low. I'm running the XE 282 HR cam (282 duration) in my 406 (not too different from your 383) and my idle vacuum is 17-18 in. and like GTOguy, it pulls like a mule right off idle.

DucatiDon

What rpm does your 406 idle at?

63 340HP

It is goes to reason it now idles and does not bog off idle. A few fine tuning things may help.

The need for a restricted PCV line makes the PCV check valve suspect. The correct 63-67 PCV valve is designed for horizontal mounting, most of the later valves are designed for vertical mounting in a valve cover. Make sure you have the correct valve in the correct mounting orientation to have the correct metering restriction. The 63-67 valve is threaded 3/8 male pipe to 1/2 tube, and usually needs a 3/8 FPT to barb fitting.

The centrifugal mechanical advance needs to be stable at idle and immediately above idle. Your post indicates 11 deg at 400 rpm and immediate change off idle moving 3 degrees by 800 rpm. You need a stable advance to achieve a stable 500 rpm idle. The stiffer advance springs should keep the centrifugal advance stable at 11 degrees to about 700 rpm, to minimize the potential for mechanical advance induced trailer hitching. A worn football and/or worn advance weights at the idle speed leverage point can make this difficult.

Get stable centrifugal advance, and hook up the vacuum advance to a manifold source.

If the primary float was high, check the secondary float. Check both with the carburetor and intake at operating temperature. A perfect looking cold level can be bleeding fuel at operating temp.

You are close enough to get the engine some break in time under load. The vacuum and idle will improve with break in miles. My only concern is the 35 degrees total mechanical advance and another 18 degrees vacuum advance with the AFR heads. The aluminum will mask detonation in the fast burn chambers, and with the 383 you may not lose vacuum and vacuum advance because it is difficult to load the engine in such a light car. A few spark plug inspections during break in may be a worthwhile health check.

If you see evidence of detonation limiting the vacuum advance may be needed.

Good luck.

Westlotorn

Your leaking PCV may have been causing lean idle conditions which make hot cylinder temps, same result as running too much advance.
Both cause hot chambers in the heads.
Sounds like you are getting dialed in one part at a time.
I hope you are having fun with the battle, sounds like you are winning.
Hang in there Don.

Mark

Randy G.

I'll spill the beans on my 383.

It's an aluminum headed engine, 10:1 cr. The camshaft is a roller, .528 Intake, .536 Exhaust, 221 Intake, 226 Exhaust duration @.050, 110 degree lobe separation. It idles in drive (700R4) at 700+/- rpm with 15" of vacuum, 26* timing all in at idle (set at 10* initial with vacuum advance disconnected). Pretty lopey idle with this camshaft. Dyno's at 435HP peak with 465 ft lbs torque.

I calculated the carb required based on an estimated volumetric efficiency, 6,000 RPM max, and determined it would require just under a 600CFM carb at that RPM. Since I'm not racing it and will never rev it any higher I installed an Edelbrock 1406 Performer series carb on it because I didn't want to over carb it. I run the non ported (below the butterflies) manifold vacuum line to the vacuum can. I have an unrestricted PCV valve in the valve cover connected to the port at the base of the carb. It runs very strong with no issues.

In my experience, what you are describing sounds like a lean condition. If restricting the PCV is helping then you are putting a band aid on it and not solving the actual issue.

I would call a few friends and find out if they have another carb you could try. I have an extra 1406 Edelbrock but you are 450 miles away. At least that would rule out a problem with your Holley.


.

MarkC

Don, thanks for posting your 383 story. I have enjoyed the read. Check your valve covers before pulling the pan. Once you get the tuning done, a video would be nice.

65air_coupe

900 A/C off, 750 with A/C on.

DucatiDon

I have a Lars advance limiter plate installed, so I only get about 12*-14* Vac advance.

63 340HP

Good safety factor.

If you do not have a large diameter advance button on the centrifugal advance pin, adding one may help the idle stability. The bigger diameter will move the position of the advance weights on the football at low rpm & idle. This may be enough change to reduce the leverage for less immediate advance change off idle.

Cheers

tbarb

IMO, set the initial timing at 12-14* with a 15* V/A connected to manifold vacuum, if the engine makes 14" vacuum get a control that is pulled full at 12" vacuum.

Make sure the primary throttle blades are set even with the bottom of the transfer slot. Use a standard (for the engine) PCV valve and set the idle speed with the secondary stop screw.

Get a piece of .020 wire and twist it around the vent stack so it will not fall into the carburetor and put the other end in the idle air bleed. Do this on both sides of the primary side of the carburetor. Warm the engine and adjust the A/F emulsion screws then go for a ride and see if it gets better. Closing the idle bleed will allow vacuum to pull harder on the idle circuit allowing a richer A/F ratio.

DucatiDon

I have a B20 advance can already....0@6". And 16@12". (Limited to 12*) which should be fine.

I'll plumb in the factory PCV valve and see if it makes a difference.

Will look at the plugs too.....