Idle problems under load

[BurgundyZ28]

I have a 68 RS with original 327/powerglide, and recently rebuilt the engine.  I put in a mild cam, edelbrock manifold and 1406 carb.  The ignition is the original distributor with a MSD 6AL box.  I re-jetted the carb for my elevation (3000 feet). The engine is slow to warm up, and then finally idles ok.  Then when I put it in gear, it idles very rough, and stumbles when I try to accelerate.  Once I get it in motion, it runs fine, and cruises down the road very nice.  It's running about 12 degrees advanced, and I put in the lighter springs and weights in the distributor to try to get as quick as advance as I could.  Any ideas with this problem would be greatly appreciated.

[Sauron327]

What's your vacuum at idle? I'd change the vacuum advance can. Read John's Timing 101 very well and save it for future reference. It's invaluable. Reply #8 in this link: http://www.camaros.org/forum/index.php?topic=5534.0  It could be a combination of timing and carb adjustment. 

[BurgundyZ28]

Thank you for all of that great information.  I learned more about timing from that article than anything I've ever read.  I've already shared that with other folks.  I finally had some time to put the info to use, and this is what I found out:
The distributor was connected to ported, so I moved that to full vacuum on the carb.
My vacuum at idle with the distributor disconnected from the carb is 12", and 14" connected.
My initial timing is 14 deg (with the distributor disconnected) as it will barely run with anything less.
The timing is 34 degrees when the distributor is connected, which is strange because this is an original distributor, and my Motors manual is states that the vacuum can should only add 7.5 degrees at 12" vacuum.
With everything connected, and I rev it up to 3000+ rpm, I'm way off the the scale on my damper, which goes to 60 degrees.
It's still rough at idle when it's in gear, and now spits and sputters when I try to accelerate, but smooths out after I let up.  I've obviously still got something wrong.  Any ideas would be greatly appreciated.  One other thing that may or may not be related, is the fuel pressure is about 11.5 psi when I first start it up, and then after it gets warmed up, it drops off to 3.5 psi.

Thanks in advance

[BurgundyZ28]

Update to my last post.  When I disconnect the vacuum off the distributor, and increase the engine rpm, the max total timing I get is 45 degrees.  Where is the extra 10 degrees coming from?

Thanks

[JohnZ]

The distributor was connected to ported, so I moved that to full vacuum on the carb.
My vacuum at idle with the distributor disconnected from the carb is 12", and 14" connected.
My initial timing is 14 deg (with the distributor disconnected) as it will barely run with anything less.
The timing is 34 degrees when the distributor is connected, which is strange because this is an original distributor, and my Motors manual is states that the vacuum can should only add 7.5 degrees at 12" vacuum.
With everything connected, and I rev it up to 3000+ rpm, I'm way off the the scale on my damper, which goes to 60 degrees.
 One other thing that may or may not be related, is the fuel pressure is about 11.5 psi when I first start it up, and then after it gets warmed up, it drops off to 3.5 psi.

The 7.5* vacuum advance specs in the Motor manual must be in distributor degrees, not crankshaft degrees - you have to double them to get crank degrees. Virtually all GM vacuum advance units provide 14*-16* of advance when fully deployed. Have you used a Mityvac to verify and note how much advance the vacuum advance is supplying and at what level of vacuum it's starting and finishing? Ignore vacuum readings except at idle; at any speed/load other than idle, there are too many variables that affect it, and the readings become meaningless.

Similarly, have you verified at what rpm the centrifugal advance is starting, and at what rpm it's fully-advanced and how much it's adding? Unless the advance limit bushing has fallen off the pin, it should only be adding about 24*.

If you have 11.5 psi fuel pressure, you're overpowering the carb's needle and seat; Carters don't like more than 5 psi.

[BurgundyZ28]

Ok, I finally had some time to look under the hood again, and here is what I found out:

Initial timing is 13*  BTDC
Vacuum at idle (850 rpm in Park) 13-14"

Vacuum can: 
8" to start, and 15* max at 15.5"

Centrifugal advance: (with vacuum disconnected)
Motors says:  Starts @900 rpm, and 28* max @ 4200 rpm
Actual:
1000 rpm  13*
1200 rpm  17*
1400 rpm  19*
1600 rpm  21*
1800 rpm  22*
2000 rpm  24*
3500 rpm  28*
4000 rpm  45* (maxed out at 4000)

This car runs GREAT once it get's past the stumble when the throttle plate is opened, and the rpm is around 2000 rpm and beyond.

Do you think I need to change the vacuum can?  Thanks
 

[L22K19]

How did you set the cam timing up. Did you just use the marks on the gears or did you actually check lobe lift.

I had a very similar problem to this when I first started working on cars 30 years ago. I found out the hard way that what they supplied me was not what I asked for.  Have you confirmed that the cam is actually "MILD". What are the spec's.

[JohnZ]

Ok, I finally had some time to look under the hood again, and here is what I found out:

Initial timing is 13*  BTDC
Vacuum at idle (850 rpm in Park) 13-14"

Vacuum can: 
8" to start, and 15* max at 15.5"

Centrifugal advance: (with vacuum disconnected)
Motors says:  Starts @900 rpm, and 28* max @ 4200 rpm
Actual:
1000 rpm  13*
1200 rpm  17*
1400 rpm  19*
1600 rpm  21*
1800 rpm  22*
2000 rpm  24*
3500 rpm  28*
4000 rpm  45* (maxed out at 4000)

This car runs GREAT once it get's past the stumble when the throttle plate is opened, and the rpm is around 2000 rpm and beyond.

Do you think I need to change the vacuum can?  Thanks
 

Your centrifugal advance is adding 32*, taking your total timing to 45*, which is WAY excessive - it shouldn't be more than 35* total (initial + centrifugal). I think that distributor advance curve was originally designed for 2* initial timing (based on '67 specs - I don't have the '68 specs).

If you're going to run 13* initial timing, you need to take out some centrifugal advance so it only adds around 22*, or you'll get detonation at wide-open throttle. I'd recommend 22*, all in by 2800-3000 rpm.

The vacuum can will also need to be replaced - if your vacuum is 13"-14" at idle in Park, what is it at idle in Drive? Your objective is to have the can fully-deployed at idle in Drive, not in Park (assuming you have the can connected to a full manifold vacuum source, not to a "ported" vacuum source).

[BurgundyZ28]

L22K19 -  I degreed the cam when I put it in, and it appeared to be as advertised.  The cam specs are:

                        intake         exhaust
Gross valve lift     .462             .469
Duration at .05    218              224
Lobe lift              .308            .313
Rocker ratio         0                 0
Lobe separation     110
I wouldn't know a mild cam from an extreme one by looking at those numbers.  It's a Comp Cam.

JohnZ - the vacuum when idling and in Drive is around 6", so I see what you mean about needing a different vacuum can.  Is there a can you would recommend? Is there something in the distributor that is supposed to stop the amount of centrifugal adavance?

Thanks

[L22K19]

I would start by confirming that the TDC mark on your balancer is actually correct. As JohnZ has stated, 45deg is way over the top and would be detonating BADLY and is only going to harm your engine. I would also suggest some form of fuel pressure regulator. The presuure you are running will on flood the engine and kill the needle and seat in the carby, if it hasn't already done so.

What is the Comp Cam number. How much compression and what is the manifold & carb  (specific numbers) you have on it. What heads and are they stock or ported. Exhaust?? Do you have a standard torque convertor?

[L78 steve]

A Vac. reading of 6 in drive is darn low for a mild cam. Have you checked for Vac. leaks?

[BurgundyZ28]

OK - I finally get.  Since the original timing called for 4*, and the weights were designed to add 28* to get the desired 32*.  Since I'm running so much initial, I'm beyond what I need.  I'll find some weights to match the intial timing of 13*.

The damper is one of those that is not marked right, so I found TDC, and marked the damper in the correct location.

The Comp Cam part# is 12-238-2, Grind# CS XE262H-10
Heads are stock Dart Iron Eagle S/S 165
Carb is 1406 Edelbrock
Intake is Edelbrock Air Gap Performer
Hooker Ceramic coated headers
Standard torque converter

I've looked for vacuum leaks, and can't seem to find any. 

Thanks

[L78 steve]

Have you clamped off the brake booster hose to check for an improvment? The weights don't limit total centrifugal the slots do.

[BurgundyZ28]

Yes, since this is a RS car, I actually plugged the entire outlet off of the manifold to make sure there weren't any leaks in that area.

Thanks

[mikefam]

One simple test you can do is to hand choke the carb manually when the idle is rough. If it smooths out then it may just be idling too lean. You might try removing the idle mixture screws and blowing some carb cleaner into the holes in an attemp to blow any obstructions out. Also revving the engine to about 2500rpm and slamming the choke flap closed can suck dirt from the idle circuits that can be causing the rough idle. This will of course flood the engine badly so only close the flap momentarily and open it before the engine stalls.

Don't overlook point adjustment and general tune up. I learned the hard way to always start troubleshooting problems with a quick check of the tune-up.

Mike.