Creating vacuum leak makes engine idle better?!

[gbentley]

I just bought a '66 D100 with a 318 poly. This is my first post, and my first truck (I'm an AMC car guy, and like AMCs, the Sweptline is a "dare to be different" kind of truck, so I like it).

Anyway, the truck idled very poorly when I bought it. In park, it was bad - in drive, it shook so badly I thought it was only running on 6 or 7 cylinders. Since I bought it, I put a proper NOS Stromberg carb on it (the previous owner had an incorrect Carter on it, bypassed the well choke, and had a manual Dorman choke pull off mounted in the cab) and a new fuel pump and filter. I replaced the distributor with a rebuilt unit, and installed a Petronix Ignitor II and a Flamethrower II coil. I put a new vacuum line running from the carb to the vacuum advance unit, and a new PCV valve and hose running from the base of the carb to the valve cover. I also installed new wires and new, properly gapped plugs (the short-reach Champions ... I read another post that said the 318 should take longer reach plugs, but the plugs that were in there were short-reach Autolites).

After I do all this work, the truck started right up, but it still idled poorly. I tried to set the status timing to 10 BTDC as it says in the service manual, but the idle got worse, so I timed it "by ear", and the engine ran "best" at about 20 BTDC, but it still vibrated and shook at idle. I had to set the idle at about 1000 to 1200 RPM to get it to smooth out a bit, but it's still not right. I connected a vacuum gauge to the only vacuum port on the manifold and the needle rapidly vibrated between about 15 and 19. The engine runs very smoothly when you rev it, but at idle it's terrible, and in gear it shakes badly.

I thought it was a vacuum leak, but I sprayed a little carb cleaner around the base of the carb and the perimeter of the intake manifold, and it didn't change the idle. So I rechecked all the plugs, made sure the wires were seated, made sure the wires were in the proper order, the distributor was on right, etc. Everything was right, but the engine still idled badly. So I go to pull the vacuum gauge from the manifold vacuum port ... and the engine ran much smoother. I put the plug back on the vacuum port ... the idle speed went down and it ran like crap again. I took the plug off ... the engine again ran smoother.

So I got the engine to run smoother by creating a vacuum leak? How is that possible? Anyone have any ideas on what the problem could be (I haven't tackled the tappet adjustment job yet, but I don't think that could be causing this particular problem)?

Here are a couple of pics of the engine and the truck, including the weird split-back bench seat with an armrest that looks like it came out of a Polara or Monaco.

[wally426ci]

Sweet truck. My first thought is timing. I wonder if it is advanced enough to run with vacuum pulled but once connected it advances more?

[Jims68]

to the forums!! By creating a vacuum leak and it smoothed out idling; I wonder if it is running TOO RICH? Just "guessing"...

[Txas2step]

Nice truck. your horn ring is not on the steering wheel correctly, that may cause an idle problem. . 20btdc is too much advance should be 5-10. Plug wires correct? 18436572. 500 rpm idle? check play in timing chain?

[nytemuvr]

Take the fuel pump off and check the tension on the timing chain. Also check to make sure your distributor weight springs are intact and on their posts.

[gbentley]

The distributor is a rebuilt unit I just installed. The wires are in the correct firing order. I can advance or retarded the timing, but it still idles like crap. I leaned out the mixture screws equally, but no setting really improves the idle. After leaning it out, though, causing a vacuum leak at the manifold now has no effect on the idle.

Only advancing the timing and setting the curb idle really high (1100 or 1200 rpm) will allow me to put it in gear without dying, but even with that, it still shakes and stumbles in gear. Does this engine need the longer reach plugs?

I'm trying to eliminate all simple causes before I take the front of the engine apart and inspect the timing chain. Could a stretched chain or improper tappet adjustment cause this?

[JimE]

Step back and recheck the carburetion at idle. A rebuilt carb is only as good as the person who did the rebuild, my point being that he may have over looked the throttle plate shaft in the bottom of the carburetor. Jim

[PwrWgnDrvr]

How about doing a compression test. U may be making a rash assumption that rings and particularly valves are sealing well. That motor didn't come with hardened valve seats and if they've never been changed, unleaded gas eats them up.

[gbentley]

How about doing a compression test. U may be making a rash assumption that rings and particularly valves are sealing well. That motor didn't come with hardened valve seats and if they've never been changed, unleaded gas eats them up.

Hmm. Not so good. Here are the numbers on a lukewarm engine:

1- 100 2- 100
3- 90 4- 75
5- 75 6- 105
7- 80 8- 105

I'll do a leakdown test next. Time to take the heads in for some valve work? The oil pressure is excellent, and it doesn't burn oil, so the rings are probably OK.

[JimE]

Numbers 4, 5 and 7 aren't looking great. Jim

[Jims68]

The distributor is a rebuilt unit I just installed. The wires are in the correct firing order. I can advance or retarded the timing, but it still idles like crap. I leaned out the mixture screws equally, but no setting really improves the idle. After leaning it out, though, causing a vacuum leak at the manifold now has no effect on the idle.

I think you would want to "Richen" the mixture, not lean it out....as it already is getting more "air" than needed. Just a thought. If I remember right, The screws on my "CARTER BBL" turn IN to richen it. Not sure how the Stromberg works though...

[PwrWgnDrvr]

If u have mechanical lifters (all my polys do), as the seat erodes, the valve sinks into the head and the lash disappears, keeping the valves from sealing. As it warms up, it gets worse. Might want to check the valve lash when hot.

[gbentley]

If u have mechanical lifters (all my polys do), as the seat erodes, the valve sinks into the head and the lash disappears, keeping the valves from sealing. As it warms up, it gets worse. Might want to check the valve lash when hot.

Adjusted the valve lash cold, adding .02 to the intake spec and .03 to the exhaust spec, as some have suggested on this forum. What a difference! Set at 10 BTDC, idle at 500, pulling steady 20 vacuum at idle. No shake at idle or in gear. Result! Thanks to everyone for their suggestions.