You mentioned that you taped the choke partially closed during the break in run to keep the A/F mixture from being too lean. Could the rough running at the end simply be because the engine was running too rich and loading up?
Could be possible. However, it ran fine for 20 mins or more, then it was like something sudden happened. I expect I'll figure it out once it is running. I'm just relieved to be able to rule out something catastrophic.
Sorry for the long pause. Lots of cars and projects and only one of me! Well I finally did get back to this. I put in fresh oil and filter. Also cut an old valve cover and put a white stripe on each pushrod. Started and ran it to confirm that all pushrods on both sides are spinning as they should. They are. On cold start oil pressure is about 60, after it gets to 185 degrees, pressure at idle is around 19-20 psi, jumps up as soon as you move the throttle. Dwell is at 30, I'm running a points distributor. I set the initial timing (vac adv plugged) at about 6 degrees. I did place a mark at 30 degrees on my balancer when I built this. All in timing advance (somewhere around 2000 rpm) I have 32 degrees. Qjet carb was started with idle screws out about 2 turns. After it was warm, set the idle at 650; I turned idle screws in until I lost rpm and then opened about a generous half a turn There is still something I am missing here however. As the idle has a noticeable stumble. It doesn't threaten to stall, but it is clearly not smooth as it is supposed to be. FWIW I am running 93 octane I just bought this week. I sprayed starting fluid all around looking for vacuum leaks and found nothing. Vac is between 18-20 at idle BTW. So I took the carb back off and removed the air horn to check for issues. I rebuilt this myself, following Cliff Ruggles book. Idle tubes are clear, needle and seat were clean, nothing on the metering rods. So I put it back together with a new fuel filter. And I replaced the condenser in the distributor just because and checked to see if the points looked burned or something. All looked in order there. I have an old oscilloscope, so I hooked that up and looked at the ignition curve. There was a little variation with the peak voltage bouncing slightly but nothing that looked like any cylinder was popping in and out. I also see the timing mark jumping around about 1 or 2 degrees, if I watch it for a while. I am unsure if that is a concern, but in my memory I expected it to be a bit more stationary. ?? It has been years since I took this car apart. On and off I have probably run it for another hour or more now. It hasn't gotten any better or worse. I just don't like how it idles. This is not a full car yet, so I can't drive it. The engine and TH400 trans are mounted to the frame; radiator, fan shroud are all mounted to the frame as normal. So I figured I would put this note in and call it a day. No odd noises or anything that I have noticed. After the initial run in, when I pulled the plugs, they looked perfect. I think I will pull them again and run a compression check. Thoughts are welcome.
Others may disagree, but I think your centrifugal is coming in way too early. I'd stiffen the springs so you had full advance at around 3K. Light car with drag-race gearing, a little less, heavy car with highway gears a little higher--3200, maybe. Station wagon that tows a trailer, higher still. And--pending test drives--be prepared to advance the initial timing a degree or four. How much vacuum advance does the canister have, and how much vacuum to begin activation--and how much for full vacuum advance? This is a Q-Jet that's had the idle passages and air-bleeds verified and perhaps modified so that it's sure to pass enough idle fuel? You're not trying to run a hot-rod engine on an emissions-lean idle circuit...right? You have a proper base gasket on the carb/manifold, no uncovered vacuum passages? Intake manifold does not have an exhaust crossover passage on the carb mounting pad? Engine fully warm. Wheels blocked, park brake engaged. Connect a vacuum gauge to manifold vacuum source. Run engine at idle, in-gear. Adjust idle mixture screws for highest lean vacuum. (There's likely a range of mixture-screw adjustment that doesn't reduce vacuum. Be on the lean side of that, not the rich side.) Then turn the screws lean just enough to lose about 1/4" of vacuum each. 1/2" of vacuum total. Lean-dropping the idle mix does wonders for tailpipe emissions while hardly affecting idle quality. What is your fuel pressure? Any hints of fuel dripping from the booster venturis? Consider a cranking compression test, and a power-balance test. Maybe even do the power balance test first, to discover problem cylinders while the engine is still running. Engine fully warm, wheels blocked, park brake engaged. Get eight small finishing nails. Blunt the ends on a grinding wheel so they're still pointy but not "sharp". Add a touch of dielectric grease to each nail, and SLIDE it BETWEEN the plug wire and the distributor boot until you feel the nail touch the metal terminal of the plug wire. DO NOT PUNCTURE the plug wire insulation. This should be a "zero damage" operation. Connect the same vacuum gauge to the same manifold vacuum port as you did to set idle mixture screws. Start engine, allow to idle. Take a 12V test light, or even a plain jumper wire, connected to ground. Touch each nail in turn with the other end, watch the vacuum gauge to determine the amount of vacuum reduction for each cylinder. Short-out the spark to each cylinder for five seconds, then let the engine recover for at least five seconds before shorting the next cylinder. (This can be hard on the catalytic converter, (if equipped) and computer-controlled vehicles need to be in open-loop. Vehicles with port injection are better done by disabling injectors rather than ignition.) Ideally, every cylinder loses the same amount of vacuum. Cylinders that lose little vacuum are weak. Cylinders that lose no vacuum are dead. "Dead" cylinders at idle may run just fine at higher rpm/higher load. Don't forget to remove the nails, they'll also ground out on the air cleaner causing no end of frustration. Remove PCV valve from valve cover, remove PCV fresh-air hose from air cleaner. Allow both to breathe plain under-hood air, not engine fumes. Spray your starting fluid into a valve cover--perhaps via that fresh-air inlet hose. Does the idle change? Internal vacuum leak, perhaps via a failing intake-manifold gasket although there's other potential places (cracked castings, etc.) too. I just love ignition 'scopes. Firing lines looked good? Adequate--and even--burn-time, with reasonable slope to the burn line? Snap the throttle, firing lines jump way up? No cylinder-to-cylinder dwell variation? (HEI can vary dwell, but generally doesn't have much from cylinder-to-cylinder. They all change about the same.) Loose timing chain? Worn distributor/cam gears? Worn distributor? TOO-LIGHT SPRINGS ON THE CENTRIFUGAL WEIGHTS?
Wow Schurkey; Thanks for all the thoughts and time you put into your reply. Guys like you make this board the great resource that it is. I may not have a lot of time to put into this for the next month (out of the country). So I want to at least respond briefly. I just ordered a spring kit for the centrifugal advance. Timing chain is brand new, cam gear also. This is a complete engine rebuild. I did disassemble the dist and check it for wear. I wanted to replace the wire anyway so I took the opportunity to clean and check it. It is not HEI, rather this is a 1971 stock points type. Dwell did move a bit on the scope when snapping the throttle, but I don't think it is significant based on my experience. As I am running this engine I always have oil press, water temp, tach, dwell meter, two vacuum gauges and a timing light hooked up. While running the dwell on the meter stays +/- 1 degree. At one point I did move the timing light from plug to plug while it was idling just to see if I could detect a missing flash in some relation to the stumble. It all appeared to be normal visually. Regarding the carb comments. The intake DOES have exhaust path under the carb. This is a stock 1971 build with the original divorced style choke. As noted in the early posts this is about as close to factory stock as it can be. TA cam (112 degree intake lobe), TA double groove cam bearings and Johnson 1405J lifters. Not at all a hot rod engine. And no cat conv. I can't put it in gear as the car is not assembled yet. But I could do some idle adjustments while referencing the vacuum, which I have not done, I've always looked at rpm. The plug wires are new, but they are a cheap set I bought last year. I am thinking that when I get back to this, I will do a compression test and power balance. If nothing obvious comes from that, I'll try the stiffer springs on the advance and new plug wires. Maybe bump up initial timing. A couple photos of the setup, ... just because. I'll post with more observations and test results when I get back to it. Thanks all
looking great! My only comment is the the vac adv appears to be hooked up to the three way valve. Try straight manifold vacuum then carb vacuum, see if that helps.
Are you talking about an exhaust crossover under the manifold plenum, or an exhaust channel carved-into the CARB MOUNTING PAD? If this is a channel carved into the carb mounting pad, you MUST have a high-temp gasket on the manifold, and a stainless-steel heat shield against the carb. Failing to have the proper gasket and stainless heat shield will result in the engine pulling exhaust gas into the air-fuel stream, and overheating/damage to the carb. The exhaust gas in the air-fuel stream will kill idle quality. I was kinda thinking that the exhaust channel in the carb mounting pad was done by '71. This is supposedly a '68, the photos I'm seeing of '70 and '71 don't have the curved channel in front of the primary holes.
If the carved channel does not have openings into the exhaust plenum, you do not need a steel shield. The one in the post above does not require a shield. This one does.
Don't add zinc! Except in a non detergent oil. Modern high grade oil has high detergent that will not allow the added zinc to stick to the moving parts. Racing oil is low detergent and high zinc. Non detergent is cheap but will allow zinc and break in additives to stick. 20 minutes break in, drain, add racing oil. .
With the exhaust passage in the intake that runs under the carburetor like you have, your carb to manifold gasket needs to be the type with the thin steel shim gasket sandwiched with composite gaskets. If you just have a composite gasket with the steel shim gasket omitted, the exhaust will burn through that, and then it will run like crap, just as you described. I did that to myself years ago, and drove me nuts trying to track that down. I ended up plugging those passages (the two vertical sections) in the manifold on two different engines, and they both ran just fine. I could run any carb to manifold gasket I wanted then as well, and gas did not boil out of the float bowl near as quickly.
My intake. Not the type with the exhaust passing through a loop, but it does have exhaust coming in to the bi-metal coil for the divorced choke.
