Here's the story: 1972 Lark, 350 4bbl. 95k on car. Just converted to Crane PS20 coil and XR-i points conversion. Also installed new plugs, gapped to 40, and put on new wires and cap and rotor. Timing is set at 6. Tried setting timing all the way from 2 to 10 with no change in the stumble. Vacuum advance is working. Car now has a slight, somewhat intermittent stumble and backfire through the exhaust. The QJet is new about 10k ago, but that was before I had the car, and the ignition system was in pretty bad shape...bad wires, way old plugs, bad wiring to points, etc. My guess is the carb was adjusted for the poor ignition and that led to a way rich setting. It didn't seem like it stumbled prior to the ignition swap, but its hard to tell because it wasn't running all that smoothly. I believe its running way rich right now. I have a lot of soot around the tailpipe, and the stumble/backfire seems too intermittent to be ignition related. The miss is present all the way from idle to WOT. I tried to adjust the idle metering screws, but i turned 2 1/2 turns in each direction with little to no change in the miss. Leaning it out resulted in a drop in RPMs, but no change in the stumble. I know that you can adjust the power piston rod depth, but it would seem to me that i should be able to overcome a rich condition at idle be tightening the mixture screws, yet it doesn't seem to help. Any thoughts????
You need to check all your vaccum lines, total vaccum and timing . Initial timing might be way off. Also make sure that your new system uses .040 gap . I did a Stinger and it uses .032 I had it set up for GM .060 HEI and it ran like crap. Make sure you have no vac leaks at the carb and intake. Tune the carb adjustment with a bac guage not by ear.
Initial timing is stable at 6 right now at idle with vac advance line plugged. Vacuum is around 25 at the carb, IIRC. Crane recommends 35-45 gap. The stumble is present at idle, so that would at least point away from advance issues, correct?
Also, it doesn't seem that I have any vac leaks...sprayed carb cleaner all around with no change in idle. A quick shot down the primaries made it bog out and almost die though.
If all the other ignition stuff is new, consider putting a rebuild kit in the qjet. They're inexpensive, and it only takes an afternoon (if you've never done it before) to get the job done. Pull it apart, clean everything thoroughly, spraying carb cleaner through every port and orifice. Blow compressed air through all passages. Reassemble and install with a new float and fuel filter. Also, check your plug wire order. It is common to swap the 5 and 7 wires.
A stumble at idle sounds to me to be something lie a clogged PCV system, Idle tubes in the carb (very possible) maybe try another carb. Is this a California emissions witha EGR? Its is also possible that some gunk has formed on your valves and is holding a exhaust open a little. There are numerous p[alces to look that can affect idle quality. In your case if you have or can get a known goood carb I would try it. Is there any gunk build up in the valve covers around the springs? You might try usingt GM engine top cleaner system available form the dealer or at least it used to be . thats if you cannot get idle quality from a different carb. Another thought is to do an old test. with the engine running pull one plug wire out at a time and if you find one that doesn't drop idle rpms then you have a problem on that cylinder
Car is not emissioned. PCV valve is working and has been replaced. The stumble is intermittent.....like one or two 'puffs' every 15-20 seconds or so...not steady like a valve issue.
An easy way to diagnose a valve problem is to un hook the neg at the coil and disable the start and crank it. If it cranks and has an even sound ok but if you have a even sound and then a dip in the sound you have something to look into. remember even new plugs can be bad. Maybe pull them out and have a look at them
That really doesn't matter. Bad fuel, sitting for extended periods of time, and other things can make a carburetor gum up and not work correctly. I had a carb that would NOT idle for anything, and I spent all kinds of money trying to fix the problem in the ignition system, and it ended up being the carb's idle circuits being clogged up. 10k miles before that, I had installed a rebuild kit. o No:
I would eliminate the components I changed before diving deeper into other components. 1st I would set the gap on the plugs to .035 and see what happens. If the miss is still there I would pull the crane unit and go back to the ignition that was removed. If it was points replace the points with a good quality set. If the miss is still there then check into other engine components.
So last night I got ready to tear into the car again and pull plugs, and pull the Crane unit back out. I thought I'd go through the firing order one more time and reseat the plug wires into the dist. cap..... Well, I found the problem. #4 plug wire connector had pushed back far enough in the boot that it was only making an intermittent connection, but it was enough to hold it and the boot in place on the cap so the problem wasn't readily visible. Pushed the connector into place and she runs like a champ now. ou: Talk about a dumb thing to happen.... On a side note, the Crane Xr-i unit seems to function flawlessly. Nice, solid performance all the way up the RPM band, and now that all 8 cylinders are firing, I can really notice a difference in the smoothness of the car over the points. Installation of the unit, including distributor removal took probably 15 minutes, and it seems to have been well worth it. Seems to be a lot simpler and a lot more straightforward than the Pertronix kit. Also put in a bit hotter coil, the one recommended by Crane (PS20). Also, the unit NEEDS a resistor, whether by wire or stand alone. My car had a factory resistor wire, so I didn't have to modify any wiring. Eventually (when I decide it would be fun to run that hot wire for the HEI) I'll switch to the big body HEI setup I have have in my parts pile, but for now this was a nice, easy swap-out over the points.
