Guaranteed on-topic tuning question.

[Paul VW CLC #21348]

OK folks, heres the problem.  This is a 68 Deville with, of course, a 472, and 53,000 miles.  It has an uneven miss at higher RPMs under full throttle.  If I accellerate under full throttle for, say, 5 seconds, it doesnt happen.  Any longer, it starts to miss, kind of like it bogs and recovers.  It only happens under load.  Also, the car doesnt seem to have quite the power it should from a stop-  I cant spin the rear tire from a stop, unless Im turning right.

The car had been driven very little for 8 years before I bought it.  Right after I bought it, I put a rebuilt carb on it, and new plugs.  That solved the low rpm problems, and got it running OK, but left me with this miss under heavy acceleration.  I dropped and cleaned the fuel tank, replaced the in-tank fuel filter, the fuel pump and filter and all rubber fuel lines, and blew out the hard fuel lines.  This helped, but didnt elimminate the problem.  The plug wires look pretty new, and I cleaned them, and made sure they were all seperated with wire holders, so there isnt any cross firing.  I guess Ill replace them this spring, but I cant imagine they are the problem.  I dont see any little blue sparks when I rev it up in the dark.  The dwell and timing are correct, assuming the timing mark is in the right place, there are no vacuum leaks at any of the lines coming off the carbs, and the vacuum and mechanical advances seem to be working.    What am I missing?

[Porter 21919]

I would inspect/replace the cap/rotor/points and condenser, might as well get a new set of wires for $ 20 while you are at, preferably silicone, get a new PCV valve too.

Also check your vacuum and centrifugal advance at the distributor.

If you can adjust the idle with the mixture screws and the accelerator pump shoots gas when you crack the throttle your carb is probably okay.

HTH,

Porter

[Porter 21919]

http://www.centuryperformance.com/vacuum.asp

[Steve Preston #16375]

Well, I certainly know your frustration as I tinkered with my 68 convertible for a year before I got it right. Porter is probably on the right track, it sounds like an ignition issue. I didnt get good results with mine until I replaced the distributor with a rebuilt and installed a pertronix ignitor instead of points. I also installed an MSD 6A ignition and blaster coil.

So.. as Porter said, do a full tune-up. New wires, at least 8mm, plugs, cap, rotor, points (or electrinic conversion), ignition coil and condensor. Make sure that your new plug wires do not lay on any metal parts of the engine and use wire seperators to space them away from each other.

After that, itll be time to deal with the fuel. You may be having a problem with float level, the inlet fuel filter, fuel pump, power piston or the secondaries. But 1st things 1st, get the ignition up to par.

[Porter 21919]

http://www.recarbco.com/technical/newtrouble.html#Warm_drive

I think your problem is more likely ignition related, if you have a steady timing mark your distributor is okay, increase the rpms and see if the mark advances to check the centrifugal advance, my vacuum advance will move a little bit, I plan on switching from ported vacuum to intake manifold vacuum as many here have recommended.

I do okay with my antique breaker point igntion, it keeps me amused with my tach/dwell meter.

HTH,

Porter

[David #19063]

Paul,

Pull all your spark plugs and check the condition and the gap on each.

Also, there has been a rash of bad coils or should I say intermittent coil issues.  In fact, now I realize that one of my previous cars had a coil problem as I had the same problems you are experiencing.

David

[Lynn 10923]

It sounds to me like the carb is running out of fuel. Ive had this same problem also, at the top end of a full throttle run the engine will start sputtering. When I put a mild aftermarket cam and dual exhaust on it, the problem got worse. I havent got this fully ironed out yet, but one thing I did that helped a lot was to take the fuel filter out of the carburetor inlet. There is still a filter in the line just above the fuel pump. The little ones in the carburetor are very restrictive and wont supply enough fuel under a high demand. The 5/16 fuel line from the pump to the carb doesnt help either. Later 472/500s used a 3/8 inch line.

As far as not being able to spin a tire except when turning right, thats about normal for this car with a stock engine. Due to some factory compromises in the engine design, this engine doesnt really have the energy an engine of this size ought to have, even a stock one.

Lynn

[densie]

   I was going to write this long, involved message about my experiences with fuel filters, but I think Ill make it short.  I dont want to be controversial, but I dont recommend that you take out that filter.  Replacing it with a new one would be a good idea, but dont start modifying things to chase a problem, or youll likely end up inducing more problems.  Remember, it worked when it rolled out of the dealership.

   This idea did remind me, though, that you might follow the lines around and see if someone has installed an aftermarket inline filter that could be clogged.  Also, check any rubber hoses in the fuel system, especially the ones that connect to the tank.  If theyre soft, they will crush, or get porous and pull air into the lines.  The fuel pump itself could be weak as well.

   You can remove the hoses at both ends of the metal lines and  blow air through the lines to make sure that theyre clear.

   And, oh, gas cap.  Make sure that you have the correct one.  As Im writing this, Ive already forgotten what year your car is, but however the fuel tank is vented, make sure that its vented properly.  If it cant breathe, it cant pump.

-densie

[densie]

  From the symptoms, it sounds like an ignition problem, but since you said that it hadnt been driven much, there is a good chance that you have shellac in your carburetor.  I would recommend putting a kit in it and cleaning it out very well.  Also, check the cam that opens the secondaries to see if it has broken, and check the four tubes that come down from the air horn.  They get loose and leak or fall out.  You can put some lock-tite on them and gently tap them back in with a rubber hammer.

-densie

[Bob Chesnutt]

My experience has always been with this type of miss is that it is a spark plug.  Check your new ones for a problem, maybe a crack in the porceline or a mis-adjusted clearance.

[Mike #19861]

 To me, I would be hunting down an ignition problem. You seem to have all the bases covered, at leat from a general stock point, in the fuel sysyem.

 WOT demands a lot from the ignition system. All components are asked to perform at their peak. Since the car has not been driven much, I would go through the system and check things closely. Do a complete electrical tune up, including points, condensor, wires, rotor and plugs. Use only AC/Delco parts. Pay close attention to tuning specs as well. These engines respond well to attention to detail.

 Check the weights in the distributor. Make sure they are not loose or sticking. Check the vacuum advance with a vacuum pump. You can connect it to manifold vacuum as well rather that ported or venturi vacuum. This will help low end response and deliver better fuel economy. Set timing to recommended specs.

 Finally, did you replace the carb with an off the shelf replacement rather than have your carb rebuilt? That is a no no in the Q-Jet world. An off the shelf replacement will not likely work as well as the original. Q-Jets are calibrated for the particular application they are put on to. A replacement rebuild have a general, or middle of the road, calibration for a wide assortment of applications. You do not know where this carb came from. I would have the carb recalibrated to the specs particular to your car. A low float level may be contributing to your problem, or metering rods and jets may mot be large enough leading to a fuel starvation problem.

 The 472 when properly tuned can be a real performer. You do not need any exotic aftermarket parts to wake them up. Just close attention to tuning. For example, I can peel rubber from a start with my 70 Fleetwood and it has a controlled differential. I can attain WOT for as long as I have road, and I have found it will run with Impala SSs and Caprice 9C1s. The only mods are tuning, HEI and an open element air cleaner.

 On the thought of the air cleaner, the single snorkel will only flow 380 cfm (as tested by Al Betger of MTS) which is completely insufficient to the demands of the 472. Try opening that up as well. It will help. I just placed a smaller cover on mine which leaves the element exposed all the way around. The engine can pull all the air it needs.

  Mike

[Jerry Goclan]

If after doing the proper ignition maintenance(master tune up)and you still have the same problem, you may want to investigate if your cad came equiped with a tank filter  (tank sock). If so 1 trick I have used is to remove the gas cap, and blow compressed air through the inlet line backwards into the tank, which will usually blow the sock off the end of the pickup tube, and add an inline filter, or pull the sending unit and replace the orignal sock.  Jerry G.