Missing Cylinders

[Matthew E. Herd]

Here's another doozy for you all. Lets see what we can figure out.

It would seem that the car has a dead miss in two cylinders ('76 Midget). It runs rough cold (who has a car that doesn't) but this prompted my mechanic (a touch obsessive) to start playing. Once again, we rechecked compression and found 225 in the rear three. However, when pulling plug wires, as expected, the engine RPM dropped significantly when wire 2 and 4 were removed. The unexpected part is that the engine RPM did not drop when plugs 1 and 3 were disconnected. The high tension wires for plugs 1 and 3 were switched (even though the order is 1-3-4-2 if i'm not mistaken and that's what was reflected by the distributor) and the car would no longer want to start. Under all circumstances, the spark arc'ed at least 1/2 inch from the distributor to ground. However, the lack of a wire on 3 did not seem to affect the engine as it was revved up to about 6000 RPM.

As an aside, I'm running dual 40mm Weber DCOM's.

Observations: 1 and 3 must contribute somehow if the car would not run at idle under these conditions (it would fire off, but not keep running without gas). I seem to have plenty of spark and the cylinders looked to be burning well, as when run for a bit, the plugs are golden brown. Compression is good in all cylinders tested (not 1). If the cylinders make compression and have spark, I suspect the following problems: out of phase sparking, or improper mixture. If the order is correct and the car runs, the sparking is most likely in phase. If the mixture is bad, the plug color would not be golden brown (incidently, when only run for short periods and started cold, the plugs are black. I suspect this to be a side effect of the fuel that is dumped in when the pedal is pumped a few times to start it.) Any experience with this problem?

 

[ThomP]

Here's my guess: You may be running a tad lean. Yes, the plugs can be that brown color and be lean. My Big Healey needs to be about 1 flat (1/6 of a turn) rich from the ideal Colour Tune setting to be happy in traffic. My Mini is somewhat tuned and it is fussy about the difference in Florida summer and winter weather. In the winter, it will stumble along, even after it has reached operating temperature, if it isn't given a little extra fuel.

 

[aeronca65t]

Hmmm...almost sounds like another bent pushrod or broken/damaged rocker arm.
Or...If it's idling and you give it some choke, does this help? (indicating, as above, a lean condition).

 

[Matthew E. Herd]

Don't actually have the chokes hooked up. However, I am now leaning toward electrical. Today (clutch problem solved AND the oiling and adjustment is taken care of) I drove it for quite a while -- not small joy, let me tell you! Turns out that when warmed up, in traffic, suddenly the idle smooths out for a bit and picks up to 1300-1400 from 550-600 and rough. Then it will drop back down after 5 or 10 seconds. I'm investigating plug wires tomorrow, despite the obvious arc'ing I saw before. Also, smells to me like its burning rich, which would indicate dead cylinders, not a lean mixture. It stinks to high heaven I will keep up on the details!

 

[Matthew E. Herd]

Don't actually have the chokes hooked up. However, I am now leaning toward electrical. Today (clutch problem solved AND the oiling and adjustment is taken care of) I drove it for quite a while -- not small joy, let me tell you! Turns out that when warmed up, in traffic, suddenly the idle smooths out for a bit and picks up to 1300-1400 from 550-600 and rough. Then it will drop back down after 5 or 10 seconds. I'm investigating plug wires tomorrow, despite the obvious arc'ing I saw before. It smells to me like its burning rich, which would appear to indicate dead cylinders. It stinks to high heaven I will keep up on the details! Also, thanks a ton for the advice. Perhaps it is indeed a bit too lean on one barrel First fiddle with electric, then carbs /ubbthreads/images/graemlins/smile.gif

 

[aeronca65t]

A handy way to check plug wires is to swap them around and see if the same cylinder is still dead.
Another easy test for high-voltage "leaks" is to park the car in a dark area at night and let the engine idle. Spray pure water on the engine (with a plant sprayer).....if you see a "Chrismas Tree" of lights and sparks, you've found your problem.
I went to my local speed shop and bought solid copper wires for my Spridget(I had to cut them down to length)...no radio in my car anyway. Bonus is that it seems to bother cells phones in cars near me! /ubbthreads/images/graemlins/devilgrin.gif

 

[Matthew E. Herd]

Hehehe /ubbthreads/images/graemlins/smile.gif

 

[Matthew E. Herd]

Update: Just ran it for a 25 mile jaunt, totaling about 100 miles in 26 hrs or so! Seems that swapping plug wires does nothing (the same cylinders are still suspect). However, number three does have a marked drop in idle speed as compared with before, which was none. However, it is still less than 2 or 4. 1 still has no drop. This leads me to believe that the carbs are varnished up. The car has been sitting a long time and the carbs were also not new when purchased, so this does not seem unlikely. Moreover, idle speed (warm) increased from 550-600 to 900-1000, indicating that something is now working better ... (cylinder 3 perhaps). The changes in idle speed still occur, so not everything is cured, but perhaps more driving will continue to improve the status! (I sure hope so /ubbthreads/images/graemlins/devilgrin.gif) When manually applying the chokes, the idle speeds up significantly. I believe that this is further support of the varnished carb theory. However, even if its only making 75% of the power, its still pretty fast!

On a completely unrelated note: Gee, I'm glad I purchased the GCR, because almost every SOLO I rule is Refer to blah blah blah, exception made as follows... Pretty darned useless /ubbthreads/images/graemlins/smile.gif The SOLO II part is 90% of the book. Better get to reading ...