View Full Version : irregular heartbeat

08-16-2004, 06:21 PM
I am having carb problems. My first mistake was trying to check a perfectly functioning set of SU HS2 carbs with the official SU balancing tool. All I muddled with was the throttle set screws, but now, after two weeks of trying to go back to the basics, checking timing, spark, fuel pressure, etc., etc., I still have an extremely rough, irregular miss in the exhaust tone. The SU book says it is a lean mixture, but I'm here to tell you I have them set as rich as they'll go, to the point of fouling plugs and fumigating my garage! Following the book, I set the jet nuts so that the jets are flush with the piston deck. Then I get things started, warmed up, and adjust the leanness about 6 flats down so as to get the smoothest idle I can (a misnomer in this case). I just can't seem to adjust the miss out of the ignition, and I am now suspecting something else is wrong. I've adjusted the rockers, set the advance, checked the spark, and am buffaloed. My ear tells me I have an ignition miss, but WHERE!?!

If this is an ignition firing disorder, is there a test I'm not thinking of to check the spark other than what I've done, which is pull the plugs and turn the engine over to see a spark in the plug gap?

I'm all ears...

Tim C.

08-16-2004, 07:54 PM
How about a cracked distrubutor cap or bad plug wires?
Take a look in the dark for a stray spark around the dizzy cap or plug wires.
My last carb problem turned out to be bad plug wires.

08-16-2004, 10:42 PM
A quote form another British Car site: "Most SU Carb problems are electrical".

A lot of things can cause an engine miss. So, let's start with the basics. When was the last Major tune up? If it wasn't recent, start there; new points, condenser, plug wires, cap and rotor. Not the ignition? You say you adjusted the valves? Is one "over done" and causing a miss. Go though that procedure one more time and make sure there isn't one that is a little too tight. Still missing? How bad is the distributor shaft? If it's wobbly-bobbly, you're assured the engine is going to miss. Many people switch to electronic ignition like Pretronix, because it is less sensitive to this problem.

Still missing? OK, let's look at those carbs. You've been through the set up, so let's check a few other things before doing THAT again. Are the throttle shafts worn? If so, no amount of adjustment is going to get it right when the air is leaking in around the shafts. What about the needle and seat? Is the seat centered? Does the piston move up and down freely? Is the needle properly located in the piston?

You should have had two or three fun weekends by now and have this problem thoroughly licked. However, if it's still there, let's move on to scarey stuff. It's time to do a compression and leak down test to determine if we have a blown head gasket, burnt valve or some other take it all apart problem. I hope you don't have to do this part.

To list a few things electrical that have specifically caused missing for me:
disconnected condenser, bad condenser, worn distributor shaft, improperly installed point hardware (insulators, washers and wire were installed in wrong order), oil fouled spark plug.

Good luck

08-18-2004, 09:13 AM
As I started to think further than just the carbs, this occurred to me. I will be checking distributor and related parts soon. Thanks for the suggestion.


08-18-2004, 09:21 AM
A little more info on the engine might help here. It was rebuilt last year, installed over Labor Day, so it has less than one year on it, with about 600 running miles. The distributor didn't "wobble" noticeably, but I installed a Pertronix electronic ignition and Flame Thrower coil anyway, and it really takes off on startup and throughout the power range. The plug wires, rotor, and cap are all new, but that doesn't preclude that the problem is there, and I will be checking them soon. In my experience with engines, this miss sounds most likely like a spark problem somewhere. I need a machine that that I can hook up to the plug wires to tell me visually whether or not each plug is firing each and every time. Do they make such a thing that a local shop might have? I seem to recall the days before computer chips in cars that most shops had something like this.

As for the carbs - both rebuilt, new shafts, and leak tested this time around just to make sure that wasn't the problem (it was prior to rebuild).

I'll report back on the results of checking the distributor and rotor. Thanks for your help.

Tim C.

08-20-2004, 08:48 AM
Set the idle to about 900 RPM and remove one spark plug wire at a time. The RPM-drop should be equal as you remove each plug wire (about 150 RPM drop). If the two back cylinders make a bigger difference tha the two front ones, then the rear carb is probably OK and you should look at the front carb....or visa-versa. (Note: use insulated pliers or something similar and be sure the spark plug wires are not "stuck" to the plugs before you try this).

Also, remember that long periods of idling can foul plugs at low RPMS, but they will still fire "good-enough" in the higher ranges. You may just need plugs (and you may wish to try one heat-range hotter if you are driving the car at lower, moderate speeds).
I like spritenuts suggestion about looking for errant "voltage leaks" in the dark. When I do this, I spritz the idling engine with a water mist (from a plant sprayer). If you have any poor insulation on the coil, wires or etc., the engine will light up like a Xmas tree!
May also want to look for stuck or mis-adjusted valves, intake manifold leak (spray WD-40 near the intake gasket while the engine is idling to find leaks) and head-gasket problems (use compression tester).

08-20-2004, 09:24 AM

I have had one other instance of poor idling, poor running actually. It was from bad petrol. With only 600 miles on your car in the past 12 months, perhaps the current tank of fuel has started to turn and all you need is a fill up and a good long run (~200 miles). One of the symptoms of this problem is when you disconnect the plug wires one-at-a-time; the RPM doesn't change significantly. The RPMs change, but not like an engine running on three cylinders, there will be no one cylinder that produces no change.

08-26-2004, 06:34 PM
Just an update to the thread. I checked the distributor cap, rotor, and wires. All appear to be unchanged from new, other than a bit of to-be-expected wear from use. No carbon lines that would evidence a crack or short. I'll try the one-plug-at-a-time test next. I also suspected a partially plugged jet, so I pulled and cleaned both up this last weekend, possibly did some good, but I haven't had the time to drive it any to road test the work. Will report back in a week...

Tim C.

08-30-2004, 11:14 AM
I tried the spark plug test this weekend - taking each plug wire off as the engine ran to see how it affected the overall performance. Cylinders 1 and 3 resulted in a slightly rougher idle, cylinder 2 made it run extremely rough to almost killing it, and cylinder 4 made absolutely no difference. Good clues for sure, now what to do with that info. As I was pulling plugs, I found that both plugs 1 and 3 resulted in a long spark arching across from where the plug wire entered the disributor cap to the hold-down clip. Hmmm, possibly a loose ground inside the cap? So I pulled off the cap (shut off the engine first, of course) and checked the ground wire connections. All were tight. Then my eye caught a glimpse of the wires from the Pertronix electronic ignition magnet. Both wires were worn down by rubbing on the rotor magnet. The red wire was worn through to where I could see the wire inside the jacket. Hmmm, could this be causing the problem by shorting out every time the rotor made contact? My speculation is that on every rev the distirbutor shaft would arch at this point, which happened to be right at the #4 plug wire contact point, which would explain why removing the #4 plug wire made no difference in idle - it wasn't firing in the first place! This seems like a very logical explanation to what I have been experiencing, so I wrapped the wires in electricical tape as a temporary fix and will fire it up tonight to see what happens. Unfortunately, I was doing all of this static discovery late last night, and our home policy is no noisy/smelly car work after dark. Will report back soon with the results. Even if it isn't the answer, I would use this as a learning point to all: Make sure your EI magnet wires in the distributor clear the rotor magnet!!

Tim C.

08-31-2004, 06:10 PM
Dang! Fired up old Spot last night but the electronic ignition wire wear was not the problem. /ubbthreads/images/graemlins/cryin.gif I am back to either bad cap, bad rotor, or bad wires. Anybody have any good recommendations for types or makes that'll handle 40K volts?


08-31-2004, 10:21 PM
I am using the older style distributor cap with the wires held in place with piercing lock screws. I like these because they seem more secure (for racing). I have not had problems with the cap handling my Pertronix ignition. I am using solid-core spark plug wires (with real wire in them). I bought them and the spark plug end-connectors at my local speed shop (you can also find solid-core plug wire at Harley-Davidson shops and the "Aircraft Spruce" website). You can forget about radio reception if you use these.
I have heard of quality problems with some caps and rotors build in India and China (my cap/rotor came from Moss last year and is Italian-built).