Intermittent Rough Running

[surfguitar58]

I am experiencing everyone's favorite kind of problem: The one disappears when you try to fix/diagnose it, but reappears when you are driving and can't stop to do anything about it.

The problem is rough running, mostly during acceleration. It often happens when the car ('74 TCS) is just started cold and sometimes, but not always goes away when the car warms up. Kind of a chug chug chug imbalance between one or more cylinders. I initially thought I was running on 3 cylinders due to an ignition problem, but I would expect backfiring or a different feel. I now suspect one of the 2 Zenith Stromberg carbs is hanging-up or somehow going out of balance temporarily, then popping beck in balance on it's own. Is this remotely possible? Anyone experience this phenomenon before and know of a fix? Feeling a bit clueless.

Tom

[jbcollier]

On a Stromberg equipped car, only one carb has a starter carb attached.  It’s a good idea to warm it up first before starting out.  Having only one starter carb is a bit of a cheat and results in one cylinder pair receiving a richer mix than the other.  This might result in acceleration issues, especially with a marginal ignition system (iffy plugs, high resistance wires, marginal coil, dirty points, etc).

Not saying this is the issue, just another thing to check out.

[surfguitar58]

Thanks JB. The car came with the choke cable bent and disabled, and while reconnecting the choke is on my long list of to-dos, I haven’t gotten to it yet, and therefore I never use the choke. But, I seem to remember reading somewhere that some ZS carbs have an automatic choke with some sort of bi-metal temperature sensitive spring. Could it be that I have an automatic choke that sticks ON sometimes? That would explain the rough operation of half the cylinders.

With that in mind, I pulled the plugs for a look-see. If the forward carb choke was sticking ON I would expect to see rich fouling on plugs 1 and 2, but instead I found this (see pic below). You know those charts that show different plugs indicating too rich, too lean, too hot, too cold, etc? It looks like I’ve got one of each! Number 3 looks particularly bad. Not sure if this is related to my rough running issue, or if I just opened some new Pandora’s box.

Tom

[BDA]

A couple of questions:

Are you using plugs of the correct heat range? I run BKR6EIX in my BDA - a hotter plug than you are running. The 6 plug is popular for road driving on the Elan forum. For track days and extended high speed, the 7s are suggested. There is some disagreement there, of course, but it might be a good experiment after you've chased down everything else.
Have your carbs been properly balanced?

When I ran a TC, the fuel/air charge heater was disabled - as it is on your car - and I never used the choke. It started almost instantly (of the two TCs I've owned, one started instantly and the other almost instantly. I don't recall ever using the choke.) I only say that to imply that on a properly tuned motor in warm weather (I lived in New Orleans at the time) a choke may not be necessary at all. I agree with JB that it is a good idea to warm up a bit before starting out. I would also second his other suggestions. I might add, though I think he implied it, that you should check your timing and your point dwell (or gap).

As for reading your plugs, Passini (Weber guru) says that it is much more difficult to read plugs unless they are new or almost new. Starting with a white insulator is a big help, otherwise, the color could be associated with events that happened prior to your "cut." The "cut" should be taken at the load/rpm that you want to test and the cut should happen by shutting off the ignition with your foot off the gas and the transmission in neutral or the clutch disengaged (this presumes you're testing the mixture while running but the process is essentially the same if you're checking the idle mixture).

[jbcollier]

1 & 2 are running leaner than 3 & 4.  TC carbs do not come with adjustable needles.  They can be converted though.  I’m pretty sure I posted a long detailed description of Stromberg carb overhaul and modifications but I’m travelling right now and can’t find the post.  Strombergs work rather well after fettling.