General Tech Engine Misses Only at 2200 rpm

[KVH]

My recently rebuilt TR4A has been running great. I have a new coil, new distributor, points, carbs--near everything. Over the past two weeks, however, a strange thing started happening. The engine misses rather badly at 2200 rpms. It's fine lower and higher. Regardless of which gear I'm in, the engine misses at 2200 rpms until I go slower or faster. On some occasions there is no missing at 2200 rpms, but most of the time there is missing at that point.

I don't know enough about coils or condensers to say they are likely culprits, and I can't imagine the issue is plug wires or plugs. I'll check the point gap assuming that's a possibility, but that also seems unlikely.

I hope it has nothing to do with vibration or balance, as that would seem to be a rabbit hole. I also don't see how timing could be implicated. I checked that after rebuild and I was good on that score.

Any thoughts out there?

 

[Popeye]

I’m grasping at straws here - smarter folks will chime in: could your distributor advance be “sticking”? Below 2200 rpm it runs ok without, and some point above 2200 the vacuum is strong enough to overcome the resistance?

 

[KVH]

I’m using a Moss “Race” distributor that I believe has a limited advance. I'm uncertain whether that could be a part of the problem.

 

[CJD]

It sounds more like a sticking carb dome to me...not lifting easily as the speed transitions.

 

[KVH]

That sounds promising. I’ll pull the filters and take a look. Amazingly (or not) my new SUs really required a lot of polishing and careful fastening to avoid sticking on first instal.

 

[sp53]

I take out the brass piston filler plugs when doing a drop test. Does your racing distributor have the micro adjust for fine tuning the timing? I have seen coils that miss at a specific spot

steve

 

[KVH]

I have no micro adjust, and if I were to bet I’d say it’s the coil. It just seems like a dead-on electrical skip.

 

[poolboy]

You think it's the coil ?
Then here's the test.

Is my coil bad? : TR6 Tech Forum : The Triumph Experience

I’m a brand new tr6 owner and it’s my first classic car; I’ve got a lot to learn and demystify… I’ve been reading for days it seems on ignition issues since my 72 tr6 did at a stop light with no warning. Eliminated fuel system issues good flow rate to carbs. Now i started

www.triumphexp.com

 

[Sarastro]

That test in the Moss video is largely useless. You often can get a good spark in air, but not in the high pressure of the cylinder where the breakdown field is much higher. Furthermore, it's normally not possible to come up with a test that identifies a bad coil, as the coil usually seems fine at low voltages but arcs internally, or has some other problem, that shows up only at high voltages. Finally, if you try this, KEEP YOUR HANDS OFF THE METAL PARTS! There will be 150-250V on the primary. The guy in the video, who seemed to be touching the alligator clip, didn't get the shock of his life, only because he wasn't touching the cylinder head simultaneously.

The only definitive way to test a coil is by direct replacement with a known good unit.

That said, I doubt it's the coil. I agree that it's something like the carb piston or maybe the advance mechanism, which could malfunction at a specific speed. If the problem occurred at 2200 RPM and higher, the coil would be somewhat more likely.

 

[PAUL161]

Over the years I've had spark plugs that miss-fired under compression. Just saying. PJ

 

[KVH]

I’m really only misfiring at 2200. I’m going to check those carb pistons first. Then, I have a new coil I’ll try. I’ll report back. I’ll get this figured out. And I’ll check the plugs while I’m at it.

 

[KVH]

Following up, I removed and inspected my HS6 SU carbs and found that the piston in the rear carb was sticking in the housing. The carbs are new, so I was quite surprised. It seems that the manufacturing was a bit "off," to say the least. I had to rub and polish the inside of the housing. About ten times, each time noting where the abrasions were occurring. I finally got it. Piston moves freely up and down. I wonder if the heat from operating over the past few months caused some expansion or warping of the metal. Whatever the case, I'm no longer sputtering at 2200 rpms. The call that the issue was probably the "carbs" sure seems right on. I'm now running smooth. Great again. Such fun cars.

 

[CJD]

Glad you fixed it...you and Steve should be getting some good cruising down before it gets too hot!