Cold cylinders 4+6. Rough Running after warmup

[Complete]

Hello CBXers,

I recently purchased a 81 CBX with 4800 miles. Beautiful bike. Last week I was riding the bike was hard to start all the sudden. Before, it always ran great and started up great. As I was riding from a cold start the bike suddenly lost power and got a stumble under load as I slowly opened the throttle from 3k rpm. It felt like it was being held back… and then it would break thru the stumble and pick up speed.

As the bike got warmer, it ran worse until finally it wouldnt stay running without working the throttle and it was way down on power.

I stopped and felt each pipe and cyls 4+6 were could enough to grab ahold of. The others I couldnt touch for more than an instant.

I pulled plug 6, photo attached.

After reassembly and scratching my head, I took off. The bike ran good, until it got warm again and the same symptoms came back.

Mike Nixon did the carbs on this bike last year. It had half a tank of fuel.

I’ve read about coils going bad and igniters(spark units)failing. Short of just replacing everything with TimsCBX Dyna kit with new coils… what are my options? What troubleshooting methods would you recommend?

I have the fuel tank and seat removed and I’m contemplating my options at this point.

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[daves79x]

Problem is 4 and 6 are not connected electrically. I'd be suspecting carbs, as in almost all cases, you lose the cylinders in pairs, 1-6, 2-5, 3-4 if its a spark unit or pulse unit or coil. Once in a great while one coil output will fail, but to have two different coils do that at once is not likely. Check your coil connections and spark plug wire and caps carefully.

Dave

[Complete]

Thanks Dave. I was thinking the same thing about the coils… 1+6, 2+5, 3+4 are electrically connected and wasted spark.

The plug wires are like nothing I’ve ever encountered before.
The coils have screw on caps and the wires have just a butt end on the end of the lead.

Why would the bike run good when cold and then run rough as it warms up if it was the carbs?

How could the carbs all the sudden get fouled up after Mike Nixon just went thru them for $1400?? I’ve only ever run clear gas and run the bike frequently.

[Complete]

I just took a look…

So now I understand that the plugwires screw on to the ends with a little woodscrew looking end.

Tonight cylinders 5+6 are stone cold. I checked spark and both have spark. It’s a weak looking spark compared to what I’m used to seeing on modern machines but I suspect its normal for the era.

Here’s a video of #6 firing.

https://youtube.com/shorts/cLVmnE6uvo8?feature=share

The hole is stone cold… so it must be a fuel issue. The cylinder has compression, I felt with my finger quickly (not a gauge but the compression is enough to generate some heat if fuel was present. And the bike runs better with the spark plug out of the cylinder..

Is there a way to address this without removing the carbs?

Any threads on here that you’d recommend?

[NobleHops]

If you have weak spark, let’s chase that down. You can safely snip 1/4” from the leads and remake the connections at both ends to eliminate that as a problem. I’d do all 6. It’s also worth probing the power wires to the coils to measure their voltage when the bike is warmed up. On an old CBX when the wiring hasn’t been serviced it’s not unusual to see as much as a 2V drop in voltage at the coils, comparing the charging voltage at the battery terminals. That current makes a long circuitous path up to the rotary kill switch, through the ignition switch base before arriving at the coils, and those are both notorious spots for crud to accumulate and introduce resistance.

We were in the habit of making coil power relay harness on the bikes we built for this very reason.

This may well not cure your ills, but even so, is all good stuff to baseline.

N.

[NobleHops]

One more thought: If you can get your shop nice and dark, remove the tank and run the warmed-up bike in the dark, looking for old cracked coils arcing to ground once they warm up. Or, if they are visibly cracked, just replace them for good measure.

[daves79x]

Mike's the best in the business - not doubting that. However, turning dirty gas into freshly rebuilt carbs will negate all that good work. Are you sure that's not the case? I've had guys do that a few times, despite my admonition to make sure the tank is clean.

Eliminating that as the problem, follow Nils' excellent advice about measuring voltage and tracking down any problems there.

Dave