02 Nighthawk Compression issues - Page 2 - Honda Rebel Forum
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post #11 of 15 Old 11-13-2019, 05:15 AM
Join Date: Nov 2014
Location: NYC
Posts: 176
Engine turns freely. Compression is good enough. Tolerances are good. No warp issues. Spark is good, even after warmup.

Could it be fuel flow? You tried an auxiliary tank, but fuel would still go through valve, line, float bowl, etc.

Have you tried starting with a can of ether or whatever, to bypass that fuel flow chain?

Honda Rebel 2005; Yamaha TX500 (prior)
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post #12 of 15 Old 11-16-2019, 12:44 AM Thread Starter
Join Date: Nov 2019
Posts: 7
Figured out the issue (more or less).

First, accelerant wouldn't fire after the engine hit the no run wall. While not stellar, compression is still high enough to fire. Spark was still good (present and in time). Fuel physically present. That leaves air flow.

More specifically, controlled air flow. What I had failed to catch previously is the rear portion of the carb cooling while running. My end of the shop runs 45-50 degrees this time of the year. Humidity varies from 40-60%. Reasonable conditions for ice clogs in the air inlets. I have checked the crankcase breather lines (which should counteract icing by warming the airbox). Air travels freely. Also, did another round of checks for intake air leaks. Sealed tight. Being a US model, this one wasn't equipped with a carb heater from the factory.

To confirm the icing theory, I used a heat gun on its lowest setting to keep the rear of the carb warm while running. Had no issues when I did that. Obviously that's not exactly practical while riding. What did seem to help was switching to the recommended cold weather spark plugs. While it didn't stop the icing outright, it did prolong the runtime significantly. As so far I've only been bench testing, I have to assume adding airflow over the head will help keep the carb sufficiently warm (though it's no guarantee).

I still don't love the compression. If it were mine, I'd probably throw new rings in it. However, it's a client's that just wants it in reasonable shape to sell off. Given a limited budget to fall in and the remaining fault being environmental, think I'm calling it on this one.

Really appreciate the input. Always helps to have folks to bounce ideas with.
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post #13 of 15 Old 11-16-2019, 08:22 AM
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Location: Maine
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I am impressed with your thoroughness. Great work.

1985 CMX250 ... 1986 CMX450 ... 2005 Ruckus 50
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post #14 of 15 Old 11-16-2019, 01:01 PM
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Join Date: Jun 2014
Location: Nevada
Posts: 1,489
I've suspected carb icing at times when experancing performance issue..
Some have scoffed, but pilots have to be aware even in summer here under the right conditions..

96 VS800 Intruder (sold 2018 ), 87 CMX250C Rebel,
79 CB400 Hawk (sold 93), 75 CB350 (sold 83), 71 CB350 (stolen 74)
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post #15 of 15 Old 11-16-2019, 01:33 PM Thread Starter
Join Date: Nov 2019
Posts: 7
The only other time I've dealt with it was with a boat engine. Similar setup with each carb feeding 2 cylinders. Humidity was up around 90% at that time. Any bike I've had in with an obvious propensity for it had coolant lines run to the carbs. Oh well. It's been a good lesson.
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