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Have an 02 Nighthawk 250 with 7k miles in my shop right now. It's being quite uncooperative. I'm hoping, given the similarities with the Rebel, someone here can help shed some light.

Starting from cold, bike runs great. Once warmed up, idle becomes rough and will stall. The longer it runs, the faster the rpm tanks when you release the throttle. At full operating temp, it becomes impossible to restart, regardless of choke/throttle position. This is the case no matter where the idle adjustment screw is and using a free flow auxiliary fuel tank.

I've also tested all applicable electrical components for faults, both cold and after running. Charging is solid, spark is strong at all points.

Compression was a fair bit under spec. ~130psi each cylinder (manual for the year puts spec at 200psi or so). Leak down was 1-3% at 100psi. Set valve lash to .08mm. Compression came up to ~150. Did a run test in case specs were misprinted. Same issue. Tried a range of lash measurements. Only way I could get compression up was at 2mm(and god, do I wish that was a typo), and even then it was only 170. Oh, and did verify cam timing was correct.

Pulled the engine. Removed the head and started giving my calipers a workout. Cam lobes were good. Valve stems were good and straight. Springs were in tolerance. Seats had a little carbon buildup, but nothing major. Only real issues were valve seals having hardened a little, and some drag when removing the exhaust valves. Both likely due to the bike sitting for too long.

Replaced the seals. Drag seemed to be gone after removal, so guessing part of the cause was oil debris. Lapped the valves. Assembled and set lash to .08mm. Compression was still 150, same running issues. Did a leak down cold and hot tonight. 0-1% both ways.

Dropped the engine again. Applied heat to the head to see if it would cause any of the valves to drag. Was only using a heat gun to minimize the risk of warping anything. Still moved freely.

I'm kinda stumped. Also tired, so probably missing details, but I think I touched on all the major points. If you have read this far, thanks. If you have ideas (especially in terms of common issues I may be overlooking) I am all ears.
 

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does the enricher plunger move freely?
 

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Using throttle to start shouldn't be done according to Honda, and hasn't been needed on any of my Rebels. Choke (fuel enricher) should be used as needed. The Rebels vary greatly in how much choke they need and will tolerate. None of mine would start on full choke, and two didn't need any in warm weather. About 1/2 to 2/3 choke is optimum in my experience.

Are you sure the carb is clean? You mentioned that the bike sat for a while, and if the carb wasn't drained beforehand, there have to be some deposits in it. That might account for the starting/running issues.
 

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Discussion Starter #4
Carb has been through an ultrasonic cleaner twice and all passages blown clear after. Plus a standard solvent cleaning. Float level is spot on. Choke moves freely.

Starting cold, doesn't need throttle. Choke lever out, and pops right off. As it starts to warm, choke lever can go in and run/idle fine. The problem is when it approaches and hits full operating temp.

Carb issues wouldn't account for the compression running ~50psi low cold and more hot.
 

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Carb issues wouldn't account for the compression running ~50psi low cold and more hot.
true - you didn't mention the carb so it was worth a shot

what about the specs on the cylinders, rings and pistons? trying to figure out why it would behave that way when hot

wear causing blowby? - it's a mystery
 

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Discussion Starter #6
Haven't measured the cylinders/piston yet. Cylinder is clean, though it will get a quick hone before reinstalling. Hadn't given thought to the pistons being an issue given no leak during both hot and cold leak down testing. Checking that end is on today's agenda.
 

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Discussion Starter #7
Quick update:

Measured pistons, rings/end gaps, and cylinder. All in standard tolerance (tighter than listed service limits). No blockage in piston oil ports.

Got a little experimental last night. Put the head and valves in the oven. Wanted to see if any of the metal would expand in such a way to cause any temporary warping/valve drag. Service manual suggested heating to 300F for valve guide removal, so that's what I brought it to. No issues. Valves had full range of motion and no warp/bend while hot.
 

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I really hope you figure this out and let us know - sounds like you have covered all the usual suspects
 

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What's the update?

You write, "At full operating temp, it becomes impossible to restart, regardless of choke/throttle position. "

Question: When in this "impossible to start" mode, does the engine turn over freely, with the starter?

Question: You apparently wrote that compression was OK in this hot mode. Correct?
 

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Discussion Starter #10
Put it on the back burner. Checked over the oil pump in case there was a lubrication issue (grasping at straws). No obvious problems.

Engine turns freely after the warm/wont start. No vapor lock.

Compression dips when warm/when the issue arises. Have done a hot leak down. No leak. Like I said, ran the head in an oven. No valve binding/bending.
 

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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?
 

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Discussion Starter #12
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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I am impressed with your thoroughness. Great work.
 

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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..
 

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Discussion Starter #15
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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