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  Topic Review (Newest First)
11-16-2019 01:33 PM
ProgenyPowersports 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.
11-16-2019 01:01 PM
SoakedKarma 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..
11-16-2019 08:22 AM
01-7700 I am impressed with your thoroughness. Great work.
11-16-2019 12:44 AM
ProgenyPowersports 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.
11-13-2019 05:15 AM
reb2005 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?
11-12-2019 03:44 PM
ProgenyPowersports 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.
11-12-2019 12:03 PM
reb2005 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?
11-06-2019 09:58 AM
01-7700 I really hope you figure this out and let us know - sounds like you have covered all the usual suspects
11-06-2019 01:32 AM
ProgenyPowersports 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.
11-04-2019 02:32 PM
ProgenyPowersports 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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