Hi all,
I took a chance on an old NS90A and I'm kinda stumped.
I have it on a test stand, and it runs on the top and bottom cylinders at all speeds. It will sort of idle and every 5 seconds a big pop comes out the exhaust, like the fuel charge for the second cyl is getting pushed into the exhaust/leg and igniting.
Compression 125/115/125 cold, can't really get it warm.
Carbs are clean (spotless)
Grounds are OK
Tried it with rev limiter disabled /enabled (black wire at CDI connected/disconnected) with no change.
I figured that I had an ignition issue, but I put an adjustable gap spark tester on that cylinder and am getting a good looking spark that spans a 7/16 gap while cranking. It does not look intermittent. I swapped coils between cylinders with no change. Spark on other cylinders looks the same as #2.
After verifying that I had spark, I figured maybe I had a reed issue (not pulling fuel charge in), so I checked those and they are perfect. Checked the bearing drain check valves and they are fine. No obvious signs of a vacuum leak anywhere.
After all that, I started thinking that maybe I had a water leak into the combustion chamber. The #2 plug didn't look wet but I was running out of possible explanations, so I pulled the head, and the exhaust cover, but didn't see anything unusual. The cylinder does not have any signs of water or steam.
So, I'm not sure anymore. I ordered a DVA adapter and will run through the ignition test sequence in a few days. Perhaps I am getting spark while cranking but not while running, but that seems unlikely. The only other thing that comes to mind is a major vacuum leak at the block seam, or that the timing on #2 ignition is off (CDI?).
Did I miss anything?
Thanks...
I took a chance on an old NS90A and I'm kinda stumped.
I have it on a test stand, and it runs on the top and bottom cylinders at all speeds. It will sort of idle and every 5 seconds a big pop comes out the exhaust, like the fuel charge for the second cyl is getting pushed into the exhaust/leg and igniting.
Compression 125/115/125 cold, can't really get it warm.
Carbs are clean (spotless)
Grounds are OK
Tried it with rev limiter disabled /enabled (black wire at CDI connected/disconnected) with no change.
I figured that I had an ignition issue, but I put an adjustable gap spark tester on that cylinder and am getting a good looking spark that spans a 7/16 gap while cranking. It does not look intermittent. I swapped coils between cylinders with no change. Spark on other cylinders looks the same as #2.
After verifying that I had spark, I figured maybe I had a reed issue (not pulling fuel charge in), so I checked those and they are perfect. Checked the bearing drain check valves and they are fine. No obvious signs of a vacuum leak anywhere.
After all that, I started thinking that maybe I had a water leak into the combustion chamber. The #2 plug didn't look wet but I was running out of possible explanations, so I pulled the head, and the exhaust cover, but didn't see anything unusual. The cylinder does not have any signs of water or steam.
So, I'm not sure anymore. I ordered a DVA adapter and will run through the ignition test sequence in a few days. Perhaps I am getting spark while cranking but not while running, but that seems unlikely. The only other thing that comes to mind is a major vacuum leak at the block seam, or that the timing on #2 ignition is off (CDI?).
Did I miss anything?
Thanks...