Bike - MC28 NSR 250SE
Mods - Tyga pipes, UNI air filter
Jetting - Nothing was changed since it was running perfect and now
Considerations - I just recently rebuilt the top and bottom ends of the motor, but please read on: (1) Bike had the same issues before the motor build, (2) Leak down test was strong at 6psi for 10 min, (3) I can get the bike running perfectly with carbs from a friend's MC28
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After 2 years of good performance, my NSR250 (PGM4) has recently been running rough, which I think I've isolated to the carbs.

Symptom 1-
Idle, 1st and 2nd gear pull great. Climbs the power band smoothly with no flat spots. However, once you get into 3rd, the entire bike goes flat, like there's no gas. In this case, I can either keep the throttle open, in which it'll stay flat and feel like it's going to die for a few seconds before suddenly having a surge of power, or I can pull in the clutch and rev it freely. If I come to a stop (ie stop sign) after doing either of the above, it will idle, but once I start to go again, the bike dies like there's no gas. After stalling, it won't start on the first kick, but will always start on the second kick.

Symptom 2-
Again, the bike will pull hard in 1st and 2nd, but if instead of shifting, I actually slam on the brake and come to a stop, the bike again (not moving) will idle, but once I start to go again, it dies out like no gas. First kick doesn't start, but second kick fires right up.

Clues 1-
1. Cleaned the hell out of my old carbs, every jet and every passage allows fluid through
2. Set the floats to 13mm per FSM
3. Cleaned the float needles and valves
4. Synced left and right carbs together

Clues 2- This is where it gets interesting
After running in circles with my own carbs, I decided to test the carbs off of a friend's NSR using my old jets. With his carbs, my bike ran PERFECTLY. Absolutely zero issues.

Clues 3-
So, knowing that it was an issue with my old carbs, I got a used set from Japan. Cleaned em up a bit, put my old jets in, and used my old float bowls because the ones from Japan got damaged during shipping. Fires up first kick from cold but, still the exact same problems and symptoms as above.


I feel pretty confident that the motor is starving for fuel, but I can't wrap my head around where. I'm thinking it's drawing a bunch of gas while climbing the power band and not refilling the bowls fast enough. Open to any other ideas too.


TIA!

ps - to make it all the more interesting, I've also just rebuilt the top and bottom end of the motor. However, I believe I can rule this out as a cause since the bike was running perfectly with my friend's carbs in place (clues2) and it had the same symptoms before the motor work.

You could cross-check the float heights between the good and bad carbs
Have you tried swapping the floats from your friends carbs.
Both floats 16013kv3950 and float valves 16011km9711 are still available
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Why do things simply when you can complicate them

RobH wrote:You could cross-check the float heights between the good and bad carbs
Have you tried swapping the floats from your friends carbs.
Both floats 16013kv3950 and float valves 16011km9711 are still available

Hi Rob - thanks for your reply. I have cross checked float heights, but I did not try his floats in my carbs. I am however using the floats that came in the carb assembly I just got from Japan that are exhibiting the same problems. I guess it's possible that both sets of floats were bad - though seems unlikely.

After reading more about this, one thing that was different between my carb setup and the loaner setup is the carb vent hose. In the set of carbs that ran well, it used a short stock hose that ran from the plastic T connector above the carbs and down the RHS into a red plastic holder.

In my own set, I'm running a very long hose that connects from that T connector down the middle of the carbs, in between the air box boots to where the rest of the drain tubes behind the right foot peg. I wonder if my tube is either too long or getting kinked somewhere and not allowing the carbs to vent and restricting fuel flow?

Well, after 6 months, I finally get to put this topic to rest.

I'm happy to report back that I've resolved the issue above. I stupidly though that the carb vent tube from the top T connector was a drain tube and replace it with a long 2 feet hose and routed it to the back of the bike. The extra length of the hose and routing was restricting air flow and not allowing the carbs to breath. With the vent tube restricted, fuel from the gas tank wasn't properly flowing into the carbs and not filling the bowls after long draws on the throttle.

Well done. It's always worth going back to basics and checking for the obvious differences
_________________
Why do things simply when you can complicate them