XS250 Engine fade, power loss

[Deltic]

When out and about on my XS250 it goes all day at 50 - 55mph, but once you open it up the bikes accelerates to 70mph with no effort. After about 15 seconds at 70 the engine starts to fade and loses power, speed reduces to around 45 for about 30 seconds then the engine picks up and runs good at 50 - 55mph. Then the cycle continues. Any ideas?

 

[CaptChrome]

Sounds like a fuel delivery issue. There is a restriction between the tank and the carb float bowls.

 

[Deltic]

Sounds like a fuel delivery issue. There is a restriction between the tank and the carb float bowls.

The ‘what had I done since’ in this episode of keeping a 46 year old bike on the road. Previous problem unable to tune the engine, tick over spasmodic, poor starting, carbs and cylinders flooding plugs soaked in petrol. I found the Petcock had been assembled wrong (my fault) and the carb float valves were leaking past. So righted the Petcock and renewed the carb float valves.

Now the problem in this thread.

Yesterday I had the carbs off yet again and looked at the new float valves, the internal needles are 2mm longer than the originals, so I’ve subtracted that 2mil from the float height and now have them set at 24mm.

I’ll give it a run today, fingers crossed.

 

[Deltic]

The ‘what had I done since’ in this episode of keeping a 46 year old bike on the road. Previous problem unable to tune the engine, tick over spasmodic, poor starting, carbs and cylinders flooding plugs soaked in petrol. I found the Petcock had been assembled wrong (my fault) and the carb float valves were leaking past. So righted the Petcock and renewed the carb float valves.

Now the problem in this thread.

Yesterday I had the carbs off yet again and looked at the new float valves, the internal needles are 2mm longer than the originals, so I’ve subtracted that 2mil from the float height and now have them set at 24mm.

I’ll give it a run today, fingers crossed.

Give it a run with the 24mm float heights, the bike is still the same, engine fade/loss of power above 55 mph. Got back home, carbs off (again) and stripped down.
I’ve honed and re-seated the original float needle valve using Brasso as a valve grinding paste, built the carbs back (again) up and fitted (again).

Give the bike another test run and bingo, the bike is now running perfect.