No power at low rpm

[Drizzle]

Stock motor with bs38 carbs with higher jets. Don't remember which jets in all honesty. Running exhaust headers i made, hhb full cdi kit and velocity stacks. My problem has been that I have no power at low rpm. Bike seems to act like a 2 stroke power band. Once I reach higher ram's, it takes off like a new bike. Interstate speeds are good. I have tried rebuilding the carbs, cleaning them. I am lost right now. I wish I could afford vm34's but currently I can only work with what I have. I've tried searching the forums with little luck. Anybody got any ideas? Thank you.

 

[Kevin Werner]

I had "running like a 2 stroke" on my undeveloped/unsorted '71 750 bbk. The mains were 3 steps too rich and the pilots were 2 steps too lean. Fortunately the jets are less than $5 each and some source have them at less than $5 a pair. If your carb jets are far from the specs in the chart in this thread you might need to reconsider. Know that each year/series carb set is built around the included needle jet, needle combination and what works with 72 BS38s is irrelevant to 78 BS 38s.
https://www.xs650.com/threads/carbs-carburetors.43/

 

[Drizzle]

I had "running like a 2 stroke" on my undeveloped/unsorted '71 750 bbk. The mains were 3 steps too rich and the pilots were 2 steps too lean. Fortunately the jets are less than $5 each and some source have them at less than $5 a pair. If your carb jets are far from the specs in the chart in this thread you might need to reconsider. Know that each year/series carb set is built around the included needle jet, needle combination and what works with 72 BS38s is irrelevant to 78 BS 38s.
https://www.xs650.com/threads/carbs-carburetors.43/

Thank you. I will be taking the carbs off again and checking the jets. I am uncertain on which year bs38's I am running. I have the old ones on a shelf but bought a "tuned" set years ago before I knew the vm34's existed. I am thinking it is a carb issue. Doesn't bog when reving in neutral but under load it just doesn't want to go well at low rpm.

 

[Kevin Werner]

Thank you. I will be taking the carbs off again and checking the jets. I am uncertain on which year bs38's I am running. I have the old ones on a shelf but bought a "tuned" set years ago before I knew the vm34's existed. I am thinking it is a carb issue. Doesn't bog when reving in neutral but under load it just doesn't want to go well at low rpm.

I have not removed "needle jets" but your needle should indicate which series of carbs you have. Look at the needle with a magnifying glass and get the number (assuming original stock needle). My "2 stroking" went away when I went up 3 on the pilot. I have a 750cc kit, foam pods and a pretty open exhaust with 77 carbs. I imagine my small pilots caused me to open the idle adjust until the bike was idleing on the main jets because the pilots were to small. When I went up on pilots I had to back off on the idle adjust considerably.

 

[650Skull]

This will help with identification. Pre, and including 74 carbs, were twin cable operated carbs with crossover tubes linking then.
https://www.xs650.com/threads/tentative-carb-id-layout.43392/

 

[Jim]

You said you made your own headers. What size pipe did you use?
Our XS' are known to not like larger diameter pipe on the headers. Takes away the bottom end grunt (like you're experiencing).
Mikes XS sells inserts... most people just make their own.

 

[Jim]

Sorry... the link.

https://www.mikesxs.net/yamaha-xs650-headpipe-torque-inserts-pk-2.html

 

[Drizzle]

You said you made your own headers. What size pipe did you use?
Our XS' are known to not like larger diameter pipe on the headers. Takes away the bottom end grunt (like you're experiencing).
Mikes XS sells inserts... most people just make their own.


View attachment 252001

I used to have a set made by Joe wise guy but they broke in half so I actually made my own yesterday that actually sounds better and rides better but still has the 2 stroke experience.