Quick Float Level Question

[bsdoig]

Recently finished the rebuild on my current project and having a few issues with a slight breakup when cruising in 4th-5th at around 50-55 mph. These symptoms only occur when cruising. At WOT no issues at all.

Little Background. (1977 XS650)
Got the bike together and was running great. Carbs rebuilt (oldsckoolcarbs), cam chain adjusted, valves set, ignition timing check (pamco ignition w/ new coils plugs etc.)

Had an issue with fuel overflow out of the left carb. Pulled the carbs and found a leaky brass float. Ordered a mikuni float, installed and checked float height. Found the the original was a little low around 23.5mm so i adjusted both to 25mm. Got everything back together and seemed to run great, no more leak.

Yesterday I hooped on the bike and experienced the symptoms above. So my question is can float height (to high, fuel to low) create these symptoms? I've double checked all of the above adjustments and everything is OK.

I did notice it looked like it was holding more fuel in the lines (clear hoses). Any advice

 

[5twins]

Well, I suppose re-setting your float levels could be causing that. Usually, float levels have the greatest effect when you put a load on the motor, like grabbing a big fistful of throttle. You have lowered (leaned) the fuel level by going to 25mm. The jetting installed and the needle setting may not be quite right now.

You're using the proper spec for your carb set but it has a +/-1mm tolerance. You could experiment with 24mm (richer) and 26mm (leaner) to see what that does. That would tell you if the problem is float height related.

I assume you have mods like pipes and pods. What jetting are you running (mains, pilots, needle clip position)? On the '76-'77 carb set, you usually can't go much more than mid 130s on the mains because of the stock rich midrange (Z-8 needle jet).

 

[bsdoig]

Well, I suppose re-setting your float levels could be causing that. Usually, float levels have the greatest effect when you put a load on the motor, like grabbing a big fistful of throttle. You have lowered (leaned) the fuel level by going to 25mm. The jetting installed and the needle setting may not be quite right now.

You're using the proper spec for your carb set but it has a +/-1mm tolerance. You could experiment with 24mm (richer) and 26mm (leaner) to see what that does. That would tell you if the problem is float height related.

I assume you have mods like pipes and pods. What jetting are you running (mains, pilots, needle clip position)? On the '76-'77 carb set, you usually can't go much more than mid 130s on the mains because of the stock rich midrange (Z-8 needle jet).

Main's are up from 122.5>127.5 and pilots are up from 25>27.5, needle clip position I assume is stock height. Using Uni pods and straight exhaust. Bike pulls great throughout all gears, its just this small hicup when cruising. I'll try and reduce the float height from 25mm to 24mm this weekend to see what happens.

 

[5twins]

You might want to check your needle setting 1st. You can do that with the carbs still mounted, just pop the tops off to gain access to the slide. Stock setting is the middle or #3 slot. Lean them a step to the #2 (from the top) slot. Increasing the main jet size usually requires this and the needle setting controls your midrange, the area you seem to be having problems with.

You might also be able to run larger mains. I run 132.5s in my '76-'77 carb set. I have the UNI pods and Commando exhaust. I'm thinking with your set-up, you could probably use about the same.

 

[bsdoig]

Just to follow up on my post. I pulled the carbs are re-adjusted the float level back to 24mm. Re-installed and hesitation is gone. Can't explain it, but the bike runs great now.