550 falls in face when warm - pilots?

So I have an 81 550. I took my extra set of carbs, stock, verified the fuel service level was correct, bench synced them, cleaned them and then set the air screws to 1 3/8 turns out. Bike popped right off and I went for a spin. She pulled hard and ran good, except even when warm it struggled at less than 1/4 throttle and from a start it would literally fall on its face. Add some choke and life was great....

So I'm a little stumped. I know the pilots and passages are clean. When I got home I Richened up the air screws and it helped some, but still no dice. I did not vacuum sync them, but the bench sync should get me in the ballpark that it shouldn't be this bad, right?

The only other thing I can think of is the fuel service level. I'll create another post here shortly once I photoshop my pic to get some clarification...

Could also be an air leak...

I quickly posted that thread on my phone - I apologize for the typos.

Yes, it could be an air leak, but I do doubt it - I did check for that. The one interesting thing I found, is if I pull #2's spark plug lead off, it runs fine... So I'll most likely take them off again and go through them again. I will also check for air leaks. Also, my air screws seem rather non-responsive to idle, except extremely threaded in or threaded out...

Just stumps me... if it was an air leak or plugged pilots, why does it idle like a champ? It's almost like the needles aren't right... I did verify, they are the Y77 aftermarket ones, 2nd clip from the top.

Have you tried checking your valve clearances? I heard they can cause stuff to not run right when the engine heats up

Yes for valves. One of the first things I do. I had my other carbs on this bike and it didn't do this.

So do ppl think that the pilots could be the culprit here? Or does it sound like something else? I'm open to suggestions

... She pulled hard and ran good, except even when warm it struggled at less than 1/4 throttle and from a start it would literally fall on its face. Add some choke and life was great....


If it was running ok before the carb swap then put your original jets and needle back first, sounds like either a fuel supply or jetting is miles out. What colour are your plugs?

Maybe I didn't explain it well enough. I have an extra set of carbs from a parts bike I'm working on getting running. I took the extra carbs, cleaned them and set the up properly. (They did have a rebuild kit in them, adjustable needles at 2nd clip). I never actually took jets or needles out of these carbs. I just was trying to get the carbs in working order. Just stumps me that it idles fine but almost dies when starting from a stop.

Have you ever had it running ok before with different carbs? Your main,pilot jet, needle and fuel level is correct ?

The idle circuit is working, points to your primary or main circuits or a fuel supply/level issue.

Bench synch is a blunt instrument, the whole point being each pot on a used bike has a slightly different inlet vacuum. Much better done / checked with individual vac gauges.

Hi Tyrell,
I did have the bike running OK with my other carbs, stock everything, with the exception of a small rich condition, which I have corrected with (fuel level too high). Off the line and everything was fine. Initially just had a bench sync, but then vacuum synced (not a whole lot of adjustment was needed).

Now, the carbs I started this thread about were an extra set I had, new to me. I disassembled them, cleaned them, verified all the jets were the stock size (albeit, not original jets). I did verify on carb 1 only the correct needle and clip position - assumed the other carbs would be set the same - might need to verify this. The fuel levels were all across the map, so I corrected them all, along with the air screws (they were like 2+ turns out). So, this set I'm having issues with SHOULD be clean and stock settings, with a bench sync. I did throw the vac gauges on, and they could use some adjustment, but they are all actually pretty close already. I wouldn't think a small vac sync would take a bike that CANNOT pull away from a stop to running fine, but I've been wrong once or twice before. hehe. Hence why I am stumped

The only other thing I can think of is maybe the o-rings on the air screws are leaking...or I have a vacuum leak elsewhere.

Ok. I think I'm onto something. Took the carbs off again tonight. Everything looked good, except maybe just maybe the pilots had a little plaque buildup. Took care of that issue, assembled and went for a spin. The bike started off like a dream. Then the whole fall on its face thing started again. But then I realized something...I could hear a pop on deaccel from the left side. I had just previously patched a large crack in my left pipe. Sure enough my patch gave way and it was leaking again. So I pulled the exhaust off now and will have it welded tomorrow at work. I sure hope this is the culprit.