Bike loses power when I downshift to stop

[ted630]

Hey guys. I bought a 1980 xs400 a few months ago, rebuilt the carbs, and i've been riding a little over a month now. After the rebuild it started up with ease every time, though sometimes i would have issues with high RPMs at idle (it will randomly shoot up to 3-4 grand). This was inconvenient, but it was rideable. Today, after 2 days sitting in the garage, it wouldn't start at all. I changed my spark plugs because they were totally black (they were new a month ago) and about a mile into my ride, I came to a stop sign and when I downshifted the bike lost all of its power. I rolled on all the way but it wouldn't go above like 800 RPM and when I rolled off it cut out and wouldn't start back up. I popped the clutch rolling down a hill and rode home, but it felt really weak and tired the whole way back. This same thing happened to me twice before I rebuilt the carbs, but after that I haven't had any issues.

I'm not sure what to make of this. The hanging idle problem is a sign of lean mixture, but the black spark plugs mean it's too rich. Then I replace them and it loses power. Is the mixture the problem? Is it the plugs? or am i totally off the mark?


(note: I'm running NGK BP7ES, and they're gapped correctly)

thanks guys

 

[jonjkim12]

Hey Ted,

have you checked your timing? I had similar, but not exact, symptoms with mine. After riding it for a little bit, when i'd pull to red light and pull the clutch in it would die.

After gapping/adusting my timing, it went away.

 

[xschris]

No timing to check on the 80 as it has a TCI ignition. Make sure the carbs are synced with a manometer and the floats in the carbs are set correctly (26mm) Also what jets do you have in it and what do you have the idle mix screws set at. Are there any mods to the bike? Post some pics of the bike so we can see what you are working with.

 

[ted630]

Here's Layla.
The jets are fresh. Bought them from S.C.I.
I haven't touched the mixture screws for fear of making a mistake, but it seems mistakes have been made and now I have to fix them. I'll look into that tomorrow and keep you posted.

In the second picture you can see the new bars, but more importantly the discoloration on the exhaust, also leading me to believe it's running lean

 

[xschris]

Non mikuni jets tend to be leaner metered and also less accurate than the real deal ones. A long time ago I got a set of the china made pilot jets from mikesxs. They where stamped 45 but when I measured them with a micro drill they where around 42 The main jets are worse. Always get real part Try your mix screws at 3-3.5 turns out from a gentle seating. How do the air filters look? Make sure they are still in one piece and are clean. Has there been any holes drilled in the back of the mufflers?

 

[Scorpio1963]

Id say by the color of the exhaust the exaust coming out of cylinders are getting extremely hot.
That would indicate a lean condition but with black plugs something in the carbs is out of adjustment and or you may possibly have the wrong plugs?What are plugs you are using?

 

[16VGTIDave]

Have you done a compression test? Does it sound/feel like it has reasonable compression? Checked the valve adjustments?