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Rough idle, no low-end power until HOT

Started by universalhat, October 28, 2014, 04:02:49 PM

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universalhat

As the title suggests, my not-quite-broken-in-yet engine has some problems, and I'm not sure if they're the normal "that'll go away after a few thousand miles" break-in issues.
The bike runs(!), and does pretty well on startup with full choke.  Power exists and throttle response is good, idling even comes out pretty well - until the engine just starts to warm up, and then power delivery stays good, but the bike starts idling way way too high.  Since the choke is applied, I assume this to be normal.

But this leads to an uncomfortable space between stone-cold (where the bike runs normally with full choke) and hot-as-a-pistol (where the bike runs normally with no choke - typically takes about 10-15 minutes of riding to get to this point.)
In between those two times, with no choke or moderate choke, the bike will idle low-to-death, and is practically incapable of generating power below ~2500RPM with a moderate amount of throttle.  Starting from a stop is extremely difficult when the bike's like this, as releasing the clutch even very slowly will cause the engine to stumble and stop producing power.

...But after about ten or fifteen minutes, this goes away.  The idle remains low, but the bike can be convinced to idle at a reasonable speed with gentle application of throttle - but starting from a stop isn't so much of a chore anymore.

Any idea what might cause this?  Should I just act like the problem doesn't exist until I've got my 1500 miles on the new engine?
1990 FJ1200

1979 CB750K, but we're not here to talk about that :P

Fj.itis

Not sure about your problem, but my bike can take off straight away, cold or not.

TexasDave

All of the symptoms you describe indicate a lean mixture. I would check gas supply problems first. Gas line routing and pump function.  Dave
A pistol is like a parachute, if you need one and don't have one you will never need one again.

universalhat

Quote from: TexasDave on October 28, 2014, 06:44:33 PM
All of the symptoms you describe indicate a lean mixture. I would check gas supply problems first. Gas line routing and pump function.  Dave

I'm the newb here, but wouldn't a failing supply cause a few cylinders to starve out, rather than an overall lean mixture?  I'll check lines and the pump itself (of which all I can currently say is "it makes noise when i turn the key on), but want to know why that'd cause a lean mix rather than a total loss.
1990 FJ1200

1979 CB750K, but we're not here to talk about that :P

TexasDave

Quote from: universalhat on October 29, 2014, 09:14:26 AM
Quote from: TexasDave on October 28, 2014, 06:44:33 PM
All of the symptoms you describe indicate a lean mixture. I would check gas supply problems first. Gas line routing and pump function.  Dave

I'm the newb here, but wouldn't a failing supply cause a few cylinders to starve out, rather than an overall lean mixture?  I'll check lines and the pump itself (of which all I can currently say is "it makes noise when i turn the key on), but want to know why that'd cause a lean mix rather than a total loss.
I suggested checking the fuel supply first because it is the easiest. It sounds like your fuel pump is working. If you replaced the fuel lines make sure they are routed properly and not too long. If too long they could kink and impede fuel flow. From your discription it sounds like a cylinder or two is starving out. Did you replace the O rings on the rubber manifold intakes? These tend to develope flat spots and leak air. Check the files for the many posts on carb cleaning.  Dave
A pistol is like a parachute, if you need one and don't have one you will never need one again.