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Hi All!  So I bought a parts bike for my zephyr zr550 and then realized it's in better shape than my original.  Only 8200 miles and just about everything looks pristine. I decided to save it, even though it hasn't run since 1998 and even though I know getting a CA title is going to be nightmare. I throughly cleaned and rebuilt the carbs (used a Factory Pro Stage 1 kit, and installed new "long boy" idle screws), including setting proper float level, changed the air filter (which had pretty much disintegrated), put some fresh fuel in, and voila!  I got it to start first try today. Only problem is, I'm having a hard time getting it to idle. I could keep it running playing with the choke and throttle, but it wouldn't idle on it's own. What's the best process for setting  idle from scratch (or from a long sleep)?  I had turned the idle adjustment knob such that the butterflies were pretty much completely closed to begin with. Didn't seem like turning the screw did much of anything when I was trying to get it to idle smoothly. 

Needless to say, the butterflies need to be open some small amount.  I'd start by turning in the adjustment screw/knob.  And set the pilot screws at 2.5 turns open from lightly seated, for a starting setting.  If the bike still won't idle, that suggests the carbs aren't properly clean. 

Thank you!
Related question - now I don’t remember which way to turn the idle screw to open the butterflies. Does turning the screw clockwise open (raise idle) or close them?

Turn the throttle just hard enough to take the slack out of the cable, but not hard enough to open the throttle plates. Hold it there and turn the idle speed screw. you will be able to feel if the plates are opening or closing.

Turn the screw IN, clockwise, to open the butterflies.

Oh man, I've been waging an epic battle trying to get this thing to run correctly. I went back into the carbs and made sure the pilot jets and circuits were completely clear. I also used the clear tube method to make sure the floats were sealing and that the fuel levels were correct. I installed new (Factory Pro) needles, idle screws, float bowl gaskets and vacuum diaphrams. Cleaned everything obsessively, again. Did not replace valve needles (they looked fine) and didn't really inspect the seats because everything seemed to be sealing nicely. Crossed my fingers and put the carbs back on the bike.  Still a disaster. A weird disaster. Looking for theories on what's going on and what I can try next. 
It will start up, but never really idles well unless I play with the choke and throttle. What's even weirder is it seems to stall unless I cap the vent hoses. How could that make any sense? And then the punchline is it's flooding massive amounts of fuel back into in the airbox.
Other fun facts - when I examined the spark plugs they seemed oily.  Which got me worried...someone parked this bike 25 years ago for a reason, maybe there's something more serious going on. That said, it has only 8K miles and is in overall amazing shape. Maybe I'll try swapping over the carbs from my running zr550 to try to isolate the issue. 

Well, I tried swapping in the carbs from my working zr550...and it wouldn't even start. 
Did a compression test and came back with:
Cylinder 1: 105 psi
Cylinder 2:  135 psi
Cylinder 3:  85 psi
Cylinder 4:  100 psi

I did notice oil fouling on at least a couple plugs (should have kept track of which but didn't).  I'm thinking maybe a blown head gasket? 
I may have to let the bike sleep another 20 years or so, won't have time to dig into the engine.  It's a shame, it's in such good shape otherwise. 

 

Have you checked valve clearances ?

Think valve clearance could effect compression that dramatically? Maybe worth a shot… 

 

Zaddict wrote:

Think valve clearance could effect compression that dramatically? Maybe worth a shot… 


 

Yes, if they are too tight

Well I checked the valves and for better or worse they were spot on. 
I did notice one of hoses from the emissions system had come loose, the big one going from the airbox to the exhaust ports. That could have come loose during my install or deinstall of the carbs during my last test and could explain why the bike wouldn't even start last time. I think I'll try the carb swap from my running zr550 one last time and make sure that hose is set properly.  Still doesn't explain the low compression numbers though. 
 

There's a fine line between persistence and stupidity. Not sure which side I'm on. In any case, I gave it one more shot swapping carbs from my running bike to the project bike, making sure there were no vacuum leaks and everything was buttoned up nicely.  Still ran like crap and wouldn't idle.  I'm pretty convinced there's something internal to the engine amiss, most likely a blown head gasket. I think I'm going to dig in and take a look. Will be good practice for installing the new 615cc cylinders and rebuilt head on my main bike. Think I should go ahead and hone the cylinders replace the piston rings while I'm at it? Maybe I'll know more once I see what's going on in there.  

Related question - how important is it to stick with OEM piston rings? I see some cheap ones on eBay, but I have no idea about quality.