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Discussion Starter #1
I picked up an 87 650sx for the kids to play with that was listed as "needs some carb work" (it was priced accordingly). Seller was able to fire it up and idle it for a few seconds. I also checked compression (145psi rear, 148 front) so figured it couldn't be a total disaster.

Cleaned and rebuilt the stock 28mm Keihin carb. Checked the pop-off pressure: 23 psi. Cleaned out the fuel lines, pickups, reserve selector valve, vent check valve, and checked that fuel flows through the return line when cranked. However, it doesn't seem to ever draw fuel into the carb past the needle valve by itself. Like the fuel pump is not generating enough pressure, or the diaphragm in the carb is not pulling in with enough pressure to open the valve. If I push the needle valve open by depressing the diaphragm through the vent hole, it will run for a few seconds, and sometimes even stay running for a while, but eventually it runs dry and won't re-start without opening the needle valve manually again.

If I pull the return line off, it pumps fuel out, but it doesn't seem to be very high pressure.

Just checked the crank leak-down and it seemed very tight - 10 psi for half an hour and I couldn't see any drop on the pressure gauge.

Any opinions on what to check next?
Replace/rebuild the fuel pump?
Pull the carb apart again and mess with the pop-off setting?
Reed valves?
 

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How do you have the lines hooked up to the carb and fuel pump ? Make sure the pickup filters in the tank are not plugged. Might try bypassing the selector valve and see if that helps. Do one thing at a time so you know what fixes it. Make sure the mixture screws are set right. Don't rely on someone else.
 

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Discussion Starter #3
All good suggestions, thanks. I already checked the pickups, selector valves, and that the lines are all connected to the right places. Some of it needed to be cleaned out, but still not getting good fuel flow to the carb. If I pull off the return line, fuel is flowing, but not with a lot of pressure. It seems like there is just not enough vacuum pressure on the diaphragm to open the needle valve.

Is there a priming procedure to get the carb initially full?
 

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No priming needed. Get a vacuum pump and check the fuel pump for leaks. Be easy on it. Just like 5-10 psi max vacuum.
 

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Discussion Starter #5
OK, learned something about this machine and these types of carbs. Closing the choke helps produce enough vacuum to pull in the diaphragm and let the fuel in. As soon as the battery is recharged from all the messing around, I will be putting her in the water to get her tuned up a bit running under load and with water cooling. Hopefully once running and warmed up it won't be an issue to keep the fuel flowing with the choke opened back up.
 

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Discussion Starter #6
Was able to get her running, but still seems like no amount of tuning makes sense. Unable to sustain idle, and not a lot of power at WOT. Checking the plugs, they seem to be somewhat wet fouled even after running. Makes me think the plugs themselves may be bad, or something ignition or timing related causing misfiring. Going to try some new plugs and check how strong the spark looks.
 

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Discussion Starter #7
New plugs, but not really any improvement in how she runs/starts. Spark looks acceptable (but not super fat).
Just keep coming back to feeling like the carb is never opening its needle valve to let in any fuel. Is there a spec on what the spring pressure should be on the valve arm?
 

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Could be the fuel pump is bad. Where do you have the mixture screws set at ? Might try replacing all the fuel lines and pulse line. Could be an air leak somewhere. The fuel pump does not supply the full psi to unseat the needle. It works with atmospheric pressure and engine vacuum in the carb.
 

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Discussion Starter #10
Looks like the needle valve spring was way wrong. Could not get it down to 12 lbs pop-off with the spring that was in there. Ordered a lighter spring and was able to get the pop-off down to about 8 lbs (one article I saw recommended 8-12 lbs). With it setup like that, I am able to get it to keep idling once it is warm without the choke on and without seeming like it is starving for fuel all the time.
Ran out of daylight to tweak the tuning and test ride. I will work on it some more in the morning.
 

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Discussion Starter #12
Carb is adjusted for best idle, and seems to rev up reasonably at full throttle. However it seems like it should have quite a bit more power when actually riding it.

The ski had some evidence of past damage from ingesting some debris. I would imagine a sudden jamming of the drive shaft could sheer the flywheel key and mess up the timing? Is it possible to at least check the timing without pulling the fuel tank and flywheel cover? Does the fuel tank really need to come out to get the flywheel cover off merely to adjust the timing if I am not pulling the flywheel?
 

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There is a inspection plug, but to be honest I've never paid any attention to look for any marks anywhere, and I've not worked on any stand ups, so I can't tell you about pulling the tank or not, sorry.
 

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Discussion Starter #14 (Edited)
Flywheel key was banged up a bit, probably messing up the timing a degree or two. No good way to put it back together with any precision on the timing without a tight fitting key. No local Kawasaki dealers had the part, so had to order one. I will come back to precisely setting the timing after I have the right key.

Even only running at 25MPH its a fun enough toy that we wanted to get it back in the water for the holiday weekend. I put it back together, splitting the difference on the slop on the key as I tightened down the flywheel bolt. It ran about the same as before.

So meanwhile I continued to mess with tuning the carb and learned some more about it.

I can get the popoff adjusted so it will start easy and idle well, but then the high speed is too rich even with the adjustment needle full closed. If I adjust the pop-off pressure up, I can get the high speed closer to the right mix, but then the carb stops pulling fuel altogether at idle (making it hard starting and stops running when throttle released). I think I need to go to a smaller main jet size. Maybe somebody put in a larger jet to get it to run right with the extra-high popoff spring they had in there.

Question if anybody knows: What thread size are the jets? I find jet kits available in 4mm and 5mm threads, but no kits say they are specifically for the KeiHin CDK carb.
 

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Pop off should be set So it will start easy, idle well, and provide a smooth transition up to 1/4 throttle. You shouldn't even be anywhere near being able to make difference of idle quality with the high speed screw. That is the last 1/4 of the throttle. You shouldn't even be on the low speed jet at idle. If you are, that indicates that the butterfly is open to much, and your off the idle circuit. The only screw that should do anything at idle is the low speed screw. The one on the bottom.
If your getting fuel from the high speed circuit at idle I would be looking for some kind of blockage in the fuel return system or the check valve in the carb is bad, or installed wrong. The gasket should be against the carb , then the valve, then the cover plate.
 

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Discussion Starter #16
Yes, agreed. When I set the popoff where it should be for easy start and good idle (and adjusting the low speed needle to achieve best idle), it runs too rich at high RPM even with the high speed needle fully closed so looking to go smaller on the main jet.

Checked the check valve in the main jet circuit and it is installed correctly and looks to be functioning correctly.
 
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