650sx no fuel getting into carb - PWC Forum: The best hang-out for Personal WaterCraft enthusiasts
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post #1 of 17 (permalink) Old 06-15-2019, 06:08 PM Thread Starter
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650sx no fuel getting into carb

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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post #2 of 17 (permalink) Old 06-16-2019, 06:58 AM
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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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post #3 of 17 (permalink) Old 06-16-2019, 12:55 PM Thread Starter
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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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post #4 of 17 (permalink) Old 06-16-2019, 01:37 PM
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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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post #5 of 17 (permalink) Old 06-16-2019, 05:31 PM Thread Starter
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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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post #6 of 17 (permalink) Old 06-18-2019, 01:16 PM Thread Starter
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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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post #7 of 17 (permalink) Old 06-21-2019, 02:57 PM Thread Starter
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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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post #8 of 17 (permalink) Old 06-21-2019, 11:05 PM
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Did some googling on your behalf, looks like the pop off should be 12-14 lbs
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post #9 of 17 (permalink) Old 06-23-2019, 06:42 AM
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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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post #10 of 17 (permalink) Old 06-29-2019, 10:55 PM Thread Starter
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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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