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  1. #1

    87 650sx no fuel getting in 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?


  2. #2
    Quote Originally Posted by LakesideRec View Post
    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?
    Figured out how to get fuel flowing on these types of carbs - need to use the choke.

    Spark plugs look to be wet fouled, even after running it a bit. Maybe its just not getting good spark and not firing every cycle? Going to try some new plugs.

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