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Message started by Savageman on 01/05/16 at 13:31:17

Title: Pop POP Poof
Post by Savageman on 01/05/16 at 13:31:17

The carb has a 150 main and a 52.5 pilot jet. Also the white spacer replaced with 3 thin washers to raise the needle a notch. Also installed a manual petcock. Valves are adjusted to .004 in and ex. The bike has a Harley Sportster muffler on it now. (Just installed that)

The problem is it still has popping when the throttle is released and when I turn the bike off a poof pow sound is emitted from the muffler.

What can it be now?

Timing, ignition coil...??? Carburetor still too lean? :(

Title: Re: Pop POP Poof
Post by Dave on 01/05/16 at 13:54:31

Normal.....it is the nature of the bike.

When you close the throttle the fuel mixture leans out....this happens with all carbs and the only cure is to buy something with fuel injection. The stock muffler can mask the sound really well - change the muffler to a more open design and it becomes more noticeable. You can make the bike run much quieter by not allowing the throttle to close quickly or completely when you shift - or when you are slowing down. Keep the throttle open just a tiny amount and the noise will be reduced significantly.

When you shut the bike off, the ignition is turned off - but the engine is still pulling air through the carb and getting some fuel, and the engine doesn't ignite the mixture and it just pumps it into the hot exhaust pipe...and the unburned fuel/air mix is heated by the hot exhaust pipe and ignites in the header/muffler. If your idle speed is too high it makes this problem worse as it pumps a lot of air/fuel into the exhaust (but don't get the idle speed too low or you will ruin your cam bearings). Also letting the engine idle for a 10 - 15 seconds before you turn the ignition off can sometimes help. I have wondered if cracking the throttle open at the same instant you turn off the ignition would help.....as it will move the throttle butterfly away from the ports for the idle circuit, and the vacuum may not be enough to pull fuel out of the circuit.

Title: Re: Pop POP Poof
Post by Kris01 on 01/05/16 at 17:42:24

Have you tuned the air screw?

The cold air will cause it to pop like that as well. It might be less when it gets warmer outside. Cold air will cause the carb to run leaner than warmer air.

Title: Re: Pop POP Poof
Post by Savageman on 01/05/16 at 21:24:22

I didn't think this carburetor was so sensitive to temp and pressure /elevation. It's 45f and 150 ft elevation here in no cal.

Guess i'm use to a ECU doing all the adjustments for me.

I did let it warm up good and adjust the pilot mixture screw. Seemed to help when I opened it up about 90 degrees ccw.

thanks 8-)

Title: Re: Pop POP Poof
Post by Rodger on 01/06/16 at 17:01:45

@Dave....

After a 30-mile ride today, I tried your 2 ideas, letting the bike idle in the driveway for about 30 seconds, then lightly cracking the throttle as I turned the key off. Nothing for a couple seconds, then a quiet "belch" out the exhaust.

I've decided to view it as a Charming Idiosyncrasy. ;D

Mind you, the bike is still in break-in, is box stock, & it was about 40° out.