7 replies to this topic

[mcman56]

When my Rev 3 engine winds down off throttle, I'm getting a lot of Pop Pop Pop that sounds like gas going off in the exhaust. You don't have to rev it very high to get this result. Otherwise runs fine and starts very easy but this is annoying. Idle mixture screw does not have much affect until getting down to about 3/4 turn out when idle starts to drop. Plug looks tannish almost lean. This is a 250 with Keihin and stock jetting. Needle at mid position. What could cause this? Would it be a good idea to try a leaner pilot?

Edited by mcman56, 29 January 2012 - 01:18 AM.

[copemech]

I thought all Revs had the pop!

On a Kiehin, 1/2- 1 and 1/2 is a preferred range on the mix screw so that sounds normal to run it at that range of about 1 for a good idle.

Edited by copemech, 29 January 2012 - 04:51 AM.

[BillyT]

mcman56, on 29 January 2012 - 01:17 AM, said:

When my Rev 3 engine winds down off throttle, I'm getting a lot of Pop Pop Pop that sounds like gas going off in the exhaust. You don't have to rev it very high to get this result. Otherwise runs fine and starts very easy but this is annoying. Idle mixture screw does not have much affect until getting down to about 3/4 turn out when idle starts to drop. Plug looks tannish almost lean. This is a 250 with Keihin and stock jetting. Needle at mid position. What could cause this? Would it be a good idea to try a leaner pilot?

Classic signs of pilot jet being to lean. Richen pilot jet until symptom go away.

Edited by BillyT, 29 January 2012 - 04:56 AM.

[lineaway]

Billy is correct on the pilot, but you probably ride a wide range of altitudes and temp.

[NZRalphy]

Also sign of carbon build up on cylinder head and top of piston. It could be time for a de-coke and polish.

[0007]

Technically a pop in the exhaust means that there is both unburnt fuel and a near perfect amount of oxygen as well
There should not really be enough oxygen in the exhaust to sustain combustion, nor should there be enough fuel

It's possible there is a misfire that puts both of these directly from the cylinder into the exhaust, misfire can be from a bad plug, or oddly enough a lean condition can cause a misfire and is usually the cause, hence the pilot jet comments, and most often you do not hear the misfire, just the after fire (pop)
Make sure you have no exhaust leaks as that can bring oxy into the exhaust

[pindie]

I second the air leak in the exhaust as the first place to look. Is it sealing well onto the barrel? Are the O rings in joints in good condition?

[mcman56]

pindie, on 31 January 2012 - 09:27 PM, said:

I second the air leak in the exhaust as the first place to look. Is it sealing well onto the barrel? Are the O rings in joints in good condition?

I sealed the exhaust and went up two sizes on the pilot jet yet the pop is still there. At 60 degrees F, it started without the choke but did sound lean until warm. Could it need to be richer?