Ported intake question

[k.chandler23]

I'm experimenting with a buddy's Shindaiwa 488 that he wanted me to "push the limits" on. I got it back together and after the first fire it started to idle very high like it was lean. If I back out the low jet it will load up and spit back. The saw has good throttle response and gets on the pipe quick. I pulled it back down and pressure/ vac tested it to find it good and tight. My question is can too much intake duration cause a high idle? The duration is at 156 degrees on the the intake.

 

[huskihl]

Too much intake duration will eventually just cause more spit back and it reduces case compression

 

[k.chandler23]

I ordered a new piston to go back to a lower duration. If I tune the carb to where it should idle normal it spits back terribly and loads up. So obviously it's too much intake. I even tried lowering the metering lever a tick to see if it would slow it a bit. We'll see what the new piston does.

 

[huskihl]

I ordered a new piston to go back to a lower duration. If I tune the carb to where it should idle normal it spits back terribly and loads up. So obviously it's too much intake. I even tried lowering the metering lever a tick to see if it would slow it a bit. We'll see what the new piston does.

156 duration isn’t going to cause excessive spit back. You would need more like 170–180°. Piston could be worn out thin and allowing it to happen. Could also be another issue and your tune is just masking it

 

[k.chandler23]

156 duration isn’t going to cause excessive spit back. You would need more like 170–180°. Piston could be worn out thin and allowing it to happen. Could also be another issue and your tune is just masking it

There was definitely some wear on the intake side of the piston skirt. I'll find that out with the new piston coming. I'll be shooting in the dark if that's not the issue. Other than a carb issue.

 

[k.chandler23]

Well the new piston didn't change anything. Intake duration is at 136 with the piston as is. Still idles extremely high just before spitting back terribly. I am only running one ring on the piston. Could the open bottom ring gap be causing this? I'm about to look into the carb next. Possibly a plugged low jet?

 

[farmguywithasaw]

Well the new piston didn't change anything. Intake duration is at 136 with the piston as is. Still idles extremely high just before spitting back terribly. I am only running one ring on the piston. Could the open bottom ring gap be causing this? I'm about to look into the carb next. Possibly a plugged low jet?

What carb dose it have ?

 

[k.chandler23]

What carb dose it have ?

I swapped to a Walbro HDA 79 so I'd have the high side adjustment. I rebuilt it with a Walbro kit.

 

[farmguywithasaw]

You could be having an issue with the high speed nozzle check valve. It will act like and air leak at idle. Also check and see if the metering diaphragm metal cover has a hole in it. If not drill a small hole in it so it looks like other walbro covers

 

[k.chandler23]

You could be having an issue with the high speed nozzle check valve. It will act like and air leak at idle. Also check and see if the metering diaphragm metal cover has a hole in it. If not drill a small hole in it so it looks like other walbro covers

Ok. I will tear it down and check it. I'm pretty sure the cover has a hole in it.

 

[k.chandler23]

And all this for a slightly clogged micro screen Now I want to know how it runs with the increased intake timing like I had. It's always something.