F90 Backfire

[Timbuktu]

Hello everyone.

I'm new to the board and excited to learn new stuff.


I picked up an 2004 F90 that had very dirty carbs and did not run although it shows almost no signs of wear. I cleaned them up by taking them apart and spraying carb cleaner then air through all circuits.

The main needle jets were completely plugged, and there was lots of varnish.


Now the motor starts right up and runs, however, It has random backfire through the carbs at idle maybe 7-15 seconds apart sometimes a few in series. I coughs when I rev it too. I looked at the plugs and there apears to be unburned fuel on them(they are new).

At this point I'm wondering wehether it is an ignition/timming problem or if I just need to do a more thorough job on the carbs.


Thanks for any help
Tim

[99yam40]

my thoughts are a better job on the carbs.
But you can always do a compression/ leak down test and spark test

[rodbolt17]

that back fire is a lean condition.
do the carbs again, this time clean the fuel lines as well.
wht type of manometer did you use to sync the throttle shutters?

[Timbuktu]

I will clean it again, this time I will soak it (yamaha carb cleaner) and do all fuel lines as well

Other than removing the outter connecting linkage that pops off
I did not mess with the shutters. I did not change or check any of the settings
Except the piolit screw. It was 1 3/4 turns I reset it to 1 1/4 turns.

I was thinking that the shutters would not need to be set up again unless
I disasembled the parts. (Is this right ?)

I was only cleaning. My factory manual is on the way so next time
I will check as many settings as I'm able.

Oh yea.... do the caps over the pilot screw need to be replaced? Or sealed at least

Thanks

[rodbolt17]

by letter of the federal law in the US, yes the tamper proof caps must be installed.
there is NO spec on setting the pilot air screws.
I find 2.5 to 3.5 turns works well as a starting point.
if the carbs were removed from the intake, the ONLY way to reset them is with a 4 channel manometer and the service manual.
period.
that back fire is most likly the pilot air screws are set to lean as well.

[Timbuktu]

Thanks

Thanks Rodbolt! I set the pilot screw to 2 1/2 and the motor ran like a dream

When the manual gets here I'll just double check the shutters.