US 90 carb issue need some input

[bigcrunch]

I have a 1970 US 90, it is a complete machine.

I finally got it running but here is what it does when idling sometimes it runs up the rpm apprx 3000 sits there for
a minute then drops back down to normal, does it cold or warmed up.
I thought it was a vacuum leak so I checked by spraying carb cleaner around the intake to head and carb to
intake mating area with it running no rpm change. But I went ahead and changed the gaskets on the intake
to head side both of them. I also checked between the carb and intake O ring looked fine and also checked for warpage, I added a temorary gasket with antiseize just to make sure it was sealing.
I also taped up the boot where the throttle cable enters the carb to ensure it was sealed against vacuum leaks.
Still did it,

So I took carb apart and cleand it by soaking it in a can of carb cleaner, I checked it for cracks and blew
out the holes and orfices with 80lbs of air.
When I do put it in gear no power with the choke off, flip it to on has power warmed or cold.

Still does it, any ideas???

I want to keep the original carb and not replace it with newer.

[shortline10]

Try running it with out the gas cap , sounds silly but it might be a pluged gas cap vent and the carb bowl isnt getting the correct amout of fuel so it runs lean and than back to normal as the fuel flowes . Make sure the boot between the carb and the air box is perfectly sealed , this can cause issues also . A worn carb slide can cause it also .



I have a 1970 US 90, it is a complete machine.

I finally got it running but here is what it does when idling sometimes it runs up the rpm apprx 3000 sits there for
a minute then drops back down to normal, does it cold or warmed up.
I thought it was a vacuum leak so I checked by spraying carb cleaner around the intake to head and carb to
intake mating area with it running no rpm change. But I went ahead and changed the gaskets on the intake
to head side both of them. I also checked between the carb and intake O ring looked fine and also checked for warpage, I added a temorary gasket with antiseize just to make sure it was sealing.
I also taped up the boot where the throttle cable enters the carb to ensure it was sealed against vacuum leaks.
Still did it,

So I took carb apart and cleand it by soaking it in a can of carb cleaner, I checked it for cracks and blew
out the holes and orfices with 80lbs of air.

Still does it, any ideas???

[03 ORANGE SHEE]

try cleaning the pilot jet and pilot jet circuit. my gas cap was rusty, try blowing out the vent holes, mine would run for a while, then vapor lock from the tank not running.

[bigcrunch]

Pilot jet is the smaller one inside the bowl screwed upwards?

[shortline10]

Pilot jet is the smaller one inside the bowl screwed upwards?

Yes its the smaller of the 2 jets , make sure its clean .

[bigcrunch]

Took carb apart and double checked carb is all clean from prior cleaning.
I did do two adjustments

1. On the needle I moved the E clip down one notch per the "84 110 carb adjustment " post here.
2. I found a 1974 ATC 90 maintenance manual online, gave details on how to adjust the air mixture screw. Set idle to 1200 rpm run air mixture screw in and out until it stumbles find best setting in between mine was 1 and half turns out from seat. Reset idle. manual is own74atc90.pdf pg 39

Took it for a short spin with choke off and warmed up pulled almost as good as my 1978 90.

thanks for the help, now I need some input where to get these tires fixed.

[atc350xer]

also, watch the liberal use of carb cleaner... I found that it was melting my altitude button