The idea for this came from a paragilder err- ultralite aircraft site. On the well lighted side of the bore on the 990 you can see a dot. There are 2 more dots hidden in the upside down U shaped shadow. These " dots " are holes- the fuel jets for the low speed -midrange circut on the 990. The shadow you see is the reflection from a polished HY 30 cc factory carb - y if you are in AU. The Hy carb has a U shaped notch. The idea is- the notch causes a stronger air flow across the 3 low-mid circut at idle and slightly off idle openings. This is what the ultra lite fellow says happens - improved hot-cold starts, You have to lower the idle speed screw- slow the rpm down and feed more fuel with the low speed adjustment screw. -- less low-mid starvation of fuel. I will see -- as soon as time and weather allows. This factory notch is small - but i have dremel-red loktite and screws. - never pry-pull on the butterfly valve- when r-r is correct the valve will fall out- drop in to place easly.-- In the morning i will wet test with viberation and read the pop off pressure.
Good morning. I dry tested the 990 -- pumped air only into the fuel inlet on the carb. The pop off valve released at 36 1/2 psi. The HY carb blew off at 25 psi. From what i have read on the HPI site, 25 psi is normal- the sweetspot is 15. -- but all the HPI threads were dry tests. Later i will purge all the air out of the carb and fuel line and wet test. This seens logical-- stay tuned folks----
O.K. everyone-- heres the skinny. When the mighty WT 990 was wet tested it popped at 42 psi. -- this is very high. I tested agan with a gas-air mix and it came down to 31 psi. So, i robbed the short spring you see in the photograph from a 26.2 weed wacker and installed in the 990. the results -- wet test-no air in the carb or line and filter- 30 psi. air-gas mix- 20.25 psi. The HPI fellows say the sweet spot is 15-17 psi. The 990 is a hot rod carb, for the time being i choose to err on the side of caution. I will bolt up the carb at the 20.25 setting and fire it up when the relentless eastern Oregon winds stop. Also- lots of people cut the stock spring down with toenail clippers. bad idea because the solid coils you see in the photo are intended to be a seat- stable the spring in the carb. The stock spring - WT-990. - # 48 on the Walbro 990 exploded view. replacement part # 98-3248. cost-0.46 cents. The proper way to experment is to buy a Walbro spring kit. Walbro part # 400-606. Go to- mowpart.com in Texas - enter the part # in there search feature and there it is- no photo.-- $37.76 with a 5% discount. I have looked at this spring kit somewhere else in cyberland- comes in a little fishing lure clear box with lots of springs.
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