alfred e numan
Senior Member
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I bought the Walbro 400-606 pressure spring kit and started testing saturday- finished 01/31/12.- Engine. 30.5 HY. Jetpro rear. DT-1 three stage filter. 3/4 inch velocity stack. ADA manifold. Maxima 927 @ 25 to 1. Non ethanol 92 octane gas. copper base gasket - 0.30 mm, permatex ultra copper. Toqured to 8 N/M, resulting to .078 mm squish- safety for the 2 bolt-carbon build up. O.K, here we go. I cut a 3/4 mm deep square notch with a Dremel thin cut cutoff wheel on the 990s butterfly plate. this 3rd generation notch straddles the idle jet in the bore. there are 2 jets behind the idle jet. these 3 jets operate in three stages. 1st jet for idle- as the throttle is opened slightly -air enters the rear 2 jets to help atomize - feed the idle jet- as the throttle is opened further gas only is drawn out of all 3 jets. - this is more complex than i discribe -- not enough room - time to detail further on this post. The pressure spring. This spring has 2 functions. It controls when the needle valve opens to feed more fuel to the jets- nozzles in the carb. AND it controls the reseat pressure. The fuel pump produces approx 5-7 psi with a rubber over silk type of diaphram- this is what we use - as stock part. The reseat pressure should be 10 psi. To be more stable running in my engine i settled in with 12- 12 1/2 psi. The reseat pressure tends to go down in a running engine. The "pop-off" pressure tends to go up in a running engine. Before you start expermenting , you must have a carb in perfect working order that has 2/3 rd to 3/4 full fuel bulb as the engine is ideling. I eventually settled with pressure spring # 98-189. interchange # 98-3255. In static testing, this spring released # 18 psi. Reseated # 12 psi. this was a happy medium setting. The hi-speed fuel screw-- 3/4 turn- slightly rich, but safe. This setting is because there is so much more fuel comeing out of the 3 idle-mid throttle jets AND the main fuel nozzle. On the mild mannered XB with 6000 rpm spring- 4 grp bulldog m compound narrow tires on 3 year old chip sealed asphalt road, in 3 feet this car either lifts the front tires slightly OR spins out of control. The mid range is quite a bit more powerful also. there was a slight improvement in the top end power. this is a torquer engine. With a hot-rod engine i suspect there would be even more gains. DONOT cut -strech springs. Be safe and buy the Walbro # 400-606 spring kit and the proper pressure tester and use water to test the pressure tester for air leaks before you dive into this most interesting way of building horsepower.- from idle -up to overrev. There is far more to this than i have posted here- P-M me provide me with a phone # and i will talk deeper on this topic. yea- i am a "newbe" but ya gotta start somewhere!!! Thank you for reading this.