WT 990 Test results.

[alfred e numan]

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.

 

[alfred e numan]

me agan---

The 3 stage DT-1 filter is too much. I have a 2 stage on the way from DDM. I also tried the 1" tall velocity stack with pressure spring # 98-343. This blew off at 15 psi static test, reseated #11 psi. The car launched even harder, BUT--- I could not control the high speed fuel needle set- just a Alfred guess- I think that the tall velocity stack caused so much air-flow that it overrode the high-speed check valve in the main fuel nozzle. just a guess-- but it did launch hard!! The combonations are endless- agan- pardon the spelling.

 

[Deluge]

Nice. You got any pics of the throttle plate mod? I've been thinking about experimenting with a spring kit.

 

[alfred e numan]

plate mod

I photographed the first two plate mods, they were a small U shape. This idea -cut was replaced with a new plate with a right angle notch. 3/4 mm deep- 1/2 mm wide. I used a Dremel thin cut 1 1/2 cutting wheel mounted to a old Rockwell radial drill press - rotated the drill head so the cutoff wheels disc was vertical spinning about 4500 rpm, and very carefully held the plate with both hands- forefingers and thumbs and cut. - I did not photo this cut- sorry, next time i pull the carb i will photograph. After the cut, i just removed the burrs, left the 90 degree angles alone. To add to my short 2nd post- It is highly likely that the 3 stage filter was 99% of not being able to get the correct spark plug color on the high speed fuel needle -- just a guess until my next shipment arrives from DDM. Should be a 2 stage filter in the box. I have to wonder -- do the people that make the high horsepower engines mod the carbs? The 990 i bought blew off at 30+ psi. this is way to high. Mass produced. We have to do the adjusting, just like fiddeling with the fuel screws. As much as this helped the HY engine- I wonder what would happen to the hand built 5-7-8 horse power engines, those really would get with the program. There are only 3-4 types of springs in the kit that are useful- the rest are too heavy.- It was worth the 52 dollars shipped!!! The kit came from N.C. ---

 

[alfred e numan]

home built tester

Nearly all the 50-60 dollar pressure testers are a joke- no thank you. Little tiny dials, the pump is a hypo needle for a horse or some bovine. The dial on the left was bought many years ago. Made in Posen, IL. The zero to 60 psi can be bought at a decent auto parts store for 10-15 dollars- made in C='?a it is a high quality unit thats right with the old unit. Clean all fittings- remove every speck of copper burr fron the copper tubeing- use teflon tape to seal- very short section of 1/4 od -1/8 od tygon fuel line to the carb. On the input side- 3/16 fuel hose short as possable. slide a valve stem into the input end of the hose. Forget the air compresser. Use a long stroke bicycle tire pump with a quick release fitting. Learn to use this on a old HY carb. Very slowly pump air and listen to the carb while looking at the pressure dial. at the moment you here a pop- release the fitting on the tire pump. the dial will slowly lose pressure and stop at the reseat pressure. When you are ready to test the Walbro carbs- drain the gas tank and use the primer bulb to purge the gas out of the carb -- saves a big mess- puddled gas in the air filter- engine when the valve opens. Also- It is very important to pressure test this homemade set up under water- do not immerse the dial its self- just the fittings and valve core.