Hey.

I have run into a minor problem with the IDA weber for my racecar.



At 5psi fuelpressure there is fuel seeping out of the pump jet (#76406.--)



Im not a carburettor guy and from the diagram http://www.webercarburator.com/48IDA.pdf i cant really tell whats up. I can take it apart and so on, but have no idea how the work.



Any suggestions?



-Kim

 

Its probably the ball valve that stops carb from flooding when float level is up. With out pulling it down to clean or unstick it, get some welding wire eg 1.2mm and poke it through the breather hole and gently move the float up and down a few times, this should unstick ball valve. If not you might have to take the top off and free it by hand.

Even with fuel filters very small particles can cause it not to seat and fuel pressure causes fuel bowl to flood over spilling out main fuel jets.

 

K thanks.



Odd thing is that it is only the "rear" jet that leaks fuel.

 

I am not familiar with that number.



Are you saying that the accelerator pump nozzle is dripping or, that the booster venturi

(the tall tube down through the center) is dripping?







Lynn E. Hanover

 

I find any more than 3.5psi blows the needle valve open (3.00 needle seat) and fuel will run out of 1 or both booster venturis depending on carb angle.

 

Accelerator pump, thats the word i was looking for.

Yes its dripping or more correct running out of the AP nozzle.



the number is what i found it should be according to this http://www.webercarburator.com/48IDApl.pdf



Should i just try to run lower fuel pressure?, my Holley regulator makes a weird noise when going too low.

 

Either the carb is assembled incorrectly, (both of the ***** are missing from the accelerator pump well) or we have a nomenclature problem.



My instructor on 20 MM cannons always said, "know your nomenclature"





The big tubes down the center of the hole is the booster venturi (American talk) or may be called the auxiliary

venturi.



The accelerator pump jet is the little nozzle that sprays for just a second or two when the throttle is opened.



There are two styles. The older style has a brass screw holding it in place. That screw has a ball in it, and the assembly (screw and ball) is called the delivery valve.



The newer style is just the nozzle with "O" rings and it pushes into the hole, and the tight fit of the "O" ring holds it.



Dripping boosters is just the float level too high. This is common if you get a bounce into the car, like a big piece of apex or an off the road experience.



If you were happy with the performance before this problem, just adjust the float level to 25MM or one inch.



Depending on what type of racing is involved, the normal setup would be to adjust the level down until the dripping just stops. So it sits there idleing just fine, and if you tap on the carb, it drips a bit and then stops dripping on its own. Your first thought will be that if that is the case, it will do that all of the time, while racing.



Once you start using the engine hard, the float will seldom get very high in the bowl. So the boosters never get to drip anything at all. And, even if they did, the amount of excess fuel involved would be nothing compared to what the boosters are dropping into the chokes. So you would never notice it.



On spooldown under braking it will come back up as fuel refills the bowl. Other interesting things happen under other situations. Under braking, the front hole goes way over-rich, and helps make the big fireballs out the tail pipe. Under acceleration, the rear hole runs richer, and the front hole runs leaner. Running different air corrector jets is common. (I dont do it, it's just a pain in the ***).



The stock needle seat assembly is OK for street but will not flow enough fuel for racing. It will not hold back much over 4 pounds of fuel pressure without the dripping boosters. So there are two problems that show up as dripping boosters.



I use a 350 Gross jet. The one that uses 2 ***** to shut off the fuel. We run 6 PSI measured right at the carb.



Right inside the carb cover is a cavity that contains a plastic screen filter. While you have the cover off remove that screen, and throw it away, drill out the land it is mounted on. Just double the hole size, whatever that is. Clean out the cover and install the Gross jet.



The reason for all of this is that there is not enough fuel entering the carb to support a long run of WOT.

The float bowl is just emptied, and the needle seat becomes the main jet. We shift at 9,600 RPM, but this problem also shows up at much lower RPM.



We video tape the gages in each session, and you could see the EGTs go up to say 1,700 degrees and then begin to drop near the end of the straight, as the carb ran out of fuel.



With a good ignition system, the engine will not lean miss so as to warn you about this problem. It will not hurt the engine at all to run overlean, or, "Lean of Peak" as the airplane people say it. So some guys race for years without getting full power from their rotaries.



Another gag, is to drill through the bottom of the unused accelerator pump well. Only one is used. The other just takes up bowl volume. So drill, or die grind through the side of it close to the bottom of the bowl and make a notch at the top so it cannot seal against the cover gasket.



If you can tell me what kind of racing you do, and what choke size (main venturi) you use, maybe I can help you with jetting.



At the end of a long pull at top RPM, you should see 1575 to 1600 degrees on the EGTs with probes 3" out from the port. That will be your best power mixture.





Lynn E. Hanover

 

Thanks Lynn.

You have been a big help in most of my questions, remind me to buy you a beer if I ever come to Ohio.

I had to look up "nomenclature" haha



I might just get the float bowl extended, the Competetion manual suggests this. http://www.wankelkim.net/rx7/comp.manual.pdf



I will try and check what you have suggested.

This setup is very similar to a previous setup by Lasse Wankel of rx7club.com.

Carb has 44mm chokes

260 main jet

160 air jet

F7 emulsion tubes

Once the new bearings are broken in, it will be dynotuned at a Danish Weber wizard.



Im going to do some Special Saloon racing (like the Super Unlimited class), everything goes, when the car is ready and the friggin snow is gone



Thanks once again

-Kim

 

The extended bowl idea is IMSA people killing an ant with a hammer. It works, but it is just covering up the fuel feed problem as above. Try it with the mods as above first, without the bowl addition first and I think you will like it.



You must be able to hold 6 PSI all the way to the end of the longest straight. If pressure sags before then, you need more fuel pump. We use two sliding vane pumps, and a regulator right beside the carb.



The stock bowl shape gives you better control over the fuel in corners than the wider version. Although the bigger bowl does work.



The 44mm chokes are the limit for a 48mm carb. The booster signal will be poor, and tuning a bit touchy.



When last we ran the Pport with 44mm chokes, we had 235 mains F-7 tubes and 100 or 110 airs.



Anything over 240 mains with a poor booster signal and you just have no control of main jet flow, and tuning reverts to just the airs.



The 44MM chokes should be enough for 275 HP without a lot of work.





Lynn E. Hanover

 

The Weber is a Mazdatrix raceprepared 51 mm, does that change your reccomendations?