Need to reduce warm control pressure
 

Anyone out there ever modify a WUR to reduce the warm control pressure? Not to be confused with beating on the pin to change the cold control pressure.

I'm trying to cure a lean condition in my 1980 euro 911. The lowest warm control pressure I can get is 51 psi. I would like to get it down to around 40 psi. These values are without vacuum, to simulate full throttle. I beleive this is because my car has the U.S. 1980 version WUR. What I really need is the WUR for a '78-'79 model, since I've had no luck locating european WUR's. I'd rather try to modify the one I have first, before buying a new one.

The car is not running near as lean as it was when I first purchased it. The warm control pressure was actually equal to the system pressure. The WUR was not creating any control pressure. Luckily a dyno run brought this to my attention.


Thanks for any help,
Rob
1980SC euro

Warm control pressure is adjusted by adding or removing shims from a plug on the fuel distributor body, unless I miss my guess. Frankly, I've never actually heard of anyone doing this. It is set at the factory. I believe warm control pressure and system pressure are commonly the same. I'm not sure, though. Also, I am not so sure that adjusting fuel pressure is the way to deal with a lean condition.

My advice is to use the mixture control screw on the airflow sensor arm to richen the mixture, if needed. As far as system pressure goes, I would verify it is within factory specification, then leave it alone.

You can buy a euro WUR from www.berlynservices.co.uk
I am not sure if the price is competitive with the US but at least you could get the exact part you require

I think you may be able to change control pressure by dropping your system pressure, but I would really recommend getting the correct WUR. The system is designed to run within a specified range of pressures, so just adjusting your mixture won't give you the best solution for the entire operating range of the motor.

Have you posted on the 911 parts board?

BTW, the correct link for the above post is

www.berlyn-services.co.uk

Thanks for the responses guys. Unfortunatly, that website doesn't want to work for me right now.

I was hoping someone had done some modifications to a WUR , or perhaps knew how they can be changed to yield a lower control pressure.

Removing shims from the fuel distributor only changes system pressure. At least that is the case with my setup. I've done lots of testing, and can vary system pressure between 71 and 77 psi, but warm control pressure always remains around 51.

This is beyond turning the little mixture screw. That doesn't help. I've got to get my fuel pressures within spec. Oh, and control pressure should never equal system pressure. That is way too lean!

Thanks,
Rob

Try the link in my earlier post, it works. They have a WUR for your car listed in the parts list.

"control pressure should never equal system pressure" Seems like I've heard that before.

I take it that you have said that before Bill. You seem to be one of the more knowledgeable folks on this board when it comes to CIS.

You can make your WUR adjustable
 

There is an article in the Tech Articles section on theis website on how to make your WUR adjustable. Seems like it's not too tough to do.

That article is just for making the cold control pressure adjustable. The warm control pressure isn't affected.

I'm chasing the same thing at the moment.

The system pressure works with the control pressure to control how easy the air flow sensor plate moves. Your on the right track.

If you drop the system pressure the plate will move easier. Or if you drop the control pressure the plate will move easier. Just a gut feel but I don't think you want to drop your system pressure to far outside of the spec range. Maybe just put it at the lower end using the shims. I'd be interested in how to modify the WUR also (so I'm no help....just would like to know).

My own problem is two fold. My system pressure is at the high end of the spec (99 psi). The low limit is 89 psi. I had a paint on my cleaner (long story) which I think was causing a high vacumm in the air box, which the WUR uses for altitude adjustment. So at high engime RPM's, the WUR altitude compensation was kicking in increasing my control pressure .8 bar (11psi) and leaning it out. So between these two factors I'm off about 20 psi for potential full load enrichment. This is assuming the plate still has room to move but can't due to the pressure.

At least that's what I think. I'll be doing another dyno run after I tweek the sys pressure. I'm crossing my fingers hoping this is the problem.

Hey, thanks for the explanation Jeff. John Walker told me that removing a few of the shims would help richen the mixture, but when I removed them, only the system pressure dropped. I knew he must be correct, but I didn't understand why. So by reducing the system pressure, you allow the sensor plate to move easier, just as if the control pressure was reduced. Cool.

With no shims I get 71 psi system pressure. The specs are 65-75, but I can't get it down any lower.

Thanks,
Rob
1980SC

Can somebody explain how only reducing the system pressure, with no reduction in control pressure, will richen the mixture? I thought only the control pressure determined the back pressure on the sensor plate. Is something else going on here. It seems like a reduced system pressure would also result in a lower pressure to the injectors for a given sensor plate position, resulting in a leaner mixture.

I also thought that reducing the system pressure should reduce control pressure accordingly. I haven't had the need to mess with my system pressure....yet.

According to the Bentley manual, the control pressure is low when the engine is cold and higher when the engine is warm. Control pressure is the pressure at the top of the plunger in the fuel distrutor. Lower control pressure results in more of the metering slit exposed, yielding a richer mixture. So it sounds like that the mixture would be richer if you lower the system pressure but each shim removed from the piston on the side of the fuel distributor lowers pressure by 2.2 psi so you can't lower it much.

The range of warm control pressure for a US CIS is 48-51 psi. It would be interesting to know what iit is supposed to be for a Euro/ROW system but I bet it isn't down in the 40's.

Sure is. 39-45 psi.

I understand how a lower control pressure will enrichen the mixture. I don't understand how a lower system pressure WITHOUT a lower control pressure will do the same thing.