ShopTalk Forums | Aircooled VW Tech & Restoration Help

No ....there is ...as of yet, no substitute for the .015" thick copper diaphram. The suggestion of alternate materials does bring up some interesting thoughts though. Years ago, I tried alternate materials just for fun. I tried rubber....it cannot work.
When rubber vacumes down, it is not stiff enough to have the force to actually push the metal rod/plunger in...because the inner spring and vacume is stronger than the rubber is stiff....make sense? The copper...in comparison acts like a solid...though it has some flex. So when say.. 20" of mercury hits it...it can actually be pulled in...pushing the rod with it. The rubber...or plastics...will begin to move the rod...but then actually start pulling in around the keyhole intermediate plate without moving the rod because the rod spring is stiffer than the rubber.
In order to work, they would need a metal lattice...or scrim to act as "rebar" for the rubber diaphram. I then thought or punching or drilling holes in the copper plate to make a flexible lattice out of it. I added 100 micron (just about .004") latex sheet to the copper. It worked...but was ugly....and latex is not good with heat. I mused...that rubber is rubber (so to speak) which is when I decided to try just repairing the cracked rim with a "cast" on bead or gasket. That works very well depending upon the extent of the crack.
I have recently aquired the ability to "print" very thin coatings of warmed up high temp RTV onto surfaces. I may try this soon and see if I can coat a copper diaphram and then build a gasket edgw without sacrificing sensitivity of flex.
I have found that there is merit to be had in varying the thickness of the copper dfiaphram...say going down to .010"...or up to .020" for different levels of tuning sensitivity...but one thing at a time. Ray

Rocket science! After all that reading about MPS, it seems to me that most of you guys is engineers or scientists. Anyway, I was offered the used MPS (from carbs conversion), I guess it'll serve for a few years. Thank you all for your help. I'll keep you posted on this matter till the "winning" end.

[This message has been edited by knabini (edited 07-24-2002).]

Always nice to get a free MPS. Just save the old one. If the body and the windings are good...it has value. Ray

KISS. you know keep it simple...

For diaphragm repairs use aluminuminized self-adhesive duct tape available from FLHS (Friendly Local Hardware Stores)

I have tried this with diaphragms that were completely separated from their outer rings and it works great, is durable, fast and easy. Put a layer or two on each side of the diaphragm (cut the patch out around the screw) for extra strength. The additional thickness may alter the mixture adjustments, so you will need to readjust the screw settings, but it will extend the life of the MPS if that is the only problem.

Ray, the MPS, I mentioned, ain't for free: it's for sale for 150.00. It's a sealed adjusting screw, so it's been adjusted at factory (does it matter what displacement it's been calibrated for?) OR to get a rebuilt one...Do the rebuilted ones differ/behave much from factory? Anyone knows the reliability of these rebuilt units? Thanks...

I have actually tried that too. You said it all. It will extend the life...its more of a patch than a repair. It will alter the settings and sensitivity as well. The settings are an easy fix, the sensitivity of the plate flex can be a little worse on some engines. Because it is aluminum, it will sooner or later crack as well. I got another 5 months out of the only one I did this way. The tape is commonly known as furnace tape. Its just very thin sheet aluminum with adhesive. It does not stand up well to heat or solvent, before you get it adhered well, so be careful. Good suggestion. Ray

Do NOT...pay that much for a used MPS. They are not that scarce. It may be seconds...or years away from failure. All of the units were factory sealed in the beginning. When you take the epoxy out...be sure and mark the screw/cover before you remove it. Count the turns. That is the outer stop for the copper plate. Its not just a cover/ plug. Yes, different engine configurations used different part #'s. The main difference was the factory calibration level, and on some models, there were differences in the main load spring for the main stack on the inside. Most of them are so similar that they can all be adjusted to work on anything. Also be sure it is not an earlier type 3 unit...with no copper diaphram plate. Those can also be used...but there are a couple of mods that need to be done to the FI system...and a lot of adjusting...so don't throw those away. I would not pay more than $50 for a used pressure sensor. They are just not that scarce, considering the # of people actually using D-jet. You can buy new ones for in the $400 range...with no wear on them. I still sya itwould be cheaper to repair what you have. Ray

That's right - the remaining life on the used one could be one day or 5 years; you just never know. As for repairing my MPS, I can't take it off and admire it's beauty on my bench, cause I drive my 914 every day. I'll get the rebuilt one, install it and, only then, admire my broken one and see what I can do about the damage. George...