Thread: A new car!
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Unread 12-15-2016, 01:55 AM   #138
GlenL
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Been a while in process, but I've gotten the CIS worked out enough to put a '79 engine into the '80 S. It starts and runs but won't "take the gas" when I press the throttle. I've checked a lot but can't find the false air leak that I'm sure is there. The car will idle at 1200 RPM fairly smoothly and do so until I turn it off. It's unresponsive to changes in the idle setscrew. I'll get this figured out. At least it runs without blowing fluids about.

I did spend a lot of time working to make the CIS workon the bench and wanted to share those results. In the end, I couldn't get the flow rates balanced as well as I liked but they were pretty close and I had to get the project moving along. A bit of math came to form my struggle:

8 x 8 x 8 = 512

There are 8 metering valves and 8 springs and 8 shims. When I took the fuel distributor top-up, all the bits fell out and got mixed up. That leaves 512 combinations of hole, spring and shim to work out. Plus aging and diaphragm damage that may make it never have the cylinders get balanced. Avoid this first mistake and:

Always disassemble the fuel distributor up-side down and carefully track which parts go where.

I've posted a few times on this. I'll rehash that a bit for completeness.

Using a spare CIS fuel pump, I set up a bench-test system for the CIS. It's easy enough. My initial goal was to flush gas through the fuel distributor and injectors and get the system cleaned out before putting it into a car. The initial work was problematic with the system being clogged. So I resolved to rebuild the fuel distributor and clean the injectors. And that's when the fun started.

I've lost track of the number of times I had the fuel distributor apart. Likely 20 or more. Pull it apart. Clean it a bit, put it back together and try it out. A few things have come clear to me on this.

Always use a fuel filter going into the system.

It's OK to re-cycle the gas that come from the return line. I found real trouble putting the gas back into the system that was caught coming out of the fuel injectors or leaks. I suspect those were getting contaminated from the tubs I as using. Something was going on as using fresh gas got good results put pouring the "caught" gas back into the supply gas can often lead to clogging of the distributor and WUR.

The thin, simple "washers" below the springs are shims and are more critical is setting flow rate than the springs themselves. I started to track flow rates versus shim height and it was a very clear trend of the thicker shims producing higher flow rates. By moving the shims around and comparing flow rates from the injectors with no other changes I could see that some springs were stiffer. Or the diaphragm was more compliant there.

By moving the shims to get a gross balance and then moving the springs based on which were "stiff" and "soft" I could get the fuel distributor back into some sort of balance.

Be very careful when installing the 8 small o-rings around the central core. It's very easy to have one pop off, slip up, break or have an abrasion. The result being an injector that comes on full blast and the trial is wasted. It was handy to have a bag of 100 o-ring to use. The more assembly cycles the o-rings are used in, then more likely they are to fail.

One effort that went well was characterizing the fuel injectors. I swapped them, one-at-a-time, to a line that flowed well. This lead to finding 8 (out of 17) that had comparable flow rates and good patterns.

Something quite interesting was that the fuel distributor on the "S" car is an adjustable distributor. The model number matches the one off the '79 and is correct for the car: 0 438 100 027. But! Each valve in the fuel distributor has an Allen-key adjustment on its side. I hadn't seen this before, or heard it mentioned, and could find this feature on other 928s. It looks like something for a Ferrari 400. Carl at 928 Motorsports sells an alloy version of this type of unit for strong money. Picture below.
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1980 928S Euro x2
1987 944 N/A
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