hupp

Definately rich. I missed this before. May want to try a bit more than a quarter turn.

Eliminating the hot start valve is not an issue.

The vacuum limiter essentially prevents the throttle body from slamming shut during deceleration, a problem here would result in the rpm's dropping quickly each time you take your foot off the gas, not just when cold.

Dennis Wilson

The best way to hand set the A/F mixture is to turn it to lean until it starts to falter, mark that point and turn it to rich until it falters. Then turn it back to the midway point or just to the rich side of the midway point.

Dennis

jpitman2

FWIW - my hot start issue was a complete failure to start after being turned off for between 10-20 minutes, give or take, but sometimes more. I could fill it with gas, and restart OK. I could stop for 30 minutes, no problem. In between, it would fire once and die, then I had to wait another 5-10 mins before it would go again. Embarrassed me a couple of times. The hot start switch on the heater water outlet at rear of LH head had a minimum resistance of 20ohms, enough to stop the connected solenoid from firing. The solenoids job is to drop control pressure ONLY while cranking when temp was in a certain half hot condition - 35C IIRC. As Dennis tells it, I got a 924 switch (M10x1 thread), adapted it via a plug (M14x1.5) into the head fitting, and all is now well again.
jp 83 Euro S AT 51k.

SharkSkin

Great stuff, Scott. Always good to have someone who's BTDT. Reading back, I see all the signs that point to your diagnosis, I think it was the high idle that led me astray.



um... well, pending Keith's results of course...

tailpipe

Some hot start probs with CIS are caused by a leaking one way valve at the fuel pump under the tank this valve with the accumulator keeps the system at regualr running presure for up to 3 hours. If the presure leaks out you get the tell tail symptom of a little burp of a start then nothing untill you manually add fuel or wait till the temprature of the motor cools enough to let the cold start vavle add enough fuel for start up. Other things that can contribute to this is a plugged fuel filter, a collapsed fuel accumulator diaphram or even just alittle bit of crud in the inlet filter screen in the WUR. Look at the cheap stuff first check the WUR for crud a little carb cleaner in a spray bomb should clear it out, Next cheapest is the one way valve it can be bought at your loacal VW dealership for about 15 bucks you will need a cap nut with it also just add everything on top of the original porsche valve on the long end of the fuel pump, might as well change the fuel filter at the same time as you have the system apart already. As for the fuel accumulator thats the last thing to look at its a replacement part only. I have heard some rumors that ethenol and fuel accumulator diaphrams do not get along maybe someone can confirm this

I hope this helps good luck

paul with 4 79 928s

928ntslow

Paul I assume you are noting this for JP..my car has no hot start issues.

So Dave, when I get a chance, I will pull the hose between the AAV and the plenum just for ****s and giggles.

I had heard that the A/F maybe be an issue as Scott had said. Obviously, the motor appears to be "hunting" as it rises and falls to find a happy medium between too much A/F or not enough. The question is what's causing the mixture to vary?

Dennis, WSM says to turn the mixture all the way clockwise (IIRC) and then back just a snitch until the motor idles smoothly. Sounds like you are saying to get the adjustment right in the middle of the two lock points....is that correct?

Dennis Wilson

Keith,

Yep. That has always worked for me on the CIS cars. Actually, I go a little to the rich side of center.

Dennis

928ntslow

Had a chance to take the AAV out of the loop this morning to see if there are any changes. NONE is the answer. I did this at the point the car started acting up.

I will re adjust the A/F mixture and see what that will do. However, I do recall that after the rebuild and install, getting the car running and fine tuning it, I had the same trouble keeping it running while it was warming up. So, I am not sure this is the issue...but we'll try it.

Dennis Wilson

Keith,

Have you checked the air metering plate lately to see if it had a build up of oil/dust which could affect the balance? Also, have you checked the setting on your throttle cable to make sure it isn't adjusted too short and affecting the throttle plate when cold?

Dennis

928ntslow

I had cleaned the throttle plate quite well at rebuild as I had mentioned. it responds nicely to the slightest tap of a finger...of course with the FD not installed.

I set the throttle cable properly, but have not checked it in relationship to this issue. I can't imagine that I had overtightened it, but hey, Chit happens. I will check it.

SharkSkin

That was intended as a basic troubleshooting step... it sounded like Hupp had nailed the problem. Not so?

tailpipe

Hey Kieth check you WUR see if there is any rust or crud stuck in the inlet screen on top it might be something as simple as that, almost every one I have taken apart has had crap in it, if there is no screen in it I would suspect the the crap has invaded to diaphram inside not sure if its cleanable. Easy and cheap to check easy to clean too.


paul

Dennis Wilson

While you are checking the WUR try pulling a vacuum on the vacuum inlet(s) to make sure the internal diaphrams are not leaking.

Dennis

Fabio421

Kieth, what did you end up finding was the cause? A local Rennlister is having the same issue with his 78. He just rebuilt the fuel distributor as you did so I'm hoping your resolution will help him.

jpitman2

FWIW my pump and check valve are relatively new, as is the accumulator. My change of the temp sensor firing the pressure dumping solenoid seems to have fixed the hot start issue.
jp 83 Euro S AT 52k , P110 Brian Long.