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Whistling Kettle: 924 Turbo


Lesworth

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Time for an update. While looking at the hot start issue I discovered that I should have a one way 'check' valve after the main fuel pump which was missing. I fitted one and also changed the fuel accumulator at the same time, along with replacement fuel lines. This resolved that problem.

So now its just down to hot versus cold idle which is what I have been trying to get right all along. The guy at Rallytec got the warm idle bang on right by blocking the air passage from the boost tube to the three throttle body bypass routes: AAV, Decel Valve and Brake Booster venturi device. He also removed vacuum to the distributor retard vaccum device. As well as stabilising the warm idle, it allowed idle speed adjustment at the idle screw and also significantly improved performance and drivability.

The issue is that while cold starts are fine, the idle when cold is awful, hunting and almost stalling until the WUR leans out the mixture as the car warms. I am really not happy living with this compromise. The Rallytec guy did a lot of smoke testing and found there are no external air leaks so there must be air passing through one of the three bypass routes listed above based on blocking the air passage and the effect that had. Simple logic?

Blocking just the route to the booster venturi had no affect. That makes sense because its designed to allow air to pass the ensure you get booster vacuum even when on full throttle / boost when there is no vacuum for the booster. The mixture spec must already account for that air to pass. There is also a restrictor (pinhole size) so its only a tiny amount of air anyway.

Next I connected the AAV directly to the air outlet on the boost tube and used some rubber bungs to plug the other open outlets so that only the AAV had air from the boost tube and the other two bypass routes were blocked. On a cold start I immediately had about 1300 rpm and this slowly lowered as the car warmed. Hmm, think I am onto something here. The problem is the idle wasn't stable with a lot of misfires. That could be because the bungs I used weren't totally sealing the other open pipes/ hoses. I connected everything up again and re-instated the block and drove the car to make sure the misfires had gone and they had so I put that down to the bungs.

Next test was to isolate the remaining bypass device, the decel valve. This has an inlet and outlet for air and in its default state its closed. It also has a signal vaccum hose to the manifold. During vacuum peaks, such as when you release and close the throttle, it pulls open a diaphragm to allow that air passed. This is to avoid a low of unburnt fuel causing backfires and also allows the revs to drop more slowly, improving drivability, for example between gear changes. Its been isolated for a while along with the AAV and booster venturi of course and you do notice its absence a little. there is a little pop between gear changes as you accelerate but it really is barely noticable.

I replaced one of the rubber elbow hoses that feeds air from the boost tube with a pair of silicone bungs and remove the blockage at the boost tube so I now have air to the AAV and booster venturi. Cold start; boom! 1300 rpm, stable with no misfires. Reduces nicely as the car warm to that stable warm idle. I also now have an adjustable idle for the first time. I now just need to drive it for a while to make sure its not just a fluke. I'll be taking it to the Silverstone Classic and it'll be parked on the 924 owners club stand all three days if anyone wants a closer look at it.

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Today I thought I would tackle the last remaining 'bodge' that was introduced to get the right level of performance / running / warm idle. This was the removal and plugging of the vacuum signal to distributor timing retard vacuum device. With this reconnected, cold start was fine but the revs were lower (800 rpm or so) but nothing like they were before removing the main air bypass blockage mentioned above. It felt a little sluggish off boost and not as harp as before. There was also a noticeable increase in popping on deceleration. A little timing tweak (slightly more advance) resolved this and it now drives smoothly, with the right level or performance and reduced popping.

I now have all the right things connected and working properly, with the exception of the isolated decel valve. They're not cheap! I would like to re-instate this function but not at the price the new ones are going for (500 EUR on Rosepassion). I suspect a second hand one will be a risk if I can find one. I'll just keep my eyes open and just use the car as is for now.

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Ran a last test today with a start from cold. It now fires and idles strongly at 1200 rpm and slowly lowers to 950ish as it warms. Sorted! In fact it runs better than the 911 now.

This car has taught me a ton about CIS. Now to start planning the next phase - paintwork.

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Well done for persevering and achieving that end result, must be a great feeling.

After 924 paintwork, a refresh of the 911 CIS :whistling:

 

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I assume you’ve seen that 924 on Car and Classic auctions Les? It’s listed as for spares or repair & ends in a couple of days. Currently at £100 with no reserve. Appreciate you need next to nothing parts wise for yours, but just thought I’d mention it in case you hadn’t seen it. Looking forward to seeing the continued updates on yours re the paint BTW. 👍🏻

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  • 1 month later...

Although the cold and warm starts and idle are now great, It still has some hesitation for the first few minutes of driving from a cold start. Using a gas analyser, the fuel mixture reads 0.5 which is right at the bottom of the 0.5 to 1 range according the the workshop manual. One thing I noticed is that when I was trying to measure and adjust the mixture using this analyser a few months back before a lot of the work I have recently done, I could not get a stable reading at all. This time it was completely stable and adjustment was a simple process.

A little tweak to 0.8 has had a noticeable positive improvement to warm running. It might be the placebo effect but it seems to perform better too, especially on boost. Also noticeably less popping on overrun given its not running as lean as it was. Next I just need to try a cold start to see if the cold running hesitation has improved.

In other news, just awaiting a quote for the repaint. Hoping to have that done early winter if possible.

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On 02/09/2021 at 10:09, Lesworth said:

In other news, just awaiting a quote for the repaint. Hoping to have that done early winter if possible.

It will be the end of an era when the red front wings are finally covered up, unless you are planning to retain the pseudo Marlboro colour scheme, Lesmonde.

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On 03/09/2021 at 11:12, hagarep said:

Let me know when you're done with the gas analyser Les.

I am going to do my pressure checks this weekend and then a bit of fine tuning.

Jason

All done now - thanks! Send me a PM when a good time would be for me to bring it back - same place as last time?

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