Hi Guys,

Trying to diagnose a strange issue. Here are the vital stats: 2056 with Djet, raby 9550 cam. Fuel pressure at 29-30, timing spot on, new plugs/wires, 123 dizzy.

When I start the car, whether dead cold or warm, the engine turns over and catches right away, but it stumbles as if only firing on 2 cylinders for about 5 seconds. A few pumps of the accelerator and the stumble goes away and car idles fine.

There's a slight smell of unburnt fuel after start.

Other item which could possibly be related: new CHT sensor from 914Rubber which is said to conform to 0 280 130 012 spec.

Thoughts? Thanks!

At one point I measured a new in bag 012 sensor vs 914Rubber. Chart below.
Click to view attachment

Thread with background:
http://www.914world.com/bbs2/index.php?showtopic=351065

You could start by measuring your CHT when cold vs the chart. The largest changes in CHT values are at cold start. Maybe something had gone wonky with your CHT or you may have a high resistance connection between CHT and vehicle side wiring?

Quick and easy to check.

Yo Ian!

I've had two engines with Raby 9550 cams. One was 2056 w/Djet, and now a 2258 w/Microsquirt. Same behavior you are describing, particularly when cold. I don't notice it much when warm, but for the first start of the day I may need to feather the throttle or goose it once or twice until the CHT hits over ~120 or so.

So...it might be normal

@ClayPerrine has experienced something similar with that cam too, IIRC.

I bet it is a cht issue. I wonder if you could set up a dual resistance path? Something that switches from a little less rich start up to the 914rubber. If it had a pot it might be really useful for people with mods that result in these issues.

The heat soak issue most people get, where the cht doesn't tell the motor good things if you shut down and try to restart hot.

Thanks, guys. I was thinking CHT too, but it only takes about 5 seconds for the engine to run well. Could the CHT sensor could be that responsive?

Yes. Look at the exponential change between 50 and 100 degrees.

Good point. I was just thinking it would take longer than 5 seconds for the heat transfer to happen / register. I'll check the resistance and see if all seems right.

my 75 does this too.. and I have to blip the engine to get the charge excited too and the charge light goes out..then runs fine

FWIW...

CHT measured at 1500 @80 degrees, so it looks in line with @superhawk996 ‘s findings…

Betty's car with the Raby 9550 and L-Jet wouldn't idle dead cold and had to have the throttle feathered at first start up. It took less time for it idle than it did to back out of the driveway. So it was maybe 30 seconds of blipping the throttle.

After the engine got a little heat in it, it idled fine.

It's the cam. Once warm it runs like a raped ape. You might try advancing the static timing a little. Are you running vac advance?

I too am running a 9550 cam, same issue, but great response after it warms up.

Might have to get me one....

Interesting. Can anyone explain the process by which the cam causes rough running for a few seconds. Would love to learn.

I would suspect it is on the fringe of confusing the computer due to valve overlap etc at idle. My mustang's old speed density system has some of the same stuff. You can't put a big cam in these first gen fuel injection systems
Without some problems.

I’m running it on Microsquirt where basically all the inputs are tunable. Still have the issue. Just a tiny bit of throttle and it idles fine. I’ve spent quite a few hours trying to figure it out and solve it with the tuning tools I have. It just seems to want a lot of air when stone cold.

I’d be happy to have someone (Jake, maybe ) explain why.

Me, too. The Experience … and the hope.
Didn’t used to do that; started recently.
1.8 stock FI.

@iankarr
@beatnavy
@geezer914
Ok; got the data:
Click to view attachment
+ 1.8; stock FI
+ For test, I let it run in driveway until normal idle reached by itself - about 6 min. Didn’t rev throttle.
+ Normally settles in after driving just half a block or so. Otherwise, runs great!
+ If I reset the idle screw to get a good rpm at the start, then I end up with a final idle of 1400+ rpm.
Questions for group-think is:
1. AAR? (Bench tests ok, though old so opens/closes about 80% in each direction)
2. CHT? (Haven’t yet checked resistance)
3. Adjust idle TB air screw to get 950+ rpm at start, then counter adjust AFM to lean static and richen AFM idle screw to reduce final rpm (too much “beating around the idle bush?”)?
4. Just Old & tired? (Not unlike me; tough to get started but once up and moving get along just fine.)

Other option is to just leave well enough alone and get it on the road half a block and keep going.

Hey Steve, not sure what exactly you're asking here. Are you having the same symptoms with stock L-Jet? Your cold start idle does look pretty low if that's what I'm looking at in the picture above.

Maybe start another thread so we can get some L-Jet experts onboard? (Of which I am not one).

Great idea, Rob! Will do.

I run a 9550 cam 2056. Do not have that problem. However, I did install an 02 sensor, and recalibrated the mps with my innovate LM1.

The CHT extension piece from 914ltd is said to improve warm up drivability..it supposedly delays the change in resistance during warm up - whether that would help or not with the fuel small I don't know. you didn't say if you were using one, but maybe it would help.
https://914ltd.com/product/porsche-914-4-cy...nsor-extension/


.

I made one of these, it didn’t seem to make much difference in the initial start as the impedance would be the same. It did extend the rich running time some, but that turned out to be unnecessary because the car would run fine after the first, say, minute without it. So I removed it. Was a fun experiment anyway

There was a resistor that AA sold to attach to the CHT wire to richen up the idle circuit.Click to view attachment