DJet Problems, 3000-4000 RPM Stumble Options

[yeahmag]

I'm helping a friend with a DJet issue and I use to consider myself pretty good with this system, but I'm running out of ideas with this one. There is a considerable stumble at steady-state, 3000-4000 RPM. So far I've done the following:

* Replaced his dizzy with one of my good ones - re-timed.
* Replace his spark plug cables, rotor, and cap known good ones.
* Disabled the TPS to rule that out.

He's running a Petronix Ignitor that we swapped over to my dizzy. I'm starting to suspect it, but what keeps me from changing that out is that it runs well at WOT. It really feels like a spark/timing issue...

What am I missing here???

-Aaron

[ChrisFoley]

I doubt its the pertronix.
Maybe one injector is weak.

If its too lean at light throttle settings, that could cause misfiring.
You could try adding a resistor to the head temp sensor to richen the A/F mixture and see if the stumble goes away.

[yeahmag]

That's not a bad idea. I'll have him pull the injectors and we can measure their output. If I remember right I just turn the ignition on and off a few times and then compare the cc's. Correct?

[76-914]

Ign on and pushing the throttle will activate inj's. Hey Chris, wouldn't pulling temp sensor II do the same thing; i.e enriches the mix?

[TheCabinetmaker]

Temp sender two is the head temp sensor. Pull it and it won't run. You can ground it out to make it run full rich.

[76-914]

My bad; temp sensor I. Air intake sensor..... and they do run poorly, if at all, with CHT sensor unplugged

[TheCabinetmaker]

You said you disabled the tps to rule it out. Did it run good with tps unpluged?

[ChrisFoley]

... wouldn't pulling temp sensor II do the same thing; i.e enriches the mix?

I'm not sure how much enrichment occurs with the IAT sensor unplugged.
I noticed a small difference on my N/B AFM when I moved the sensor from laying on top of the engine to inside the cold air intake on my dual TB setup.

I recently had a symptom of low CHT on one cylinder. At idle I could pull the injector plug and the rpm wouldn't change, although the engine seemed ok when driving, and the cht only lagged by 40 degrees or so. With the injector disconnected the engine clearly was only running on 3 cylinders and the cht stayed cold. When I bench flow tested the injector it seemed to spray just fine, but I didn't measure the flow rate.
Since I replaced the injector with another used one the engine runs much better with all chts close together.

[yeahmag]

No. It did not run better.

You said you disabled the tps to rule it out. Did it run good with tps unpluged?

[914_teener]

No. It did not run better.

You said you disabled the tps to rule it out. Did it run good with tps unpluged?

1. Check fuel pressure.

2. Check fuel delivery on injectors.


You didn't say if you had done this.

[yeahmag]

Fuel pressure was set a bit ago, but warrants re-checking.

[pbanders]

Sounds like ignition to me. You did everything but the Pertronix, swap in points and see if it goes away. If not, check injector spray patterns and delivery. I'm assuming this engine pulls a good intake vacuum, has good compression on all cylinders, and has had the valve clearance recently set.