Backfiring, rough running, after rebuild

Boxer 6 · 06-24-2007, 03:45 AM

I need some pointer from some of our experts that reside on this board. Let me give some background on the rebuild first so you get a good picture of the situation.

The engine is a 3.2 liter Motronic engine with JE 10.5:1 pistons, DC 20 cams, twin plugged (using a 964 distributor and Andial signal splitter), Magnacor 10 mm wires, Steve Wong chip and Kennedy aluminum pressure plate. The good news is that the engine fires right up....but the idle is fairly rough and the engine sounds a little "flat". The engine will easily rev, but when the throttle is "let off" it will usually stall with a backfire. I originally thought the rough idle was due to the weight of the aluminum pressure plate and the lack of inertia. Where should I start looking? Thanks in advance.

svandamme · svandamme's Garage

the light flighwheel will loose revs faster then the normal flywheel
this is normal

but is the chip configured for this??

the faster the engine spools down
the less time the motronic has to compensate the idle stabilizer...

combined maybe with maybe a bit lean mixture down low...

but that's an assumption i'm making here
it's all hard to tell on a custom thing, you'de need some data... exhaust o2 measurements to see what it really does...

was that chip made specifically for this engine setup??

Boxer 6 · 06-24-2007, 06:38 PM

Hi Stijn,

You have some very legitimate points. I appreciate it. I had Steve Wong burn me a custom "baseline" chip for this setup. I'm not sure whether I mentioned the lightned pressure plate to him??

Steve W · Steve W's Garage

Could be either a miswired set of spark plug cables, an intake air leak of some sort, or the base idle mixture being too lean. It almost sounds like there is a intake air leak such as from a misconnected hose. A lightweight flywheel should not be an issue, as the chip already is modified for accomodate such.

You can check for miswired spark cables by disabling one set of spark plugs by disabling one of the plugs at the ignition module at the Andial splitter. Disconnect one of the modules with the engine off, start the car, see how it idles. Turn off the car, reconnect the plug and disconnect the other module and repeat. They should idle the same if connected properly. If they do, then ignition is not the problem.

svandamme · svandamme's Garage

tx for the feedbacl Steve, wasn't sure if it mattered, i was just thinking out loud..

would it matter if say there was an ultra light flywheel involved, like those really skinny ones that Wevo has.. pretty much just a thin frame for the starter ringgear?

Steve W · Steve W's Garage

Even with as light as the Wevo racing flywheel, the idle should not be rough and sound flat - really does sound like it's idling very lean.

svandamme · svandamme's Garage

maybe not the idling
but the idle stabilizing after spooling down, would you need to compensate for a very light flywheel??

Boxer 6 · 06-25-2007, 06:46 PM

I have some troubleshooting to do over the next few days. I disconnected one coil at a time and there was no change in idle, so I'm pretty sure that I wired it correctly. I will put the engine into TDC and pull the distributor caps to make sure that the rotors are under the number one terminal. If that's ok I'm going to pull my valve covers and check to make sure that I did not screw up the timing when I did the cams.

Thanks for your input guys. As far as the chip goes, no one does it better than you (Steve). I just want to get my car to the dyno so I can get the data sent to you. Thanks!