As the title says, my stock 72 1.7 is popping a lot on deceleration. I thought it was an exhaust leak, but everything is tight. I thought it could be the stocker muffler wearing out, so I swapped it out for a fairly new Bursch that came on the new to me 71. Still pops, but now it's a louder, meaner sounding pop instead of the wimpy pops from the stock muffler. Any ideas on what's causing it? Injection misadjusted? Timing? Help me, please, it's driving me nuts! Not that it was much of a drive....

I'll be watching this thread as mine does the same thing.

My 74 2.0 had this problem on decel as well, with the "trailer hitching" effect. The engine was not stock, but this might apply (D-Jet car only). If I leaned the mixture out a little on the ECU, it decreased, then leaned it out until it was gone. It ran so smooth, and great throttle response. I also lowered the fuel pressure (within published limits of 28-32 psi) which helped some as well. I had been running it around 35 psi on recommendation from a couple people. I believe it was being caused by an over rich fuel condition that was passing by the exhaust valve and burning/igniting in the exhaust, causing the pop and a little buck. Counter clock wise on the ECU knob=more lean. If you decide to try this take note of the knobs current position for reference, before you move it. 72 may not have the knob. Never owned that early of a car....yet.

Martin hit on it... but he was probably masking a leaking fuel injector. One drop of fuel from an injector that isnt supposed to be firing and it will pop "right now".

Pull the injectors and coil wire off (pos. lead from coil not the coil wire) and spin the engine over and let them fire into a container of some sort so you can watch the pattern of the injectors while they are firing. Do this on both sides.. watch for a nice cone pattern AND watch to see if they dribble after firing.


B

I was led to believe the knob on the ECU was strickly for idle. Was I spoken to by someone with a forked tongue?

.......b

I think this applies because during decel the throttle is closed (obviously), the signal from the Throttle Position Switch would be idle, as you are off gas.

@Bruce:
Hi,
adjust the idle speed only at the throttlebody screw, the knob at ecu adjusts the idle mixture.
Both interference together. This means if you adjust the idle mixture you have to control the idle speed and vice versa.

So the fix here is to set the ECU at center, and adjust the throttlebody screw. Right?

That will work. The *proper* thing to do is get the car to a 4 gas analyzer and tune it at idle.


B

Thanks, Brad. Once I decide to spend some more time at the shop, instead of trying to get the 71 back on the road, I'll have the shop next door put it on his machine & we'll dial it in. First thing I'm gonna do is dump the Bursch. It makes the car sound like a ricer.

This thread is exactly what I needed! I have been dealing with an anoying popping on decel for over a year. I swapped the heat exchangers and made sure they were not leaking, replaced the muffler--and still the same thing. I ran a fuel injection cleaner through and drove it hard and that seemed to help, but didn't eliminate. Sounds like it could be as simple as a bad injector. How long will fuel injectors last?? I replaced them in 1996, but the car sat in storage for much of the time since then. I will also try the ECU recommendation.

I set the ECU at center (22 clicks from one end to the other), got the car good & warmed up, adjusted the throttlebody screw for the idle at 1k. Car has just a tiny bit of poppoing to it still. Have to really listen to hear it, so it seems like the next chance I get I'm going to do some checking on the injectors. Thanks all, for your advice and help.