Popping on decel, fuel pressure regulator? Options

[PDXMike]

Hey all,

My 75 1.8 L-jet has always popped on decel. I've looked for exhaust leaks, including dropping and reinstalling the entire exhaust and getting it smoke tested. My decel valve is working correctly. This thing makes a LOT of noise, even when just letting the engine spin down with the clutch in. It sounds like some crazy-tuned dirt track stock car or something. This is fun for the first 5 minutes or so, then anybody within 100 yards of me wants to run me off the road.

While testing my decel valve, I noticed that the same vacuum line that controls the decel valve also runs to the fuel pressure regulator. Is it supposed to cut the fuel supply off when vacuum pressure goes up? Could this have anything to do with my popping? If the injectors are still firing I'd be pumping gas right through into the exhaust, right?

I have a Monza muffler, which probably isn't helping.

Is this a crazy theory? What else can I try?

Thanks,

Mike

[TheCabinetmaker]

FP regulator only has two nipples. One is fuel in, the other fuel out. Decel should not be hooked up to it. Picture?

[Grump]

My 75 1.8 also has this vacuum hose connection. It's also indicated on Jeff's vacuum hose diagram.

Some members disconnected and plugged the decel valve. I've done this also but I didn't like the backfiring on occasion and hooked it back up. One time I did notice while watching the hoses on deceleration, one of the hoses pinched together due to weak hose wall. Replaced it with new hose.

I've also noticed decel valve does not always let idle to come down to normal. I just put a slight load on the engine by lifting clutch a little and idle comes down to normal.

Cannot comment on your exhaust. Some due exaggerate the sound. Others here probably can provide input.

Good luck,, Bill

[catsltd]

My 74 1.8L Monza exhaust popped a lot on decel,(no decel valve.)

I had a hole where it joins the muffler,fixing the whole did not stop the popping.
My timing was the problem.

Even though the mufflers leak is back,there is no more popping.

My timing is set to 7.5 BTDC from the red mark on my pulley.
I downloaded the PDF and printed the template and went with that.

Not sure what a 75 is tuned to,just trying to help.
Grant.

[PDXMike]

Grant,

7.5 BTDC sounds familiar. That's pretty easy to check so I'll move that to the top of my list.

Mike

My 74 1.8L Monza exhaust popped a lot on decel,(no decel valve.)

I had a hole where it joins the muffler,fixing the whole did not stop the popping.
My timing was the problem.

Even though the mufflers leak is back,there is no more popping.

My timing is set to 7.5 BTDC from the red mark on my pulley.
I downloaded the PDF and printed the template and went with that.

Not sure what a 75 is tuned to,just trying to help.
Grant.

[Mikey914]

If not A&P isn't too far away.

[porschetub]

[quote name='PDXMike' date='Sep 27 2015, 01:40 PM' post='2243633']
Hey all,

My 75 1.8 L-jet has always popped on decel. I've looked for exhaust leaks, including dropping and reinstalling the entire exhaust and getting it smoke tested. My decel valve is working correctly. This thing makes a LOT of noise, even when just letting the engine spin down with the clutch in. It sounds like some crazy-tuned dirt track stock car or something. This is fun for the first 5 minutes or so, then anybody within 100 yards of me wants to run me off the road.

While testing my decel valve, I noticed that the same vacuum line that controls the decel valve also runs to the fuel pressure regulator. Is it supposed to cut the fuel supply off when vacuum pressure goes up? Could this have anything to do with my popping? If the injectors are still firing I'd be pumping gas right through into the exhaust, right?

I have a Monza muffler, which probably isn't helping.

Is this a crazy theory? What else can I try?


Never dealt with these but have heard of issues,don't think your injectors will be firing fuel out the exhaust however ,your throttle is closed so that's covered by the AFM throttle positioner ,as mentioned I would look @ the exhaust unless you a lean condition on deceleration which is not that likely.
Has the exhaust blown its guts out like the old beetle glasspacks did ? don't know.
No the FPR cannot shut the fuel off.
How did you go with the timing ?that's a key issue if not right,check your advance plate on the dizzy they can stick up ,very common issue, good luck.