excessive "popping" on deceleration - FIXED
 

83 SC Engine
Top End rebuilt last August

I have recently obtained a very stong "burble/backfire" when i lift off the gas at moderate speed. I like a small burble but this is beyond that

Recent changes before moving from gentle burble to strong burble

New CDI installed
Backdated Heat
Replaced low oil pressure switch by removing top of CSI, re-install of boot appears to be tight and well fitted
Exhaust has been checked and is tight with new gasket and no discernible leaks. What are other sources that can contribute to this condition?

One suspect is one of my injectors seems a bit loose (the orange insert portion) and i can move it up and down but doesnt pop out..perhaps this needs a new o ring and or sleeve replacement if its resulting in poor gas burn or am I just hoping in vain its something that simple?

Checked all vacuum lines..and found this one loose, I am not even sure what that component is (air restrictor?)...could this be a culprit?

http://forums.pelicanparts.com/uploa...1599840123.jpg


Mixture was not changed from prior to work above

any ideas?

Thank you ALL

Bob

That is a decel valve. A 1983 engine would have to be a euro model (930/10) as the US models have a different decel valve type. The function description below is helpful.

https://jimsbasementworkshop.com/CIS/pages/comp_page.html#decelvalve

so skier...

if that vacuum line was nt attached would that result in a lean deceleration and popping?

My assumption would be "maybe" (very helpful right?). The valve working correctly adds air to prevent a temporary over rich condition. I believe an over rich symptom can be popping in the exhaust as unburnt fuel ignites. If the valve isn't working perfectly because that line is loose and not providing correct variation in vacuum, it seems possible. I would zip tie that line to the top connection and see if things improve.

The other standard troubleshooting stuff should make your list: smoke test, check your pressures at the WUR, etc. As you likely know, lots of things work together on the CIS system to run smoothly. I would be surprised if that line being loose is the issue.

i ll do a test run tonight and see :-)

as mentioned..adjust your fuel mix.it is too rich

Ivan

I think that valve is used to prevent stalling when you let off the throttle quickly. The Throttle plate almost closes and engine speed drops below normal idle. This valve is controlled by intake vacuum after the throttle plate and when the vacuum builds There it opens the valve causIng air to bypass the throttle plate so the engine slowly decelerates and as vacuum where measured drops the valve modulates closed to bypass.

Its the other way around. Popping in 99% means an actual too lean mixture at the spark plug in the moment of spark ignition.
Cause at deaccel. the CIS will drop to a very lean condition, then the mixture in its AFR is that high that the fuel quantity within the mixture wont be sufficient to start the combustion. So as not ignited the mixture will be moved towards the exhaust system. In there with up to 800°C even in its very lean condition the mixture will be ignited by the exessive heat.

@ Bob
1. Is your engine/car a ROW or a Euro model?
2. Did you change your heat exchangers to SSI ones?
3. How is your ignition timing? If your car left the factory as a US model then you should have a 930/16 engine with also a vacuum retard line at the dizzy. In combination with SSIs a retarded timing at idle will result in excessive Poppings when deaccelerating from mid or higher revs.
4. Also a wrong adjusted sensor plate could result in a lean mixture at mid and high revs

andrew ..nope it is the other way around,if it is too much unburned fuel it shoot into the exhaust;-)))

Ivan

If too lean, the fuel dosnt ignite propperly on decel, and pass through to the exhaust. Happened to me. More fuel sorted it out.

I want to ad that I have a 1982 sc that has a correctly operating CIS system with O2 sensor connected etc. I have an AEM wide band air fuel sensor so I can monitor fuel mixture under all driving conditions. When cruising along below 1/3 throttle the ratio fluctuates around 14.7 as it is designed to do, however with the decel valve disconnected and vacuum line plugged, while decelerating, the ratio goes up momentarily into the 15 - 16s perhaps a little higher, and there is some popping.

ok boys;-) the q is where is the popping coming from ?The OP did not specified it ..exhaust or intake?

Ivan

Thank you all

It may be a euro version as Ski mentions given some of the components layout (was not listed by PO as a euro version though)

regardless, re doing that vacuum line fixed the popping completely (just a nice burble now instead of announcing to the world here i come). Even idles better....I would love to measure mixture but dont have the tools and not too interested in installing AF gauge (unless I place in engine bay like a did a secondary oil pressure gauge off pressure sended)

Thanks ALL for your guidance

bob

Ho Bob ..glad it was so easy to fix...well back in the 1970 we used a match-torch below the muffler tip to find out if the car was rich or lean;-)..nowadays i do it by feel;-)if the engine was lean the flame would suck in if it was rich the flame would go away from the tip.......it was no not on Porsche engine thou;-)

Just do a test ... for example with a 930/16 controlled mixture by duty cycle.
When on deaccel. you change the duty cycle to 5% minimum, means a very lean condition, then you will have exessive popping on the exhaust. If you rise it to 90% for a very rich mixture, .... no popping occurs.

I did prrofe it with an own lambda control unit where you can setup everything individually. Means also a lean micture at deacceleration like explained above.

Greets to tasty bohemia (where my great grandmother was born)

Andrew,i totally get you point ..just did not know where was is popping from??
So, your grandma must have been one of those evicted after the war...totally shameful stuff..it is still big conversation subject here in Bohemia alias Sudetenland..where i live ,Karlsbad..where was she from?

Ivan

I know it has been a few years since this thread was last posted on but I am experiencing the same problem in my car. 82SC US with SSIs

Rebuilt the whole CIS system and all pressures are in spec and no vacuum leaks from my smoke machine (also new airbox)

When I richen car up the popping in the exhaust goes away, turn screw the other way pops alot.

The decel valve on my car is the pancake one on the right of the engine. Should this line be going to manifold or atmosphere?

I have a whole thread about my problems with the CIS since I put it back together and am completely lost on where to look next.

Check this out:
https://nineelevenheaven.wordpress.com/vacuum-connections-on-the-k-jetronic-for-the-911-sc/
https://cis911primer.com/pages/descr_lambda.html
https://cis911primer.com/pages/comp_page.html#decelvalve
https://j+o+e-engineer.com/porsche-911-cis-subcomponents-and-testing/ (remove the +) or shortened link: https://shorturl.at/Bey72

Thomas

Just slap a BMW badge on your car and drive normally....


It's not "popping," it's unburned fuel igniting in the exhaust system, usually described as "overrun." Your fuel cutoff circuit isn't working.

Exhaust system improvements result in a freer flowing engine which produce more power. If you keep the same fueling, your system will run leaner. As others have said, this is what causes the popping/burbling. When I removed the cat for a bypass pipe, I experienced this problem.

Your options are to cut all fuel on decel- not possible on CIS but possible for those with programmable EFI, or, as others suggested, enrichening the air fuel mixture. Another possibility is more timing advance on overrun but, again, not easy on a stock car since doing that will affect your timing everywhere.