OT (somewhat) 911S 2.2l Motor maladies, Steady Throttle popping issue Options

[Cupomeat]

My Dad's 911S has an issue with intake popping while normal driving.

BACKGROUND; Engine fully rebuilt (within 1.5k miles) and valves adjusted about 2 weeks ago. Engine was an MFI engine converted to PMOs before we bought the car. (someday I dream to go back to MFI.)

SYMPTOM: Prior to the valve adjust a sporadic 'Popping" was coming from the intake as a small backfire on a regular basis.

After the valve adjust (first since rebuild), it runs better but still pops. When I go up there, I am going to check the rotor and dist cap, but anything else to check?

Sadly, the motor does not run nearly as strongly after the rebuild as it did before the rebuild (crazy, I know). The top end, just isn't there now like it was before. I don't get the needed time to spend with the car, tinkering, as I'd like to see why.

But, back to the popping, It seems to happen mostly while cruising at part throttle.

My logic says that it is ignition and that a spark plug (or carbon build up?) is firing while the intake valve is open.

Does this makes sense? David asked me if they carbs had been rebuilt, and it has been years since they have, but there are no flat spots or stumbles while driving.

Thanks

[ArtechnikA]

SYMPTOM: Prior to the valve adjust a sporadic 'Popping" was coming from the intake as a small backfire on a regular basis.

Eric's suggestions are good ones.
In addition, use a SynchroMeter to make sure the carbs are synchronized side-to-side.

Pop off both link rods and use the idle stop and (if necessary, shouldn't be) the idle air correction screws to even them up. Opinions on the idle air bleeds varies, but AT LEAST ONE must be fully closed. Some (including me) maintain that they should all be fully closed.

Anyway - get them all sync'd at idle.

Then put the linkage rods back on. If anything changed, adjust the linkage rods so that they're the same again.

Mis-sync'd induction means that one side of the engine is forcing the other side lean...

[Cupomeat]

SYMPTOM: Prior to the valve adjust a sporadic 'Popping" was coming from the intake as a small backfire on a regular basis.

Eric's suggestions are good ones.
In addition, use a SynchroMeter to make sure the carbs are synchronized side-to-side.

Pop off both link rods and use the idle stop and (if necessary, shouldn't be) the idle air correction screws to even them up. Opinions on the idle air bleeds varies, but AT LEAST ONE must be fully closed. Some (including me) maintain that they should all be fully closed.

Anyway - get them all sync'd at idle.

Then put the linkage rods back on. If anything changed, adjust the linkage rods so that they're the same again.

Mis-sync'd induction means that one side of the engine is forcing the other side lean...

GREAT Info, I have the Weber book, so I'll give it a go the next time I am up there.
I've had a lot of experience with synch of Dellortos, but none of the 3 barrel Webers like these.

So, in theory, what is causing the popping? How is the intake charge ignited to create the backfire? The lean condition is clearly bad for the car and I'll want to get it fixed asap.

[ArtechnikA]

So, in theory, what is causing the popping? How is the intake charge ignited to create the backfire?

OK - Eric's elaboration wasn't very explanitory ;-)

And probably I once knew the blow-by-blow details but they're eluding me a bit as I try to recall. But it is a factor that you have a high-overlap engine, converted to carbs. As rich as MFI needs to run to be happy, it's OK with lots of overlap for a variety of reasons.

(Some of this is coming back to me...)

A high-compression small combustion chamber (e.g. 2,0 - 2,2) engine needs a bunch of spark lead to get the mixture burning. This means there's not much time between the intake valve closing (definition of big overlap - intake closes late...) and the spark event. A lean mixture takes a long(er) time to get lit, so it's possible that some of the flame front can leak back through the open intake valve.

And that's a big deal because in a high-overlap carbureted engine (as opposed to a port-injected engine like MFI) the air actually goes through the metering system -3- times, picking up a fuel charge each time through. There's the initial intake suction, and then a good bit of reversion when the intake valve closes against the considerable inertia of the air column. It bounces back through the intake and the carb venturii (its second trip). This is what you see with the classic 'standoff' fuel fog over the intakes, especially when the intakes are cold - which they always are, due to the evaporation - below the vaporization temp of fuel (around 140ºF).

Then the intake opens again and that fog is sucked back through the carburetor for its third trip.

So the intake manifold typically has a bunch of fuel in it, in various degrees of atomization and re-condensate puddling.

Every now and then an early spark, a momentarily too-lean port (possibly due to puddling) and you get a 'sneeze' back through the intake.

At higher revs, especially when the engine is actually on the cam, there's enough inertia in the air column to minimize reversion and standoff. (The intake air column can approach Mach 0,5 at the port...)

Once the intake is warmed up enough to keep the fuel atomized intake sneezes are much less common.

[Cupomeat]

So, in theory, what is causing the popping? How is the intake charge ignited to create the backfire?

OK - Eric's elaboration wasn't very explanitory ;-)

And probably I once knew the blow-by-blow details but they're eluding me a bit as I try to recall. But it is a factor that you have a high-overlap engine, converted to carbs. As rich as MFI needs to run to be happy, it's OK with lots of overlap for a variety of reasons.

(Some of this is coming back to me...)

A high-compression small combustion chamber (e.g. 2,0 - 2,2) engine needs a bunch of spark lead to get the mixture burning. This means there's not much time between the intake valve closing (definition of big overlap - intake closes late...) and the spark event. A lean mixture takes a long(er) time to get lit, so it's possible that some of the flame front can leak back through the open intake valve.

And that's a big deal because in a high-overlap carbureted engine (as opposed to a port-injected engine like MFI) the air actually goes through the metering system -3- times, picking up a fuel charge each time through. There's the initial intake suction, and then a good bit of reversion when the intake valve closes against the considerable inertia of the air column. It bounces back through the intake and the carb venturii (its second trip). This is what you see with the classic 'standoff' fuel fog over the intakes, especially when the intakes are cold - which they always are, due to the evaporation - below the vaporization temp of fuel (around 140ºF).

Then the intake opens again and that fog is sucked back through the carburetor for its third trip.

So the intake manifold typically has a bunch of fuel in it, in various degrees of atomization and re-condensate puddling.

Every now and then an early spark, a momentarily too-lean port (possibly due to puddling) and you get a 'sneeze' back through the intake.

At higher revs, especially when the engine is actually on the cam, there's enough inertia in the air column to minimize reversion and standoff. (The intake air column can approach Mach 0,5 at the port...)

Once the intake is warmed up enough to keep the fuel atomized intake sneezes are much less common.

This is great and VERY helpful! I'll have a some time to work on it over Thanksgiving for him and I'll see what I can do.

Thanks again! (IMG:style_emoticons/default/pray.gif)