Backfiring on acceleration

[oregonmon]

I have had this symptom of stuttering on hard acceleration at rpms above 4000. Wasn't sure what to think but here is another symptom related I'm sure that might let you guys steer me towards the right remedy.
Since then I have put on a Fabspeed muffler bypass so the exhaust is much more aggressive. On this last drive home really hammering it on the back roads it was backfiring on hard acceleration. Never on deceleration.
Spark issue? Have ran two cans of Seafoam through the last two tanks too.
Thoughts?
Thanks.
P.S. The muffler bypass is sweet.

[afterburn 549]

I wouid guess fouled plugs or a weak secondary unit. KISS. TRY new plugs and report

[Flieger]

Detonation?

If so, timing adjustments may help. Make sure you are running the best fuel you can find.

Give the engine a work out to get rid of any carbon deposits that cause heat retention and sharp spots in the combustion chamber.

Check spark plugs and maybe clean fuel injectors (remove and inspect the spray pattern in a glass jar)

[oregonmon]

Originally Posted by Flieger

Detonation?

Give the engine a work out to get rid of any carbon deposits that cause heat retention and sharp spots in the combustion chamber.

THAT I've done.

[Scooter]

Check fuel pump/filter.

[oregonmon]

Fuel filter replaced a few months ago.

[Scooter]

Could still be the pump or a bad batch of gas could have clogged the filter. It might me worth a look.

[oregonmon]

Well the stuttering has been going on for over a couple months now and I think the backfiring and stuttering is hand in hand. I just never had the backfiring I guess the stock muffler, well muffled it. If I really get on it, above say 4000rpms it start to do it, then if I ease of the throttle just a bit it smoothes out. Once I ran to about 6 grand in 2nd gear and came to an abrupt stop and let it idle and is was like it was missing on a cylinder. Mechanic replaced plugs a few thousand miles ago but could it be wires or distributor? Still, relatively cheap to change plugs anyway.

[oregonmon]

Bringing it around again.

[afterburn 549]

Did you check the secondary ?

[Por_sha911]

While idling, pull one plug wire at a time and see if the idle changes. That will tell you if its an isolated cylinder.

[oregonmon]

Originally Posted by afterburn 549

Did you check the secondary ?

Come again?

[oregonmon]

Originally Posted by Por_sha911

While idling, pull one plug wire at a time and see if the idle changes. That will tell you if its an isolated cylinder.

Well it doesn't run rough at idle only at heavy load and high rpms.

[pozee]

Does yours have the RPM/crank position sensors?

[oregonmon]

Originally Posted by pozee

Does yours have the RPM/crank position sensors?

Huh? Are you guys pulling my chain? Seriously, are these real questions? If the next guy asks me to replace the muffler bearing I'm gonna be pissed?

[Paulporsche]

Sometimes going to more of a free flowing exhaust means you have to richen the mixture. If you try that, do a test drive and report back.

If that doesn't do it, then you have to start going through everything: timing, ignition, fuel delivery, dizzy advance, WUR, etc. I like to try the simple stuff first.

[RoninLB]

Originally Posted by oregonmon

stuttering on hard acceleration at rpms above 4000.

1979 911 SC

if it's ignition wild guess is failing Beru connectors

[afterburn 549]

Ok , Now I will tell. Please change out the pluds (and gap them) If that does not work change out all questionable parts in the secondary (coil onward ) I will be surprised if that does not fix it.
What you are discribing are plugs miss fireing under load,, or a weak coil. These are the things in the secondary system.You say the wires are new? Maybe I missed that.

[oregonmon]

No wires are not new but the mechanic said they looked newer. But even that was a few thousand miles ago.
I think I will do what you are saying because it seems that most people are pointing that direction. Thanks.