Low rpm back fires, 77 2,7 S

[AndreasK]

Help me narrow down my search, please

I´m getting small fuel explosions (popps) in my exhaust, at low rpms. Especially when giving throttle after coasting for a bit. But also when driving steady around 1500-3000 rpms. Gets worse with resistance, like driving up hill. Never on higher revs. Occurs while warming up and when fully warm. I sometimes, rarely, get a different pooof. Which I think MAY be from the pop off valve. But its rare and I´m not certain. I have a 2 in 2 out muffler with SSI headers. (964 cams) My left tailpipe is sooty. My right gets warmer and has a lot more airflow.

I got the car a year ago. Ran quite well. Got a new fuel pump and acumulator, fuel pressure checked by a Classic Porsche shop before I took the car home. Last fall, it started occasionally not to fire on all cylinders, just before winter storage. Backfires started around same time. Changed all seals around injectors and new spark-plugs. Car runs and sounds better (faster) than ever, but backfires worse. Got fuel mixture adjusted yesterday. Was lean, but no change with backfires after adjusting.

So far on my list of culprits, in no particular order, is;

1. Further air leaks. Especially airbox (I have a popp of valve) and hoses to intake manifold.
2. Exhaustleaks.
3. Valveadjustment. Could exhaust valves not be closing properly?
4. Headstuds. But broken headstud-bacfires are more common at high revs???
5. Other. What haven´t I thought of?

Where would you recommend I start my search from here?

Regards Andreas

[scary driving]

You should consider checking the timing as well, like testing with the vacuum line on the advance disconnected. Just put it on your to-do list to check your timing.

[Bob Kontak]

My only experience with this (poof-poof) was a leak where the intake runner met the head. One cylinder only.

Check leaks with carb or brake cleaner at these flanges.

[kach22i]

Changes in the weather affect the fuel/air ratio mix, will need I think a long 5mm Allen wrench to adjust - look it up. I keep one in my owner's manual so it's always in the car.

This might explain the cool weather to hot weather performance differences.

Also I think the pop-pop sounds are a too rich of a mixture. Be careful too lean and you will run hot and bad things could happen - look up lean engine damage. Better too rich than too lean in my opinion. If the exhaust burns your throat a bit, or smells like gas when you put your hand to the pipe then smell your hand it's running rich. The exhaust should smell close to any other car, best to use a meter not your nose but I have good, eyes ears and a good nose so I put them to work if nothing else is available.

I recall increasing the gap on my spark plugs, slightly larger than spec, this helped top to bottom rpm combustion but read it can stress the coil. The popping is un-burnt fuel being burnt somewhere in the exhaust system. If the popping comes out of the intake at the top of engine you could crack the airbox, and that can be a pain in the butt.

I also agree with "scary driving", you want to recheck your ignition timing, it's gotta be just right on these cars or they will do odd things, no six computers onboard to compensate (like on a modern Porsche).

I had this problem before, will look up my old posts for you, could be of help.

EDIT-1:

Found an old thread about MSD and points:
Where to find distributor number?

AndreasK, do you have the original "points" system on the car or an aftermarket kit like Pertronix Ignitor magnetic pick-up or Crane Optical Ignition?

All you have to do is pull off the distributor cap and take a picture, the guys here will know.

EDIT-2:
Quick fix for me was increasing the spark plug gap slightly, and leaning up the engine a tad.

I also redid the timing, but had to re-index my distributor to do so - a scary thing for me to do at the time, but Walter Mitty walked me though it with his excellent posts as I recall.

Are you running hot or any other symptoms?

What spark plugs are in your car?

EDIT-3:
The solutions I proposed might be better suited for popping/burbling/crackling when lifting off the gas quickly after hard acceleration. That was the problem I had years ago, not the original posters situation. Oh and I know nothing about re-gaping his style of spark plugs or even if that can be done. Old fashion NGK copper plugs are the only thing I've found my car likes.

[T77911S]

check the thermo time valve(TTV). you should be able to blow thru it.
check the vac line from the WUR to the TTV and to the TB.

air leaks., control pressure.
sounds like it is too lean

[kach22i]

Originally Posted by T77911S

check the thermo time valve(TTV). you should be able to blow thru it.
check the vac line from the WUR to the TTV and to the TB.

air leaks., control pressure.
sounds like it is too lean

Wow, just learned something new or just forgotten about the TTV.

thermo time VALVE.

1977 Porsche Thermo Valve Location

At one point I replaced all my vacuum lines except the really big fat one on the back of the engine that you have to do a partial engine drop to get at.

Once all the really minor leaks were cleared up it sort of magnified anything else wrong and I had to readjust things like the fuel air mixture again.

Just a heads up for anyone trying to fix things only to have things running worse afterwords, it's maybe not your fault, just more adjustments ahead as you woke a sleeping dog.

[AndreasK]

Wow. Lots of good stuf here. Timing was actually on my list, but i forgot to include it.

I did just yesterday get my mixture professionally adjusted. TTV and hoses to it and timing next.

Can't remember exactly which spark plugs I got. But Bosch super4 something, heat rating 5. Car fires beautifully on all 6 after installing. Haven't had them gaped, perhaps I should. Anyone know the Super 4 plugs? How should they be gaped?

https://in.bosch-automotive.com/en_GB/parts_and_accessories/service_parts_1/spark_plugs_1/bosch_super_4/bosch_super_2

Sorry bad link. I'm on my phone
Will get a picture of my distributor. It's not the original, but I'm not sure which. Installed by P.O...

[Bob Kontak]

Did the problem go away?

[AndreasK]

Originally Posted by Bob Kontak

Did the problem go away?

Nope. Adjusting mixture had no obvious effect on the popping. Al tough I only got the short drive home from the shop to go by.

[T77911S]

you should do the tune up before mixture adjust,

timing and such.

I really don't think that is a good plug for Porsche.
use copper plugs.
bosch supers or NGK.

those you cant gap.

if it pops at steady RPM you are lean/ have air leak.

if under acceleration it is the vacuum portion of the WUR. the CP drops under acceleration.
been quite a while but when I tested mine with the vacuum removed it popped and sputtered under accel, but drove fine at steady speed

[AndreasK]

Originally Posted by T77911S

you should do the tune up before mixture adjust,

timing and such.

I really don't think that is a good plug for Porsche.
use copper plugs.
bosch supers or NGK.

those you cant gap.

if it pops at steady RPM you are lean/ have air leak.

if under acceleration it is the vacuum portion of the WUR. the CP drops under acceleration.
been quite a while but when I tested mine with the vacuum removed it popped and sputtered under accel, but drove fine at steady speed

I really hope the bold statement is true. Ill check my timing. Check TTV. Search for further air leaks. Try different plugs. Check mixture again. In that order. #fingerscrossed

[AndreasK]

I´m stumped. Ignition timing is spot on. Mixture is OK. Can´t find any major air-leaks. (By spraying brake cleaner) But I don´t get a change in RPM by removing oil-cap, so i´m guessing there is something I´m missing.

TTV seems ok. Al tough I´m not a 100% certain how to test it. I´m finding conflicting advice. Disconnecting vacuum hoses from it has no apparent effect. Is it supposed to be open or shut, when car is cold?

Could it simply be the spark plugs I got in her? Runs really well on higher revs, may be a bit sluggish on lower revs. Tiny bit of random bucking. Goes poof-poof on steady-ish revs up to 2500rpm.

[T77911S]

make sure you have vacuum on the line going to the WUR.

check the advance. make sure time advances with RPM

[fintstone]

Check for a leaking fuel injector.

[AndreasK]

Originally Posted by T77911S

make sure you have vacuum on the line going to the WUR.

check the advance. make sure time advances with RPM

Found this in an old thread of yours. If I understand correctly the diagram is for 78-79 MY. Do you have one for the 77?



I´m around 27-30deg with 4000 rpm. Spot on at idle.

[AndreasK]

Originally Posted by fintstone

Check for a leaking fuel injector.

This whole search started after replacing o-rings and sleeves on all 6 injectors I definitively had some leaks there as the car ran rough before and runs great after. (Also changed pugs as they where gone after running a bit with misfires and such) Only issue now is the fuel passing through and igniting in the exhaust on low, steady, RPM´s.

[AndreasK]

My vac connections are like this:


Al tough I dont think i have the T (nr 21) and hose to the distributor (nr 25) I think there is another vacuum hose going to the distributor, coming from the front og the engine. Car isn´t here, so this is from memory so I may be mistaking.

[T77911S]

on the diagram you got from me just swap the 2 lines going to the WUR.

vacuum should be on the TOP port to the WUR which goes to manifold vacuum . if you have the 033 WUR.
you should be able to blow thru the TTV when it has power, after about 20sec, and you should not be able to blow thru it when cold.

you can just bypass the TTV for now if you have doubts.
mine went bad and I just bypassed it. it is nice to have as it makes the cold starts a little richer and can prevent some backfiring when first started.

[AndreasK]

Originally Posted by T77911S

on the diagram you got from me just swap the 2 lines going to the WUR.

vacuum should be on the TOP port to the WUR which goes to manifold vacuum . if you have the 033 WUR.
you should be able to blow thru the TTV when it has power, after about 20sec, and you should not be able to blow thru it when cold.

you can just bypass the TTV for now if you have doubts.
mine went bad and I just bypassed it. it is nice to have as it makes the cold starts a little richer and can prevent some backfiring when first started.

Excellent! I´ll try this tonight. If TTV passes inspection I´m down to spark plugs. But I will be surprised if my plugs are causing the backfiring.

[allaircooled]

But I don´t get a change in RPM by removing oil-cap, so i´m guessing there is something I´m missing.

You won’t on a 77.

On the diagram you posted, just to be clear, the red line goes to bottom port for vacuum. Blue line goes to top port for atmosphere. It’s hard to tell in that diagram.

You said you had fuel pressures checked. What were they?