Low rpm back fires, 77 2,7 S

[fintstone]

Originally Posted by AndreasK

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.

If you do a search, there should be info on testing injectors. Essentially they should have a good flow/pattern when open and no fuel should come out when closed. If one leaks or is failing, you get extra (or insufficient) fuel to a cylinder. Variations are more noticeable at low RPMs.

[T77911S]

what plugs are in it

[AndreasK]

Originally Posted by allaircooled

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?

Oh... nice to know Thanks!

Had pressures checked by a Porsche classic shop last year, before I took it home. Don´t remember, but it was ok by them.

[AndreasK]

Originally Posted by fintstone

If you do a search, there should be info on testing injectors. Essentially they should have a good flow/pattern when open and no fuel should come out when closed. If one leaks or is failing, you get extra (or insufficient) fuel to a cylinder. Variations are more noticeable at low RPMs.

Aaa... that kind of leak. Hadn´t thought of it. On my list.

[AndreasK]

Originally Posted by T77911S

what plugs are in it

Bosch WR56 Super 4. Recommended a few places, but can´t really be gaped.

[T77911S]

nope!! not those

get the bosch super coppers, nothing fancy.
or NGK copper.

[AndreasK]

TTV is good. I definitely have vacuum at the WUR end. Bypassing TTV does little or nothing on the warm engine. Ordered a set of original spark plugs from Porsche. Fingers crossed.

[Splitbus]

Hi

I had similar problems with my -75.

A local mechanic (with a lot of 911-experience) recommended me to check/replace the injectors. I pulled the injectors and the fuel filter, and found that i had to replace most of it. I tried to clean the injectors, but the problem is that they have to be back-flushed. It can be done, but it's time consuming, and difficult to get a good result.

After replacing 4 injectors & filter, the car ran almost perfect. Further down the road i found that the ignition timing was difficult to set because the distributor shaft bushings was worn (impossible to get a stable dwell angle). It's been fixed now, and the next step would be to install a Pertronix ignitor.

Injector testing:
Buy a syringe at the local drug store. Fill it with cleaning fluid, push it firmly into the injector, and flush the injector into a glass jar
You need the injector to open and make a nice spray pattern at low pressure. You WILL feel and see the difference between good/bad injectors. Nice sunday-evening-garage-project.

Good luck

[boyt911sc]

Originally Posted by T77911S

check the thermo time valve(TTV). you should be able to blow thru it.

Ty,

If you could blow through it (cold) that is a defective TTV. The TTV is normally closed at room temperature and opens up when heated.

Tony

[fintstone]

To check out fuel injectors, the easiest war is just run one at a time into a glass jar. Just us a small glass jar and watch it...or get fancy with a baby food jar rig where you can see all at once.
.http://forums.pelicanparts.com/porsche-911-technical-forum/574957-excellent-way-test-injectors-78-911-a.html

[allaircooled]

Originally Posted by boyt911sc

Ty,

If you could blow through it (cold) that is a defective TTV. The TTV is normally closed at room temperature and opens up when heated.

Tony

He describes that well in post 18.

[Targalid]

You do not say what ignition system you have. If you still have points, check the gap (should be 0.012 inch) and the distributor timing. It sounds like your timing might be retarded which will give exhaust backfires as will collapsed points.

[T77911S]

Originally Posted by boyt911sc

Ty,

If you could blow through it (cold) that is a defective TTV. The TTV is normally closed at room temperature and opens up when heated.

Tony

mine broke closed,.
but yes, cold is closed,. hot is open.

I was assuming he was testing with every thing operating.

[AndreasK]

Originally Posted by T77911S

I was assuming he was testing with every thing operating.

Your assuming correctly But I´m not having much luck so far. Appart from finding a lot of things that are in good working order

Like I said, I´m waiting for new plugs.

I´ll also check out my distributor gaps. Distributor was changed by the PO recently, but I´m not really shure whats in there... Needs to be checked out. Questions will probably follw.

[AndreasK]

Spark plugs was my main culprit

(New ones are Bosch/Porsche plugs gaped to spec.)

I do stil get the occasional backfire, but not bad. Stil mainly low rpm and perhaps more frequent after deceleration. Should I have my mixture adjusted again? Had it adjusted with the bad plugs. Would I be lean or rich? Engine is running slightly cooler than before the new plugs.

[petrolblue83911]

A bad or fouled plug can cause the car to backfire/hesitate/ run rough

I’d check the plugs first - even though they are new .

Check gaps, coloration, look for debris around the electrode

[AndreasK]

Originally Posted by petrolblue83911

A bad or fouled plug can cause the car to backfire/hesitate/ run rough

I’d check the plugs first - even though they are new .

Check gaps, coloration, look for debris around the electrode

1st plugs was not the best ones for the car. Coloration after use seemed OK though. Second set, original from Porsche (Bosch). Gaps perfect. They improved things, seemed almost good, but after 10-15 miles backfires are getting more frequent again

Finally got a pic of my distributor. Some suggested adjusting points. Dont think I can with this one. Not a regular, original, distributor. Never seen one like it. Is it an OK one for my car? PO said it was "upgraded"...

[fintstone]

No points. Looks like the right rotor...but in the photo (assuming it is not just camera)...looks to be in bad shape (dirty/burned contacts and chipped plastic). ...what does the inside of the cap look like? Is the rotor pushed all the way on? Is there slop in the distributor?

[AndreasK]

Originally Posted by fintstone

No points. Looks like the right rotor...but in the photo (assuming it is not just camera)...looks to be in bad shape (dirty/burned contacts and chipped plastic). ...what does the inside of the cap look like? Is the rotor pushed all the way on? Is there slop in the distributor?

Evryting feels nice and tight. I think the camera/lighting lies a little. Not as filthy as it looks A bit dusty. Looks quite new. But some plastic chips yes. The pin in the yellow circle doesn´t look too good? Maybe a bit rusty.

Inside. Some scratches on the terminals. Otherwise OK?

[fintstone]

Looks like some chips in the plastic inside the cap too. Maybe someone turned engine over without getting cap on tight. Both cap and rotor look suspect to me. I would replace (I do when I put on new plugs). If it does not fix...you have a spare for future.