911 backfiring

[Carolyn's 911]

I've asked this before and appreciate the help I've gotten. Have a 1970 911T. Cannot stop the backfiring thru the carbs. Timing is set, Webers rebuilt with Pelican kit. Jets at .055. Backfiring is random thru the carbs. Very dramatic flames, noise. Many threats from neighbors and Sheriff. Some have suggested mixture. How do I check? Any other ideas? I have a feeling I'm missing something basic, given my skill level (non-existent). Thanks

[ruf-porsche]

intake valves adjusted correctly or are they too tight?

[Carolyn's 911]

Haven't touched the valves. Wanted to make sure I covered carbs first. Would the intake valves cause this? I already have the rebuild kit for this project, but time is an issue.

[ruf-porsche]

If the intake valves are too tight or not closing all the way, some of the mixture will go back through the intake manifold when the spark plug fires the mixture. The valves has to be completely seated in order to prevent the mixture from back firing into the intake manifold.

The other cause could be the timing is off, you could have the timing too far in advance or perhaps the cam and distributor (also known as a dizzy on this board) are not in sync.

[Carolyn's 911]

Thanks, we may be getting somewhere on this. I have adjusted the timing to spec at 6000 rpms. Pretty scary when your not used to that level (please remember, I'm learning as I go...always wanted a Porsche). Didn't seem to matter. I also played with the timing, advancing and retarding to see the effect. Didn't seem to matter, the backfiring continues no matter the timing. Need help on this, as it's embarassing to drive, not to mention several visits from Sheriff's Dept. They have no sense of humor/adventure. Suggestions on confirming valves as a problem?

[afterburn 549]

did this problem come on slowly or over night???
i'm inclined to belive sticky advance in the dis. or lean mix. or a whole in muffler.
under what cond. dose it do this???
like off throttle dwn. hill?
or just stepping on the ga s from a stop?? or///////??? you tell us

[Carolyn's 911]

OK. Bought the car on a whim. Prior PO was past president of local Porsche club. Several other owners after him who did not care for car. Car was backfiring when purchased, but thought I could solve. Backfiring begins at around 3000 rpms. It is constant and random, through all cylinders. Flames through the webers...would be appropriate for display on 7/4. Car was built as a "track" car by prior Club owner. Feels that way on the road (stiff and fun). Backfiring ceases on very hard acceleration, so to keep the neighborhood peace, I'm forced to speed (Sheriff didn't but that one either). When I went to time car, dist was frozen (Bosch). Took a lot of work to free it, but I'm not sure how to check the advance mechanism. Thanks for your help

[Allenk]

Are all the throttle valves aligned? Does the back fire take place in all barrels?

[Carolyn's 911]

allenk, as you can see, I'm still learning. Backfire is in all barrels. Can you descibe what you mean by "throttle valves aligned" before I reveal any more of my ignorance?

[Allenk]

The plates that open at the base of the carbs when the throttle is given. My Zeniths had a bad spitting and popping problem and it was due to those plates not all being aligned. I took them off, backed the idle adjustment screws out al the way and flashed a light from the bottom and instead of darkness, they all had varied amounts of light passing. I aligned them and all was good. Honestly, I would review your ignition system again first if you having neighbor disturbing backfires.

[Garrett]

Your idle mixture is off. If all is well under full throttle, that means you now have the main circuit going; so with no problem under the main circuit, and only a problem on the idle, and under transition phases, I would work hard on making sure your floats are at the correct level (use the PMO float bowl gauge - very simple and effective) and make sure your adjustments are correct - idle mixture (jets aren't clogged, too), air balance, etc.

I assume you have Webers - right? I have them too, and sometimes they are a bear to get working right, but eventually you'll get it.

Do a search on this board for more on weber tuning if you've never done it before, or buy a weber carb book.

[Carolyn's 911]

Thanks. I'm having difficulty picturing your solution. Where exactly did you shine the light? Were the plates off when you did this test? What do you mean, specifically, when you say "review your ignition system"? I guess I'm at the low end of understanding this.

[Carolyn's 911]

Garrett, thanks. Do you discount the valves as a source? The little bit I've learned tends to lead me towards valves or mixture (exhaust leak follows these). What is a PMO guage. I've read about measuring the the volume of gas going into the bowl. I'll obviously have to read more about Weber adjustment. Jets are clean

[Allenk]

Maybe some more info is needed first. Have you always had this problem or is it new. If it's new then it's probably not the plates. If its new then when does it happen, at ide only, when giving throttle, all rpms? Have you messed with adjustments or changed anything?

[Carolyn's 911]

Allenk, Have always had this problem. Hard throttle seems to control backfiring (very hard throttle). I have played with timing to no end. After I rebuilt carbs, I had a day or so of no backfires, but then it started again. I have been so frustrated with this, as well as having little time, that I have not touched the car for months (also have a boat that takes a lot of time). All the toys are fun, but I need to get this one right. It's important to me to solve this. Thanks

[ruf-porsche]

I believe she said that when she brought the car it was already back firing.

You set the timing at 6000 but did you set it at idle? Timing may be correct for 6000 but incorrect for idle.

The distrb. was frozen? what do you mean by frozen? The nut to adjust the distrib. was hard to undo or the distrib. was hard to turn?

Have you remove any of the plugs to check the coloration? Black sooty plugs indicates a rich condition while a while tipped plug indicate a lean condition. The plug should look a little tan.

[ruf-porsche]

When you rebuilt the carb did you set the floats? What condition were the floats in? Sometime they leak and gas will get into them so the float will not float as high, which in turn will cause carbs to be too rich.

[Carolyn's 911]

Ruf, I understood from reading that idle timing was not important. Dist could not be turned after loosening nut. Seemed to have been frozen, but I finally freed it, even removed it and it seems OK. Plugs have been generally tan, with a little soot. Haven't looked at them for a while, as the car has been sitting due to owner's technical paralysis

[Allenk]

I think you should start from square 1 with those carbs. Take them off, turn the idle adjustment al the way out and flash a light from the bottomand see if all the plates are blocking the light. If not loosen the linkage between barrels and adjust. Next put back on with spec idle screw ajustment and set float level. Next syncro barrels then mixture adjustment.

[Allenk]

Tan= a little lean=pop.