Massive backfire; distributor in wrong??

bfinley · 10-21-2009, 12:09 PM

I just installed the Carrera chain tensioner kit in a 1971 911T, and... is it possible my distributor is in backwards?? I mean 180 degrees off.

The car runs well enough, but I will get GUNSHOT style backfires, and the sort of distributor I have can go in any old location. I put it in with the Z1 mark and the mark on the distro in the proper spots, but.... I would think the whole thing can be out by 180 degrees!

Any ideas on that? I don't have a helper, so trying to think how to know I'm at TDC on cylinder #1 not the exhaust portion of the stroke....???
Thanks.

deathpunk dan · 10-21-2009, 12:34 PM

The symptoms are consistent with a distributor 180 degrees out. Ask me how I know

dtw · 10-21-2009, 12:35 PM

If the car is running, it is not likely to be out 180. You should put a timing light on it though, and ensure you are timed to specs.

Are you sure your cam timing didn't get fouled up when doing the tensioner swap?

bfinley · 10-21-2009, 01:10 PM

Thanks for answers so far! Very helpful to know there are people out there! I'm pretty new to this car, etc. I'm a fairly good by the book mechanic (I follow the book), not so good at diagnosis though, never was, and I know there can be LOT of reasons for this backfiring, but I'd REALLY like to clear it up mostly.

I'm willing to live with some compromise in this old car/old carbs, but...

I did remove the distro and try it at 180 degrees and the car ONLY backfires and does not start/run in that position. It does, however, crank much more easily in that position.... (any ideas on that?).

However, thinking there are only two possible positions where the rotor will face the slot on the distro rim and the crankshaft pulley will be at TDC mark, I'm now back to the "it runs well enough" position... and it just backfires like a GUN (rare in Canada). Sometimes on accell, sometimes on decell.

So is it safe to not worry about the distro for a second and just say, what's with the gunshots? And yes, I've got 40 year old Weber carbs, vacuum MOSTLY balanced but cylinder 6 was higher than all, and higher than I could get any of them. The rest are all at 5 on my scale but #6 was over 7. Could not bring it down.

I can go on.

But is there a likely cause of MASSIVE backfiring (not constant or frequent, but MASSIVE when it does happen).

Thanks.

moneymanager · 04:37 PM

Why not actually be sure where you have the distributor and rule out an easy-to-make mistake? Loosen spark plug number 1. Turn the motor over by hand. Listen for the hiss from spark plug 1 that tells you #1 is on its compression stroke (ie the valves are closed and the cylinder, if the car were running, would be getting ready to fire.) Watch the distributor rotor as the hiss occurs to make sure it it pointed to cylinder #1. If it isn't, you are 180° off.

dtw · 10-21-2009, 05:31 PM

Distributor is only rough timed. You need to get a timing light on it.

911quest · 10-21-2009, 06:34 PM

You might have a plug wire swaped...if you didn't recheck them.

ItzaMe · 12-07-2021, 10:23 AM

Another one...

Did you ever resolve this bfinley??

Last time logged on 2012... oh bother.

Follow up is important people! Thank those who gave you some insight!

boyt911sc · boyt911sc's Garage

Mario,

Go to the other thread you just bumped up. What is your reason for bumping these old threads?

Tony

ItzaMe · 12-07-2021, 11:36 AM

Quote:

Originally Posted by boyt911sc

Mario,

Go to the other thread you just bumped up. What is your reason for bumping these old threads?

Tony

Just surprised so many people don't follow up is all..

My bad if I caused any disruptions.

Locker537 · 12-07-2021, 11:59 AM

Quote:

Originally Posted by 911quest

You might have a plug wire swaped...if you didn't recheck them.

Start with this.

bahman · 12-07-2021, 12:16 PM

Also check your points gap and dwell angle. I had a similar issue with my '72E. It was the last item I checked/replaced in a long line of trial & error steps/parts.