The long semi sad tale of a poor running 911...

lateapex911 · 10:55 AM

OK, my 73 911E has been with me since 92 or so. While living in California in 96, the engine slipped a timing chain. My company was paying me with checks that bounced, and my Mother was sick on the East coast, so the car went on the trailer and I moved east.

Once here, I got back on my feet financially, and my cousin mentioned a friend who was magic with Porsches. So, he had just started his business, and did me a bit of a favor, and we put together a 2.7 9.5:1, Solex cams Weber engine with SSI. Nothing special, but workable. The MFI had been giving me fits, and the additional cost of the headwork and recalibration made the Webers a tempting option. The vast info on the net about MFI just didn't exist back then, and Gus was about it as a source. I figured I could tune a Weber if need be.

So, I go to pick up the car, and it runs great at high revs and under load, but pops and backfires at low loads/idle. He tunes on it with jets and tweaks for a bout 2 hours, and gets it better, (some) but, he sold me the used Webers cheap, and I was out of cash, so I said I'd just fine tune it myself.

yea...famous last words! I got the Weber book, invested in jets and such, and spent the next few years dicking with it occasionally. Of course buying a house, racing another car, instructing, jobs, charity swims and life got in the way, and the car didn't get the intense attention it needed.

At one point I'd done everything possible with the carbs, the ignition had been converted to a Crane optical and MSD AL6 with appropriate coil setup, so I decided to bite the bullet and get fresh PMOs, assuming the Webers needed rebuilding. Heck, threads on here suggested the same.

LOL, I bolt the PMOs on and viola! The EXACT same problem!

So, (sorry for the long winded story)
here's what I've done:

Dicked with the Webers until half my wardrobe stunk of hydrocarbons
Dicked with the PMOs until a quarter of my wardrobe stunk
Replaced and timed the ignition system, wires, distributor firing method (optical) and MSD/coil.
Changed plugs numerous times
Pulled carbs and intake manifolds searching for leaks, using ether spray, etc.
Checked valve lash
Checked compression
Replaced the gas dozens of times.
Two batteries over the years.

So, I ask you, the gurus of 911s, what have I missed. What do I go back and redo?
(If you say, "You suck, start over, read the directions and tune it right", I'll accept that, LOL, but I'm trying to think out of the box here!)

Essentially, it runs poorly at idle and under light loads. Richard at PMO set it up for the built specs of the engine, so I expected it to be close, or at least to have a different set of issues, LOL. The popping and backfiring are the major issues.

I've considered taking it back to the guy who built the engine, but he's become very successful, and works mostly on track support and water cooled versions, and the last carb he touched....was mine.

Thanks!

docrodg · 07-03-2009, 10:57 AM

Cam timing maybe?

Rot 911 · 07-03-2009, 10:59 AM

Is the backfire in the exhaust (too rich) or the intakes (too lean)? Could also be a timing problem.

Rot 911 · 07-03-2009, 11:01 AM

Quote:

Originally Posted by docrodg

Cam timing maybe?

I first thought he might have that problem too, but then the backfiring should persist and even get worse as he ups the rpms.

docrodg · 07-03-2009, 11:06 AM

If just a hair off with a solex cam it can just be from excess reversion.

lateapex911 · 07-03-2009, 11:09 AM

Ooooo! good stuff here!

OK, the 'popping' is from the intakes, and the backfiring is from the exhaust.
Yes, it is running rich...when I lean it out the popping increases. It's been a back and forth duel with no happy compromise.

The cam timing issue is something I had vaguely thought of, perhaps I should get the good book out and try to figure out how to check that. IIRC it isn't easy. (I last built a 911 engine in 94, IT ran fine, LOL)

nhromyak · nhromyak's Garage

Quote:

Originally Posted by Kurt V

I first thought he might have that problem too, but then the backfiring should persist and even get worse as he ups the rpms.

This was not my experience with my '73.5 911 T (CIS) with the Cams too advanced. I got a little popping at idle, it went away at higher RPMs. It would take off like a ra*ed ape and overheated too.

Good luck.

Turboo934 · 07-03-2009, 12:05 PM

Bad dizzy.

Goth · 07-03-2009, 12:10 PM

Check which stack the popping is coming out of:

If it is the same one all the time; then check for vacuum leaks or fuel circuit blockage on that stack.

If it is one bank or all; then perhaps cam timing.

Just my $0.02 / hope it helps...

Goth · 07-03-2009, 12:12 PM

I'm guessing your no longer running points in your distributor.

lateapex911 · 07-03-2009, 12:17 PM

Doug, it seems like it's all the stacks, very difficult to nail it down. Yes, I removed the points and replaced with a Crane optical unit that fires an MSD.

Tyson Schmidt · 07-03-2009, 12:25 PM

What idle jets did Richard at PMO set you up with?

My experience has been that he supplies them with too small idle jets. You probably need 60's.
If it has 50's in there now, try 55's first. The 60's if needed. You'll have to readjust the idle mixture screws after each jet change. (obviously)

Goth · 07-03-2009, 12:33 PM

Are you running good spiral core wires or something else?

A long time ago: I had a 1978 Celica that acted as you described. It turned out to be the coil wire. It idled fine, coughed and sputtered in the mid range, and ran like a champ in the high RPMs. Took me a while to figure that out as I thought the symptoms would be much different.

Perhaps you could just check the coil wire, or, switch it with a spare (if ya got one).

lateapex911 · 07-03-2009, 12:50 PM

Tyson, good to hear frm you!!! I'll have it in the shop tomorrow and will report back. Doug, it happens at idle too. (It's actually embarrassing to drive anywhere near others unless I'm really giving it the boot, obviously impossible for even a quarter of the time.)

Goth · 07-03-2009, 01:04 PM

Just tell em it has a super radical cam

Best of luck at the shop!

lateapex911 · 07-03-2009, 01:07 PM

Quote:

Originally Posted by Goth

Just tell em it has a super radical cam

Best of luck at the shop!

LOL, by "The shop" I mean MY shop. The race car goes out and the 911 in.

RSTarga · 07-03-2009, 02:37 PM

Check to see if the throtle plates are completely closed when you are off the throttle, for the backfire. Are you sure you are not getting crossfire on your ignition system? I had the same problem on a car with optical triggering when I put an MSD on it. You changed so many things at the same time that you are going to have a hard time finding the problem.

GaryR · GaryR's Garage

Jake - you live in/near Bridgeport? You may want to give Dan Jacobs a try if swapping out the idle jets doesn't cut it.

Zeke · 07-03-2009, 03:29 PM

It always seems silly to suggest the spark plug wires. And, I assume when you did the 2.7, it got new wires. If so, nevermind. But, I won't go to the next step, or any step, if I don't know the wires are good.

lateapex911 · 07-03-2009, 04:20 PM

Quote:

Originally Posted by RSTarga

Check to see if the throtle plates are completely closed when you are off the throttle, for the backfire. Are you sure you are not getting crossfire on your ignition system? I had the same problem on a car with optical triggering when I put an MSD on it. You changed so many things at the same time that you are going to have a hard time finding the problem.

It happens at idle, and it's been with two completely different sets of carbs, so I'm thinking, it's something to check, ...gotta check it all, LOL, but I've dicked with it too, LOL.

Actually, (and this is embarassing to admit) the changes have taken place (I gasp when I actually think about it) over a 9 or so year period. So, no, I haven't done them all at once. Lots of them have been the "I doubt this is the problem, but it's a good thing to do anyway" type of thing.

But, you're right, I'll have a hard time finding it...safe to say on that!