Erratic running below 3800 RPM; above 3800 OK- HELP!

lazerander · 05-23-2008, 12:42 PM

The engine was running beautifully. I stopped for 20 minutes, then attempted to restart the engine; A couple of cranks and it responded with a tremendous backfire. I waited a minute and tried again; this time it started normally. I let it idle, then blipped the throttle and it still seemed fine (much to my relief). However, when I put it in gear and tried to drive off, the engine balked and sputtered; I could barely keep it running, up to about 3000 RPM, where it began to run better. By 3800, it was running as smoothly as ever! This is the way the engine has been running for over two years now, and I've spent all this time trying to find out what's wrong and fix it. I've boiled out the carbs, replaced the distributor, plugs, and plug wires, taken a compression check, and installed a new coil...all to no avail. If the backfire was the result of the problem, it seems to me that it would have occurred again; if the backfire caused the problem, and had damaged the engine internally, wouldn't the poor running be evident throughout the RPM range, and not just below 3800 while in gear under load? I've done all the work on all my Porsches (11 to date) for 46 years, and built several engines (including this one), but this has me stumped! Any ideas?

rusnak · 05-23-2008, 01:40 PM

vacuum leak? Did ya try the starting fluid test? Don't bother with carb cleaner, try starting fluid.

Flieger · Flieger's Garage

What kind of 911 are we talking about here? CIS? Motronic?

Jeff Higgins · 05-23-2008, 02:24 PM

Since it has carbs, I'm assuming it's an older 911.

Does it have a capacitive discharge ignition system? The CD box may be on the fritz.

Flieger · Flieger's Garage

Sorry, missed the note about carbs

Flieger · Flieger's Garage

To me, it sounds like a rich air-fuel mixture.

My BMW E21 320i (1983 model year) had a similar problem.

It turned out that the engine was pretty tired in the cylinder head. The valves needed to be adjusted, which brought compression up. The valve guides were worn and leaking oil into the cylinder. There were also some vacuum leaks. The mixture adjustment screw had also been set too rich.

It went from a 20x gross polluter to a smooth, powerful, responsive, and efficient powerplant.

It was choking on oil and too much fuel, combined with bad breathing. This made starting terrible. It would crank a while, then cough, pop, sputter, and shake while I gave it throttle. It would run rough with no power until temps were up and revs were 4000 or so.

Maybe this story can give you an idea. It sounds like you have done all the standard checks.

What's your fuel mileage? Mine went from 18 to 26 mpg in the BMW.

rusnak · 05-23-2008, 04:33 PM

I had a 1982 320i in college. I had to turn the ac off every time I went over the mountains into southern calif., or wait until the stupid ac relay blew by itself, which was about once every 3 months. I'd call the engine a lot of things, none of them being powerful, or smooth.

Zeke · 05-23-2008, 05:41 PM

I have to ask if there are non metalic spacers between the carbs and the heads. What is the oil temp "after 20 minutes"? We'll go from there, if I can help at all.

2.7RACER · 05-23-2008, 05:49 PM

Talk to us Lazerander. Tell us what you are running.

Flieger · Flieger's Garage

Quote:

Originally Posted by rusnak

I had a 1982 320i in college. I had to turn the ac off every time I went over the mountains into southern calif., or wait until the stupid ac relay blew by itself, which was about once every 3 months. I'd call the engine a lot of things, none of them being powerful, or smooth.

I think the engine is smooth, however, it is also "gentle" There was a big increase in power from a percentage viewpoint.

I have the AC removed. It made changing my fan belt a BIG PITA when it broke. The AC never worked anyway, just needless weight. Engine bay much more open now.

The motor is durable and strong, though. BMW won the Formula 1 World Championship in 1983 with a turbo M10 making like 1000 hp out of 1.5 liters

lazerander · 05-24-2008, 04:30 PM

Quote:

Originally Posted by rusnak

vacuum leak? Did ya try the starting fluid test? Don't bother with carb cleaner, try starting fluid.

Thanks for your input!

lazerander · 05-24-2008, 04:36 PM

Quote:

Originally Posted by Jeff Higgins

Since it has carbs, I'm assuming it's an older 911.

Does it have a capacitive discharge ignition system? The CD box may be on the fritz.

Thanks for your input...I'll install a new Permatune and let ya know if it fixed it.

lazerander · 05-24-2008, 11:10 PM

Quote:

Originally Posted by 2.7RACER

Talk to us Lazerander. Tell us what you are running.

1977 911 / 2.7 Euro Carrera pistons (8.5:1) / E-cams / Webers / PMO manifolds (with Brake Vacuum Booster line) / Permatune C.D. / Carrera chain tentioners.
Thanks for your interest!

lazerander · 05-24-2008, 11:16 PM

Quote:

Originally Posted by Flieger

I think the engine is smooth, however, it is also "gentle" There was a big increase in power from a percentage viewpoint.

I have the AC removed. It made changing my fan belt a BIG PITA when it broke. The AC never worked anyway, just needless weight. Engine bay much more open now.

The motor is durable and strong, though. BMW won the Formula 1 World Championship in 1983 with a turbo M10 making like 1000 hp out of 1.5 liters

Thanks, but my 911 has no AC (we live near the beach!)

lazerander · 05-24-2008, 11:25 PM

Quote:

Originally Posted by milt

I have to ask if there are non metalic spacers between the carbs and the heads. What is the oil temp "after 20 minutes"? We'll go from there, if I can help at all.

Yes, I put phenolic blocks under the manifolds when I built the engine (in 1986).
However, it has never run hot (max temp 180 on a warm day). Thanks!

2.7RACER · 05-24-2008, 11:31 PM

I suspect since the engine runs well at 3800 and higher, you may have one or more idle jets plugged up, or leaking o-rings in the idle circuit.
The engine runs strictly on the idle jets up to 2000/2500 rpms.
The idle jets are small and plug easily.
I would check the transition ports as well.
They are under the cap screw just above the idle mixture screw.
You'll find three very small holes. These are the transition ports.
Fuel flows through them as you open the butterfly's.
Usually the idle jet will plug first, but it is nice to know the transition ports are clear.
The o-rings at the idle jet and the idle mixture screw must be in great condition for the engine to pull emulsified fuel through the idle passageways.
If in doubt replace the o-rings. They may be older than you.
Remember to oil the o-rings before installing the jet screw or idle mix screw to the carbs.
Once the idle jets are clear and new o-rings in place, you may find it necessary to block the carb inlet for a second or two to help pull the fuel through the idle passagways.
I just put my hand over the inlet horn briefly.
Bad o-rings are like sucking through a broken straw.
Keep asking questions, you'll find help here.