please help diagose my miss - webers

[jwetering]

3.0 L, weber 40 IDA, MSD ignition

Ran great at the track, then during a long hot drive home it developed a miss. The miss seemed to go away under heavy throttle, but lots of popping from the exhaust while on the idle circuit.

No problem - I take the carbs apart and use a can and a half of brake clean and some low pressure compressed air to blow every passgae clean. I was surprised and a little dissapointed to find no obvious signs of crud, sediment or the like anywhere - they looked pretty good. I have three fuel filters in the car, one on the main line and one on each carb. I replaced all three. I re-installed the carbs and took a couple of hours to set them up. Happy as hell to find the car idles smoother and runs better than it ever has. Hurt my shoulder patting myself on the back and go to bed a happy guy.

Well a few days later I take the car out to demonstrate for a guy who might want to buy it (long story) it runs fine on the test run, most but not all of which is done at big throttle, but toward the end of the about one hour drive (moderate temperatures) the exhaust popping returns, and slowly develops into a full blown miss again.

At that point I go home and cut open the three fuel filters, and find no signs of sediment or crud in any of them. I buy six new spark plugs, and just got home from installing them. The ol plug each looked the same, and were neither fouled or burned looking. The new plugs didn't help at all. I notice a crappy crimp to the black wire on my coil, but don't have the tools with me to fix right away....and I don't think it's the root problem anyway.

So - here's some questions:

1) I'm assuming that exhaust popping / backfiring is indicating a rich condition rather than lean - this is what was leading me to suspect an ignition miss. Is this a decent assumption ?

2) Would a bad coil ground, or negative connection result in a condition that sounds like a miss?

I wish I would have taken a closer look at the needle valves while I had the carbs apart. Tomorrow I will check the float bowl levels.

Any help is appreciated people

[ianc]

Don't suppose you've still got the CIS laying around, eh?

ianc

[jwetering]

Nope - and I don't miss it....usually. Really the carbs shoudl be easy to troubleshoot. I'm just being a little lazy.

[ruf-porsche]

Next time it miss, start pulling spark plug wires to determine the cylinder that is missing, the cylinder that doesn't drop in rpm when the wire is pulled is the missing cylinder. Once you have the cylinder identified, I would check valve clearance. Could be a little tight and once the engine warms up, the clearance goes to nothing and could be just keeping one of the valve open.

I would also check the High tension spark plug wire ande the spark plug.

EDIT

exhaust popping would indicate a tight exhaust valve, where as popping through the carbs would indicate a tight intake valve.

[JMatta]

I had the same problem with my '73 S spec. I would constantly clean the needle jets in the idle circuit, only to have the problem return. Put in the best fuel filters money could buy, only to have the same problem. I finally had the fuel tank pulled and a fuel cell installed...problems gone.

[RoninLB]

I has the gasket sealing the bowl slowly falling apart and winding up on the idles.

[jwetering]

That's good stuff guys. In fact when I did my "mini rebuild" I didn't find and solids, but the idles jets did look a little gooey. I think my next move (besides confirming float bowl levels) is to take out the mixtures screws and idle jets, inspect for goo (and gasket material), clean as necessary, and blow a little low pressure air through that part of the idle circuit. If the problem clears up I guess it's a gas tank job. I shoud have mentioned that the first time it happened I was very close to an emty fuel tank, so it makes sense.

Funny that the contaminant woud make it past the filters - all I can think is that the contamination stays solubilized until it hits the needles where the fuel starts to vaporize, leaving the heavier goo behind.

Thanks

[RoninLB]

I'm not an expert with this so it's only my impression gained from others + my experience..

Webers and I guess PMOs are dirt sensitive. The routine decent carb filter is 10-20 microns. My filter is 2 microns which is smaller than the aids virus afaik. Also i always run techron before oil change to clean up any hidden carb varnish..

Also I've pulled an idle jet on a missing cylinder and couldn't see any dirt so I reinstalled after doing the air only in carb passages. It still missed. I pull only the idle again and can't find new or old dirt. I put compressor air only thru the idle parts and reinstall then aok. As of now I don't trust my eyes and trust compressed air.

[2.7RACER]

Jasper,
Ron is being modest. He knows Webers well.
I will add my recent experience with my Webers on my 2.7 used mostly for tracks.
I had issues with missing and poor performance. I was cleaning the idle jets when I noticed the small holes were not all the same size.
I carefully checked each one with a magnifying glass. Sure enough a significant difference. All were marked as 45's.
I gathered all my jets together and dropped by my local independent Porsche perfomance shop run by Russ Kelso in Corrales, New Mexico.
Greg checked each jet with a tapered gauge and found 2 of the idle jets to be closer to 55's than the marked 45.
He found errors with my 52's as well.
Put the correct jets back in and the idle and mid range smoothed right out.
Bottom line, look carefully at your idles, they may not be as marked.
Oh, we routinely run smaller jets at 5,000 feet than you would find at sea level.

[jwetering]

Originally posted by 2.7RACER
Jasper,
Ron is being modest. He knows Webers well.
I will add my recent experience with my Webers on my 2.7 used mostly for tracks.
I had issues ...All were marked as 45's....dropped by ...Russ Kelso...he found errors

The car has run both very well and exceptionally well with the jets I have, so I don't think it's a sizing or mismatch problem. My jets came from Richard Parr, and he made a point of telling me he checks them all before he sends them out.

Ronin's notes are well taken. I wouldn't trust my eyes either given the things they've seen over the last 40 yrs

Do you agree then that pulling the idle jets and the mixture screws and putting air in the idle jet passage should get the job done? I guess a better fuel filter is indicated too (as well as cleaning up my fuel tank). Any specific recomendations for fuel filters?

I hate to think the carbs are that sensitive to contamination - I though CIS was bad.

[2.7RACER]

Jasper,
I have read somewhere on this site that compressed air into the passages can risk collapsing the floats.
I have no personal experience with that.
I generally disassemble the carbs on the bench and check all passages with a light and some air.
The idle jets are the smallest point in the fuel circuit, so they generally are the first to plug.
I can see where small particles could possibly cause the float valves to not fully close, thus overfilling the bowls. Again I have not had that problem.
Float levels are critical with Webers. I use the external gauge that reads the float level when screwed into the side of the float bowl.
It is simple to monitor the float level with this external level indicator screwed in to the side of the bowl.
The PMO carbs use a sight glass in the side of the carb to show float levels. It's that important.
When I have the top of the carbs off I always blow into the fuel inlet to check the control valves to make sure they are working properly.
Be sure to safety wire the float pivot screws when you are done.
It sounds as if you have done all the carb work well. Still could be an ignition issue.

[trekkor]

When my car has the symptoms you describe, I spend ten minutes and clean the idle jets.

KT

[jwetering]

I have read somewhere on this site that compressed air into the passages can risk collapsing the floats.

> Yes - I've heard that too. My asumption is that with the idle screws out the air has somwhere to go besides into the float bowl. I'm using relatively low pressure air too.

I generally disassemble the carbs on the bench and check all passages with a light and some air.

> Did that - they looked ok. I don't want to tear them all the way down *again* aargh. I defintely don't want to take them apart without having a new set f gaskets on hand. In fact mine are a little tattered and remember Ronin had a problem with deteriorating gaskets plugging the idle circuit.

The idle jets are the smallest point in the fuel circuit, so they generally are the first to plug.

> With three fuel filters and no obvious sediment in the filters I cut apart I am suspicious. The passages aren't less than a tenth of a millimetre are they? I think I would have noticed gasket material....

I can see where small particles could possibly cause the float valves to not fully close, thus overfilling the bowls.

> I want to have a good look at these.

I use the external gauge that reads the float level when screwed into the side of the float bowl.

> I have one of these and plan to use it - it makes such a mess though, each time I check a bowl and then remove the guage I have to try to capture a float bowl full of fuel in a cup. Is there a trick to this?

When I have the top of the carbs off I always blow into the fuel inlet to check the control valves to make sure they are working properly.

> This would check if they open - but it is possible too that they aren't closing completely, although I suppose I'd observe fuel overflowing the bowls. Does popping from the exhaust not mean a rich condition ie overfilled float bowls?

Be sure to safety wire the float pivot screws when you are done.

> done

It sounds as if you have done all the carb work well. Still could be an ignition issue.

> My plan is to do the carbs again, although not a complete teardown, at least not at first. If the problem goes away then it's carbs. Otherwise it may be ignition. But you know it can't be foulled plugs since a new set didn't help. The wires are brand new Magnecors and I can surely rule them out. Would a failing MSD box sound like a miss? Or would it just run weakly on all cylinders.

In fact I haven't had the time to investigate all weekend Tomorrow night I'll try some of the carbs tips we've discussed. Thanks for the support gents!

[trekkor]

I remove all six idle jets and spray them and the jet holders with carb cleaner into a paper towel.

Spend the ten minutes before you get to deep.


KT

[jwetering]

Originally posted by trekkor
I remove all six idle jets and spray them and the jet holders with carb cleaner into a paper towel.
Spend the ten minutes before you get to deep.
KT

Thanks for the reminder to think before I do anything rash trek. Good advice.

[jwetering]

Well - the solution was simple. The #5 idle jet was blinded over with.....a small fleck of hardened JB weld which I used so many years ago to cement in some baffles when I did the Jerry Woods style float bowl mod.

HOPEFULLY this doesn't turn into a regular event where pieces of the JB weld fleck off of the job.

I shoulda known.

[jeffgrant]

At least you figured it out Jasper!

Hopefully it hasn't discouraged your prospective purchaser.


...jeff

[jwetering]

Originally posted by jeffgrant
At least you figured it out Jasper!

Hopefully it hasn't discouraged your prospective purchaser.

...jeff

Prospective purchaser my eye ! I had one call so far and although the car ran great for the test drive, the prospect was a little put off by the bare bones set up. He's going to check out Jorians white car.

I guess putting the carpets and some of the other stuff back in would help me sell the car...except I have had no calls or emails expressing interest thet than the one guy. In the end I'm not to worried about it. I'm second guessing the decision to get a 996 daily driver anyway. The M96 motor has always worried me. A late model M3 would be an interesting car though...

Probably I should just remember why I built the car I did and just enjoy it.

[RoninLB]

my bro has a nice LA street 1988. 993/996 guys pull up next to him and ask if wants to sell. They are bored with newer cars and wish that they never sold their old 911.

Racor makes some nice fuel filters. They have a 100psi inner cartridge unit for less than $100. I use a spin on but it's designed for low psi but it has a water trap with an optional water level sensor that triggers a bulb. The consideration is that it's another fuel gasket that may someday leak.

[Jeff Alton]

Have a Racor like that on my boat!! They do make very nice fliters.


Cheers