Webers - stuttering and sort of missfire

[sfh3l]

Hi Chaps,

I am writing in the hope that the collective wisdom might be able to help me out. Left home this morning for the PCGB trip to Stuttgart, Prague etc, so a fair old way ahead of us. Car went beautifully in the crisp morning air on the way down to Dover, but toward Dover developed a fuelling problem.

Symptoms are rough running between about 2000 and 3500 rpm, particularly on part throttle. More throttle seems to clear its throat, but there is definitely some starvation going on. The car has a new fuel filter and it feels as if one carb has a blockage in whatever deals with that zone of running. Tickover is fine and the fuel consumption appears bang on what it normally does. The car is a 2.4T, running on webers.

I'm in Belgium now, safely at our first stop and on the way to Stuttgart area tomorrow. The car can cruise at 75mph without any difficulty, but there is a sort of "berrrrrm------berrrrm-------berrrrm" on and off droning, as if the fuelling is momentarily good and we get back on song and then poor again and it is off beat again.

Are whatever jets are responsible for that running anywhere simple and easy to get at (like on the side of the carb throat), or is it a carbs off job for an expert, just to see if there is something simple and obvious? Alternatively, from the symptoms, should I be looking somewhere else completely?

By the way, plugs are new, good quality ones (can't remember here and now precisely what, but about a year old and no prior issues). I run MSD ignition, with Pertronix ignitor in the distributor, both on the car for at least 2 years and never any issues. The sparks are as strong as anything, so I don't think the problem lies there.

Any ideas would be most gratefully welcomed, or I may just have to slog it out and wait till I get home.

Thanks in advance lads. I'm flying the flag here and need all the help I can get

[mycar]

Sounds like it could be blocked jets.

Idle jets are in positions 3, 19 and 11 or 13.

Main jets, 8, 17 and 21.

They undo easily and can be blown through with carb cleaner or compressed air.

[KS]

Classic blocked idle jets - Webers rely on the idle jets for a substantial part of the throttle opening, relying on the accelerator pump circuitry to richen up the mixture when load is applied. Only at almost WOT do the mains take over completely, then using the air correctors to fine tune at the top end. DCOEs and IDAs are notorious for this, having a relatively unsophisticated progression circuit to smooth things out in the transition between running on the idles and the main jets.

[sfh3l]

I bloody love you guys!!!!!

Thanks so much. After enjoying the 911 for 7 years now, I suppose it is time to be properly initiated into Webers. I have just been out and taken all six jets out. I don't have any air or even brake cleaner in fact, but blew them out with my own breath. I now have E10 aftershave - not recommended, but I will see tomorrow whether it has worked or not.

Thanks again. It really helps to know there is help out there! I'll report back. I reckon we did 320 miles today (Buckingham to Chiny, Belgium) and there are a few more tomorrow as we have to be in Ludwigsburg by the evening, so it would be good to have won out with this. You're fantastic - thanks!

[fetuhoe]

The normal operation of a Weber tends to rely on the main jet circuit as opposed to the progression circuit and if the car runs well at above 3500rpm this would tend to suggest that it may be the progression.

The first hole that is uncovered by a very small opening of the throttle plate controls idle and the small gap between the throttle plate and this hole creates a Venturi effect and the pressure drop draws the fuel/air mixture through the idle jet and it's associated air corrector.

As the throttle continues to open this venturi effect is lost and the fuel/air delivery would reduce and the car would tend to stumble. The next progression hole allows the venturi effect to be re-established so fuel continues to be drawn and so on depending on the number of drillings.

A typical IDA3C has three progression holes and in general progression should be smooth. Delorto carbs have, historically, been provided with more progression drillings than Webers and 'Spanish' made 40DCOE151 carbs had only two drillings which led to the poor 'progression' reputation.

If the idle jets are blocked then it is fair to say that progression will be poor but also the car will not idle in a stable manner and this would tend to suggest that the idle jets may not be the cause.

I would think that the progression circuit starts to become ineffective at engine speeds much above 2750 rpm although there is some suggestion that it may be effective to 3500rpm. Once the third progression hole is completely uncovered this part of the circuit will become redundant and the main jet will take over.

Under normal operation the idle jets are not delivering much in the way of fuel as fuel flows through the main jet into the emulsion tube well where it mixes with air from the air corrector and exits through the holes in the emulsion tube and then to the auxiliary venturi.

If the car progresses with very gentle throttle opening but stumbles with abrupt changes in load the issue is likely to be associated with the accelerator pumps but if it just doesn't run well above 3000rpm I would be concerned with the main jets.

Poor progression is also commonly associated with debris blocking the fuel transfer galleries in the body of the carbs and these will need significant work to clean them out.

The other problem areas may be associated with poor balance and if the linkage has wear then this could cause quite poor running as some of the cylinders could be 'leaning out' if the system is not well synchronised.

It is difficult to re-balance without some tools but it may be worth inspecting the drop links and linkage parts.

I would also check to see that the advance mechanism in the dizzy is working. If you take off the cap and twist the rotor and ensure that it can move to increase the advance and that it returns correctly then this would suggest that the ignition is OK.

[KS]

48IDAs only had two progression holes, too. Drilling a third is a common modification for street use. My experience with IDAs has been that blocked idles still allow the engine to idle as in reality they're rarely ever completely blocked and still allow a dribble of fuel through, sufficient for the engine to run. Pulling away requires more throttle, thus calling the pump jets into play to mask a problem, but as soon as the engine goes into light throttle 'cruise' mode, the (partially) blocked idle jets mess things up again. Blocked progression holes, in my experience (albeit limited largely to 48IDAs on VW race motors going back to the 1970s), tend to make the engine stumble while the throttle is being opened slowly but have less of an affect on a steady throttle.

100 per cent agree about distributor advance and carburettor balance, but assumed the problem in this instance was one which has occurred on the journey, suggesting some crap fuel picked up along the way (how many garages flush their tanks these days?).

Ah, the joys of screwdrivers, listening to carburettors through lengths of rubber hose – and the taste of petrol after blowing out jets by the roadside... How I miss it. I think...

[mycar]

Forgot to mention, the handbook recommends cleaning any obstruction from the idle jets with a horse hair.

[911hillclimber]

I used fuse wire.
This brings back memories of my fist engine, a 2.2T on Weber side, lot of trouble until the rebuild was done.

[sfh3l]

Brilliant! All of you - thanks again.

To report back, having driven a further 250 miles or so today. The little car was definitely different, with cleaner running at some times but still clearly not right. So I went to go an have another look at the jets again this evening, once it had cooled down a bit. I started with the idle jets on the RH bank and did them, before moving to the other side.

When I cleaned them yesterday, the car was parked with the setting sun in front of it and I was bit blinded by sunshine coming through the cabin and through the rear window, through the gap between the open engine cover and the car body, so didn't see too clearly. Today, the car was the other way around, with sun beaming into the engine bay from behind me. I spotted a brass screw head, lying on the tinwork by the rocker covers, where the rubber seal fits. I picked it up and had a good look at it, trying to work out where it had come from. In the pic posted by "mycar" above, it is plug number 6, that was effectively completely missing from the carb itself! Needless to say I have refitted it. I am not 100% sure of what it does, but I suspect that it allows air into the carb in an uncontrolled manner and stops one of the jets working, perhaps at all.

Not sure, but we will see what happens tomorrow. Only driving from Ludwigsburg to the Stuttgart factory tomorrow, but it will be far enough to see if the little car is back on form again. Do we think there will be anyone in the factory who knows about carburettors, or are they all "Enspritz Only" now, since Porsche hasn't made a car with carbs since about 1973 (I think!).

Thanks again, to all of you for the help and support. I love classic cars. The cars are great, but the community is even better. I will report back.

[sfh3l]

Forgot to mention, the handbook recommends cleaning any obstruction from the idle jets with a horse hair.

I did see a few gee gees beside the road today, but none that looked as if they'd let me shave them Thanks though.

[mycar]

No probs. The bit you found floating about on the tinware is an idle progression inspection screw. The car will run much better with that back where it belongs.

Here's an old thread so you can identify any other bits that fall off.

viewtopic.php?f=1&t=55284

[fetuhoe]

If the progression ports are open to air the car should run weak at any time when you are using the progression circuit as air is normally only drawn from the idle air corrector which is around 1.4mm diameter.

[sfh3l]

Thank you guys - normal service has been resumed

You are a great help and I'm not 100% sure that there wasn't some crap in one of the idle jets as well. The car definitely ran better yesterday after my first poke about, but today was perfect, so the inspection bung/plug was the main culprit.

I wasn't able to photograph the old car in front of the Porsche Museum, as they were doing a "multimedia experience" there. Making and selling cars these days is all marketing B@ll@cks.

Off from here to Prague tomorrow, so very glad the little car is back on top form again.

Thanks again fellas.

[sfh3l]

Greetings from Prague, where the little car is safely tucked up in the underground garage at the hotel.

All sorted. It ran like a dream, as per usual today, so the carbs are indeed sorted now. I shall give them a bit more attention once I'm back home, but all good now.

Thanks again and I hope I can help as much when my time comes.

[hot66]

Road trip photos needed