Engine running poorly at lower revs (2.0 911)...

[Herr Kuult]

Hello all,

I've been about posting this question for a few months, but a recent drive of a couple of other 911s has highlighted how badly my car's engine is running. I believe that the car is not running correctly below about 3000rpm, and this manifests itself in three symptoms.

Firstly, it is impossible to hold the engine between 2000 and 3000rpm when sat in neutral. It doesn't matter how carefully I gradually open the throttle, as soon as the revs creep over about 2000rpm the engine picks up and rushes way over 3000rpm. A similar thing happens when I try to lower the revs back down. This can't be right, can it? It'd almost be a good fairground attraction - "Win a goldfish if you can hold the engine at 2500rpm" - a sure money maker.... It makes me think there is a (Weber 40 IDA) carburettor issue, perhaps the transition to the main jets...?

Secondly, the car really has no torque below 2500~3000rpm. I find myself cruising around in at least one gear lower than I'd expect. I've driven another 2.0 911 on Webers recently (albeit a with a modified engine) and the difference is ridiculous. I daren't imagine how long it would take to go from 40 to 70 in 5th. I'd probably run out of road first...

Finally, I really struggle to pull away, especially if it's uphill. I end up having to slip the clutch like billy-o, whereas this manouevre in other similar cars is a dream.

The combined effect of this is somewhat diminishing the driving pleasure.

So, does this description trigger any memories of similar issues for other DDK members?

I've thought about a few things myself, and I find myself focussing on either fuel or ignition. I've rebuilt the distributor in the past, in order to bring the advance curve back in line with the Bosch spec for my car, but it's quite difficult to check the whole advance when you can't hold the engine in the rev range where the trouble lies. Also, I reconned the carbs last year, but this was limited to a general clean up, new gaskets etc. I didn't rebush the spindles, or do any other serious work.

Any advice greatly aprreciated....

Cheers,
Paul.

[912uk]

Nick and Alan are your best bet for flat 6 motor questions

[Barry]

Without knowing much about your carbs I can only give some general ideas:

1st have a quick check of your ignition timing: not because it's the main culprit (it's almost certainly fuel related) but because it's so quick to check. Just put a strobelight on with everything set as normal: i.e. vacuum pipe on. Just take the engine up through your 'troubled' rev range whilst watching the timing. If it suddenly starts diving about then you might have terminal dizzy shaft wear, sticky advance wieghts or failed advanced springs. Also just check your actual timing is more or less right: at this stage a degree or two won't make that much difference, just make sure it's not wandered 20-30 degress out or anything.

I know you say you've checked this, but worth doing: I had exactly this problem on my 911, and it was the dizzy (that has appeared O.K.) was suddenly changing the timing, resulting in the revs leaping up from nowhere. I appreciate not easy to do if you can't hold the engine at those revs, but just try to take it from idle to say 4000+ in a clean sweep to get a rough idea: you should be able to see if something really alarming happens.

If it behaves more or less O.K. then it's back to the prime suspect: fuel. It sounds as though the car is seriously leaning out through these revs. Is the idle reliable, or is it tricky to set and then moves all of the time: if so spindle wear, carb to manifold air leak, or manifold to head leak. Also possible porous manifolds (it does happen).

I've heard that a quick test for air leaks is to start the car and let it settle to idle, and then introduce another fuel around the suspected area: i.e. gas from a blow torch: personally I've never had the nerve, and rather suspect it would be an ideal way to get a nomination for the Darwin Award . Anyway the theory is that if the engine speeds up as you pass a certain area (carb spindle for instance) then you've got a significant air leak there. Eitherway check all round for air leaks.

After that a lean mixture is either too much air or not enough fuel so: blocked jet? Float level? Fuel pressure and flow (highly unlikely if higher performance is O.K.).

After that you might have to look further: has the car always behaved like this, or has something been done to the car recently that may have brought on this fault? Does it check out mechanically? Compression, valve clearances, cam timing?

Hope this helps, although I'm sure you'll have been well ahead of me on all of these anyway. Bottom line, it really does sound like it's running very lean at those lower revs, but well worth double checking the easy-to-check basics first. Keep us posted!

[Barry]

Forgot to say, when I got my 914 it seemed to be running a bit lean. I did all of the usual tests, and in the end took the carbs and manifolds off of the heads.

It was only then that you could see that the manifolds had been put on half over the engine tinware . As you can imagine, this did not help achieve the very best airtight seal, causing what must have been the most spectacular airleak. The amount of low-down torque available after that was fantastic, and once I corrected the 20 degree timing error it just got better .

Having followed Brendan's 2.0 911 this last weekend on the Northern I was amazed how well it performed, so it's well worth sticking it out..

[912uk]

common 912 and 356 problem encountered from turkey engine builders in the US home made gaskets for the head to manifold and 30% of the intake was covered up with the wrong gasket ( prob VW )

best bit was half the cars were with performance exhausts and performance air filters giving oh 8 BHP at best and there was about 25BHP sitting there waiting to be uncovered...

[Barry]

Also ( ) forgot to say: since you car is so very unhappy, don't get bogged down with things that are a fraction out: i..e if your timing is within a couple of degrees, or your dwell is a fraction out don't worry about them for now: you are looking for something miles off from the norm.

Same principle with the air leaks and carbs: if there's just a tiny fraction of play in the spindles move on: you're looking for something worse. By all means go back and get everything to spot-on factory spec later, but it's very easy to get bogged down with stuff that simply won't matter to a huge degree. Yes they might cost you a couple of BHP here and there but would no way cause such a large running problem.

Always a tricky balance, this: knowing what is acceptable, what you can scrape by with, and finally what is bad enough to actually cause a fault, as opposed to merely not letting an engine perform to 100% of it's potentail (very different).

That's it: I'm done .

[Nick Moss]

Barry has covered most things. Just check the timing, points, plugs and tappets are right before you start changing things. Is the car totally standard?

[Herr Kuult]

Thanks all (esp. Barry for his essays... ),

I don't think this problem has suddenly developed, but I'm sure it wasn't this bad a year or so ago. BTW, the engine is completely standard, other than the removal of the US emissions gear.

I haven't checked the timing recently, so you're right to remind me about that. I checked the compression a year ago and everything was perfect. And I need to seet the valve clearances, because one or two of the tappets sound a little out.

But the main thing that I have doubts about are the carbs, and Barry's comments seem to add up. In fact, I find setting the idle mixture very difficult. The mixture tends to be either incredibly rich at idle, or too lean at around 2000rpm. If I adjust the needles with the idle set at around 1800rpm, when I return the idle to the normal setting I disappear in a cloud of noxious fumes. Since those needles control the idle and progression mixture, I don't exactly know what to do about it. It almost seems as if some of the progression jets are blocked... Furthermore, I'm pretty sure there is some play in the throttle splindles. I'll try to measure the play.

I'll do some investigation and report back if I make any progress.

Thanks again,
Paul.

[davidppp]

no

[davidppp]

Hello, Paul.

Sounds pretty typical of blocked idle jets to me:

What size are they? do you have proper air filter, and no air leaks round the carb trays?

Dirt gets into the idles from the air as well as the fuel..

Drop the mains out, and watch the colour of the petrol as it pours all over the hot exh manifold(!)..is it rusty?

if so clean out and seal the tank, and lines.. and fit really good filters..

This is not rcket surgery..youve just got to follow a logical sequence as outlined by Baryy etc above..

Good luck./
Kind regards
David

[hot66]

I had this on my 2.0 '67 & that turned out to be a blocked idle jet. Cleaned & rebuilt the carbs then got then set up properly by a carb wizard & it ran really strong Also change the fuel filter.

[Barry]

I agree: those jets have got to be the prime suspects really. Will hopfully be one of those lovely fixes, almost no cost and total transformation .

[Herr Kuult]

Thanks again for the input....

I checked the timing the other night, and all seems to be in order. It's running about 2° ATDC at idle (spec for my engine is 2-5°ATDC) and it advances pretty gradually after about 1500rpm to somewhere between 25 and 30° from 4000 or so. There's no sign of random advance fluctuations in the problem range, so I think it's unlikely to be the cause.

As for the idle jets, they're all clear but that's not to say that there's no blockage somewhere in the drillings. When I rebuilt the carbs last year, they had a thorough cleaning in both the hot-wash and ultrasonic cleaner at work, and I gave them a good seeing to with an air-line. They did end up looking pretty sparkling, but this doesn't mean there's nothing in the way now.

The air cleaner is the standard item, but the paint on the inside of the housing is looking tired and it's not unreasonable to expect that some of that has ended up in the idle circuit. Especially as it gets the abuse from all the carb spitting....

The thing is that the car never really feels like it's running unevenly. The idle is smooth (but of varying speed) the progression is also smooth, but it doesn't feel like a 911 until you've got it well into the smile zone. It really does feel like a generally lean mixture at lower engine speed, rather than a single cylinder that's not firing correctly. On the other hand, do I really know what I'm talking about...... ?

Perhaps I'll get all "technical" and resort to welding 6 lambda bosses onto the exchangers.....

Paul.

[Barry]

I was just going to suggest either an Easy-Tune (see through spark plug) or gas test just to get an idea of what's happening at those troublesome revs ...

Still think it sounds lean though .

[Alan @ CanfordClassics]

Hi Paul,

I have seem similar symptoms a few times before and I think you may have already said what it could be

"not to say that there's no blockage somewhere in the drillings"

On the last that did this my friend drilled the plugs out to reveal a load of crud, which was causing the problems. Then used some correct sized fishing weights to plug it.

Work fine after.

I hope you manage to sort it out

Alan