DDK - Die Deutschen Klassiker

Got a 72 911T with an E engine running on Zenith Carbs. Under normal driving conditions the car never misses a beat, it's fantastic. But:

If I accelerate hard (pedal well down), then back off and coast, when I accelerate again it misfires very badly. It can still be driven by putting my foot to the floor and it'll accelerate through it. To cure the misfire I need to dip the clutch and let it idle for 10 seconds. It then drives fine.

Any clues?

Petrol starvation - crap in the fuel line perhaps?

Hi, Maybe the float levels are low, or the fuel pressure is down?

if yti was MFI I'd say your fuel cut off microswitch needs adjusting (btw how do you get an E engine with carbs, they were all injection?)

Surely a diaphragm problem to do with accelrator pumps?

Hi Nigel: just to let you others know, this is my old car .... so therefore well dodgy .

Nigel has mentioned this to me before and we struggled to come up with much other than poo in the carbs. You may remember when I was struggling to get the car to rev, I was paraniod about muck getting to the carbs. Therefore the fuel goes through a pre-pump filter (two in line in fact), then through the four carb filters (one on each carb inlet).

The fuel pump was new about five months ago, and after all of the filtering, flowed at 1.6 litres per minute. I have been told the factory minimum flow for a 911T was .9 LPM, so its running near double.

The carbs came from a running car that Nick Moss had at his place (owner changed to a full 'E' setup).

The fuel overun switch assembly was never connected, either when I bought the car, or after I had built it: don't know whether this would have an effect. I was under the impression that most people disconnected this lot anyway?

Diaphrams may well be worth a look .

To be honest, thinking about it, I'm not sure I ever gave it some serious beans ( ), so perhaps the problem was always there? Unlikely though, I'm sure something would have shown before.

BTW, Nigel, have you had a chance to strip the carbs at all, or are you exploring all options first?

Actually thinking on this: what would be the effect of simply disconnecting that overrun switch? Did I read somewhere that there were other mods that could/should be done?

Like I say, I really can't remember nailing it, and then going straight into heavy overrun, so perhaps this is something that simply hadn't surfaced until Nigel (lead foot ) bought it?

Thanks for all your thoughts.

Barry, I've not had chance to look at the carbs as yet. I'm back in Glasgow, and next week move up here so frantic packing when I am at home. I think (hope) the cars ok to drive up here (470 miles is a long way to hold your breath), as I'm not going to get chance to look at it before I come back down to pick it up in a few weeks. I'll store all the responses and take a look once I get it up here.

I'll give you a call re the auto jumble in Crawley if that's ok?

The overrun enrichment device won't work if the switch is not wired. As such this whole circuit is redundant. I am not sure why it is even there ! I removed mine (See link to useful web info on Gary71's sig) My car has run so much better since, but then one of the diaphragms did look like this

I wasn't getting your symptoms though, more just getting fed up with the rubber pipes popping off all the time .
Cheers
Tim

Perhaps it was my imagination then, Tim .

Nigel, yes, give me a ring: be happy to help. I'm sure the trip will be fine, when I took it to Beaulieu earlier in the year, I ended up doing nearly 300 miles in one day, and it didn't miss a beat, just got into it's stride, and off it went.

The obvious thing would be to check the jets and stuff to see if they maybe clogged. I had a similar problem and it was dirt and gunk in the idle and main jets.

Yes, you would normally expect this, wouldn't you? Will be interesting if the carbs do come apart: particularly as the car is basically a four-wheeled fuel filter transporter .

Perhaps I could of sqeezed another one in: still space in the glovebox .....

Perhaps it is fuel stavation then....
The fuel pump just can't push the fuel past all of those filters !