The 'E' Type Forum

I have an engine problem that's unusual. I’m hoping somone can shed light on it. The car is a 1968 4.2 on SU's. The problem is this. On starting it will fire instantly and tick over without misfiring. But it won’t respond to the accelerator. It’s as if it’s gasping for fuel. After a couple of minutes the problem gradually disappears and the engine will run properly. On the road it runs almost perfectly apart from an intermittent misfire at high rpm.

Initially we thought the problem was caused or at least exacerbated by the engine being cold. But exactly the same thing happens when starting it from hot, when it’s been left for say 10 minutes.
Here’s what we've checked and adjusted where necessary:

Compression test - all cylinders good.
Ignition timing - it was out but resetting it carefully made no difference.
Carbs - cleaned and carefully set up. Twice. No difference.
No sign of old fuel in the carbs.
Checked fuel pump and that fresh fuel is getting to and into carbs. It is.
Replaced plugs
Isolated inlet manifold from brake servo.
The choke is working but makes little or no difference to the problem.

The problem occurred after the car was removed from an air chamber where it had been for 12 months. The petrol had been drained from the tank when the car went in.

We're perplexed. Any thoughts?

Sounds like sticking carb pistons to me . Can you move them up freely when cold? and do they drop with a click after having been lifted? If not, then either they are partiaally seized in the bell chambers , or else ( even more likely ) the jets have been interfered with and not properly centred

Pistons definitely not sticking, checked and double checked.

It’s been suggested to me that there are diaphrams is these su's? Is that correct? This might be the problem - destroyed by the new fuel?

Hi Jonathan.....take off the air filter and plenham so you can look into the carbs.....engine as you say starts ok....now what happens when you apply throttle......butterflies should all open together and pistons rise.......you say all is ok after a few minutes....so whats happening in those few minutes?......can you hear air being sucked in?Steve

Yes diaphrams on the jets....but you say youve cleaned the carbs? What does that mean...stripped down?

Hello Steve,
Re carbs we checked pistons, recentred jets and retuned. We didn’t check the diaphragms however, because I wasn’t aware of their existence...now I am. Tomorrow I’ll remove carbs.

Yes definitely hear air being sucked in and pistons rise equally.

Sounds like it's lean.

Seeking advice I’ve been told that if the diaphragms are split, the carbs will leak fuel. What if they are just hardened? Might that be the cause of the problem?

I had a similar problem. Turned out to be insufficient fuel delivery caused by internal corrosion of the main fuel line. The poor pump was only able to deliver just enough to full the float bowls on light running. After that the engine stalled.

To check, pull the fuel line off the glass bowl and see how much petrol is delivered. You may well have the require 3.5psi pressure, however you should witness a minor torrent of petrol. You should certainly be able to fill a mug in a couple of seconds.

The diaphram is attached to the jet and forms a seal keeping fuel in the jet.....splits will cause leaks. .hardening i dont think will be a problem.....but you could check and replace if needed.....the jet only moves up/down when you operate the choke lever.....beware if you loosen screws in the butterflies they are a once only use really.....the end is split and splayed after setting....so there is no chance of them loosening and getting sucked into the intake.....if you loose them to reset butterflies ensure the splayed ends dont break off......Steve

Sounds like fuel starvation to me
Joe

No one has yet mentioned the fuel. Old fuel looses its oomph, and behaves like the engine is running weak. Is the OP using fresh fuel?

Original poster says no old fuel in carbs...and fresh fuel used....Steve

Yes Steve, old fuel removed and fresh fuel being used. Have now removed carbs from car to check diaphragms. First one looks just fine...