Kangaroo

[aggie]

I have a strange problem with my MK 1 Spitfire.  After a rebuild lasting quite a number of years and many setbacks ( not unusual ) I have finally taken to the road only to be hugely disappointed. When accelerating, especially in 2nd gear, it lurches forward ( rapidly ) and then dies down and again and again.  Engine has been completely rebuilt, carburetters have new jets, needles, throttle rods,discs, float needles and jets.  I have spent hours tuning them and she will tick over sweetly and rev to the limit as long as she is sitting in the garage.   We drove this car for many years for 100010 miles  all across Europe and never missed a beat, now I can barely make it to the roundabout a mile down the road.  Only thing I haven't tried is a stronger return spring on the accelerator arm. I'm beginning to think that she is scared of going back to the road !  Any ideas please.

[dougbgt6]

If you're happy with the timing, sounds like fuel starvation,  not enough fuel getting through.. So fuel blockage, dodgy pump? Do you have a fuel filter? The dread rubber slivers blocking inlet valves?

Doug

 

Edited August 27, 2023 by dougbgt6

[johny]

yes agree with Doug. Fuel needed without load is minimal so will rev fine when static but flow not enough for driving especially acceleration. You could test the pump by pumping fuel into a jar which should produce good squirts unless its non return valves have failed.... 

[Wagger]

3 hours ago, johny said:

yes agree with Doug. Fuel needed without load is minimal so will rev fine when static but flow not enough for driving especially acceleration. You could test the pump by pumping fuel into a jar which should produce good squirts unless its non return valves have failed.... 

Yes, just dangle the fuel pipe into a jar and crank the engine over. It should deliver about a tablespoonful per pulse. Pumping by hand with the lever does not prove that the cam is working.

[DanMi]

do you have the return spring from the throttle arm down to the chassis. I find that mine jumps if this is not present

[Ian Foster]

Do not discount a blockage in the fuel line...ask me how I know!

[aggie]

21 hours ago, dougbgt6 said:

If you're happy with the timing, sounds like fuel starvation,  not enough fuel getting through.. So fuel blockage, dodgy pump? Do you have a fuel filter? The dread rubber slivers blocking inlet valves?

Doug

 

Timing should be ok.  I've tried electronic distributor and points with the same result so will check the issues that you have raised.

Thanks, Alun.

[aggie]

20 hours ago, johny said:

yes agree with Doug. Fuel needed without load is minimal so will rev fine when static but flow not enough for driving especially acceleration. You could test the pump by pumping fuel into a jar which should produce good squirts unless its non return valves have failed.... 

Thanks Johnny will try this.

Alun

[aggie]

4 hours ago, DanMi said:

do you have the return spring from the throttle arm down to the chassis. I find that mine jumps if this is not present

 

2 hours ago, Ian Foster said:

Do not discount a blockage in the fuel line...ask me how I know!

I do have a return spring from the arm to the chassis but I'm sure it's not OE.  It's one I got from Rimmers and is the same as the return springs on the throttle stops.  I reckon it should be stronger but, naybe I'm wrong.

Ian, I think I know how you know !

[johny]

but surely if you can control the revs smoothly when stationary the pedal, linkage, springs and carb butterfly valves must all be working well?

[DanMi]

return spring was the same as the mk2 return springs 136835 and not very strong

19 minutes ago, johny said:

but surely if you can control the revs smoothly when stationary the pedal, linkage, springs and carb butterfly valves must all be working well?

the movement of the car can make your foot jump on the pedal if the spring is too weak hence when it accelerates your foot moves back slightly so you decelerate which makes your foot move forward, hence the kangaroo effect. I had to make sure that my foot was resting on the A post carpet when my spring was missing to prevent this.

[johny]

Reading the original post I think its a bit more than just throttle control especially as it used to drive ok....

[dougbgt6]

6 hours ago, Ian Foster said:

Do not discount a blockage in the fuel line...ask me how I know!

Ah! The old pink sludge problem, I dynoRoded my fuel line with nylon strimmer chord. 

Doug

[aggie]

Okay, so here's an update.  I have two fuel filters, one in the boot close to the tank and one just before the fuel pump, neither show any sign of debris.  Fuel pump is working fine, good strong delivery.  There was no appreciable debris in the float chambers but the front needle valve appeared to stick intermittently so I have renewed them both.  Carb pistons operate smoothly.  I changed the spring on the accelerator arm for a much stronger one and, guess what,  IT'S WORSE THAN EVER !!!    to the point of being uncontrollable.  I'm close to giving up on this, anybody got any further suggestions. O, the only things I haven't changed are the piston return springs, but I wouldn't have thought they would make that much difference.

Alun.

[JumpingFrog]

If you're on SU's, are you sure you managed to install the jets without kinking the pipe that goes to the float bowl? Been there, done that, you have to throw the jets away, there's nothing you can do with them once they've been kinked. Really easily done if you've been incorrectly sold the black coloured jets for Minis (and other cars with SU's at 45 deg), they have longer pipes than the red jets for Triumphs (and other cars with SU's at 90 deg).

I know it was touched on before, but timing will also make a big difference too, if you're running too retarded it will appear to run okay and rev without load fine, but have absolutely no power. If you've got a timing light, also worth checking that centrifugal and vacuum advance are working correctly. Best way to set timing is by ear, trying to chase the factory setting doesn't usually work out for me, modern unleaded doesn't burn the same as fuels of old.

[Pete Lewis]

and dont discount engine mounts allows the whole thing to shift about as you apply load

Pete

[DanMi]

42 minutes ago, Pete Lewis said:

and dont discount engine mounts allows the whole thing to shift about as you apply load

Pete

particularly with the throttle with levers and poles rather than a cable. Maybe convert to a mk4 throttle pedal and cable. 

[Iain T]

5 hours ago, aggie said:

, the only things I haven't changed are the piston return springs, but I wouldn't have thought they would make that much difference

They can as without them due to the piston rising too quickly the mixture will run very lean under loaded acceleration and stall. First off I assume they are the correct springs installed and 20/50 oil in the dash pot? Is there resistance when you try and lift the air piston with your finger? If OK I would try richening up the mixture to see if that helps. 

If its still bunny hopping I would start on the ignition. 

Iain 

Edited September 13, 2023 by Iain T

[johny]

yes is it possible its losing the ignition repeatedly somehow everytime the engine rocks under acceleration (less movement so no problem when just free revving) and then regains it to do the same thing again. Check all engine electrical connections including earthing?