accelerator racing under load only ...

[RichC]

okay so it's off the blocks and i'm driving it a few feet backwards and forwards for the first time .
whereas the webers are happy idling and revving with the engine stationary, once i drive a few feet forwards or backwards the throttle races for a few seconds before it dies down , not a situation i'd be happy to experience out on the road.
Any bright ideas?
i did check the thackeray washers to check they weren't too loose but they seem okay (20thou ).
does it sound like extra air being drawn in somehow?

[types26/36]

Check you have a little "free play" in the accerator cable when the linkage is in the idle position.

[Dag-Henning]

- do also check that the carbs are properly syncronized. Maybe one of them is a little "ahead" of the other. May show symptoms like that.

Dag

[JJDraper]

Being more pessimistic, it could be a very slight twist in the shaft that the butterflies are mounted on. This causes an even more pernicious problem than straightforward carbs being out of sync.. check that both close at precisely the same angle (carbs off).

Jeremy

[reb53]

If it has a fast response when you actuate the throttle at the carb, i.e. in the engine compartment, but a slow response when using the pedal, I'd be checking the cable to ensure it isn't full of sticky old oil/grease.

Ralph.

[rgh0]

A slow return to idle speed is usually due to one or more of the throttle plates exposing the progression holes to vacuum. You can see the throttle plate through the progression holes by removing the plug above the holes and shining a light in the carb throat. Adjust the butterlfly so the first hole is just upstream of the butterfly. Then adjust the idle mixture with the screw to get the right idle mix.

The other less common reason can be leakge of air through the seals at the ends of the carb throttle shafts

regards
Rohan

[andyhodg]

I had that problem when some of my well intended but ignorant tweaking of the idle and mixture settings ended with the exactly the same symptoms as you described.

I ended up taking the car to a carbrurator expert - the best thing I ever did. He adjusted the setup and recommended some mods the the progression holes which I subsequently asked him the carry out. My car has never run so well.

Good luck

Andy

[bill308]

In additiion to the two concerns Rohan writes about there is also the possibility of a premature ignition adavance or failure to retard. This could be do to friction in the advance mechanism or weak springs.

Bill

[RichC]

ah - good one Bill
i shall get my 16yo son to check this distributor plate out...

[RichC]

well, 6 months later and with the good weather with us again , we actually got round to putting the distributor back in after an overhaul which included checking the advance timing mechanism for faults !
i'd decided to replace the distr. drive as the cog had serious cracks & there was no thrust washer behind the brass ring so the end float was excessive (1-2 mm)... I thought the latter might be a contributing factor why the revs didn't come back down as you'd expect after taking your foot off the throttle, but no ... the problem still exists ...
I know once the carbs are removed and replaced there's a bit of fine tuning to do to balance them up again so I'll be looking to sort that out next. i

Thanks Rohan, Bill & Andy. I had a look at the progression holes and they looked fine . Are there seals at the ends of the carb spindles then?
looks like i'll have to track some down if there are .


I've ruled out timing & sticky throttle as being the cause already.

Next, I'll be looking for a good Weber specialist in Essex!
RichC

[oldelanman]

Hi Rich,
As this only happens when you are shunting backwards and forwards could it be a faulty brake booster perhaps ? Try blanking off the vacuum pipe and see what happens - or use the handbrake to stop it......maybe not !
Regards,

[RichC]

uh?
sorry old bean but you have me there !
are you saying it might be due to servo leaking in air??
if you are i'd be v interested if you could elaborate 'cos the original servo was serviced with replacement parts.

cheers,
RichC

[RichC]

Roger,
I'll try your suggestion at the weekend when i can empty the drive of other cars .
sounds very interesting ... I'll take the blanking off option rather than handbrake !
RichC

[oldelanman]

Hi Rich,

are you saying it might be due to servo leaking in air??

Precisely so.

Thinking outside the box it occurred to me that one thing you are doing when you are shunting the car which you are not doing when just revving the engine with the car stationary is applying the brakes. It is not uncommon for servos to malfunction particularly after standing for a while and even following rebuild - do a search on here and you will see what I mean. This is one of the reasons why some people get rid of the servo altogether. This may have nothing to do with your problem but it is simple to eliminate it as a possible cause before you start pulling carburettors apart.

Regards,

[Galwaylotus]

At the risk of stating the obvious, is the throttle return spring the correct one and installed correctly?