1098 Carb/throttle_slide/return_spring problem

Post by **rayofleamington** » Mon Feb 05, 2007 5:40 pm

Hi folks,
Not sure if anyone knows their SU carbs like an expert, but just in case:

POD came up with a new problem for me yesterday on the way to Salisbury. Coming into a town on the Fosse way, the engine was running rough. When declutching to change down to 3rd, whilst slowing for the traffic lights ahead the engine stalled.
When re-engaging the clutch in 3rd the engine sprang back to life, but would only run at high revs.
I had to let the engine die as I needed to come to a halt at the lights with my foot on the brake I wasn't quick enough to blip the throttle and keep the engine running.
The engine would not start again - it would not even try to start.

The root cause? The throttle slide has risen up in the carb/dashpot far enough that the slot had gone beyond the locating pin. It had then mis-aligned and therefore was stuck fully open.
After fitting the carb back together it was apparent that that this was going to happen again as there was nothing to stop it! It could go around 3mm too far

I made a bodge and packed the end of the return spring, so that with the spring fully closed up, the slide could not travel beyond the pin.

So...

What should normally stop the throttle slide going beyond the pin?
Should it be the spring - if so are there different springs (different fully closed lengths)?
Is the pin likely to be at fault (a bit missing or worn away)?
Should it be something else limiting the throttle slide travel?

At the moment, my bodge will stay in place but as it was done whilst stuck at the the traffic lights on a busy A-road, it was not very 'accurate' and this limits the possible throttle slide travel.

Post by **Willie** » Mon Feb 05, 2007 7:12 pm

RAY, not something I have had to check but,if the schematic of the carb is to scale in the workshop manual then the Piston Damper rod would
hit the piston before it reached the top of the chamber. There are different springs, identified by their colour but don't ask me which is which! I wouldn't think it was the spring which stops the piston rising as to do so would probably cause the coils to lock over each other.

Post by **bmcecosse** » Mon Feb 05, 2007 7:49 pm

Aye - the piston damper should stop it - well diagnosed - and it does indeed sound as though this car is 'possessed'!!

Post by **rayofleamington** » Mon Feb 05, 2007 7:59 pm

Aha - thanks guys.
I will try some other piston dampers to see if they are different!

Post by **Willie** » Mon Feb 05, 2007 8:44 pm

Hope you find a longer rod. I have had one fault caused by the damper
but it prevented the piston from rising past a certain point, not a 'sticking
up' problem such as yours.

Post by **Willie** » Tue Feb 13, 2007 2:01 pm

RAY, don't know what your outcome was for this problem but I did a check on my carb today and the damper rod is NOT what stops the piston from rising! I raised it as high as it would go and then unscrewed the damper which made no difference. With the damper removed the piston stops lifting with a solid metal to metal clunk.

Post by **Packedup** » Tue Feb 13, 2007 2:37 pm

Has POD got an HS4 instead of the HS2?

I know various HS4 carbs had different height dashpots, could it be someone's mix and matched a short dashpot piston and damper with a tall dashpot? Not sure what, if any differences there are, just a random though.

Post by **rayofleamington** » Tue Feb 13, 2007 2:43 pm

Hi Fred,
Yes - I found the same on another carb I stripped down - the throttle slide stops 'metal to metal' on the dashpot casting.
At the moment I am assuming that the fault lies with the key in the carb body - if it is too short, it would allow the throttle slide to go beyond the key before hitting the stop.
The only way to check that would be to measure the carb body and compare to others.

It's been very cold (and I've got a cold), so apart from removing and replacing the waterpump and fitting a better radiator, I've not done much to the car.

The roadside bodge-repair of packing the spring to create a stop has been working for a few hundred miles (already) so it may get left like that. This probably reduces the top speed a little, but as the car is in need of TLC, this is not such a bad thing at the moment.