Brakedown - Carb problems ?

Post by **vincenth** » Mon Feb 05, 2007 1:55 pm

I had the pleasure of breaking down on a dual carriageway but luckily for me I stopped directly behind a 3 car crash which was being attended to by a police car so I was protected from the worst of the traffic. I was going along quite happily in the very heavy traffic (idle speed when in neutral was very high though) and as I was pulling past the accident she started to splutter/chugging. I pulled over to have look and she wouldn't start again - it sounded like misfiring. It also 'Ran on' when I switched off the ignition. Battery was brand new, new spark plugs, new air filter. The dials on the dash said everything was fine, water temp was ok, amps was ok, oil pressure was a bit low but nothing unusual. I've got an alternator which is new so that was fine as was the fan belt.

After about 10 minutes an AA van passed and stopped to have a look. The nice chap asked me to turn her over and she worked first time. How embarrasing was that. He then escorted me about 300 yeards to the next slip road where he had a quick look. His opinion was that it wasn't getting enough air in the heavy traffic and in effect got flooded so it looks like a tune up is needed. I had a quick look at the carb and the oil in there is pretty much thick/congealed. Should it be like this this ?

I've never done anything with a carb before so I don't know how to tune it up or how it should run. I'm not very good on the mechanical side so shoudl I be able to tune it myself or should I get somebody to tune it for me - can you trust a garage to do it. I had a look at my small autodata book but that doesn't help with what it shoudl sound like. I suppose that is one of the downsides of not having another moggie to compare it with.

Any help appreciated.

Post by **alex_holden** » Mon Feb 05, 2007 2:45 pm

If the oil in the dashpot has congealed, take it off and clean it out thoroughly with petrol or white spirit or something, then put it back together and refill it with new engine oil. The tuning procedure is described in this old thread.

Post by **bmcecosse** » Mon Feb 05, 2007 7:27 pm

If you had been going along very slowly in the heavy traffic - it may have been the dreaded fuel starvation which some on here report. Usually this is in the high heat of summer - and not in the depth of winter. Did you notice if the fuel pump was 'racing' - rapid tick-tick-tick noise ? Otherwise - it could have been the beginnings of the pump failing - and after a wee rest it got going again. Or (I know - there's lots of possibilities) it could be condenser or coil overheating - again they recovered after a wee rest. Check out the pump first - if it seems healthy - then consider an ignition system overhaul. The parts cost hardly anything.

Post by **rayofleamington** » Mon Feb 05, 2007 7:42 pm

Fuel vapourisation causing fuel starvation is pretty common, but only on hot days, when the underbonnet temperature gets very high - only after a long time in a traffic jam / queue.

Fuel starvation itself is extremely common - normally when the petrol pump decides to 'have a rest'. This is followed by the feeling of running out of petrol (stop start jerkyness, and only running with a lot of throttle for the remaining seconds before it dies completely).

Running on and missfiring, however are symptoms usually related to completely seperate faults (i.e. not a short term lack of fuel) Running on can have many causes but generally things linked together with overheating

Congealed oil in the dashpot damper - 2 screws to remove the dashpot. Remove the spring and then the throttle slide.
Turning the throttle slide upsidedown will normally empty out all the oil but if it's thick then maybe use wd40 or petrol to wash it way.

Make sure everything is VERY clean before re-assembly.
Put some new oil in (20/50 is commonly used)

To check opertation:

lift the throttle slide using your finger inside the carb mouth (from where the air filter would sit)
The piston should rise slowly against some resistance (the oil acts as to damp the movement)
When you let go the piston should 'plop' back down all the way. If it gets stuck part way, more cleaning may be needed.

Post by **Axolotl** » Mon Feb 05, 2007 11:12 pm

As above re cleaning the oil out and topping up with fresh, but make sure you mark the dashpot before you remove it so you can put it back on the same way round. Wrong way round and the piston will bind and you'll never get it to drop with a satisfying thunk.

Also, unscrew the cap and take the dashpot piston out before you dismantle the dashpot from the carb.

I'd prefer meths or petrol to clean it with, as WD40 leaves a deposit that isn't supposed to be there. Reassemble with bare metal surfaces only, no oil except where it should be, inside the central bit after you've put it all back together.

Post by **bmcecosse** » Tue Feb 06, 2007 7:54 am

Gasp - shock - surely not!! 20W50 in the dashpot!! Surely only ever ever use the official SU damper oil in the carb - after all that's what it is designed for !!!
If that theorey is good for the dampers - it must be good for the SU too ? Yes ? No ?
Just to add - the Works used EP90 in the Rally Mini SU dashpots!!

Post by **bigginger** » Tue Feb 06, 2007 1:49 pm

Not quite as many stresses transmitted through the dashpot as there are through the dampers, so no, oddly enough.

Post by **rayofleamington** » Tue Feb 06, 2007 11:08 pm

Gasp - shock - surely not!! 20W50 in the dashpot!! Surely only ever ever use the official SU damper oil in the carb
If that theorey is good for the dampers

Fortunately 20/50 will give pretty similar results to the OEM dashpot oil, and is not a likely cause of complete front suspension failure / loss of steering.

Post by **bmcecosse** » Wed Feb 07, 2007 7:47 am

Neither is 'adjusting' the damping rates in the Minor dampers !

Post by **bigginger** » Wed Feb 07, 2007 4:53 pm

And yet it has happened. And yes, we know it hasn't to you, and it's anecdotal, but not a risk I'd be prepared to take

You've never had a wheel stud break either, but it doesn't alter the fact that they do.

Post by **Rasputin** » Wed Feb 07, 2007 5:02 pm

According to my workshop manual, they recommended 20/50 for the carburettor.

Post by **rayofleamington** » Wed Feb 07, 2007 6:08 pm

According to my workshop manual, they recommended 20/50 for the carburettor.

So do most people - although it seems Roy only recommnds the SU original oil, despite 20/50 not having any increased-risk-of-major-accident side effects when used in the dashpot damper.

Post by **bmcecosse** » Wed Feb 07, 2007 9:40 pm

It 'may' cause the piston jam just when you need that sudden burst of acceleration to escape from the wheels of a large truck!!
I was of course only stirring up some interest with my original remark - and it worked!! But it is true that the thinnest oil that works without 'gasping' from the carb should be used to get the best throttle response. Indeed - years ago there used to be a 'quick lift' conversion that was done on carb pistons - involved drilling new holes and plugging others - but 3 in 1 works just as well! The 'Works' used thick EP90 in the rally Minis simply to stop the SU pistons bouncing up and down on rough special stages - this had been causing the engines to splutter and lose power - and was a 'fix' that worked on rough roads - but did reduce the throttle response.