12 replies to this topic

[jakejakejake1]

Hi Everyone,

I have had this problem with my car for a while now and its starting to annoy me so thought I would ask.

Basically whenever I let off the throttle, the revs drop well below the normal idle speed (below 4-500), then gradually pick back up again (up to 20 seconds normally), but sometimes they drop right off and the car stalls, obviously this can cause some problems when driving haha

So what do you think could be causing the problem?

I'm running a HS4 carb, the idle speed is ok (900-1000ish), there is oil in the dashpot (20W50) and the car starts fine and drives fine on throttle, its just when letting off the throttle that I get problems.

Cheers,
Jake

[mk1leg]

have you ajusted the mixture properly.....................

[se_juggles]

sounds like it could be a vacuum problem.... there may be a air leak. make sure the breather cable is connected, oil cap is on properly, hoses dont have any breaks in them etc etc

[CityCharlie]

Silly question, is your dash pot spring present?

[MiniAbbott1993]

Why is it people use engine oils such as 20w50 in their dashpot?
I had the same problem not so long ago with my mini.
Turns out my car ran far better with the piston damper loosened off a bit.
After taking the piston damper out it was clear that an engine oil was used.

I changed the oil for a much lighter oil (similar to the classic haynes everyman)
Oh and make sure its filled correctly, the volume of oil should be about the same as the volume of the barrel on the end of the piston damper.
After doing so and refitting the piston damper mine didn't have the problem, may work for you?
I guess with a lighter oil the piston is able to move more free..
Anyway I won't be using anything but a light oil from now on, it seems much better

[freshairmini]

Why is it people use engine oils such as 20w50 in their dashpot?
I had the same problem not so long ago with my mini.
Turns out my car ran far better with the piston damper loosened off a bit.
After taking the piston damper out it was clear that an engine oil was used.

I changed the oil for a much lighter oil (similar to the classic haynes everyman)
Oh and make sure its filled correctly, the volume of oil should be about the same as the volume of the barrel on the end of the piston damper.
After doing so and refitting the piston damper mine didn't have the problem, may work for you?
I guess with a lighter oil the piston is able to move more free..
Anyway I won't be using anything but a light oil from now on, it seems much better

Really depends on the engine and its spec. A standard engine would work better with the normal "lighter" SU oil. Where as a more tuned engine, like mine, has worked with 20w50 better. All depends on the engine, so no one solution fits all. same with changing the spring in the dash pot that will have an effect as well. I think you can even change the piston for a different damping rate!

But you are right, its worth a try at least, as well as checking or any air leaks.

Edited by freshairmini, 11 December 2012 - 10:40 AM.

[jakejakejake1]

I have tried a lighter oil in the past (3in1), but found that it hesitated on acceleration and the thicker 20W50 help and acceleration was much better, the dashpot spring is definitely there, and I have had a brief look for vacuum leaks but have not found any yet - but will have a more in depth look later.
The thing that makes me think its not a vacuum leak is that after a while it idles fine, and when driven lightly it idles fine too, but when accelerating then letting off the revs drop really low - suggesting its the piston/ piston related.

And as far as I can tell the mixture is ok, but with the recent cold weather maybe a flat or so richer would help?
I might take the dashpot off later and give it a clean as well just in case, then put some nice clean oil in there.

Cheers,
Jake

[RetroRay]

It does sound like its a vacuum leak/air leak, ive had similar problems in the past alot of the time it was down to the joints on the vacuum pipe or the pipe itself thats been split or it was the pipe thats connected to the breather on the back of the engine to the carb.

[freshairmini]

I've heard that WD40 is useful for finding leaks, as the when the oil is drawn into the crack or leak area it causes a momentary raise in the idle speed. worth a go...

[jakejakejake1]

Well I went out and had a look, and I think I have narrowed down the search a bit:

Tried cleaning the dashpot and putting more oil in - didnt help
The old 'wd-40-around-the-inlet-manifold' trick didnt seem to find any vacuum leaks there
The breather hose from the engine to carb seems to be causing the problem, as when I pinch it the idle speed rises, and there is no drop after revving, and it didn't seem to matter what part of the hose I pinched - same results. Which indicates a leak where it joins the breather, but the hose seems fine there and is still connected fine. Also taking the oil filler cap off seemed to affect the idling as well.
The plot thickens...

[se_juggles]

Sounds like the little L shaped rubber connection may have a little hole in it? Just buy a new breather cable as there not very expensive. And taking the filler cap off will effect it... all air leaks do. My stupid chrome rocker cover had a cap that poped off lol x

[CityCharlie]

In a futile attempt to not divert the thread too much regarding dashpot oil, some cars of mine have worked well with ATF, where others have not (Stag when it was on Strombergs was fine on ATF, my old Dolomite Sprint was great on ATF in comparison to 20W50 oil) although my Spitfire seemed more suited to 20W50 in comparison. One classic car garage I know of swears by using 20W50 in the dashpot of any car with an SU carb.

IMO it depends on your driving style to a degree as to what suits you. My current Mini seems to be running on ATF, although it is purple in colour (could it be specific dashpot oil?) I have to say, whatever it is (it seems similar in viscosity to ATF) it does shift very well IMO for a stock 998.

[Ethel]

The oil filer is a restricted air inlet so the crankcase pressure is lowered (air goes out the carb connection than it gets in).

Rover specified engine oil in the dashpot, so it's the best starting point. It should only effect the mixture under acceleration (like the accelerator pump on Webers etc). The thicker the oil; the slower the piston rises ; the faster the airflow across the jet; the greater the Bernoulli effect; the faster petrol flows; the richer the mixture.