26 replies to this topic

[goat64]

Spec:

 

Mini 850 1976. No mods.

Mechancial fuel pump - chucking out fuel nicely

Vented tank (clear)

SU HS4 Carb

All new Bosch plugs (BOSCH WR7DC+), which are equivalent to the original Champion N9YC

Running on unleaded with valvemaster added.

New coolant, water pump and thermostat stat working fine. ALl flushed through recently - nice and clear.

 

Symptoms:

starts OK from cold (usually!)

once heated up, goes very smoothly for the most part, but intermittently tending to stutter and stall, even when rolling.

This is fixed by pulling out choke a little bit. This clears the stuttering and choke can be returned to hot position with very brisk running for 5-15 mins more.

When you stop, the engine will die spontaneously after a few mins, unless you tap the throttle pedal.

If it dies, or you turn it off, it will NOT restart.

Starter turns over fine (until battery flattens).

There is a spark at all the plug gaps (except the time i flooded it !)

Once it's cooled down a  bit, it can usually be pursuaded to get going again and runs fine, for 15-20 mins more.

 

What I've checked:

I've used the piston lifting pin method to set the mixture to what i think is correct.

Timing, dwell , points, condenser, rotor arm, distributor cap: all good or renewed.

On examining plugs, they look sooty to me - smooth black carbon, but dry. No oil.

I noticed the plugs (all new, and with correct resistance on core, no earth faults) didn't smell of petrol last time i cleaned them stuck at the roadside (something to do while it returns to cold) - that despite winding the starter loads.

Suspecting fuel line / vapour lock, I risked detaching fuel feed to float chamber at carb end and wound starter - loads of pertol coming out.

 

Anxiously delved into carb (new territory for me): needle was a bit rough with deposits - polished gently with cloth, now smooth and gleaming.

Piston rises and falls freely.

Dash pot is clean and smooth and has clean SAE20 oil to required level.

Throttle butterfly opens and closes fully and has no significant play.

Breather tube from tank is free of blockages.

Choke is fully disengaged when in hot position and is set to catch throttle cam as per haynes manual.

air filter is new.

Today I risked removing eth fuel line where it goes into the float chamber when it refused to start again when hot. The engine had been off for a few minutes but the fuel came out of the end of the fuel hose under pressure (about 1 tablespoon full, over the back of a hot engine !!!)

 

So, I don't think it can be vapour lock (I'm going to change the original bare metal tube for an armoured neoprene fuel line just in case). - happy to be convinced otherwise iff the symptoms fit (positive pressure at fuel line into float chamber???)

The fuel pump seems to be delivering fuel to the float chamber.

But I'm not convinced fuel is getting through the carb to the cylinders.

The clues are: 1) misbehaves when hot, 2) positive pressure in fuel line to float chamber even when engine off.

 

It was running absolutely beautifully - about 13 yrs ago, before I SORN'd it.

It passed an MOT last week, including CO2 and HC values in the lower half of the range on emissions (I supsect the test was performed below normal operating temperature). The mechanic who got it through MOT said it was runnig " a bit rich", which surprised me in light of the emissions values.

 

I want to avoid going down the electric fuel pump route if possible - I feel sure there is a problem with the carb somewhere.

 

I've looked at vapouyr lock / fuel pump threads on here - nothing seems to quite fit the symptoms.

 

Where else should I look?

 

Happy to provide more details if needed.

 

Thanks in advance for any assistance.

 

Goat

[Wiggywithahailo]

Hi
Not an expert by any stretch,but if the car has sat for 13 yrs the first thing I would do is buy a rebuild kit as they are pretty inexpensive for the carb and clean it thoroughly

[imack]

Have you checked the valve clearances are correct? Too tight may cause these problems when hot.

[goat64]

OK. Sounds like reasonable advice. My only concern is the only moving parts I can see in the VERY simple HS4 SU carb are the piston and the needle, both of which look clean and are moving beautifully freely, even when hot. The kits I've seen are mostly for twin carbs (mine's a single) and even the few single kits I've seen are £35-£65. If it fixes it back to the way it was, happy to invest in it.  I remember rebuilding a renault 5 carb once - lots of tiny little springs and diaphragms - nightmare! The SU is a much more straightforward looking thing - might give it a go.

Thanks for advice.

[Magneto]

Sometimes a bad coil will cause this, they're pretty inexpensive.....

[goat64]

To imack:

Thanks. I will try this, but would be surprised if anything wrong with tappets, as they were fine when mothballed years ago, and no knocking / tapping under load etc. No-one'e been under the tappet cover since then.

I guess some of them could have gummed up over the years off road. From what you wrote, this might only manifest when hot??

I'll geta  new cover gasket (they always tear!) and take a look.

Thanks

[goat64]

Sometimes a bad coil will cause this, they're pretty inexpensive.....

I thought of that, bute the spark is Sooo strong when cold, and it starts like a bomb unless hot. Even when it fails to start, I've taken a plug out and checked the spark when turning it over - there is a spark there, but now you come to mention it, it doesn't look quite as fierce as when cold. I would have thought any spark would have been sufficient to blow up petrol vapour. I'm still not convinced fuel-air mix is getting that far.

 

I've put the ohmeter across the coil windings (several web sites describe how) and the resistance of 1y and 2y coils seem to be ball-park, and there is no shorting. I haven't tested it when hot mind you - that might be illuminating - I'll add it to my "to do" list and report back.

 

 

Another clue which may mean something to someone clever:...

When the engine speed starts to die before it stalls (or I tap the accelerator to keep it alive, or pull the choke a bit out), there is an odd noise from the engine. The closest I can describe it is like a quiet asthmatic wheeze, but continuous. It seems to come from the alternator end of things, but I could be wrong. It goes away if I can pursuade the engine speed back up to normal idle.  Mean anything to anyone?

 

Thanks to all

 

Goat

[Cooperman]

Check the timing, the condenser, the rotor arm, the plug leads, the mixture and float level, the carb piston operation, the coil and the distributor cap. Check also for an inlet manifold air leak.

 

If it won't start when hot get some 'Instant Start' spray and give it a squirt. If it then starts you have a mixture or fuel delivery problem.

[goat64]

Check the timing, the condenser, the rotor arm, the plug leads, the mixture and float level, the carb piston operation, the coil and the distributor cap. Check also for an inlet manifold air leak.

 

If it won't start when hot get some 'Instant Start' spray and give it a squirt. If it then starts you have a mixture or fuel delivery problem.

Comprehensive list - float level and coil (mentioned also by Magneto) had not crossed my radar until mentioned on here. Will check them too.

 

When you say "give it a squirt" - excuse my stupidity, but where do you squirt it? Down the air inlet?

 

Thanks

[Cooperman]

Yes, just give it a squirt down the inlet and it should fire up.

[midridge2]

The symptons you describe sounds like a faulty float valve, clean it out and check to see if it opens and closes.

[goat64]

The symptons you describe sounds like a faulty float valve, clean it out and check to see if it opens and closes.

This sounds appealing - I've checked the rest of the carb as best I can and all seems well.

One thing though - why would a dodgy float only play up when hot?

It is behaving like a well-behaved thing when cold.

Thanks

 

I'll let you (and everyone who has suggested things) know the outcome once I've had a chance to work thorugh them.

[midridge2]

Heat and expansion.

[Rorf]

You might have a dud tank of petrol - maybe it is old petrol?

[Wim Fournier]

Well, there is the phenomenen of heat build up when you have driven the car to an normal engine working temperature. When you shut it off, there is no cooling no more. No driving wind, the fan is stalled, the waterpump is stalled. So, the heat from the last seconds of driving is now dissipated from the exhaust manifold going upwards, where the inletmanifold sits and the carburettor. 

So when you start the engine again within a few minutes, its inhaling the warm air under the bonnet and sucking fuel from the carburettor bowl that is quite hot. Resulting in total evaporation of the fuel already in the inletmanifold, before passing the inlet valve.

And, as you know at ambient air pressure 1 gram of liquid fuel only takes very little room compared with 1 gram of evaporated fuel.

Isn't that so?

As a result there is also little air (oxigen) entering in the inlet manifold compared with the quantity of fuel, since all the fuel is evaporating already.

In these circumstances you never reach a flamable mixture of intake air. So, every thing else may be perfect, but it won't burn.

 

Now you understand the reason why people in southern countries open their bonnets to let the hot air out of their engine rooms when they take a minute on an parking alongside the highway. They don't wait for the road assistance and they are not mad. They are just experienced.

Your solution is easy. Just permit your carburettor to mix a little bit more fuel into the intake air.

When fuel is evaporating it uses up a lot of calories. The more calories that are taken from the intake manifold, the more liquid fuel will remain to pass the inlet valve, the bigger the chance is that it wil burn there.

When you have set your carb a bit richer, the end temperature of the burning gass in the engine will be a bit lower, so there is less heat to dissipate when you shut off.

May be it would be good to find out if the carburettor is leaking air alongside the axle of the butterfly in the carb. And, yes. have a look if the inlet manifold is leaking a bit air alongside the sealing. Both lead to making the mixture leaner. And you should enrich it. That is what you do when you play the throttle pedal when coming to a halt and that is what you do when you pull out the choke to prevent the engine stalling. You should drive with a little richer mixture.

Tell us the result please.