how many bubbles?

[stuffedpike20]

How many bubbles can you have in your petrol before it affects the running of your car?
I have a symptom which makes me wonder if I have got a split in my fuel pump diaphragm. I drive along up to a steady 30 mph , then it is as if I have jabbed my foot on the brakes for a split second. Slowing down, everything is ok again. I am wondering if the float chamber is not filling fast enough. I have not tried any roads faster than 30.
All joints are tight, but when I pump some petrol into a jam jar there is a stream of tiny bubbles. The pump is ticking as normal, not any faster.
Any ideas? Pump is a 6 year old genuine SU.
Thanks, John.

[geoberni]

IMHO, bubbles in the feed from the pump can't have any affect on the mixture being ignited since it is then held in the float Chamber on the carb for a minute or so before being atomised into the engine.
With the viscosity of the fuel, the bubbles will disappear in seconds once in the float chamber.

But I'm happy to be corrected if anyone has evidence to the contrary.

[stuffedpike20]

Thanks berni
I wonder what is causing the jab on the brakes symptom then?

[geoberni]

I should add that there have been times when I've taken samples of fuel from the pump, I've even drained the last gallon out of the tank to check how accurate the gauge is and it's always looked 'bubbly' when coming out.

[myoldjalopy]

I tend to agree with 'geobernie'. If the pump is good it will pump as fast as it needs to keep the float full where any bubbles will 'pop'. It might be worth checking the carb moves freely and there is sufficient oil in the dashpot but it is more likely to be an ignition issue.

[stuffedpike20]

Thanks myold.

[les]

Your description makes me think of an electrical issue, a momentary loss of spark.

[stuffedpike20]

You may be right Les.
I will replace the points tomorrow with a new old stock set.
Previous ones were in a Lucas box. Probably made in China.

[myoldjalopy]

When you replace and gap the points it would be worth doing a few other checks - make sure all electrical connections in the ignition system are firm and clean, examine the condition of the king and plug leads, make sure the dizzy 'pigtail' lead is in good condition. Examine the plugs and their gaps and check the dizzy cap, the four metal segments and the spring-loaded carbon bush in its centre.
If all these look fine and the new points don't make a difference, I would swap over the condensor, and then, if still no good, the rotor arm. If you do buy any new ignition parts, Distributor Doctor makes the best.

[stuffedpike20]

Thanks myold, very helpful!

[oliver90owner]

Did changing the points make any difference as I cannot imagine any scenario where this could be the case, if the gap is correct? They are, after all, only a simple switch in a DC circuit.

Small bubbles of light alkanes will form in fresh petrol under the slightest reduction in pressure and will unlikely redissolve in the liquid fuel. A consequence of modern fuels - and one reason why fuel can deteriorate quite quickly these days.

[geoberni]

Small bubbles of light alkanes will form in fresh petrol under the slightest reduction in pressure and will unlikely redissolve in the liquid fuel. A consequence of modern fuels - and one reason why fuel can deteriorate quite quickly these days.

People say that, yet I have a 2 Stroke 30cc Garden Power unit that accepts different attachments, Strimmer, Tree Pruner etc.
The instruction booklet says to use 'fresh fuel'.
Seeing as I don't use it much, I'm just getting to the very end of a 5L mix I made up 3 or possibly 4 or 5 years ago and it's started every time I've used it.
I've not been out in Basil this year at all for a proper long drive on the open road, so the gallon or so left in the tank is from about a year ago. He still starts first time, every time, I give him a start and move him out of the garage for a quick drive in the village as a sort of mini road check for brakes.

[oliver90owner]

I find no problems with a securely sealed plastic container (can’t lose too much without the pressure increasing) but vented containers can definitely be a problem - especially if the fuel gets quite warm!

[stuffedpike20]

I replaced the points yesterday, but have not test driven yet.
I agree that points may not be the problem. I thought about why the points might fail, and could only think that maybe the gap would close as the plastic heel wore away, so not really a failure. And that would mean that the car would not run, not just jolt and carry on. Could points fail in any other way?
The fault could well be elsewhere, if the problem is jolting then carrying on.

On the subject of petrol; my mate bought a lawn mower last year that will only run on pure petrol, at £25 a gallon. If he puts brand new unleaded in it, it will not start. The manual states pure petrol only, which has to be bought from a lawnmower shop. I think the mower was mostly produced for the New Zealand market, he bought it because of its size and weight (he has to carry it down steps to get to his lawn).

Thanks, John.

[myoldjalopy]

Well, I suppose anything is possible but I agree it is unlikely the points are the problem. Points trouble usually manifests as difficult starting as they wear away. To me, the jolting suggests an intermittent failure of a connection or component in the ignition system and, if the problem is still persisting, I would next undertake the checks I suggested earlier.

[philthehill]

https://www.bartrammowers.co.uk/what-is-aspen/

[geoberni]

https://www.bartrammowers.co.uk/what-is-aspen/

Nice to know.

Thumbs1.jpg (11.91 KiB) Viewed 1383 times

Back to John's issue though, I'd be thinking of an intermittent electrical connection somewhere on the ignition circuit.

[stuffedpike20]

Thanks guys.
Might go for a drive today. Will let you know.

Just a thought, I have got silicone HT leads. Are copper better?

[geoberni]

Thanks guys.
Might go for a drive today. Will let you know.

Just a thought, I have got silicone HT leads. Are copper better?

I think you'll find Silicone refers to the insulation, rather than Rubber or Plastic, not the conductor core.

Old HT Cables were Copper strands back in the day, and that was why they interfered with the radio, once radios became popular.
Modern leads tend to compressed carbon, spiral wound copper/carbon fibres and/or other modern conductive materials.

Traditional Copper HT cables are best left for far older vehicles using magnetos that need the low resistance.

Sit's back and waits to see if anyone disagrees....

[stuffedpike20]

Every day is a school day.
I vaguely remember some years ago buying a set of silicone leads that arked (?) from new. I could see them arking if I lifted the bonnet in the dark. They were expensive too, with 'bendable' ends. Took them back for a refund.