Float valve

[Marty ULC]

Are there any disadvantages to having too large a float needle valve? I.e. flooding or wasting loads of fuel through the overflow etc.

What are the symptoms of having it too large or too small?

[Knowledge]

There are fuel starvation issues with having a too-small float needle, which can manifest themselves in detonation noises from the engine (like having a 10mm spanner rattling around inside an empty leg-shield tool box).

I have also wondered if there were any issues with having a float jet which is too big. If there are no issues, then why do you need a small float jet?

[johnnyXS]

I would imagine the sensible answer to that question is likely to be ' choose the size of needle valve jet that is appropriate for your highest engine demands with a small extra safety margin..'

If the float needle jet is incapable of flowing sufficient fuel at WOT at max speed then it needs enlarging . This assumes that there are no flow restrictions between the tank and fuel bowl of course.
I read somewhere (probably MB Dev site) that some of the early 22mm Jetex carbs were supplied with very small restrictive float needle valves and needed to be drilled out to 2mm dia

If the jet was too large it should still perform as well as a small valve and shut off properly but that said I've never tried it. You might find at slow speeds and low fuel demands that a larger valve might not control the fuel flow precisely enough

A simple high speed test would soon tell you if the engine was getting sufficient fuel flow.

[Warkton Tornado No.1]

Unread postby Knowledge » Wed Jun 15, 2016 12:55 pm
There are fuel starvation issues with having a too-small float needle, which can manifest themselves in detonation noises from the engine (like having a 10mm spanner rattling around inside an empty leg-shield tool box).

I have also wondered if there were any issues with having a float jet which is too big. If there are no issues, then why do you need a small float jet?

I often wonder if Lambretta owners are the only users of Dell’Orto products, but that is not so. Apparently….

But even some Lambretta engines use pulse pumps to deliver fuel to the carburettor. As a consequence, smaller float valves are a necessity to avoid engine flooding.

[johnnyXS]

But even some Lambretta engines use pulse pumps to deliver fuel to the carburettor. As a consequence, smaller float valves are a necessity to avoid engine flooding.

which classic Lambrettas use 'pulse pumps ' ?

[Marty ULC]

I would imagine the sensible answer to that question is likely to be ' choose the size of needle valve jet that is appropriate for your highest engine demands with a small extra safety margin..'

Yes but how do I tell what's appropriate?

I've just gone from a 250 to a 300 on a phbl25 and I'm wondering if it's way too big. I'm a bit paranoid about my float bowl running dry on the autobahn.
Main jet is a 108.

[Eden]

I use 400 seat valves in all the phbh carbs I use on the motors I build, there is nothing worse than killing a top end due to fuel starvation at high rpm!

[Warkton Tornado No.1]

by johnnyXS » Wed Jun 15, 2016 3:25 pm

Warkton Tornado No.1 wrote:


But even some Lambretta engines use pulse pumps to deliver fuel to the carburettor. As a consequence, smaller float valves are a necessity to avoid engine flooding.



which classic Lambrettas use 'pulse pumps ' ?

Having answered a question, I’m not sure what your point is as I'm not certain what your interpretation of 'classic' may be, having not used the term.

However, in response to your question:

Various Group Five & Six ‘classic’ race bikes utilised pulse pumps years ago as they had the fuel in the frame. Similarly, ‘classic’ sidecars needed some kind of pump.

[johnnyXS]

Having answered a question, I’m not sure what your point is as I'm not certain what your interpretation of 'classic' may be, having not used the term.

However, in response to your question:

Various Group Five & Six ‘classic’ race bikes utilised pulse pumps years ago as they had the fuel in the frame. Similarly, ‘classic’ sidecars needed some kind of pump.

thank you.
However I thought we were discussing a road going scooter with the op so I was surprised that you brought up electric fuel pumps ?

I have a 750cc 4x stroke bike with twin 38mm carbs and they feed quite adequately by gravity feed at 100mph+.so i can't see the need for an electric fuel pump for a scooter

[Marty ULC]

So in short I shouldn't have a problem with the larger float jet.
Unless I get even more paranoid than I already am about fuel starvation and decide to fit a pump

[johnnyXS]

paranoia is good Marty

I'm really paranoid about a too weaker mixture when deaccelerating . I quite often slip the clutch and keep a little revs on if I drop from flat out to standstill.

[Knowledge]

However I thought we were discussing a road going scooter with the op so I was surprised that you brought up electric fuel pumps ?

WT1 didn't bring up electric pumps, you did Johnny. The pulse pumps works off crankcase primary compression to push petrol into the supply loop to the carb, but it does not need electricity

I have used one on a fuel in the frame race bike and I have seen them used on large frame Vespas where there is little head on the standard tank.

You can also get diaphragm operated fuel taps. Although they use similar principles as the diaphragm pumps, they are just a fuel tap that will only flow fuel when the engine is running. I have one on my road scooter, so I don't need to turn the fuel on and off.

[Warkton Tornado No.1]

Knowledge » Wed Jun 15, 2016 10:10 pm

johnnyXS wrote:
However I thought we were discussing a road going scooter with the op so I was surprised that you brought up electric fuel pumps ?



WT1 didn't bring up electric pumps, you did Johnny.

shoot.JPG

[johnnyXS]

However I thought we were discussing a road going scooter with the op so I was surprised that you brought up electric fuel pumps ?

WT1 didn't bring up electric pumps, you did Johnny. The pulse pumps works off crankcase primary compression to push petrol into the supply loop to the carb, but it does not need electricity

I have used one on a fuel in the frame race bike and I have seen them used on large frame Vespas where there is little head on the standard tank.

You can also get diaphragm operated fuel taps. Although they use similar principles as the diaphragm pumps, they are just a fuel tap that will only flow fuel when the engine is running. I have one on my road scooter, so I don't need to turn the fuel on and off.

yes that was a typo ...I meant pulse pumps, which was what was being discussed.

However you and that verbose wittering walkton have taken this thread completely off topic for your own ends and your posts are unhelpful and irrelevant to the OPs question.

Why don't you both add something to the thread that addresses the OPs question and is helpful (if you can )... instead of taking every opportunity to make personal attacks on other members who are trying to be helpful.

As your posts are unhelpful I have added both of you to my 'foe' list so I don't have to read any more of WK's inane rambling so you'll both be wasting your time adding any further unhelpful off topic comments for me to read because I'll no longer see them

[Marty ULC]

'Kinell that escalated quickly.
Best I don't ask any questions about oil...

[johnnyXS]

now oil....well......deep breath

I know its silly really . Someone challenges something you say and you get all irate and upset over nothing and before you know it you've got some kind of personal vendetta going on and it all becomes very unpleasant.

At the end of the day we are all trying to help each other the best we can and it helps no one to start being critical of anothers contribution .
Would you like some more popcorn ? ......

[Warkton Tornado No.1]

by johnnyXS » Thu Jun 16, 2016 8:28 am

As your posts are unhelpful I have added both of you to my 'foe' list so I don't have to read any more of WK's inane rambling so you'll both be wasting your time adding any further unhelpful off topic comments for me to read because I'll no longer see them

Thank f*ck for that!

[kennystag]

Jonny boy xs, is that you being childish again, I find the tornado very helpful, he takes time to answer our questions, maybe you are finding his comments to technical for your level of engineering skills.
Cheers
Stag

[johnnyXS]

Sour grapes Kenny ???:lol:

You pm'd me some months back and asked me for my help and advice which I freely gave .......giving you much of my precious free time for several weeks.

It was only after you continuously neglected to answer my questions and ignored much of my suggestions and advice that I told you I could no longer help you.
Now you are taking the posters thread off topic to vent your spleen .

[kennystag]

Jonny boy sx,
One of your problem is you are always right and everybody's suggestions are wrong, I do not know what the other problem is, as I am not qualified to answer that one . Yes it was all going well until I made a suggestion and you shot me down. I did answer you questions, but you just did not want listen if it was not to your liking. You gave me grief to the Point I told you not to bother emailing me again. And you are doing the same on the forum, they are all wrong and you are right, you just do not like to be told. You go off on one when I make a suggesting, I have many friends on the list that are very helpful and listen to what I have to say. That is all I am saying on this.