Needle valves

[Logg]

Hi people I've got a few questions about the needle valve inside the GTT carb!

Im I correct in thinking the stock 1.7mm becomes the limting factor at arounnd 200bhp and 105 mph trap speed?

Would there be any running issues with putting a good sealing 2mm needle valve into a carb that didn't need it yet?

What power/trap speed will the 2mm needle valve be at it's limit?

From what I've read after 2mm it seems a 2.3-2.4 seems to be the limit that the float can hold the needle shut!

Is there anyway round this?

Random ideas off the top of my little head

1) Would a float from a carb that runs a stock 2mm or 2.5mm be able to hold it shut if some how fitted into a gtt carb?

2) Some how filling it with a more buoyant substance than it already is inside the float?

3) maybe have a custom float made ?

[Mr 5]

i shall be keeping my eyes peeled on this post,good one logg

[Rob@Backyardracing]

Yes a 1.7 will limit you at around that power on a good pull of course (eg 1/4 mile) not a single gear pull on a RR...

A good 2mm sealing needle wont course any issues on any car from the cars ive tuned and seen over the years...

2.3-2.4 is the limit on the needle and what it will seal, much after that it just wont seal unless you modifi a larger 2mm needle, then the float is unable to seal the extra flow behind the needle much after 2.3-2.4 even with tried and tested methods...

As said before, there are other better ways then looking in to a better float (its been tested alot over years by me and scoff and other members i guess) a raising rate fuel reg is your better option and increase the fuel pressure on boost...

[Sparkie]

i always used to blow up at anything over 19psi at the pod.

what i will be doing to keep the float chamber topped up is one of a few things.

either boost operated solenoid with a 1mm jet to feed fuel into the carb base.

or, my favourite... just use a grainger valve/ dawes device (although you would have to solder up the small hole they seem to have), set the open pressure to be about 20 psi, which would correspond with about 15-16psi of boost + 4psi fuel pressure. the ball internally doesn't allow fuel to flow backwards, its fairly cheap, and mechanical (so less to go wrong...)

this way you dont need to worry about large needle jets etc etc.

[Logg]

Thanks for the reply Rob!

Shame a 2.5mm needle is a no go since I bought one to try when I bought the 2mm for me and the lads. Will post some pictures of them when home later!

So if I ever get to the dizzy heights of needing more fuel to try a bit more fuel pressure!

[Mr 5]

on the needle subjecy, if i drill it out to 2mm is that all i do? ie do i just take the top off the resevoir and drill the whole in the valve to 2mm? that simple?

[car.crash]

take the needle valve out the carb then drill it carefully, or just buy a 2mm one off ebay for a few quid.

[Logg]

i always used to blow up at anything over 19psi at the pod.

what i will be doing to keep the float chamber topped up is one of a few things.

either boost operated solenoid with a 1mm jet to feed fuel into the carb base.

or, my favourite... just use a grainger valve/ dawes device (although you would have to solder up the small hole they seem to have), set the open pressure to be about 20 psi, which would correspond with about 15-16psi of boost + 4psi fuel pressure. the ball internally doesn't allow fuel to flow backwards, its fairly cheap, and mechanical (so less to go wrong...)

this way you dont need to worry about large needle jets etc etc.

Cool a different way round the problem! sounds interestinig!

Some pic's of the Ebay needle valves

[dangerous dave]

there nice, wish i had one........


oh wait, i dooooo

[Mart]

Another way of getting more fuel to the carb would be to 'T' off the main feed from the pump (ie, pre-reg'), run the pipe to a boost triggered solenoid (you could use the over-boost sensor as the 'trigger'), then through a restrictor (something like a main jet would do), then 'T' back into the carb fuel inlet line (post-reg').

Set the over-boost sensor to 'trigger' the solenoid open at say ~20psi, and job done.

Obviously you'd need to play around with the restrictor size, so that A) you get enough fuel present, and B) that it isn't excessive enough to 'flood' the carb, but that's not hard to suss out - A wideband meter would help there.

If 'A' was the case, and the afr's were leaning off again, simply increase the size of the restrictor hole (say +0.1mm at a time).

And if 'B' was the case, you'd probably either see the afr's go silly rich, black smoke from the exhaust, and/or the engine would start to splutter/misfire/hold back, hence the need to then reduce the restrictor size.

[Logg]

I'm a bit confused Mart

What you describe is this away to increase fuel pressure at a x-psi or do you mean to tap the main jet into the carb top to help keep carb bowl full?

[Scoff]

mart, I don't think that would have any effect, pressure before and after the regulator is the same.

[Mart]

Scoff, pressure pre-reg' would be pump output pressure. Pressure post-reg' would be 4psi + boost pressure.

Logg, what I mean is that you'd be effectively bypassing the fuel reg' when the solenoid opens, hence pump output pressure would be at the restrictor, which would then additionally feed the carb fuel inlet/float chamber.

[Scoff]

mart, it doesn't work like that mate. the pressure pre reg is the same as post, just like an EFI car. look at the cross section of a gtt regulator, in and out come from the same chamber, reference pressure from above acts on the diaphram to block the return. the more pressure you apply to the reference the more the return is blocked, and pressure both before and after the regulator is increased.