HS2 - problem centring jet

[gs.davies]

Encountered a problem with the carbs…

New Burlen kits fitted and instructions for centring have been followed. However, with the jet higher than 50 thou down the jet holder, the needle sticks. At 50 thou however, the needle and piston move freely. This means that if the mixture needs to be any leaner, it’ll stick.

Is this acceptable? Is there any solution as I thought the jet should go flush with the bridge..

[Peter Laidler]

I fit the needle into the piston. Then assemble the jet but leave it fairly loose in its eccentric housing with a bit of oil to aid its l-r up-down movement. Yes, jet head should be level with piston. That is the datum. So that when the piston and needle drop, the tapered needle will (you hope...) centre the fairly loose jet. You might need to jiggle the adjustable jet around a bit..... But the needle WILL centre the jet. Tighten up the lock-nut and lift the needle via the spring loaded lifter and adjust until the jet drops down with a distinct clunk.

That is the adjustable jet accurately centred on the fixed needle. The needle is tapered so it necessarily follows that as the needle is lifted - by you manually or by the vacuum when the car is running - then the needle will always be clear of the sides of the jet.

If the needle does bind after this - as yours seems to be doing - then it would seem to me to be that the needle is slightly bent somewhere down its length. Easy to test by spinning it in a lathe or drill chuck and hold a mirror against the needle to emphasise any distortion.

It might take a few goes Gary but always remember that it is a FIXED needle and the jet is ADJUSTABLE for a degree of sideways movement in its hole

Just done a load of 'em!

[mk1]

I have found issues like this when people has accidentally (or deliberately in some cases) mixed dash pots, pistons & bodies. Sometimes if you are working with a mismatched set you will never be able to centre the needle accurately.

[Peter Laidler]

Yep, mismatching is a problem as I have found, building a few matching pairs for a big bag of bits last year I couldn't get a couple of them to function properly. I learned many years ago to never but never ever mix up parts of assemblies, especially optical assemblies

[gs.davies]

Been fiddling with these now for a few hours and I’m getting inconsistent results. How can I rule out mismatched suction chambers and pistons? These carbs were unknown to me when I bought them.

I’ve already swapped out a jet bearing on one of them for another I had amongst my spare carbs and that’s improved matters greatly align with an alternative jet bearing locknut.

[Peter Laidler]

Ah, we're getting to the nub of the matter now. The carb set was an unknown or a mix 'n match. You CAN match-up a piston and chamber - or a pair of pistons and pots. But it is more a monkey see, monkey do job a few times before it hits home. I don't think that I am capable of explaining it in writing. If you bring a bag of bits over and a flask of coffee - and a packet of ginger nuts - we'll have cracked it in an hour.

Not a speciality of mine except that I have done a few sets of singles and twins and understand the theory and mechanicals. That said, when centring after getting a decent matching pair of pots and pistons with the body, there is uusually enough play in the jets to cater for..... anyway.

[Dan Tweed grey]

I had one that was a pain to get right, I played with the dash pot and when rotated it 180 it sorted the problem.
Basically they can be a ball ache to do but keep trying.....

[Peter Laidler]

As with Dan, I matched a pick-and-mix piston and pot but discovered that the radial position of the pot on the carb body affected the performance of the piston and needle within the jet. The pot can be bolted down onto the body in any of three radial positions don't forget. Once I'd got it dead right, I dobbed a bit of paint on the body and pot to mark the correct assembly position.

Good informative threads here

[gs.davies]

Ah, we're getting to the nub of the matter now. The carb set was an unknown or a mix 'n match. You CAN match-up a piston and chamber - or a pair of pistons and pots. But it is more a monkey see, monkey do job a few times before it hits home. I don't think that I am capable of explaining it in writing. If you bring a bag of bits over and a flask of coffee - and a packet of ginger nuts - we'll have cracked it in an hour.

Not a speciality of mine except that I have done a few sets of singles and twins and understand the theory and mechanicals. That said, when centring after getting a decent matching pair of pots and pistons with the body, there is uusually enough play in the jets to cater for..... anyway.

Thanks Peter, if you were nearer I’d be round with them, armed with all sorts of biscuits!

I’ve managed to get them working (for now) but I think I’ll order up a pair of new jet bearings, new spring and needles (which I really need anyway) and see if that improves matters. The rate at which the pistons fall isn’t matched either, not miles apart but with unknown springs, can’t be sure it’s right.

I think the other remaining variable is the cleanliness of the of inside of the pot and the piston. They now don’t stick or jam but sometimes the piston seems to hang up a little on the inside of the pot.

[richardACS]

I don't know where you are in Essex - I'm not far off Junction 25/26 on the M25..

When my car arrived in boxes from New Zealand I discovered around 6 peices of the HS2's in various states of assembly. So if you need to find a unit with a smoother action you would be welcome to see if one of these might better suit?

[andy1071]

There is enough alignment available in the jet to allow for the possible positions of the piston in the carb body, so if the piston moves correctly with the dashpot fitted (but without the needle), then any mixing of pistons/parts won't be the problem.

You quote 50 thou for the jet distance down, I'm not sure how this relates to the "number of flats" on the mixture nut... (the usual method of mixture setting)
But, it is unlikely you will have less than "6 flats" / 1-turn down...
I centralise the needle with the jet flush with the bridge, so it's not ideal to lower the jet before centring it...

As a bit of a compromise, you could move the needle slightly (and I mean only a small amount, say 20 thou) into the piston..?
Then you wouldn't have to move the jet down so much...

Aren't there 2 sizes of jet..?
I thought there was something like 90 thou and 100 thou diameter... (or am I thinking of something else completely.... )

Of course, this is working on the assumption that you have matched (or at least, functioning) parts