Lotus Elan

Stromberg needles

PostPost by: miked » Sat Nov 15, 2008 6:49 pm

This is a bit of a continuation from my poor starting investigation thread. Will conclude that when I have smile on my face (problem fixed).

Having stripped my carbs right down I find I have spindle & bush play and also the seals had hardened and cracked.

Such is the volume of work on these that I had look a spare set that had stashed. Having parted these I find them in "like new" condition. Cant believe how good. Make my look like sh*t. Spindle and bushes are sound. Everything is real nice. Only point is that they have the bias adjustable needle that can not be adjusted from the top (outside). Just side screw adjustment by slide removal from carb. Jets are mint and the needle are too. So I thinking I am going to lightly overhaul these (seals and gasket etc).

My original carbs are fitted wih the B2AR needles, top adjustable.

The spare set in good nick are fitted with BIY needles. Having read Miles Wilkins and my webber book (with Strombergs) and found the 13 different dimensions for these two types of needle, I find they are quite close. Miles Wilkins only refers to years of manuacture (and position of balance pipe) not actual spec'. Workshop manual does not even show B2AR. I know that 118Hp SE spec was only uncovered by Miles Wilkins.

I have this 118 HP Super SE spec' with the 40 though head skim and sprint cams (as standard). Off the peg!

If I use the BIY needles will I be ok or loose some performance? Are B2AR required for my spec'?

If so, I dont really fancy doing that conversion business (I have seen on the net) and drilling the slide to fit the B2AR's as if I make a balls I could end up with the "O" ring leaking business from dashpot oil. Maybe I am being a tart!

So can a B2AR be supplied in the non top ajustable form?

I am not swapping air slides on stromberg intruction as I believe the carb, lid and slide are air leak tuned as an assembley.

Can anybody help me here and shed any light on this.


Thanks Mike :)
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PostPost by: oldelanman » Sat Nov 15, 2008 11:02 pm

Hi Mike,
The parts list on the SU website shows B2AR as standard on non-emission versions and BIG with emission engines. My '71 S4 also has B2AR adjustables. I think these were introduced later and the BIY's referred to in the workshop manual were only used on earlier vehicles without adjustable needles.

http://www.sucarb.co.uk/ExplodedImageClassic.aspx?ProductId=8519

The air valves are matched with the covers to give a good sliding fit but are a clearance fit in the carb body so I would think you are OK to use the air valve and cover assemblies from your original carbs with the bodies of your spares, then you can retain the adjustable needles. The needles themselves are spring biased to one side so centering in the jet is unnecessary. I would do a trial assembly and if the valve lifts and drops freely you are home and dry.

Hope this helps.
Roger
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PostPost by: miked » Sun Nov 16, 2008 7:36 pm

Roger,

I will have look and try the fit. I was bit nervous about doing this because of the script with the leak balance screw for two carbs matching (each other) on the air leak around the pistons.

I will have a try. Failing that I may try these carbs with the BIY needles. Whilst they are more painfull to adjust, it is possible.

I searched the archives and only found one post that said about the B2AR's being better than BIG & BIY.

Looking at the needle charts they are for most parts within 1.5 thou'. With my leaking spindles and open mixture screw I'll bet I was all balls anyway.

Mike


:)
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PostPost by: steveww » Sun Nov 16, 2008 10:48 pm

The air valve should be able to swapped over. The rubber diaphragm provides the air seal unlike the SU which used a very close fit between valve and body. The B2AR is the better needle, the B1Y had a slight flat spot on my S4 SE. The B1DG and B1DF are also popular as alternatives but I have not tried them yet.
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PostPost by: miked » Tue Nov 18, 2008 7:12 pm

I had go at swapping the air valve (on one carb) and it felt like it was binding. Tried it with its own top and the screw holes looked out of line and it still did not feel right. The business (from the books) with air leak balance screw is not done to either set of carbs. There is a tapped hole at the front of the carbs but nothing in it. So much for that in the books.

After a lot of thought I decided to keep with my old carbs and keep the top adjustable correct needles. Ordered the bits from Burlen. New spindles, bushes, seals and end caps etc. I have checked with them about the fixed jet height and it is 100 thou'. Bit of reluctance to say. I have refitted the jets at this. Same as the magazine article I mentioned. Both them and the needles are in good condition as they were actually changed.

Just a point about dashpot oil. Peolpe who I talk to keep saying about thinner oils giving a richer mixture for acceleration on application of throttle. This is not correct by my understanding and reading. From the Stromberg book, after description of dashpots;

"When the throttle is snapped open, the immediate upward movement of the air valve is resisted by the plunger and for that period of time until the damping resistance of the oil is overcome, the suction, or depression, at the jet orifice is increased and this has the effect of enriching the mixture".

So the thinner the oil the quicker the air valve reaction but less time with rich mixute to help you accelarate. So thicker oil gives you your equivalent of weber pump jets. Or am I wrong. Sounds right as when the butterfly opens, the suction must increase which must pull more fuel from the jet until balance is achieved when dashopt resistance is overcome and the air valve lifts to correct the mixture.


Mike :) :) I will get there in the end!
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PostPost by: Frank Howard » Wed Nov 19, 2008 3:41 am

Mike,

You are correct sir!
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PostPost by: steveww » Wed Nov 19, 2008 4:03 pm

Yep you are correct.

However if the air valve opens too slowly it slows acceleration and throttle response by restricting the air in to the engine. The trick is to find the right balance :roll:

Oil too thin and not enough enrichment
Oil too thick and not enough air flow

I am currently using ATF in mine but there is slight hesitation on slight throttle openings at low rpm but this could be ignition timing.

An engine is a complicated system and it is never a case of looking at just one thing, all systems should work in harmony.
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