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Dodgy diaphragm?


Iain T
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Dear Forum

Still trying to sort out why my Vitesse Mk2 is a pig to start from cold. I have managed to work out how the choke mechanism disc on the CDS is oriented so that when rotated fuel goes through the disc slot and then the (now cleared) two sets of graduated holes let fuel through to enrich the carbs. But on refitting the choke mechanism I noticed the piston on the front carb is not sitting on the bridge but some 3+mm high. The rear piston sits on the bridge. I took the caps off both carbs and both pistons fall to the bridge but as soon as I put the caps on and lift the pistons the front piston does drop to the bridge with a clunk but then 'rebounds' back to about 3mm high of the bridge. The rear carb piston drops, clunks, and stays there. Although this may not resolve the starting problem but to me both pistons should be equal??? I have seen posts saying the thickness and quality of the diaphragms varies but both are from new Burlen kits. I have double, double checked the diaphragm tabs are correctly located it just seems the front diaphragm is too strong.

Am I being too nitpicky?

Iain

front carb diaphragm.jpg

front carb piston.jpg

rear carb diaphragm.jpg

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thats strange, think the piston should definitely sit on the bridge. My carbs are older than yours and if the piston doesnt clonk down nicely it means the jet needs centering because the needle is binding on it. However Ive never had a rebound problem - your carbs do have springs pushing down on the pistons dont they and oil in the dashpots to dampen the movement? 

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Yep, jets are centred and springs fitted, although I haven't checked they are the same length, strength etc. Just read Pete's posting on 'spot the carb' thread and it seems I have one diaphragm made from 'elephant condoms' because it clonks down then rebounds. Even with the dampers out the front piston is harder to push down than the rear. I should never have changed the old ones they were gossamer thin and no holes, the new ones are from WW2 condoms!

Burlen are supposed to be a good supplier, any idea where I can buy some thin and floppy diaphragms as I threw my old ones away.....grrrrrrrr.

Iain

 

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Just spoken to chap at Burlen who said 'never heard of that before and I've been here 12 years'. Well he has now!

I changed over the front and rear piston/diaphragm assembly and the rear carb with the front piston assembly drops then bounces back..? The front carb with the rear piston assembly drops down and stays on the bridge so it must be a problem with the front diaphragm. With the diaphragms removed both pistons drop to the bridge. I assume as both kits came from Burlen the diaphragms are made in the same injection mould tool but perhaps the material mix varies giving different properties.

I have ordered one new diaphragm from Burlen as I don't know what else to do. Shouldn't have played with the carbs but I get bored and I wanted to give them a good clean up so I thought why not. Well they are now cleaner but don't work properly!

Iain

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go and search  there were some OE spec on ebay in packs of too many  doug bought some 

cant believe burlen have not come across this its pretty fundamental   ( experts   ah  wot)

one more tip if they have gone baggy and swollen a wash in petrol should revert them to size   ... wont help thickness 

they must be gossamer flimsy or you do get just what you have found    .... aftermarket crap 

doesnt solve the problem but you may find with the engine running they will get pulled down    live in hope 

pete

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I have had an issue (as have others), where even though the jets are centered a piston wont drop freely and this was using the thin diaphragms. Don't know what causes it.

Maybe worth trying for your issue. When screwing the top caps down, try and do it equally and in small amounts of a turn while checking the piston still drops at each bit of a turn. If it sticks then try backing off other screws a tad until it drops. It's a bit time consuming, but has worked for me and others. 

Edited by daverclasper
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Just remembered today is Sainsbury's day! There's also a Halfords on the same estate.

You can see from the photos the ridge in the diaphragm is a series of flats (remember the old threepenny bits?) so to my mind this gives more resistance to the required up and down motion of the piston. This ridge should be round in plan view to give even resistance. This raised piston and dodgy diaphram problem probably accounts for my uneven tickover?

Doug, looked on Ebay etc and can't find any OE spec parts if the diaphragm from Burlen doesn't work can I purchase two from you? PM me if it's possible.

Cheers

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like iain I brought 2 new aftermarket diaphragms and was surprised how thick they were and they haven't settled smoothly without creases, but my pistons drop and stay on the bridge I haven't started the engine yet to see how they perform.

Like Iain I've tossed the old ones they were gossamer thin and no tears, Bummer!.

I too will be looking for OE to replace again, I brought the thick ones from a reputable supplier, I think having to pay the AUS Govt GST & import tax a second time pi++++ me off just as much!

There's no QC on aftermarket these days

Peter T

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