Triumph herald Stromberg 150cd

[Martin mon]

Hi everyone,

Just took top off my 1969 herald 150cd Stromberg carb and noticed there is no piston spring fitted. Got myself in a muddle now as haynes manual does not show one!!! Am I right to say it's missing? The engine will not idle without choke!!

Any help appreciated

Cheers Martin

[Pete Lewis]

guess this is a 13/60   

there is no damper spring the diaphragm retaining ring may be a heavier thicker than other models 

please check the diaphragm has no splits , give it a light stretch   a wash with petrol will return any flabby wrinkled to as new shape 

make sure the location lugs on the diaphragm locate in the  relief in the body ,there are some on the inner hole as well   this ensures the air piston has two holes in its base do face the engine  not anywhere else   when its  assembled does the air piston drop  freely with a clunk on the bridge ??  (with dashpot removed)

if it wont idle without choke is this just the idle is set to slow or you need to alter the basic mixture ???   

is it lumpy and misfires or goes all wooly and stalls ??

what air cleaners are fitted  std or pancakes 

Pete

[Martin mon]

Hi Pete,

Yea it's a 13/60. Thank you for clarifying it does not need a spring really appreciate that. The piston moved fine although there was a bit of build-up on the needle which I rubbed off gently with wd40. The diaphragm was fine but there is a bit of a DIY job done on a pan filter which has the breather hose from the rocker fixed to the inlet filter at the top (I am worried that condensation will get into carb when engine cools off).

Only had the herald a week and it's been stood a while so step by step checks I suppose. Thanks for your help Pete

 

 

 

[NonMember]

4 minutes ago, Martin mon said:

there is a bit of a DIY job done on a pan filter which has the breather hose from the rocker fixed to the inlet filter at the top

I'm not quite sure what you mean here. If you just mean there's a rubber hose from the rocker cover stub pipe to a stub pipe on the air filter housing, that's original fitment as per the lower image here.

On the idling issue, try nudging up the idle speed (half a turn on the throttle stop screw) first. With the idle speed thus elevated you can set the mixture much more easily, then reduce the idle to give it a final fine trim. You do need to warm the engine up properly (drive it a few miles!) before you can accurately set the mixture and idle.

[Martin mon]

Hi sorry, yes it's an after market k&n style filter and a mesh has been screwed to the filter with the rocker pipe slotting into the mesh. I was just worried water may drop into the carb when cooling down after a run. Will probably just get an original filter box put on as the pipe is offset correct.

[PeteH]

The 13/60 "Stanpart" spares catalogue shows no Spring Either. My understanding is the spring was necessary to delay the rise of the piston under hard accelleration?. Allowing a richer mixture?. If that is correct?, how/what controls the rise. A heavier diaphram?.

Pete

[johny]

I think its the oil damping that slows up the rise of the piston when accelerating and the spring is to ensure it comes back down smartly....

[NonMember]

Given the orientation of the carb, gravity will return the piston. The spring just helps. If you have a spring, the piston sits a little lower for a given air flow, so you need a different needle profile. So it was part of the mix-and-match to get the right mixture behaviour, and some models did have springs but some didn't.

[Pete Lewis]

agree the spring is used to balance the fuel ratio /needle position for given vacuums   others that dont have a spring is the 1600

if you fitted a spring  it will richen the mixture by lowering the air piston but this increases the air velocity over the jet and out comes more fuel

with pancackes you weaken the mixture as you lower the intake  vacuum and with this you need to either add a spring  change the needle profie

or fit a std air filter and paper element 

it is the dashpot damping that slows the ait pistons rapid rise and act as the accelerator pump to give a burst of richness when accelerating

its nothing to do with the spring that aids general fuel ratio under normal running 

use a engine oil in the dash pot   any thin oil will not dampen the rise  and you get a flat spot when you open the throttles 

Pete

 

[Peter Truman]

Pete thanks for the thorough easy to understand explanation, why do some Strombergs use a spring and others a bolt on heavier weighted annulus on the piston, but not both.

Ref your comment re use engine oil I've always used auto trans as the engine oil looked a little heavy compared to the Penrite special SU dashpot oil.

I need a couple of springs for my Spit 1500 dual 150CD carb conversion, who has the largest range & any idea which spring to start with, I currently have the extra weighted pistons.

[Colin Lindsay]

Burlen have a good selection but the list does not explain which one you need for which carb; so you have to peruse the site a bit.

http://zenithcarb.co.uk/cd-piston-springs.html

A good article on adjusting Strombergs here, it might give Martin a few ideas:

https://www.howacarworks.com/fuel-systems/adjusting-a-stromberg-carburettor

[Pete Lewis]

12 hours ago, Peter Truman said:

why do some Strombergs use a spring and others a bolt on heavier

all down to getting a balance fuel ratio across the throttle range  a weight is constant  a spring has a hysteresis so its varies  

strombergs /triumph use engine oil   SU says straight 20   but  engine oil is temperature/viscosity supposedly stable  ???

what ever you use it must damp the rise or acceleration will be flattened 

1500 looks as  std  needle ABT  with two spring choice 

https://www.canleyclassics.com/?catalogue=triumph-spitfire-mkiv/1500&diagram=triumph-spitfire-mkiv/1500-carburettor-service-kits-twin-carb-engine

Pete

 

[Martin mon]

Hi all, just want to say a very big thanks for all the help and advise. In the end I went through everything again, checking plug gaps (engine was running rich), cleaning HT leads and dizzy cap, then carb diaphragm position and condition, re-centralised the needle and set the mixture 3 turns off top for basic set up. Put original air filter on too. Harold the herald (named by the kids) fired up 1st turn of the key and with some slight adjustments when up to temp sounds perfect. Thanks guys 😊