Carburettor bypass diaphragm

[gbb483]

Prior to pulling my carbs apart I've been watching all the videos on servicing them. One mentions the bypass diaphragm. This does not seem to be part of the servicing kit (I'm getting the one from Robsport via ebay though it hasn't arrived yet). A web search doesn't appear to show anyone that supplies it.

I guess I am putting the wrong words into my search - can anyone help me find a pair?

[NeilR]

What is a bypass diaphragm & where does it fit ?. I have never seen one in 50 years of Strombergs on various cars.

[gbb483]

It allows a small quantity of air/fuel to 'bypass' the throttle butterfly when the throttle is closed but the engine is still going full chat. Once the engine slows and the manifold vacuum drops it stops operating. Supposed to stop back fires.

[DJT]

Check the videos posted on YouTube by church house classics. There are a few on Strombergs and he knows what is what. I've done a few Stag carbs myself and the only diaphragms are the main ones on the air valves (pistons). Later Strombergs did have poppet valves in the throttle discs, but these are often soldered up as they are more trouble than they are worth.

[jbuckl]

There are bypass diaphragms and bypass valves on strombergs.

Uk stags did not have bypass valves with diaphragms.

99.5% + that your stag won’t need a bypass diaphragm, unless it’s had a non U.K. stag stromberg fitted.

the throttle discs sometimes had anti pop valves fitted to some (early) cars. These maybe called bypass valves by some.

the kits do not have these type of discs supplied either.

[gbb483]

OK thanks. It was the video by Midwest motoring. But like a lot of companies with a web presence, there is no indication of where in the world they are.

[trunt]

Yes, probably Federal, Midwest is an area of USA. if you go to the technical guides there is a BL document on all of the emissions gizmos on the carbs including those that made it to Uk cars. Page 26 describes the bypass valves.

https://socforum.com/forum/forum/stag-owners-club/technical-section/fuel-and-exhaust/776922-1973-triumph-emissions-control-systems-training-pdf

I have blocked mine as I found that it makes the revs slow to settle down to idle and with my manual I find it a bit disconcerting.

[trunt]

Hopefully you found the Buckeye Triumphs site? its USA but a good guide. covers that pesky vent valve.

https://www.buckeyetriumphs.org/carburetors

[gbb483]

Having now received the kit, I've been checking the components to work out what goes where. There is a pair of O rings, slightly smaller than the O rings for the float chamber bung and a pair of fibre washers of a similar size which I cannot fathom where they go. I am assuming that the other 2 pairs of fibre washers are for the temperature compensator. There also appears to be a pair of aluminium washers for the fuel cut-off valves, which seems strange. If anyone has used the kit can they enlighten me?

[Rigid]

You need O-rings in the needles - there will be 2 of them in a service kit.

[sujitroy]

I have a few carbs sets. here in USA. I too have blocked off the bypass valves. I did buy some replacement diaphragms. they were not cheap

[gbb483]

You need O-rings in the needles - there will be 2 of them in a service kit.

Yes, I have them, but they are quite small - nowhere near the size of the float chamber bung, which is the size of the 2 spare O rings I have.

[APS]

2 o rings for float chamber bung
2 small o rings for the metering needle
2 alloy washers for the needle valves
the other 2 o rings are I believe for the top of the dash pot plunger, the part that screws into the top carb casing.

[gbb483]

Today I bit the bullet and pulled the carbs apart. The left hand carb had a) the floats sitting too high b) the needle covered in a green deposit (scrapes off with a finger nail) c) the orifice for the needle sitting proud of the threshold and d) the star washer to hold the adjustment screw missing (there appeared to be a second O ring to hold it in place!). The right hand carb had a) the fuel cut off valve stuck shut and as the other one b) the star washer to hold the adjustment screw missing (also with a second O ring). How the car ever ran beats me.

[jbuckl]

Today I bit the bullet and pulled the carbs apart. The left hand carb had a) the floats sitting too high b) the needle covered in a green deposit (scrapes off with a finger nail) c) the orifice for the needle sitting proud of the threshold and d) the star washer to hold the adjustment screw missing (there appeared to be a second O ring to hold it in place!). The right hand carb had a) the fuel cut off valve stuck shut and as the other one b) the star washer to hold the adjustment screw missing (also with a second O ring). How the car ever ran beats me.

IMHO the carbs can be quite often rebuilt without the full zenith kit… unless there are leaks at the throttle spindles.

These days many strombergs do have air leaks at the spindles though.

Failure to address throttle spindle leaks is causing people to ditch strombergs for other ‘optiona’ that might not be necessary.