New carburettor?

[rosie]

pm incoming

Hey James,
I’ve sent you a couple of PMs but they’re stuck in my outbox for some reason. Is your inbox full maybe?

[rosie]

Looks like there’s a problem with my sent message folder so I can’t PM you. Could you maybe PM me with your email address please?

[mark1gls]

I know the message stays in the outbox until they read you message so he may not have read you message yet?

[rosie]

Ah ok, thanks Mark.
For some reason my sent folder says it has no messages in it, but is also 98% full, so I thought there was something amiss.

[james butler]

apologies rosie i have been really busy of late and have read your messages
please have a look and give me a shout and let me know what you think
regards
james

[rosie]

So… fitted the new custom carb flange replacement last weekend (shout out to James Butler for going above and beyond and patiently walking me through it).
Unfortunately it hasn’t fixed the original problem. The car is still stalling when I brake. It runs ok for 10 minutes or so, then starts playing up. The revs drop sharply when I brake/take my foot off the throttle. Sometimes they slowly rise back up again, sometimes it stalls. The revs fluctuate rather than a steady idle. I have to pull the choke out to keep it from repeatedly stalling when I’m in slow moving traffic.

So it looks as though the flange wasn’t the culprit this time. What else do I need to look at? Fuel pump? Something else?

There seems to be a couple of hoses and clips missing. Could these be having an impact? The two clips on top of the airbox (shown in red) are missing (side clips are intact). Anyone know the parts number for those?
The vacuum hose (?) in the picture is connected wrong - the green end is connected directly to the back of the carb and bypasses the airbox. I need to get some hose to connect via the airbox. Is 3mm vacuum hose suitable for this?

[rosie]

I thought that the missing airbox bits might be responsible it there’s too much air in the mix. But maybe that’s a red herring.

(Incidentally, ignore James’ link to the BMW flange upthread as we discovered it isn’t compatible)

[GT_II]

Does the Weber carb have a bypass jet for idle control? If so, the symptoms might suggest a partly blocked bypass jet.

The device on the airbox which is currently bypassed by the vacuum hose is a thermal switch to control whether vacuum reaches the flap in the air box intake, in order to determine whether the air box takes pre-heated air from the exhaust manifold or cold air from the front of the car. I haven't figured out in which direction it operates, so with the thermal switch bypassed, the carb would either be getting permanently heated or permanently cold air, which could cause running issues at particular temperatures.

The parts diagram suggests the braided vacuum hose is 3.5mm - N0202911

The part number for the air box clips is shown as 030129614. An air leak here may have a subtle effect during warm-up (once the thermal switch is back in the vacuum circuit), but I'd be more concerned about dust bypassing the air filter if the airbox isn't sealed properly.

[rosie]

Thank you.
That’s interesting regarding the possibility of temperature being a factor. It does appear to run ok when cold, and get much worse once the engine is warm, so maybe the missing hose is implicated.
I guess I’ll start with these easy fixes and see if it makes any difference.

[GT_II]

The reason I mentioned the bypass jet (assuming the Weber has one), is that the throttle flap would probably be designed to be fully closed when the engine is idling and the choke is off. The bypass air and fuel jets allow the idle mixture to be controlled more precisely in order to meet the carbon monoxide limit required for the MOT emissions test, which is done at idle speed. If a jet is partly blocked (maybe just some grit), that would affect the mixture, and could cause the engine to stall at idle. When you pull the choke out, as well as reducing the air supply to the carb to enrich the mixture, the throttle valve opens up slightly, depending on how far you pull it out, to increase the idle speed during warm-up.

[rosie]

I’ll have to do a bit of research re: bypass jet. My carb knowledge doesn’t extend that far! Thank you for the suggestions.

[james butler]

The reason I mentioned the bypass jet (assuming the Weber has one), is that the throttle flap would probably be designed to be fully closed when the engine is idling and the choke is off. The bypass air and fuel jets allow the idle mixture to be controlled more precisely in order to meet the carbon monoxide limit required for the MOT emissions test, which is done at idle speed. If a jet is partly blocked (maybe just some grit), that would affect the mixture, and could cause the engine to stall at idle. When you pull the choke out, as well as reducing the air supply to the carb to enrich the mixture, the throttle valve opens up slightly, depending on how far you pull it out, to increase the idle speed during warm-up.

there is no bypass valve as the throttle control screw on the front of the carb holds the valve open slightly to maintain idle
when this screw is wound all the way out the engine should stall.
but i do agree it sounds like a partial vacuum leak somewhere else or potentially contamination in the carb maybe blocking a jet or idle mix screw
defo go back to basics

[james butler]

out of curiosity are you saying it will stall when you press the brakes? im wondering if the brake servo could be compromised?
quick way to test is to remove the servo vac pipe and see if the same problem exists

[GT_II]

Taking out my old filter for service, I noticed there was a trail of fine black grit along the intake side of the filter seal, so I'm guessing that this is the kind of stuff that potentially can get sucked into carb jets if the air box isn't clipped up securely, though the side clips on their own seem to do a decent enough job. Looking at the parts list for the Weber 32/34 DMTL carb here:

https://www.carbparts.eu/exploded_view/WEBER_32-34_DMTL

there is an idle jet, which I called a bypass jet in my earlier post, so that would be worth checking for debris I think, assuming I have picked out the right Weber carb model.

[james butler]

Taking out my old filter for service, I noticed there was a trail of fine black grit along the intake side of the filter seal, so I'm guessing that this is the kind of stuff that potentially can get sucked into carb jets if the air box isn't clipped up securely, though the side clips on their own seem to do a decent enough job. Looking at the parts list for the Weber 32/34 DMTL carb here:

https://www.carbparts.eu/exploded_view/WEBER_32-34_DMTL

there is an idle jet, which I called a bypass jet in my earlier post, so that would be worth checking for debris I think, assuming I have picked out the right Weber carb model.

idle screw jet is a good shout as its such a tiny passage muck could lodge in there and cause some issue especially when off choke
but retuning would be required after doing so but if your competent enough to give it a bash then why not