Clementine's Garage
Clementine the Cat
 
Image of flower
Yellow R4
 
Réparateur d'automobiles

Carb not happy

Pete Isom

Enthusiast
Messages
179
Location
Raunds, Northamptonshire
#1
Guys one of my snags is to investigate the carb. I think is a zenith 28 that’s fitted

I don’t think it’s running right
She won’t run until warmed up. Black smoke on choke. But feels like lean when running. It was like it before the resto which I put down to a leaking valve. But full engine rebuild. Electronic points etc later means that’s unlikely.

I’ve read the zenith can be a pain. Any one recognise my symptoms

Worth getting a Webber fitted?

Pace seems to me slow too. Like it doesn’t want to go

Anyone got any blatant ideas?
 
Messages
321
#2
It's a bit difficult to diagnose without seeing the car, but I'll have a go. Assuming the timing is correct (the electronic ignition system may still not be correct...) the mixture seems rich when the choke is out. Is the air filter clean and is the winter/summer adjuster working properly? If this is clean and working correctly I think the choke butterfly might not be working properly, causing the engine to run excessively rich. The slow running may well be because the carburettor is running lean and this can easily be adjusted by finding the mixture screw and enriching the mixture by screwing the mixture screw outwards slightly. A good rule of thumb is to screw it gently in, until it can go in no further, then screw it OUT about 2 to 2 1/2 turns. A more accurate method involves using a gas analyser, but that's another matter altogether! Is the engine ticking over at the right speed once it's warm?

I've mentioned the Carburettor Centre in Leighton Buzzard on this forum before. You're not too far away in Northampton, so it might be worth a try, but I'd suggest having a go yourself first. I have both an exhaust CO analyser, stroboscopic timing light & a dwell angle meter, so if you're unsure how to proceed, PM me for more advice if you want!
 

Pete Isom

Enthusiast
Messages
179
Location
Raunds, Northamptonshire
#3
It's a bit difficult to diagnose without seeing the car, but I'll have a go. Assuming the timing is correct (the electronic ignition system may still not be correct...) the mixture seems rich when the choke is out. Is the air filter clean and is the winter/summer adjuster working properly? If this is clean and working correctly I think the choke butterfly might not be working properly, causing the engine to run excessively rich. The slow running may well be because the carburettor is running lean and this can easily be adjusted by finding the mixture screw and enriching the mixture by screwing the mixture screw outwards slightly. A good rule of thumb is to screw it gently in, until it can go in no further, then screw it OUT about 2 to 2 1/2 turns. A more accurate method involves using a gas analyser, but that's another matter altogether! Is the engine ticking over at the right speed once it's warm?

I've mentioned the Carburettor Centre in Leighton Buzzard on this forum before. You're not too far away in Northampton, so it might be worth a try, but I'd suggest having a go yourself first. I have both an exhaust CO analyser, stroboscopic timing light & a dwell angle meter, so if you're unsure how to proceed, PM me for more advice if you want!
Thanks. Only just had a bit and bolt resto. Carb was cleaned but no new parts. Engine was rebuilt.

Your right. It could be as simple as timing.

It was like it before the rebuild.

I have to say I thought the mot emissions were low at 0.6 which is why I’m drawn to the carb
 
Messages
321
#4
FAR too low for a non-catalyst car! The going rate's about 3-4.5% CO. You could damage engine valves running at that weak a mixture. Turn the mixture screw out 1 turn & see if it feels happier. Then another turn. Don't go mad and drive it a few miles each time, along the same route if you can, to note any difference. Good luck......
 

Pete Isom

Enthusiast
Messages
179
Location
Raunds, Northamptonshire
#8
The carb looks quite basic.looks like two screws. One for idle speed the other for mixture?
 

malcolm

& Clementine the Cat
Staff member
Messages
4,098
Location
Bedford UK
#9
The vent hose that goes from the T piece to the manifold should have a plastic restrictor inside with a 1mm hole. Without that the carb will run way too weak. You could plug the hose completely for a test.

The mixture screw is the one in the carb body. 1.5 to 2 turns out from fully in is a good starting point.
 

harbourseal

Renault R4 Lunatic
Messages
809
Location
Breda The Netherlands
#16
If you replace the hose and put the restriction back in. You have a good base to start tweeking.
 

Pete Isom

Enthusiast
Messages
179
Location
Raunds, Northamptonshire
#17
There is something inside the hose at the manifold end the picture may not be clear. But it’s bigger than 1mm
 

harbourseal

Renault R4 Lunatic
Messages
809
Location
Breda The Netherlands
#18
The actual hole in the restriction is 1.3mm

You can make one yourself.
Take a 8mm bolt file the tread of and make it fit snugly in the new hose. Then shorten it of to about 6mm an dril a hole of 1.3mm down the middle.

And presto there is a new restriction.
 

Melissa

Enthusiast
Messages
131
Location
France The Lot
#20
Does anyone have a hose with restrictor fitted. And a known good carb?
I don't know as much as most on here but I did have a problem on my carb, someone had screwed in the mixture screw in too much & the fine tip had broken off inside the carb body, mixture then way too rich, worth checking this wee hole is clear when you look at the carb, I maneged to push out the brass plug & then the tip & fit a new one.
 
Top