Joined: Jun 2020 Posts: 7 Threads: 3
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Location: Surrey
Car type: 1927 Chummy
08-12-2024, 11:46 PM
(This post was last modified: 09-12-2024, 01:36 PM by akharding.
Edit Reason: typo
)
I am helping someone sell her late father's 28 Chummy. The car was running fine about 6 weeks ago (admittedly in milder weather) but when I tried to start it yesterday it started ok but would only keep running with prolonged use of the choke It has a bronze Zenith carburettor. Even when warm the engine always wants to die if revs are allowed to drop below c 1500 and has to be caught by using the choke . I have had the two jets out which were not blocked and I have checked for obvious air leaks.
What have I missed - it must be something simple given that it was going well so recently. and has not been touched.
Thanks in advance - Andrew
Joined: Aug 2017 Posts: 60 Threads: 4
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Location: Northland New Zealand
Do you have good fuel flow to it. I had muck from the tank causing a semi blockage at the tap.
Joined: Aug 2017 Posts: 778 Threads: 33
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Location: Beautiful Northumberland
Car type: 1933 RP Saloon (aka Mildred)
Old fuel? It doesn't take long these days for some fuels to tip over the edge.
Perhaps was just OK last time and not now. At least easy to rule out with a fresh bowl of fuel.
Enjoy yourself, it's later than you think!
Joined: Jun 2020 Posts: 7 Threads: 3
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Location: Surrey
Car type: 1927 Chummy
09-12-2024, 01:34 PM
(This post was last modified: 09-12-2024, 01:35 PM by akharding.)
Will double check - thanks
Worth trying - fuel is only 3months old but I know it goes off quickly now. Thanks
Joined: Aug 2017 Posts: 1,474 Threads: 26
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Location: North Yorkshire
You say you've had 'the two jets out'. Do you mean the main and compensating jets to the underside of the carb? If so, did you also remove the slow running tube, split it and clean the jet inside? As the name suggests, if that small jet inside the slow running tube is blocked then it will spoil the slow running. First thing to check particularly if an engine that was running OK won't tick over after being left for a period.
Steve
Joined: Aug 2017 Posts: 1,474 Threads: 26
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Location: North Yorkshire
Until you've ensured that the slow running jet is clear (inside the pointy bit to the right in Dave's photo) trying anything else is approaching the problem from the wrong direction. I agree with Dave's comments about old petrol and 22FZ carbs.
Steve
Joined: Jun 2020 Posts: 7 Threads: 3
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Location: Surrey
Car type: 1927 Chummy
Yes I only removed the main and compensating jets and blew through them, I didnt remove the slow running tube or split it. Does sound as if it could be the problem. Will try and report back.
Thanks guys.
Joined: Aug 2017 Posts: 622 Threads: 19
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Location: Hampshire UK
Hi
At idle there is pressure drop across the nearly closed throttle plate, giving a partial vacuum downstream of the plate. This "sucks" fuel from the idle drilling on the engine side of the plate via the slow running jet. This idle circuit is the only fuel supply when idling.
At higher speeds, the increased airflow through the venturi (restricted cross section) upstream of the throttle causes its own small depression, and at a certain airflow this is enough to "suck" fuel from the main jet, the discharge point being a few mm above the float chamber level.
With careful design, the idle circuit and the main circuit "hand over" seamlessly from one to the other.
If you use the cold start strangler, the pressure drop across the strangler artificially increases the depression on the main jet and allows it to function even at low airflow, i.e. below its normal operational range. This seems to be what you need to do to keep the engine running and suggests the idle circuit is not doing its job.
As suggested above, check the idle (slow running) jet is clear. Being the smallest jet it is the most likely to succumb to blockage.
