DCOEs- one cylinder getting little/no fuel below 3000RPM

[HustleRussell]

I have a four cylinder engine fitted with twin 40 DCOEs. Yesterday it was suddenly running on 3 cylinders, with the fourth not firing at all. It works normally above 3000RPM. Am I looking for a blocked jet? Other suggestions?

[jagracer]

Check the spark plug first you may have flooded it and the plug could be soaking wet. Take it out, clean it and heat it slightly over a gas hob if you have one before you put it back. if that fails try cleaning the idle jets. What filters do you have?

[HustleRussell]

Thanks for the reply Jagracer.
This is the set-up in question:



I too suspected the spark plug, so I removed it last night to have a look. Cylinders 2,3 & 4 are the same rich slightly oily black, cylinder 1 is bone dry and burned white, which is what caused me to suspect a fuelling issue. I am going to replace the spark plugs today anyway but I thought that because it's dry and I'm getting a lot of spitting back at the carb, lack of fuel could quite possibly be the problem.
FYI the carbs were cleaned not too long ago, and since then it's had a new fuel pump.

[stevieturbo]

Doesnt matter when they were cleaned out. And you dont answer as to whether there is a filter or not ?

If it's below 3k, it's highly unlikely to be a main jet. Check idle jets and progression holes etc. They must be clogged somehow.

[jagracer]

It could be one of a myriad of things. If they've been cleaned out out then check for a blocked idle jet. Spitting back can be a balance issue, that cylinder may be out of balance, check the idle mixture on that one.

[HustleRussell]

Stevieturbo- I assumed Jagracer was talking about air filters. I discounted fuel filters as a cause because three cylinders are fine.
I have just bought a set of plugs and will remove and check the idle jets after the F1.

[jagracer]

I was talking about air filters, I once used some foam dual socks and they had a bit of foam glued in to support the centre which came unglued and sucked into one choke although that wouldn't account for it running OK over 3K. One other thing to check, although I doubt it's the problem here, is the exhaust manifold for holes or cracks. First thing after the plugs would be to check idle screw for adjustment, idle jet then progression holes.

[Simon says]

stevieturbo said:

Doesnt matter when they were cleaned out. And you dont answer as to whether there is a filter or not ?

If it's below 3k, it's highly unlikely to be a main jet. Check idle jets and progression holes etc. They must be clogged somehow.

< Agree with this, Weber pilot/idle jets have small orifices that block easily at the slightest hint of dirt or debris a decent fuel filter is essential running carbs in my opinion also check that any lead plugs/bungs for the fuel passage drilling's are intact especially on the choke in question, I have seen many a Weber lose one of these causing peculiar running problems and head scratching ie instead of drawing fuel through the passage the motor will just draw air from the missing leaded bung hole rather than a jet or metering hole

Edited by Simon says on Sunday 25th March 21:13

[jagracer]

One other thing to check could be float levels, if one is slightly high it may flood just one choke depending on how level the engine is.

[HustleRussell]

Jagracer got it in one (sort of...) a change of plugs had the engine running on all cylinders. Unfortunately the spitting back had blown one of the 'O' rings out of the mounting flange, but once that was sorted and the carbs were re-fitted and balanced the engine ran fine.

[jagracer]

Glad to hear you've sorted it.

[Simon says]

Top man, bet it sounds sweet on those DCOE's